I was reading a nice article by Andrew Shtulman* on the most common misconception people have about natural selection (that it involves not differential reproduction among genetically different individuals but the gradual and simultaneous transformation of all individuals in a population), when I came across his presentation of Darwin’s “variational” theory of natural selection. That’s the first view given in parentheses, and the correct one.
This characterization of evolution via natural selection (it’s not really a definition) involves a three- or four-step logical chain. Here’s the way it’s presented in many classes, and the way I used to present it:
- Animals and plants produce many more offspring than can survive, ergo there is tremendous mortality in nature.
- Animals and plants differ in their traits.
- Bearers of some traits leave more offspring, or survive better, than do bearers of others. (For example, those individual moths who match their backgrounds better are less likely to be eaten by birds than are their more conspicuous relatives.)
- Some portion of the differences among individuals in these traits will be passed on to the next generation. That is, some of the variation is heritable (“capable of being inherited”).
If these conditions hold, then the population will undergo gradual genetic change, being enriched in the genetic variants that give their bearers greater reproduction and/or survival (“fitness” is the word we use here). This is a variational view of evolution rather than the incorrect transformational view outlined above. The chain can also be described as “Excess production of individuals + variation in fitness + some heritability of that fitness = evolution via natural selection.”
I hope you’ve followed me so far. If you’ve taken any evolutionary biology, you’re likely to have heard the chain of logic above. It is exactly the chain outlined by Darwin in On the Origin of Species. Darwin, in fact, said that he finally grasped the importance of natural selection when he read Malthus’s “On Population”, which described the overproduction of offspring. For when Darwin realized that the huge excess of young animals and plants must somehow be culled if populations remained at relatively stable sizes, he saw immediately that the culling would probably be based on the traits that individuals had, and if variation in those traits had at least some genetic basis (remember that Darwin was unclear about how genetics worked), it would, over time, produce a predominance of the variants producing those traits. Curiously, A. R. Wallace also hit on the idea of natural selection after reading Malthus as well.
But there’s one problem with this: one of the points is not necessary for evolution by natural selection. Can you guess which one?
It’s #1—the very point that brought Darwin and Wallace to the brilliant idea of natural selection.
Why isn’t this necessary? While clearly not all offspring survive in any species—otherwise we’d be up to our collective tuchas in rabbits, beetles, or oak trees—natural selection can still cause evolutionary change in a population that is expanding (or decreasing), and in which all offspring survive. Imagine, for instance, that a lizard makes it to a luxuriant island in the ocean, one loaded with food and without predators. Imagine too that every lizard dies only of old age—after it’s already had its offspring. And further imagine that some lizards are better able to digest the local vegetation, and thus are better nourished and leave more offspring than others. Every offspring survives, but more of the next generation will carry those genetic variants that make them better at digestion. Over time, the population will evolve by natural selection, as the carriers of the “good digestion” genes overtake their dyspeptic confrères. So we have (variational) evolution by natural selection, but every individual that is born survives to reproductive age. And this will work even if individuals are immortal and never die.
Of course the situation I just described is unsustainable: eventually the population of lizards will get big enough that they’ll be competing for food, and might even kill each other. My point, though, was that natural selection can operate independently of a huge mortality—or any mortality—in a population.
Ergo, we can omit #1 above from the characterization of natural selection. When I realized this, I dropped it from teaching.
In fact, the great evolutionary biologist Ronald Fisher made just this point in his famous book The Genetical Theory of Natural Selection (1930). In chapter 2, he objects to the Malthusian point, saying that it not only ignores differential reproduction in favor of differential survival (my example above), but ignores the fact that organisms often overproduce offspring as the very result of natural selection. The vast overproduction of offspring in some ocean fish, for example, isn’t just a given: it’s likely resulted from the high mortality experienced by tiny fish, due largely to predation. If that’s the case, you have to produce more offspring just so some will survive.
Here’s the quote from Fisher explaining that. Fisher was famous for his dense and often opaque prose, but maybe you can grasp his point:
“. . . it should be remembered that the production of offspring is only excessive in relation to an imaginary world, and the ‘high geometrical rate of increase’ is only attained by abolishing a real death rate, while retaining a real rate of reproduction. There is something like a relic of creationist philosophy in arguing from the observation, let us say, that a cod spawns a million eggs, that therefore its offspring are subject to Natural Selection; and it has the disadvantage of excluding fecundity [offspring production] from the class of characteristics of which we may attempt to appreciate the aptitude. . . [T]he historical fact that both Darwin and Wallace were lead through reading Malthus’s essay on population to appreciate the efficacy of selection, though extremely instructive as to the philosophy of their age, should no longer constrain us to confuse the consequences of that principle with its foundations.”
Fisher was a very smart man. So, if you teach how natural selection causes evolution, you might want to omit point #1 above—or at least qualify it.
*Shtulman, A. 2006. Qualitative differences between naïve and scientific theories of evolution. Cognitive Psychology 52:170-194.