Natural Selection and Evolutionary Adaptations

SUMMARY:

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.

EVOLUTIONARY PSYCHOLOGY 1.1.4

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?

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

--Allen