Thursday, January 31, 2013

Ever since Darwin, evolutionary biologists have thought and written (and argued about) the evolution of cooperation. In the 20th century, at least three different major theories were proposed to explain how cooperation (and especially unselfish altruism) could evolve by natural selection. Several attempts to unify these theories have been made and are currently a topic of intense debate.

Now, as part of Cornell's Darwin Days celebration, you are invited to come to a dinner discussion to hear about, think about, talk about, and (hopefully) argue about these theories and their implications for human behavior, ethics, and philosophy.

PRESENTER: Allen MacNeill, Senior Lecturer in Biology and Evolution at Cornell and author of Evolutionary Biology and Evolutionary Psychology

SPONSORS: Liebermania! and the Cornell Human Ethology Forum (this will also be the organizational meeting for CHEF)

DAY, TIME, & LOCATION: Thursday 14 February 2013 at 6:00 PM at Risley Dining/Tammany at Cornell University. This event is part of Cornell and PRI/MOTE's Darwin Day 2013 celebration.

Please contact Allen MacNeill at by Tuesday 12 February 2013 if you plan to attend this event. Hope to see you there!

Thursday, March 17, 2011

Evolutionary Psychology: The Science of Human Nature

Having been tickled by Google Alert that my name had been mentioned in the comments at Pharyngula (P. Z. Myer's blog), I took a quick look. Just a few comments for now:

1) I became an evolutionary psychologist when studying the behavioral ecology of Microtus pennsylvanicus got boring. Those cute little field voles got boring because their ethology is relatively simple. Human ethology is a lot more interesting, mostly because it is a lot more complex. Should we not try to study it because it is more complex? Or because it might not jibe with some people's political preconceptions?

2) I assign Gould & Lewontin's "spandrels" paper to my students in evolutionary biology, along with various criticisms of it. I also assign Eldredge & Gould's "punk eek" paper and Gould and Vrba's "exaptation" paper (along with close to three dozen others, not to mention the entire Origin of Species, 1st. ed.). I also give them chunks of George William's 1966 classic, Adaptation and Natural Selection, so that they will know exactly how "onerous" the concept of "adaptation" actually is.

3) Here's the definition of "adaptation" I use:
An evolutionary adaptation is any heritable phenotypic character whose frequency of appearance in a population is the result of increased reproductive success relative to alternative versions of that heritable phenotypic character.
4) Here are the criteria I believe are most useful when one is attempting to determine if one is dealing with an "adaptation":
Qualification 1: An evolutionary adaptation will be expressed by most of the members of a given population, in a pattern that approximates a normal distribution;

Qualification 2: An evolutionary adaptation can be correlated with underlying anatomical and physiological structures, which constitute the efficient (or proximate) cause of the evolution of the adaptation;

Qualification 3: An evolutionary adaptation can be correlated with a pre-existing evolutionary environment of adaptation (EEA), the circumstances of which can then be correlated with differential survival and reproduction; and

Qualification 4: An evolutionary adaptation can be correlated with the presence and expression of an underlying gene or gene complex, which directly or indirectly causes and influences the expression of the phenotypic trait that constitutes the adaptation.
To me, it seems reasonable that if one can apply those to a specific human behavior, one can make arguments about its evolutionary derivation. Would anyone disagree?

As for the ridiculous idea that evolutionary psychology only deals with sex, has anyone making such a claim actually read a textbook on the subject? Here are several:

Human Evolutionary Psychology

Evolutionary Psychology: The New Science of the Mind (4th Edition)

Evolution and Human Behavior, 2nd Edition: Darwinian Perspectives on Human Nature

Evolutionary Psychology: The Science of Human Nature

[Full Disclosure Notice: The fourth title is indeed by Yours Truly.]

If you haven't, then please do so, and then we can discuss these questions.

While we're on the subject, Part II of Evolutionary Psychology: The Science of Human Nature (on the ethology of between-group behavior in humans) is coming out in May. My next project is an introductory textbook in evolutionary biology, entitled Evolutionary Biology: The Darwinian Revolutions, again in two parts. Part I (due out in September) is The Modern Synthesis and Part II (due out next May) is The Evolving Synthesis.

After that (if I live that long) will be On Purpose: The Evolution of Design by Means of Natural Selection (won't there be some fireworks when that comes out?), in which I present one of the core arguments for The Metaphysical Foundations of the Biological Sciences, in the spirit of E. A. Burtt's The Metaphysical Foundations of Modern Physical Science. Should be fun!


As always, comments, criticisms, and suggestions are warmly welcomed!


Thursday, March 26, 2009

The Modern Evolutionary Synthesis


Darwin was largely successful in convincing scientists that evolution had occurred, but less successful in convincing them that the primary mechanism for evolution was natural selection. This was because natural selection absolutely requires continuous variation within populations, but the theory of “blended inheritance” that was common prior to 1900 strongly implied that such variations would eventually disappear.

The rediscovery of Mendel’s theory of non-blending inheritance paradoxically made Darwin’s theory seem even less likely, as geneticists at the turn of the century believed that evolution occurred via mutation, rather than natural selection. However, the founders of theoretical population genetics eventually united Darwinian evolutionary theory and Mendelian genetics in what is now known as the “modern evolutionary synthesis.” In particular, the mathematical principles upon which the “modern synthesis” was based transformed evolutionary theory into a rigorous and testable natural science. Ever since the formulation of the “modern synthesis,” this is how evolutionary biology has proceeded: by an alternation between the formulation of theoretical mathematical models and rigorous, naturalistic field and laboratory studies.


In the years following the publication of the Origin of Species in 1859, Darwin’s theory of evolution became widely accepted throughout most of the scientific community. Other naturalists, including such “leading lights” as Charles Lyell, Joseph Hooker, Asa Grey, and especially Thomas Henry Huxley quickly came to accept Darwin’s assertion that what he called “descent with modification” had in fact occurred.

However, scientific opinion was much more divided on the subject of natural selection, Darwin’s proposed mechanism for evolution. To understand why, let’s quickly review the three preconditions Darwin proposed as the necessary prerequisites for natural selection. They are:

• Variation between the members of populations: These variations need not be extreme, as illustrated by the relatively large changes that animal and plant breeders have accomplished, using relatively slight differences in physical appearance and behavior.

• Inheritance: The distinct variations noted above must be heritable from parents to offspring.

• Fecundity: Living organisms have a tendency to produce more offspring than can possibly survive. Among those individuals that do survive, those that also reproduce pass on to their offspring whatever characteristics made it possible for them to survive and reproduce.

Given these prerequisites, then the natural outcome is:

• Non-Random, Unequal Survival and Reproduction: Survival and reproduction are almost never random. Individuals survive and successfully reproduce because of their characteristics. It is these characteristics which form the basis for evolutionary adaptations.

Considering these four ideas, we can ask the question, “What is the ultimate source of the new characteristics that are preserved and promulgated from generation to generation?” The answer is, “The ultimate source of all new characteristics is the ‘engines of variation’ – that is, those processes that produce the natural variation between individuals that Darwin emphasized as being absolutely necessary for the operation of natural selection". In a nutshell:
Variation between individuals is the key to evolution by natural selection.

However, in the Origin, Darwin summarized his presentation of his views on variation with this statement:
"Our ignorance of the laws of variation is profound."

Neither Darwin nor any of his contemporaries (that he knew of) had a coherent theory of heredity or variation. However, this was not an insuperable obstacle to Darwin. Instead of giving up his argument, he simply accepted as a given that many important traits of animals and plants are heritable (pointing again to the observable facts of inheritance in domesticated animals and plants). He also proposed that, although he had no explanation of how they arose, variations among the members of a species do indeed occur, and can provide the raw material for natural selection.

There were therefore two reasons why Darwin’s proposed mechanism of natural selection was not widely accepted, even among scientists:

• Many of Darwin's contemporaries (and, in fact, Darwin himself) believed in Lamark's assertion that acquired characteristics could be inherited through use and disuse. This process directly contradicts the blind and purposeless process of natural selection, and therefore held the door open for purpose in evolution.

• The consensus among naturalists was that inheritance worked by "blending" the characteristics of parents, which would cause any incipient adaptations to be diluted out of existence.

This second objection to Darwin's mechanism of natural selection was almost fatal to his theory. In an influential review of the Origin, written in 1867 by Fleeming Jenkin (a very well-respected English engineer and designer of the first trans-Atlantic telegraph cable), Jenkin pointed out that blending inheritance would eliminate variation within a few generations:
“However slow the rate of variation might be, even though it were only one part in a thousand per twenty or two thousand generations, yet if it were constant or erratic we might believe that, in untold time, it would lead to untold distance; but if in every case we find that deviation from an average individual can be rapidly effected at first, and that the rate of deviation steadily diminishes till it reaches an almost imperceptible amount, then we are as much entitled to assume a limit to the possible deviation as we are to the progress of a cannon-ball from a knowledge of the law of diminution in its speed.”

If (as most naturalists of Darwin's time believed) all traits were blended from generation to generation, all of the distinctiveness of each variation would be lost and the population would remain essentially unchanged. Darwin got around this objection by proposing that large numbers of new variations (i.e. mutations) occur with each new generation. He called these “continuous variations,” but did not propose a mechanism for how they might be produced.

Mendelian Genetics

However, at about the same time that Darwin was working out his ideas on natural selection and evolution, Gregor Mendel was working out a revolutionary new theory of genetics. Mendel was born in 1822 in Moravia, a province of the Austrian Empire (now part of the Czech Republic). Because he was a peasant's son, Mendel was expected to return to the family farm after finishing his education. However, Mendel was not satisfied with all that he had learned. The university, instead of answering his questions, instilled in him an insatiable curiosity about nature.

Mendel observed that some offspring of some organisms had traits that were similar to only one parent, rather than being intermediate between both. He explained this phenomenon by assuming that heredity was determined by tiny, discrete “particles of inheritance” that were passed from the parents to the offspring via the reproductive cells. This would explain how some traits could remain unblended in the next generation.

Such thinking stemmed from Mendel's physics training. In physics, all of nature is considered to be subject to laws based on the existence of and interactions between small, indestructible particles of matter. The goal of a physicist is to learn about the laws that determine the behavior of the particles. An investigator can sometimes work out these laws through careful experimentation. Mendel believed that these same methods could be used to study inheritance in living things.

In his paper, "Experiments in Plant Hybridization” ("Versuche über Pflanzen-hybriden"), published in 1866, Mendel tells how he used the garden pea plant to study the laws of heredity. His techniques differed from those of other investigators in three ways:

(1) Mendel looked at one trait at a time;

(2) He followed this trait from generation to generation over eight years; and

(3) He used larger numbers of organisms in his studies. At the end of his experiments, he had carefully observed over 12,000 plants.

In his most famous set of experiments, Mendel studied 22 varieties of plants of the same species: the common garden pea. He studied a total of seven different traits, each with two alternative forms, including seed shape, color, and seed coat color; pod shape and color, flower position on the stem, and stem height. For example, in one series of experiments, Mendel crossed pea plants that produced round seeds with pea plants that produced wrinkled seeds, and then observed what kinds of seeds were produced as the result of this cross over two generations.

Mendel observed that the two forms of each of these traits did not blend with each other. Among the offspring of the first cross, only one form of each trait showed up; the alternative form seemed to be lost. For example, when peas with round seeds were crossed with peas with wrinkled seeds, the first generation of offspring only produced round seeds.

However, in the second generation, the seemingly lost form showed up again. In our previous example, wrinkled seeds showed up again in the second generation of offspring, comprising approximately one-fourth of all of the offspring of that cross. Mendel explained this result by saying that the lost form of each trait was actually latent or cancelled by the expressed form. He called the prevailing form of a trait dominant and the latent form of a trait recessive. Mendel's definitions of dominance and recessiveness are sometimes called Mendel's Law of Dominance:
Dominant traits mask the appearance of recessive traits whenever dominant and recessive traits are combined in one individual.

In our example, the gene for seed shape has two different forms. One form produces round seeds; the other form produces wrinkled seeds. Different gene forms that produce different forms of a trait are called alleles (from the Greek allos for "other"). In this example, the allele that codes for round seeds is dominant to the allele that codes for wrinkled seeds.

Mendel observed that dominant and recessive forms of a trait did not become blended. Instead, the recessive form of the trait reappeared in an unaltered form in the second generation. Based on this observation, Mendel formulated his Law of Segregation, which states that:
The different forms of a trait remain separate and unblended from generation to generation.

Mendel was so convinced of the validity of his conclusions that his subsequent work with other plants, some of which failed to support his hypothesis, did not discourage him. As he wrote in 1866,
"It requires indeed some courage to undertake a labour of such far-reaching extent; this appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be overestimated in connection with the history of the evolution of organic forms."

Late in his life, Mendel's time was mostly spent fighting political battles for the monastery and peasants of his village. In his lifetime, Mendel witnessed a complete change in his homeland. In his later years, the focus was no longer on agricultural advances but on political advances. The rise of the Hapsburg dynasty and the consolidation of the Austro-Hungarian Empire forced different values on the people. The days of intellectual freedom, when a monk could study agriculture in a monastery garden without interference by the government, were drawing to a close. Shortly before his death in 1884, Mendel said to a future abbot of the monastery:
"Though I have suffered some bitter moments in my life, I must thankfully admit that most of it has been pleasant and good. My scientific work has brought me a great deal of satisfaction, and I am convinced that it will not be long before the whole world acknowledges it."

Mendel's belief that his work would eventually be recognized was not mistaken. In 1900, only fourteen years after his death, his work was simultaneously rediscovered by three different geneticists – Carl Correns, Erich Tschermak, and Hugo de Vries – working in three different countries. They each realized that Mendel's particulate theory of inheritance fit patterns of inheritance they were observing.

It is interesting to speculate what Darwin would have thought had he known about Mendel's work. Genes that did not blend in each generation were the answer to Darwin's dilemma, and could have put him onto the right track as early as 1866, the year Mendel's most important paper was published. A copy of the journal containing Mendel's paper was found in Darwin's library at Down House, but it had apparently not been opened or read.

Evolution by Mutation

There is an even deeper irony: the rediscovery of Mendel's work led geneticists to reject natural selection as the mechanism for evolution, in favor of mutations. Hugo de Vries, one of the rediscoverers of Mendel's work, proposed that "mutations" (i.e. changes in the phenotype of an organism, occurring in just one generation) were the primary "engine" of evolutionary change. De Vries did his pioneering work in genetics using the evening primrose (Oenothera lamarkiana), which is now known for having sudden, large mutations (called "macromutations") in its overall phenotype.

De Vries argued that these kinds of mutations were the basis for the changes in phenotype to which Darwin referred in the Origin of Species, and that therefore natural selection was neither necessary nor likely as a cause of evolutionary change. This mutational theory of evolution was accepted by most of the prominent geneticists at the turn of the century, and led to widespread public testimonials that "Darwinism was dead."

However, like Mark Twain, reports of Darwinism's death were "greatly exaggerated." In the second decade of the 20th century, three other researchers, again working separately and mostly unbeknownst to each other, proposed a theory that would eventually lead to the re-establishment of natural selection as the prime mover of evolution.

The Hardy-Weinberg-Castle Genetic Equilibrium Law

G. C. Hardy, Wilhelm Weinberg, and William Castle all proposed a mathematical theory that describes in detail the conditions that must be met for evolution to not occur. This theory, often called the Hardy-Weinberg Equilibrium Law lays out the conditions that must be met for there to be no changes in the allele frequency in a population of interbreeding organisms over time.

Recall Mendel's definition of alleles: different forms of the same gene that produce different variations of a trait. In the context of evolution, alleles are what code for the phenotypes that change over time in an evolving population. Therefore, changes in the alleles present in a population will produce changes in the phenotypes present in that population. This, in a nutshell, is the genetic definition of evolution:
Evolution is the result of changes in allele frequency in a population over time.

What Hardy, Weinberg, and Castle all realized is that for allele frequencies to not change in a population, five conditions must be met:

There must be no mutations (i.e. alleles cannot change into other, different alleles).

There must be no gene flow (i.e. individuals cannot enter or leave the population).

The population must be very large (i.e. random accidents cannot significantly alter allele frequences).

Survival must be random (i.e. there can be no natural selection).

Reproduction must also be random (i.e. there can be no sexual selection).

Notice that the Hardy-Weinberg Equilibrium Law seems to say only that there are conditions under which evolution can't happen. Aren't we interested in those conditions in which evolution can happen? Yes, but notice what the Hardy-Weinberg Equilibrium Law gives us: it outlines exactly what processes are essential to prevent evolution, and therefore by negation shows us how evolution can happen.

That is, if any of the five conditions for maintaining a Hardy-Weinberg equilibrium are not met, then evolution must be occurring. And, of course, virtually none of these conditions is never permanently met in any known natural population of organisms:

• Mutations occur at a slow but steady rate in all known populations.

• Many organisms, especially animals, enter (immigration) and leave (emigration) populations.

• Most populations are not large enough to be unaffected by random changes in allele frequencies.

• Survival is virtually never random.

• Reproduction in organisms that can choose their mates is also virtually never random.

Therefore, according to the Hardy-Weinberg Equilibrium Law, evolution (defined as changes in allele frequencies over time) must be occurring in virtually every population of living organisms. In other words,
Evolution is as ubiquitous and inescapable as gravity.

The Hardy-Weinberg Equilibrium Law provided more than just a "null hypothesis" for genetic evolution. It also provided a mathematical basis for a more comprehensive theory of evolution in which natural selection, Mendelian genetics, paleontology, and comparative anatomy were combined in what is now known as the modern evolutionary synthesis. During the 1930s and 40s, R. A. Fisher, J. B. S. Haldane, Sewall Wright, and Theodosius Dobzhansky developed mathematical models for fitness, selection, and other evolutionary processes. These models were then applied to demographic data derived from artificial and natural populations of organisms in a rigorous (and ongoing) test of the validity of the neo-darwinian model for genetic evolution. As a result of their work, Darwin's theories of natural and sexual selection were combined with Mendelian genetics, biometry and statistics, demography, paleontology, comparative anatomy, botany, and (more recently) molecular genetics and ethology to produce a "grand unified theory" of the origin and evolution of life on Earth.

The Genetical Theory of Natural Selection

Ronald Aylmer Fisher built on the pioneering theoretical work of Hardy, Weinberg, and Castle by providing mathematical models that further undermined the Mendelian geneticist's theory of evolution via mutation. He did this by showing that continuous variation could provide the basis for natural selection as proposed by Darwin. In his most important work, The Genetical Theory of Natural Selection (published in 1930) Fisher showed that traits characterized by continuous variation (i.e. those that approximate a normal, or bell-shaped, distribution) were both common and could provide all the raw material necessary for Darwinian natural selection. This is because such traits, although being continuous in populations, do not blend from parents to offspring. Instead, as Mendel first showed, they are produced by unblending "particles" of inheritance (i.e. Mendelian "genes"). In other words,
Mendelian inheritance conserves, rather than eventually destroying, the genetic variation that exists in natural populations.

Fisher is perhaps best known for what he called the Fundamental Theorem of Natural Selection. Using a series of essentially mathematical arguments, Fisher showed that the rate of change via natural selection was a direct function of the amount of variation in a population. That is,
The more variation among alleles that exists in a population, the faster natural selection can causes changes in the allele frequencies in that population.

Conversely, the less variation among alleles that exists in a population, the slower natural selection can causes changes in the allele frequencies in that population.

R. A. Fisher's work formed the basis for a mathematical theory of evolution in which the process of natural selection is modeled mathematically in the same way that Newton modeled the force of gravity. Indeed, Fisher pointed out several times that the mathematics of natural selection were similar in many ways to such physical models as the ideal gas laws and the second law of thermodynamics. According to his mathematical models, alleles that were positively selected would increase in frequency in populations in much the same was as gas molecules spread out in an expanding balloon.

To many evolutionary biologists, this meant that natural selection would inevitably result in "fixation" of alleles that were not selected against. That is,
Any allele that results in increased survival and reproduction should, if given enough time, eventually become the only allele for that particular trait in a particular population.

This presented a problem to evolutionary biologists that was almost as severe as the “mutationism” of the early Mendelians. It implied that the inevitable result of natural selection would be the eventual elimination of all non-adaptive variation in natural populations. This would then cause natural selection to grind to a halt (or to become reduced to essentially the rate of production of new genetic mutations, which is slow in the extreme, much slower than the observed rate of evolution). Fisher suggested that constant environmental change would cause different alleles to be selected for and against, and that therefore fixation might not ever happen. However, this argument seemed to be "tacked on" to his argument for the relationship between the amount of variation in populations and the speed of evolutionary change via natural selection.

Adaptive Landscapes and Genetic Drift

A solution to this problem was provided by Sewall Wright, who discovered a process that has eventually become known as genetic drift. Wright, who worked primarily with domesticated animals in controlled breeding programs, proposed that in small populations of organisms, random sampling errors could cause significant changes in allele frequencies in those populations. He showed mathematically that the smaller a population was, the greater the effect of such random events on its allele frequencies. In other words,
Evolution can proceed by at least three primary mechanisms: natural selection, sexual selection, and random genetic drift.

Wright's discovery of genetic drift solved the problem that Fisher's Fundamental Theorem posed: how can natural selection be prevented from shutting itself down as the result of fixation? Wright proposed that allele frequencies could be visualized as forming what he came to call an "adaptive landscape". In an adaptive landscape, allele frequencies formed a series of hills and valleys, in which the top of a hill represented the highest an allele frequency could reach via natural selection. According to Fisher, there is an iron-clad rule operating here: if an allele is on a slope, it can only go up the slope via natural selection.

But this means "you can't get there from here": if a trait is at the top of one adaptive peak, it can't go down through a valley to get to the top of another, even higher (i.e. more adaptive) peak. What Wright showed was that "you can get there from here" if you drift there. That is, if a population becomes very small, it is possible for it to "drift" from one adaptive peak to another, without sliding down into the valley in between. This means that natural selection doesn't get "stuck"; populations at one adaptive peak can make it to another, even higher adaptive peak, so long as they drift randomly to it.

The Causes of Evolution

John Burdon Sanderson Haldane (usually referred to as J. B. S. Haldane) solidified the revolution in theoretical population genetics begun by Hardy, Weinberg, Castle, Fisher, and Wright. In his most important book, The Causes of Evolution, published in 1932, he showed that genetic mutations could provide the raw material for Darwinian natural selection. Furthermore, he showed mathematically that such mutations could do this even when their frequency in a population was initially so low that they would be "invisible" to statistical analysis. He also showed how dominance could evolve in populations by means of natural selection, even when the original expression of an allele was initially recessive.

Haldane is also remembered for two quips that are often repeated by evolutionary biologists. The first concerns a question posed to him by an Anglican minister, who asked him (supposedly at a dinner party) what his study of nature had led him to conclude about the principle concern of the Creator. Without batting an eyelash, Haldane replied: "An inordinate fondness for beetles," referring to the fact that there are more species of beetles on Earth than any other kind of organism.

During another conversation (supposedly in a pub), Haldane was confronted with the observation that natural selection should result in pure selfishness on the part of individuals, and therefore no one should be willing to risk his own life to save another. To this Haldane replied,
"I would be willing to risk my life to save two brothers or eight cousins."

This quip is based upon the observation that brothers share an average of one-half of their genetic material, whereas first cousins share an average of one-eighth. Therefore, saving two brothers or four cousins would result in the same genetic contribution to the next generation as that represented by one's own genome. This quip was later cited by one of the founders of what is now know as the theory of kin selection in which natural selection is considered to act at the level of genes, rather than individuals. We will discuss this idea in a later chapter.

R. A. Fisher, J. B. S. Haldane, and Sewall Wright are usually recognized as having laid the theoretical foundation for modern evolutionary theory. However, many evolutionary biologists and historians of science consider that the "modern evolutionary synthesis” was initiated by Theodosius Dobzhansky with the publication of his most famous book, Genetics and the Origin of Species published in 1937.

Genetics and the Origin of Species

Dobzhansky combined the Mendelian genetics, the mathematical models of Fisher, Haldane, and Wright, and the observations of evolution and natural selection in the wild in a theory that reinstated natural selection as the primary engine of evolution. He emphasized both the scientific aspects of evolutionary theory, and the implications of evolutionary theory for education and society in general. In a famous essay entitled "Nothing in biology makes sense except in the light of evolution” he showed how modern synthetic evolutionary theory provides a comprehensive explanation for the origin and evolution of life on Earth.

Dobzhansky also grounded the "modern evolutionary synthesis" in empirical investigation. Using the common fruit fly (Drosophila melanogaster). Dobzhansky and his colleagues showed that the patterns of variation and natural selection predicted by Fisher actually occurred in controlled populations of living organisms under laboratory conditions.

Most importantly, Dobzhansky showed empirically that the "continuous variation" that both Darwin and Fisher asserted were essential for natural selection actually occurred for many traits in nature. According to Dobzhansky, most traits are distributed in what is often referred to as a "bell-shaped curve". That is, for most traits there is an average value for the trait, which the majority of the members of the population share. There is also two "tails" to the bell-shaped curve, consisting of extreme versions of the trait.

Dobzhansky then went on to identify three different forms of natural selection, which depended upon which part of the bell-shaped curve of variation selection affected:

• Directional selection, in which selection against one extreme "tail" of the bell-shaped curve caused the average value for the trait to move over time;

• Stabilizing selection, in which selection against both extreme "tails" of the bell-shaped curve caused the average value for the trait to remain where it was; and

• Disruptive selection, in which selection against the average value of the bell-shaped curve caused the population to split into two diverging curves, corresponding to the two extreme versions of the trait.

The proponents of the "modern evolutionary synthesis" asserted that this last form of natural selection was the underlying explanation for the divergence of one species into two or more different species (for this reason, disruptive selection is sometimes referred to as "diversifying selection"). That is, Darwin's "mystery of mysteries" – the origin of species – was shown to have a mathematical basis which could be studied empirically and tested statistically, thereby making it a genuinely "scientific" study.

The Historical Importance of the "Modern Evolutionary Synthesis"

What, then, was the importance of the “modern evolutionary synthesis” to evolutionary theory? Perhaps J.B.S. Haldane said it best:
"The permeation of biology by mathematics is only beginning, but unless the history of science is an inadequate guide, it will continue, and the investigations here summarized represent the beginning of a new branch of applied mathematics."

The theory of evolution as Darwin first proposed it was essentially a qualitative theory; it had no mathematical basis, and could not be tested using statistical methods. Indeed, Darwin himself was a “mathophobe,” who had neither the training nor the inclination to provide a mathematical basis for his theories.

However, the founders of the modern synthesis were all well-versed in mathematics, as was Gregor Mendel. Indeed, R. A. Fisher not only provided the first solid mathematical framework for the theory of evolution by natural selection, he virtually founded the disciplines of biometry and statistics. Many of the statistical tests that are still used to test evolutionary hypotheses (indeed, hypotheses throughout the natural and social sciences) were first formulated by Fisher.

Providing a mathematical foundation for evolutionary theory literally meant converting evolution from “natural history” into a modern science. When a hypothesis can be tested by gathering numerical data (by counting or measuring objects and events), that data can then be statistically tested to determine if it verifies or falsifies that hypothesis. This is what happens in the other natural sciences, like chemistry and physics. Since the modern evolutionary synthesis, this is also what happens in evolutionary biology, and evolutionary psychology as well.

Where Have We Been, and Where Are We Going?

With this chapter, we have come to the end of the first part of our series of chapters on evolutionary psychology. Now that we have a grounding in the theory of evolution by natural selection, it is time to take a quick look at the theories of psychology dealing with human and animal behavior. That will be our task in the next series of six chapters: “Psychology, Ethology, and Sociobiology.” In the course of those chapters, we will see how the seeds planted by Darwin, Mendel, and the founders of the modern synthesis took root in the 20th century, eventually coming to fruition in the modern science of evolutionary psychology.


Mayr, Ernst & William Provine (eds.) (1998) The Evolutionary Synthesis: Perspectives on the Unification of Biology. Harvard University Press.


Darwin, Charles (1868) The Variation of Animals and Plants Under Domestication. John Murray. Available online here.

Dobzhansky, Theodosius (1937) Genetics and the Origin of Species. Columbia University Press.

Dobzhansky, Theodosius (1973) Nothing in biology makes sense except in the light of evolution. The American Biology Teacher, March 1973, volume 35, pages 125-129. Available online here.

Fisher, R. A. (1930) The Genetical Theory of Natural Selection. Oxford University Press.

Haldane, J. B. S. (1932) The Causes of Evolution. Princeton University Press.

Jenkin, Fleeming (1867). [Review of] The Origin of Species. The North British Review, June 1867, 46, pp. 277-318. Available online here.

Mendel, Gregor (1866) Experiments in Plant Hybridization. Verhandlungen des naturforschenden Vereines in Brünn, volume 4, pages 3-47. Available (in English) online here.

Provine, W. (1971) The Origins of Theoretical Population Genetics. University of Chicago Press.


1. Why are mathematical models so important to the natural sciences? Are they necessary for something to be considered “scientific?”

2. Is the evolutionary synthesis the “final word” on the subject of evolution? Why or why not?


As always, comments, criticisms, and suggestions are warmly welcomed!


Wednesday, March 25, 2009

A Brief Note About Comment Moderation

Due to the behavior of certain unnamed and unscrupulous individuals, I have found it necessary to return to full-time comment moderation at EVOLUTIONARY PSYCHOLOGY. This means that comments will not appear here until they have been emailed to me and I have approved them for posting to the comment threads following each post. Please bear this in mind when you comment here. Thank you for your patience and understanding.
--Allen MacNeill

"I had at last got a theory by which to work"
-The Autobiography of Charles Darwin

Sunday, March 22, 2009

Darwin on Instincts and the Expression of Emotions


In Chapter VII of the Origin of Species, Darwin proposed that instincts were behavioral adaptations that had evolved by natural selection and sexual selection. Darwin provided many examples of instinctive behaviors in animals, and suggested how such behaviors could have evolved. In particular, he proposed that animal social behavior was the result of natural selection acting at the level of “families”, rather than individuals.

In his later book, On the Expression of Emotions in Men and Animals, Darwin elaborated on the idea that behaviors are evolutionary adaptations that have evolved by natural and sexual selection. He explained the roles that emotions play in the biology of animals, and extended those explanations to humans. He argued that emotions are essentially biological processes analogous to other physiological adaptations, and that the methods by which they can be studied are similar to those by which any other inherited trait can be scientifically analyzed.


Near the end of the previous chapter, I mentioned that Darwin proposed in the Origin of Species, that “instincts” were behavioral adaptations that had evolved by natural and sexual selection.

In a chapter in the Origin entitled “Instincts,” Darwin explored this idea in the light of his overall theory of evolution by natural selection. In particular, like many of his contemporaries Darwin was fascinated by the behavior of social insects, especially ants and bees. He pointed out that their highly specialized behavior and mode of reproduction posed a serious problem for his theory, a problem that he needed very much to solve.

Darwin on Instincts

In the chapter on instincts, Darwin was very careful to distinguish between the evolution of intelligence and the evolution of instincts. In the Origin, Darwin did not speculate about the evolution of intelligence at all, but rather confined his discussion to the behavior of non-human animals. However, he did make it clear that he believed that there were strong analogies between some of the instinctive behaviors of non-human animals and similar behaviors in humans.

Darwin initially avoided defining “instincts” directly. Instead, he provided multiple examples of the kinds of behaviors he was referring to when he used the term “instinct.” In other words, he used the kind of functional analysis that I described in the previous chapter. We will see this technique being used over and over again, not only in evolutionary biology as a whole, but especially in evolutionary psychology.

The examples of instincts cited by Darwin in the Origin have the following properties:

1. Instincts are not acquired (i.e. learned) via experience.

2. On the contrary, instincts can be performed by individuals who have never learned how to perform them, nor experienced the same set of stimuli before in their lives.

3. In particular, instinctive behaviors can be elicited from animals that have been raised in isolation since birth (or since hatching, as often the animals being tested were birds).

4. Instincts are stereotyped. That is, they are performed in very much the same way every time, both by the same individual at different times and by most of the members of a given species (i.e. they can be referred to as pan-specific behaviors).

5. Furthermore (and in contrast with many human behaviors), instinctive behaviors do not seem to require judgment or reason on the part of the individuals performing them.

6. By implication, this means that instincts are essentially unconscious; that is, they are not the result of conscious deliberation or intentions.

Darwin also took great pains to distinguish between “instincts” and “habits.” He pointed out that habits are stereotyped behaviors that are acquired during an individual’s lifetime, usually by constant repetition. However, Darwin also believed that some habits could be inherited, especially as the result of use and disuse. In that respect, Darwin clearly believed in the possibility that evolution could proceed by the inheritance of acquired characteristics, a theory proposed a half century earlier by the Frenchman, Jean-Baptiste Lamarck. If he were referring to anatomical or physiological characteristics, we would be safe in rejecting Darwin’s assertion that acquired traits such as habits can be inherited. As August Weismann and others showed, acquired anatomical characteristics (such as missing tails in mice, chopped off by the experimenter) cannot be inherited from parents to offspring.

However, some behaviors are learned from other individuals, and not just from parents to offspring. To the extent that a behavior can be learned or modified during an individual’s lifetime that behavior is essentially an acquired trait that can be passed on (i.e. “inherited”). Some behaviors, in other words, follow the rules of Lamarckian evolution. As we will see, there is evidence that we inherit (via Darwinian mechanisms) the tendency to learn certain behaviors (via Lamarckian mechanisms), and that we learn such behaviors surprisingly easily.

In the Origin, Darwin focused most of his attention on the instincts of “lower” animals, especially the social insects. One of the reasons for this was that such behaviors could be explained without including either consciousness or intelligence. Furthermore, Darwin took pains to show that instincts have many of the characteristics of evolutionary adaptations:

1. Although they are pan-specific, most instincts are quite variable, both within and between individuals.

2. Like other adaptations, the instincts that are present in a population (or species) apparently change slowly and gradually over time.

3. Instincts provide a benefit primarily to the individuals performing them, and especially not to the members of other species.

4. Instincts, like other adaptations, are not perfect. Rather, they are compromises that only have to function “well enough” to result in differential survival and reproduction.

5. Finally, and most importantly:
The performance of specific instincts can be causally linked to increased survival and reproduction by the individuals performing them.

Darwin also took great pains to show that many instincts are inherited virtually unchanged from parents to offspring. The best way to do this is to show that an instinctive behavior is performed correctly without any opportunity for the performer to have learned it through experience. Darwin noted that pointers (dogs used to assist in hunting game birds) do not need to be trained in how to “point” at their quarry. On the contrary, they need to be trained to hold still when a gun is fired, and not to maul a bird if it is shot and lands nearby. “Pointing”, in other words, is an instinctive behavior that has been bred into some hunting dogs.

The heart of Darwin’s chapter on instincts in the Origin is his explanation for the evolution of sterile castes in the social ants, bees, termites, and wasps. He started out by pointing out that if he cannot do this, it would be “fatal” to his entire theory. The reason for this is that he had previously and repeatedly asserted that traits that provide a benefit exclusively to another organism cannot possibly evolve by natural selection. This is because the other individuals (the ones receiving the benefit) would therefore increase in frequency in the population, while the individuals providing the benefit would decrease in frequency until they disappeared, leaving only those individuals who did not provide the benefit.

Darwin correctly pointed out that this problem would be most acute for the social insects, because many of them have what are known as sterile castes, such as workers, warriors, etc. These are often highly modified versions of the average ant, bee, termite, or wasp, with gigantic jaws, reduced or absent wings, etc. Furthermore (and most importantly) such specialized castes consist of individuals that are sterile: they never reproduce during their own lifetimes, but rather assist the “queen” ant, bee, termite, or wasp in reproducing.

Here, then is Darwin’s potentially fatal quandary: how can an adaptation like gigantic jaws be passed on at rates sufficient to make them more common over time if the individuals that have such traits never reproduce? Stated succinctly:
How can a sterile worker pass on the trait of sterility?

Darwin’s answer was surprisingly simple: he proposed that natural selection could act at the level of “families” (i.e. groups), rather than exclusively at the level of individuals. Darwin pointed out that the problem of the evolution of sterile castes in social insects is essentially the same as the problem of how to continue getting high quality meat from domesticated farm animals, such as beef cattle. After all, when a steer is slaughtered and cut up for meat, it can’t very well pass on the traits (thick muscles, marbled fat, etc.) that made it a superior source of beef. For that matter, a steer is already out of the evolutionary race even before it is slaughtered. “Steers” are male cattle that have been castrated (which makes them fatter and more placid). Like the members of a sterile caste of social insects, steers are sterile, and therefore cannot possibly pass on to their offspring the characteristics that make them so valuable as a source of food.

So, Darwin asked, how do we continue to obtain high-quality meat from animals that are castrated and then slaughtered, rather than being bred? The answer is, we breed their closest relatives, who presumably carry the same genetic traits that made their slaughtered relatives so valuable. In other words, we select for a closely related group of individuals, who can therefore evolve particular desirable traits, without all of them necessarily surviving and reproducing.

In the same way, the reproductive members of a hive of social insects (i.e. the “queens” and “drones”) can increase in relative frequency because of the attributes and actions of their sterile relatives (i.e. the “workers” and “warriors”). Darwin even went so far as to indirectly suggest that this is how the cells of a multicellular organism can become specialized for particular traits, even though only the germ cells (i.e. the eggs and sperm cells) reproduce. In both the case of social insects and the case of the cells of a multicellular organism, selection is considered to be operating at the level of groups (Darwin referred explicitly to “families”), rather than strictly at the level of individuals.

As we will see in later chapters, the question of what level natural selection operates is of crucial importance to any theory of the evolution of social behavior. It is often asserted that “Darwinian” natural selection can only operate at the level of individual organisms. In other words, populations (i.e. groups) of organisms are what change over time – that is, populations evolve. However, the process by which they evolve – natural selection – occurs when certain individuals with particular traits survive and reproduce more often than others – that is, individuals are selected. As is often asserted by evolutionary biologists:
Populations evolve, whereas individuals are selected.

However, even Darwin himself argued otherwise: that in the social insects (and, by implication, in social animals in general) natural selection can operate at the level of groups as well as at the level of individuals. In later chapters we will explore these ideas in more detail, and will see that there is a resolution to this seeming paradox. As we will see, that resolution is founded on the idea that ultimately genes are what produce the traits of individual organisms. Therefore, natural selection at the level of genes (rather than individuals) can be used as the explanation for the evolution of many social behaviors, especially the evolution of sterile castes in the social insects.

Darwin on the Expression of Emotions in Men and Animals

In one of his last books, The Expression of the Emotions in Men and Animals (published in 1872) Darwin explored the evolution of behavior in more detail. In it, he elaborated on the idea that behaviors are evolutionary adaptations that have evolved by natural and sexual selection. In particular, he explained the roles that emotions play in the biology of animals, and extended those explanations to humans. He argued that emotions are essentially biological processes analogous to other anatomical and physiological adaptations, and that the methods by which they can be studied are similar to those by which any other inherited trait can be scientifically analyzed.

Darwin used the new technology of photography to illustrate how facial expressions in humans are similar to the facial expressions of other animals in similar situations, such as anger and fear. For example, he argued that the similarities between the “sneer” of a contemptuous human and the raised lips and exposed canine teeth of a snarling dog are not accidental. Both communicate specific states of emotion, and therefore both have adaptive value in the context of social interactions within groups of animals (and, by implication, human social groups). Darwin was essentially making the same argument that William James was to make less than two decades later in his landmark text, Principles of Psychology; that humans have more instincts than other animals, rather than fewer.

In the last chapter of The Expression of the Emotions in Men and Animals, Darwin summarized what he believed to be the three mechanisms by which the expression of emotions in animals and humans have evolved. As he also did in the Origin of Species, Darwin asserted that some emotional expressions are the result of repeated habit that had eventually become hereditary. Again, this is essentially an argument for Lamarckian evolution by means of the inheritance of acquired characteristics. As I have pointed out before, while anatomical and physiological characteristics cannot be passed on in this way, it is possible for behaviors to be acquired and inherited by Lamarckian mechanisms. This would especially be the case if what were inherited (in strictly Darwinian terms) were the tendency to learn a particular behavior in a particular way.

Darwin also pointed out that the principle of “antithesis” was central to the communication of emotions and intentions. For example, the facial expressions and body postures that express dominance –

erect hair, forward-pointing ears and directed gaze, stiff and erect posture, etc. – are the antithesis of those that express submission –

flattened hair and ears, averted gaze, and downward-curled posture. As we will see later, the expression of emotions and intentions, and the ability to detect these, are central to any understanding of the evolution of human behavior.

Throughout most of the Expression of Emotions, Darwin approached the subject of emotional expression as if it were a specialized sub-discipline of anatomy and physiology. For example, Darwin began with a detailed examination of the musculature of the human face

showing the various muscles that, when contracted, produce the facial expressions that we associate with particular emotions. Darwin referred to the work of several anatomists and physiologists who had studied the muscles and mechanisms of emotional expression in humans, pointing out the essentially physiological nature of these processes.

He then compared the facial expressions of various animals,

including dogs, cats, and the crested macaque (an Indonesian monkey), showing the various similarities in expression of emotion.

From his analysis of the expression of emotions in non-human animals, Darwin then went on to examine the expression of emotions in humans. Here, he used the newly developed technology of photography to great effect, presenting photographs of children and adults expressing anxiety, grief, dejection, despair, joy, love, devotion, ill-temper, sulkiness, determination, hatred, anger, disdain, contempt, disgust, quilt, pride, helplessness, patience, puzzlement, surprise, astonishment, fear, horror, shame, shyness, modesty, and included a physiological analysis of blushing.

He compared natural expressions with facial expressions produced using electrodes attached to the facial muscles of volunteers.

Reading Darwin’s book, it’s clear that he thought of emotions as physiological responses to environmental stimuli. The bulk of the book is taken up with the anatomy and physiology of facial expression via muscle contraction, powered by blood flow through the circulatory system. His intent was and is clear; to show that the capacity for the expression of emotions in animals and people is an evolutionary adaptation, based on what could best be described as physiological processes.

Finally, Darwin asserted that much of animal and human behavior is the result of “the direct action of the excited nervous system…independent of the will, and independently…of habit.” In essence, this is an argument against the idea that human behavior is the result of “free will” or conscious intent. Freudian psychology caused a firestorm of controversy in western culture because Freud also suggested that most of human behavior was motivated by drives that were largely unconscious, and therefore not the result of “free will.” Despite a century of research into animal and human behavior, this idea – that our actions are largely not the result of “free will” – is still hugely controversial, even among evolutionary biologists.

In the last chapter of The Expression of the Emotions in Men and Animals, Darwin concluded “[t]hat the chief expressive actions, exhibited by man and by the lower animals, are now innate or inherited – that is, have not been learnt by the individual – is admitted by every one.” [Emphasis added] This conclusion would seem to have laid the groundwork for the science we now call evolutionary psychology. Indeed, had evolutionary biologists followed up on Darwin’s suggestions, it is quite possible that a detailed science of evolutionary psychology might have evolved in the first half of the 20th century.

However, just the opposite happened – instead of guiding the science of human behavior, Darwin’s theory of evolution by natural selection was eclipsed by a view of human nature that completely rejected any possibility that evolution (or even biology) played any significant role in human psychology. At the same time, a new discipline within the science of evolutionary biology was founded that eventually made it possible for evolutionary psychology to make a new start in the science of human nature. That discipline – theoretical population genetics – will be the subject of the last chapter in this first part of our course on evolutionary psychology.

Essential Reading:

Darwin, C. (1859) On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, 1st ed., Chapter VII: Instinct. John Murray. Available online here.

Darwin, C. (1872) On the Expression of Emotions in Men and Animals. John Murray. Available online here.

Supplemental Reading:

James, William (1890) Principles of Psychology. Henry Holt. Available online here.

Questions to Consider:

1. Can instinctive behaviors have learned components, and vice versa?

2. Why did Darwin focus on the expression of emotions when he analyzed the evolution of behavior in humans and other animals?


As always, comments, criticisms, and suggestions are warmly welcomed!


Thursday, February 26, 2009

Ground Rules and Moderation Policy

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As always, comments, criticisms, and suggestions are warmly welcomed!


Tuesday, February 24, 2009

Natural Selection and Evolutionary Adaptations


Like all of the natural sciences, evolutionary biology is based on the assumption that all natural phenomena can be explained with reference to purely natural causes, and that the simplest explanation for any phenomenon is the best. In particular, most scientists assume that intentions or purposes are not necessary to explain natural phenomena.

This assumption is the basis for natural selection, which Charles Darwin proposed as the origin of adaptations. This means that although adaptations may appear to be purposeful, the processes by which they come about are not. This outlook on the origin of adaptations is particularly important in evolutionary psychology, which is primarily concerned with the behavioral adaptations of humans and related primates. The capacity for human behaviors and motivations is assumed to have evolved by natural and sexual selection, operating in specific ecological contexts in our evolutionary past. A central implication of this view is that adaptations “make sense” only in the context of the evolutionary environment of adaptation.


As I stated at the end of the previous chapter, inference is the basis for all reasoning, including scientific reasoning. However, logical inference and arguments by analogy are not necessarily limited to naturalistic explanations. Consider the following:
In crossing a health, suppose I pitched my foot against a stone, and were asked how the stone came to be there; I might possibly answer, that, for any thing I knew to the contrary, it had lain there for ever: nor would it perhaps be very easy to show the absurdity of this answer.

But suppose I had found a watch upon the ground, and it should be inquired how the watch happened to be in that place; I should hardly think of the answer which I had before given, that, for any thing I knew, the watch might have always been there. Yet why should not this answer serve for the watch as well as for the stone? Why is it not as admissible in the second case, as in the first?

For this reason, and for no other, … that, when we come to inspect the watch, we perceive (what we could not discover in the stone) that its several parts are framed and put together for a purpose… it must have had, for the cause and author of that construction, an artificer, who understood its mechanism, and designed its use. This conclusion is invincible.

A second examination presents us with a new discovery. The watch is found, in the course of its movement, to produce another watch, similar to itself; and not only so, but we perceive in it a system or organization, separately calculated for that purpose. What effect would this discovery have, or ought it to have, upon our former inference? What, as hath already been said, but to increase, beyond measure, our admiration of the skill, which had been employed in the formation of such a machine?

This example of logical inference is taken from the first chapter of Natural Theology: or Evidences of the Existence and Attributes of the Deity, written by the reverend William Paley, an Anglican minister and tutor at Christ's College, Cambridge, in England. Originally published in 1794, it has been continuously in print since then. It went through twenty editions before Paley's death in 1805, and was immensely popular and greatly admired, especially among the faculty and undergraduate students at Cambridge, one of whom had this to say about it:
In order to pass the B.A. examination, it was also necessary to get up Paley's Evidences of Christianity…The logic of this book and as I may add of his Natural Theology gave me as much delight as did Euclid. The careful study of these works, without attempting to learn any part by rote, was the only part of the Academical Course which…was of the least use to me in the education of my mind. I did not at that time trouble myself about Paley's premises; and taking these on trust I was charmed and convinced of the long line of argumentation.

That student, who was so delighted by Paley's writing and charmed by his long line of argumentation, was a mediocre divinity student (and fanatical collector of beetles) by the name of Charles Darwin.

As the quotation from Paley's Natural Theology indicates, most people have a "feeling" that nature is designed in some deep way, and that this is somehow connected with religion. And, most religions agree:
The concept of God goes hand-in-hand with the concept of design in nature.

This is even true for most religions that lack a deity, such as Buddhism. Indeed, even many atheists have a “feeling of design” about many things in nature, although they generally do not credit God (or gods) as the author of that design.

As we will see, evolutionary psychology (like evolutionary biology in general) is very much concerned with objects and processes that seem to have a definite purpose. For example, both the existence of fur in mammals and the erection of fur as a warning threat appear to have “purposes”: the first exists in order to keep mammals warm, while the second happens in order to warn other animals that they may be attacked.

To a religious believer, both of these characteristics of mammals can be explained as the result of “intelligent design” on the part of their Creator.” However, as we learned in the previous lecture, one of the most important features of Darwin’s “dangerous idea” is that the theory of evolution by natural selection makes design or purpose in nature unnecessary.

As we will see, this is especially true for evolutionary psychology, in which the behaviors and motivations of humans are explained as the result of natural and sexual selection, neither of which are either designed or purposeful. That is, according to evolutionary psychology, a significant part of human behavior is just “doing what comes naturally” – behaving in a way that is not necessarily the result of conscious intentions.

Three Questions: What, How, and Why

So, let's return for a moment to reverend Paley's rock (the one he stubbed his toe on while crossing the heath). Consider dropping such a rock: once it leaves your grasp, it falls to the ground. One can ask at least three fundamental questions about this process:

Question: What does the rock do when you drop it?

Answer: It falls from your hand to the ground.

“What” questions are asking for, and are usually answered with a description. Much of what Darwin’s contemporaries did was almost entirely descriptive – they observed nature and described what they saw. This is one reason why what they were doing is often referred to as “natural history” rather than “biological science.”
Question: How does the rock fall to the ground?

Answer: It falls because of the force of gravity (that is, it falls from your hand to the ground at an accelerating rate that can be described by Newton's Law of Gravity).

“How” questions are asking for, and are usually answered with an analysis of causes and effects (hence the word “because” in the answer to the “how” question). This is what the natural sciences, and especially the physical sciences such as chemistry and physics, have traditionally been concerned with. An observable phenomenon is analyzed and the mechanisms by which it occurs are determined using controlled experiments. This is what separates “biological science” from “natural history” – the former is much more likely to involve some kind of experimental analysis, while the latter is essentially just descriptive.

Just one question left:
Question: Why does the rock fall to the ground?

Answer: Hmm…

This third question is the real kicker, because your answer to it is shaped by a fundamental metaphysical assumption, of which you may or may not be consciously aware. To get at what that assumption is, consider the following statement:
Answer: The rock falls in order to reach the ground.

Does this explanation make sense? Do you agree or disagree that it makes sense, and if not, why not?

The reason that this explanation sounds wrong to most "modern" ears is that included in it is the idea of purpose. Things that happen in order to bring about some end are purposeful things, and rocks (once you have let go of them) are clearly not purposeful things.

Or are they? What do we mean when we say that something has a purpose?

When we say that some object or event has a purpose, we generally mean that someone (i.e. an "intentional agent") has a pre-existing plan or purpose for that object or event. That is, the object or event exists or takes place because that intentional agent is actively directing it toward some predetermined end. Is that how a dropped rock moves after you have let go of it?

Almost everyone would answer "no." Rocks and other inanimate objects can't possibly have intentions or purposes of their own, and when moving (or even sitting still) on their own, their actions or existence is describable using simple physical (or chemical) laws or theories that do not include any kind of intention or purpose.

Ontological Naturalism

This way of thinking about reality is known as ontological naturalism, and is central to the way that scientists formulate, test, and interpret explanations about natural processes. Ontological naturalism is based on five primary assumptions:
1. Nature (i.e. the universe) contains only energy and matter, and the interactions between these cause all of the observable phenomena in the universe.

2. The interactions between energy and matter (and only such interactions) involve the exchange of information.

3. Information separate from the interactions of energy or matter cannot be shown to exist (and therefore is generally assumed to not exist).

4. The most productive way to analyze the interactions between energy and matter (and the existence of information) is via empirical observation (and therefore the scientific method is the best way to understand nature).

5. The simplest explanation of a natural phenomenon is assumed to be the best, until proven otherwise. This is often referred to as Occam’s Razor), named in honor of the 14th century English Franciscan friar William of Ockham, who stated: Pluralitas non est ponenda sine neccesitate"Plurality should not be posited without necessity." In scientific terms, Occam’s Razor says that explanations of natural phenomena should be limited to natural causes.

To these five metaphysical assumptions, nearly all scientists would add a sixth:
It is not necessary to assume that intentions or purposes have anything to do with natural phenomena.

This last assumption is often extended, as follows: Since purpose in nature is unnecessary to explain natural phenomena, it is assumed that purpose does not exist in natural phenomena. This is why we don't say that dropped rocks fall “in order to” reach the ground.

As Richard Dawkins has pointed out, evolutionary adaptations seem to be the result of purposeful design. In particular, behavioral adaptations seem to be the result of conscious intent. As we will see, there are many animal behaviors (and even some human behaviors) that are neither purposeful nor the result of conscious intent, but rather the simple working out of an adaptation that is the result of natural or sexual selection.

To understand how this can be the case, consider the fact (i.e. the observation) that mammals have fur; is having fur an evolutionary adaptation of mammals? A common way to answer this question is to ask “Does having fur serve some function in mammals?”, which can be simplified to:
Why do mammals have fur?

The answer seems simple:
Mammals have fur in order to keep warm.

However, this answer includes the dreaded phrase ”in order to”; can the answer be restated in such a way as to remove the implication that fur exists in mammals for a purpose? Yes:
Mammals have fur because their parents have fur.

That is, they inherit from their parents a particular variation that contributed to their ability to survive and reproduce. In long form, the evolutionary answer is: Mammals have fur today because in the past some mammal-like ancestors had fur and some didn’t (i.e. there was variation in the trait of having fur). Those individuals that had fur survived and reproduced more often than those that did not, and so having fur became more common among mammals, until today virtually all mammals have fur.

Notice that this means that the answer to the question of why mammals came to have fur is the same as the answer to the question of how mammals came to have fur. In science in general, and evolutionary biology in particular, the answer to the question “why” is the same as the answer to the question “how.” This means that evolutionary adaptations have the appearance of being the result of purpose or intentions, but need not be explained that way. On the contrary, evolutionary biologists explain the existence of seemingly purposeful characteristics of living organisms (i.e. adaptations) as being the result of a process that itself has no purpose (i.e. natural and sexual selection).

Given the foregoing, it is easy to see why non-scientists often assume that adaptations “have purposes” and are therefore the result of “intelligent design.” It should also be clear by now why scientists reject this explanation for adaptations, preferring instead the evolutionary explanation proposed by Darwin. That is, the characteristics of organisms we call “adaptations” exist because in the past the individuals who had those characteristics survived and reproduced more often than individuals who did not, and therefore those characteristics have become more common over time.

Evolutionary Implications

There are two important implications of the evolutionary viewpoint that we should take note of now, as they will become very important in later chapters:
• Given sufficient time for natural and sexual selection to operate, the characteristics we refer to as adaptations generally become so common among the individuals that make up what we refer to as a species that we say such adaptations are pan-specific. That is, adaptations are generally considered to be present in most of the individuals that make up a species.

• However, since natural and sexual selection ultimately depend on variation between individuals in populations, it is equally likely that the degree to which an adaptation is expressed in the individuals in a population is generally not exactly equal. That is, not all individuals will express an adaptation to the same degree, and it may even be virtually absent in some.

Given the foregoing, how can we tell if a characteristic found among a group of organisms is an evolutionary adaptation? The answer to this question is crucial to the science of evolutionary psychology, as virtually all of evolutionary psychology is directed toward identifying, explaining the existence of, and predicting the effects of human behavioral adaptations.

One way to answer this question is to do what most evolutionary biologists do when they observe a particular characteristic of a living organism: ask what the adaptive function of that characteristic is. Paradoxically, this means asking “what is that characteristic for?” which is essentially the same as asking “why does that characteristic exist?” This is sometimes referred to as functional analysis, and as you can see it comes very close to asking what the purpose of the characteristic might be.

There is a way to determine whether a given characteristic is an evolutionary adaptation without asking anything about its “purpose.” Since adaptations are understood to be the result of unequal, non-random survival and reproduction, it should be possible to determine if individuals with a characteristic that is suspected to be an adaptation actually survive and reproduce more often than individuals that have either an alternative characteristic or do not express that characteristic as fully. In other words, the “gold standard” in identifying evolutionary adaptations is the observation that the putative adaptation actually results in differential survival and reproduction.

As we will see in later chapters, this kind of analysis is very effective at identifying characteristics that are evolutionary adaptations. In some cases, what appear to be evolutionary adaptations clearly are not, as they do not result in differential survival and reproduction. In other cases, characteristics that do not appear to be adaptive can be shown to result in differential survival and reproduction, and so are clearly adaptive despite their appearance.

How can behaviors (which leave virtually no fossils and are not anatomical characteristics of animals) possibly qualify as adaptations? In the next chapter, we will see how Darwin argued that some behaviors – what he referred to as “instinct” – can qualify as evolutionary adaptations, and showed how complex behaviors, including some of the behaviors of humans, could be the result of natural and sexual selection, rather than conscious design or intention.

Essential Reading:

Dawkins, Richard (1986) The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design. W. W. Norton.

Supplemental Reading:

Darwin, C. (1859) On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life, 1st ed. John Murray. Available online here.

Paley, W. (1809) Natural Theology, or Evidences of the Existence and Attributes of the Deity, 12th ed. J. Faulder. 548 pages. Available online here.

Questions to Consider:

1. Is it possible for something to be not random and not purposeful?

2. How can one unambiguously determine if something has a purpose, and can this method of determination be applied to natural objects and processes?


As always, comments, criticisms, and suggestions are warmly welcomed!