December 2004

What is natural selection, and how is it central to the theory of evolution?

Charles Darwin studied beak variation of finches on the Galapagos Islands as evidence of natural selection. Illustration from BSCS, Biological Science: Molecules to Man, 1963.

Futuyma: Natural selection is the process by which species adapt to their environment. Natural selection leads to evolutionary change when individuals with certain characteristics have a greater survival or reproductive rate than other individuals in a population and pass on these inheritable genetic characteristics to their offspring. Simply put, natural selection is a consistent difference in survival and reproduction between different genotypes, or even different genes, in what we could call reproductive success. [A genotype is a group of organisms sharing a specific genetic makeup.]

The reason that natural selection is important is that it’s the central idea, stemming from Charles Darwin and Alfred Russel Wallace, that explains design in nature. It is the one process that is responsible for the evolution of adaptations of organisms to their environment.

Darwin’s book On the Origin of Species by Means of Natural Selection caused quite a stir when it appeared in 1859. Evidence to support evolution and natural selection, of course, has accumulated over time, and now science accepts that evolution is a fact and that natural selection explains very well how adaptive evolution takes place.

Is natural selection the only mechanism of evolution?

Futuyma: No, certainly not. There cannot be evolution without genetic variation in the first place. So there must be mutation and often recombination to generate the different genotypes or the different versions of the genes, known as alleles, which then may or may not make a difference in the ability of an organism to survive and reproduce. You can’t have any evolutionary change whatever without mutation, and perhaps recombination, giving rise to genetic variation. But once you have genetic variation, there are basically two major possibilities:

• First, there is simply no difference between the different genotypes or different genes in their impact on survival or reproduction, and in that case, you can have random changes of one versus the other type in a population or a species until eventually one replaces the other. That is an evolutionary change. It happens entirely by chance, by random fluctuations. That is what we call the process of genetic drift.
• Genetic drift is very different from possibility number two, natural selection, which is a much more consistent, predictable, dependable change in the proportion of one gene vs. another, one genotype vs. another. Why? Simply because there is some consistent superiority, shall we way, of one genotype vs. another in some feature that affects its survival or some feature affecting its reproductive capabilities.

Does natural selection lead to new species, and if so, how?

Futuyma: It sometimes does but not always. A great deal of evolution by natural selection can happen without the formation of new species. Natural selection is only the process of adaptation within species, and we see many examples of that. Under some circumstances natural selection does play a role in the origin of new species, by which I mean a splitting of one species lineage into two different lineages that do not interbreed with one another — for example, the splitting of one ancestral primate lineage into one that became today’s chimpanzee and the other that became the hominid line resulting in our own species. The process of splitting and becoming reproductively isolated, that is, incapable of breeding with one another, can often involve natural selection but perhaps not always.

Some take natural selection to mean survival of the fittest. How does this slogan sometimes lead to misconceptions?

Futuyma: “Survival of the fittest” is a slogan that is really very misleading. First of all, it’s not an adequate description of what really goes on in nature for two reasons:

• Sometimes there isn’t a “fittest” type. There may be several different types that are equally fit for different reasons. Perhaps they’re adapted to different facets of the environment. One is not going to replace the other because each has its proper place in the environment.
• Moreover, it’s not just a matter of survival. Natural selection is a difference in reproductive success that involves both the ability to survive until reproductive age and then the capacity to reproduce.

The notion of the survival of the fittest is also unfortunate because it has been viewed as a kind of tautology, a kind of empty statement for those who say that the fittest are those that survive and so there’s no real predictive content to the notion of natural selection. That is simply false.

How do scientists interpret “chance,” and does it play a role in natural selection?

Futuyma: Philosophers and scientists use “chance” only in the sense of unpredictability. Chance means essentially that you cannot predict the outcome of a particular event. For example, you cannot predict whether your next child will be a son or a daughter, even though you can specify the probability or likelihood. “Chance” does not mean lack of purpose or goal in science. If it did, we could say that absolutely everything in the natural world is by chance because we don’t see any purpose or goal in storms, in ocean currents, or anything else. Evolution certainly does involve randomness; it does involve unpredictable chance. For example, the origin of new genetic variation by mutation is a process that involves a great deal of chance. Genetic drift, the process I referred to earlier, is a matter of chance.

However, natural selection itself is the single process in evolution that is the antithesis of chance. It is predictable. It says that, within a specific environmental context, one genotype will be better than another genotype in survival or reproduction for certain reasons having to do with the way its particular features relate to the environment or relate to other organisms within the population. That provides predictability and consistency. So, if you have different populations with the same opportunity for evolution, you would get the same outcome.

Can natural selection select for future needs of a species?

Futuyma: No, because natural selection is not like Mother Nature watching over us. Since natural selection is totally an impersonal process that is nothing more than a difference, generation by generation, in the reproductive success of one genome over another, there’s no way that it can look forward to the future or guard against the possibility of extinction. What individuals have right now that gives them superior adaptation may lead to disaster tomorrow.

Could you give us an example of natural selection at work in the recent past?

Futuyma: There are so many examples of that! One example is the apple maggot fly. About 100 years ago it started to become a serious pest of apple orchards in New England and New York State. It’s now a threat throughout most of northern United States. It originally fed just on hawthorn fruits, but then it adapted to apple and it’s become a serious threat to the industry. That is a genetic change propagated by natural selection.

Perhaps much more crucial is an issue that agriculture has to deal with all the time: the evolution in hundreds of species of insects of resistance to various chemical insecticides. The insects then become more and more difficult to control.

Closer to home, and more serious, is the single greatest crisis in medicine: antibiotic resistance. The fact is that enormous numbers of the most dangerous bacteria and viruses have evolved to be resistant to the antibiotics or other drugs that used to be effective against them. An obvious example is the HIV virus, which, as we know, is capable of rapidly evolving resistance to drugs that once were effective against it.

The late Stephen Jay Gould has written that if we were to rewind the “tape” of evolutionary history and play it again, the results would not be the same [S.J.Gould, Wonderful Life, 1989]. Why?

Futuyma: Well, it almost certainly would not be the same. I don’t think anyone can say how different it would be, whether it would be along the same general lines or utterly, unimaginably different.

Of course, it wouldn’t be the same, because first of all, random processes are involved in the evolutionary process. For example, the origin of new mutations: a lot of evolution is dependent on particular mutational changes in genes that were very, very rare or unlikely, but that just happened at the right time, in the right species, in the right environment, but it need not happen that way. So, there’s this unpredictability.

In addition, the particular sequence of environmental changes that the Earth underwent and that organisms were exposed to over billions of years has left a long-term imprint on species as they are today. If the sequence of environmental changes were different, you would have a different evolutionary history, leading to entirely different organisms over time.

Why does natural selection pose a threat philosophically to some people?

Futuyma: The philosopher Daniel Dennett called natural selection “Darwin’s dangerous idea” for a good reason: it is a very simple natural mechanism that explains the appearance of design in living things. Before Darwin, the adaptations and exquisite complexity of organisms were ascribed to creation by an omnipotent, beneficent designer, namely God, and indeed were among the major arguments for the existence of such a designer. Darwin’s (and Wallace’s) concept of natural selection made this “argument from design” completely superfluous. It accomplished for biology what Newton and his successors had accomplished in physics: it provided a purely natural explanation for order and the appearance of design. It made the features of organisms explicable by processes that can be studied by science instead of ascribing them to miracles. The contemporary “intelligent design” movement is simply a repetition of the predarwinian argument, and of course it cannot be taken seriously as a scientific explanation of the properties of living things.

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