Some thoughts on the Unit of Selection

We've been asking: what is the unit of selection? The answer that most biologists accept is, the unit of selection is the gene.

This question can be a little unclear. This is compounded by the way we tend to talk about the issue. Even Dawkins has a way of talking about gene selection that is very confusing. He continually uses teleological discourse to describe what he means to claim are non-teleological processes. Thus, when he says that a monkey caring for another monkey's child is a "mis-fire," he indulges in teleological talk. The claim is that the gene that leads the primate to care for a non-kin child appears unlikely to be replicated if it leads frequently to this kind of behavior. A "misfire" thus means, a behavior caused by gene(s) x that is not behavior of the kind that would lead to the replication of gene(s) x and might even inhibit the replication of gene(s) x.

Perhaps a good way to think about this question of selection is as a question of reductive explanation. Imagine the tree of terrestrial life laid out before you. We agree that evolution explains how the tree came to exist, and why it has the shape it has. We can now ask the additional question: is competition between genes alone sufficient to explain how the tree came to exist, and why it has the shape it has?

In a superficial sense, the answer is obviously no. The environment always matters, and the environment is not usually just another gene. But at least the local environment is assumed to exist for any of the explanations we might cite. Hence, the question is perhaps better put: given the environmental factors that all our theories of life will assume, can we explain how the tree of life came to exist and why it has the shape it has by adding to our explanation also just competition between genes? The claim by the genocentrists is that this is sufficient.

Also, obviously, physical constraints (what chemistry can and cannot do, for example) apply to all the kinds of explanation biology might provide. It would be trivial to say physics matters -- at least for some philosophers and biologists, since some philosophers and biologists assume physics will explain everything. The point is more like: there are constraints to what the gene explanation can do which arise from chemical and other physical limitations (e.g., an organism can only be so strong, it costs such and such energy to move at such and such a speed, cold inhibits life chemistry, etc.).

Thus, our question for the genocentric explanation is, are the following sufficient to explain the biological kinds and individuals we see?

gene selection &
local environment &
physical constraints.

Genocentrism says these three are sufficient. Alternative theories claim that at least in some cases a complete explanation will require reference to some additional kinds of things (groups, species, asteroids, etc.). That is, an alternative would be if some trait, population, or other feature of the tree of life had to be explained by reference to something other than just the local environment and the physics and the genes involved -- such as the species, the group, the clade, and so on.

Note that the struggle between reduction and anti-reductive views arises all the time. It is a key kind of issue in all of metaphysics. We always try to explain a phenomenon as simply as we can, but sometimes we find that we cannot produce an adequate explanation without adding extra elements to our theories. The genocentrists are saying we can explain terrestrial life with just gene competition and a few other things. Others are denying this. We can think of the genocentrists as taking the conservative, reductive position (everyone involved in the debate agrees that there is gene selection, so that's not in dispute; it would thus be most conservative or simple if it turned out that gene selection and a little bit more was sufficient).

(For these arguments, we should remember that all bets are off when we consider human beings. Human beings seem to muddle everything up. Even Dawkins is going to abandon his genocentrism when we get to human beings, and introduce the idea of the meme. He will not describe the meme as an exception to genocentrism, but in a way it is, if we include human culture in biology. But let us set homo sapiens aside for now, and talk largely about all life but human life.)

Dawkins recognizes, rightly so, two problems: sex and death. Sex jumbles up the genes. Death terminates a copy of the gene. Both seem to require some hard thinking if we are to propose how they can be consistent with genocentrism. We discussed another challenge: cases of eusocial organisms, which (at least at first glance) appear to suggest that group selection may be occuring. Finally, we considered a problem which Dawkins did not discuss: macro-evolutionary trends which do not appear to be well explained by micro-evolutionary continual progress.

I think it's safe to say that at this point, these debates are still alive, but mostly the genocentrists are considered victors. The exception is that it seems the kind of macro-evolutionary differences described by Jablonski would, if further corroborated, be widely agreed to require some emendation of simple genocentrism. The result would be that we would want a revised genocentrism that meant something like the theory that the tree of life will be explained by:

gene selection &
local environment &
physical constraints &
some special non-local events (like mass extinction events).

Adaptationism

One other issue deserves mention, and it will arise several times again as we talk about applications of evolutionary theory to things like human behavior. This is the view called sometimes "adaptationism." Sometimes this is meant as an epithet, but here I mean the term in the following sense: the view that, because of the long history of evolution and the many selection pressures put upon the ancestors of all existing organisms, we can reliably assume that each inherited structural feature of an organism was selected, and thus performs some function. A stronger but related adaptationist claim, often implicit, is that any possible trait that would be beneficial will ultimately evolve.

Adaptationism is a controversial thesis. For our purposes, as philosophers worried about the adequacy of conceptions and explanations, it raises some concerns about whether the kinds of uses of adaptationism are appropriate. What kind of worry? Well, note that when Dawkins, to pick one example, considers group-selection accounts, he immediately responds by saying that these kinds of accounts are implausible because a mutant will destroy the account. Thus, the group selectionist says that an altruistic gene in a group spreads because it helps that group overcome competing groups. The genocentrist says, this explanation cannot work because a selfish mutant member (that is, an individual without the altruism trait) of the group will exploit the altruism of the others, benefit disproportionately as a result, and soon the whole group will contain only the offspring of the self mutant. Now, consider how this response works: it assumes that the selfish mutant is always possible. That is, it assumes some form of adaptationism.

Contrast this with Dawkins's claims about the penis in mammals without a penis bone (see in our edition the endnote on page 304ff, which is an endnote to page 158 of chapter 9). He proposes that this evolved to signal a healthy heart. Now, that's not in the interest of the males, so why would that come about? He proposes that the females learned to contrast the erect and unerect penis of an individual, and so this creates a selection pressure for males to have a strong contrast, and this can be done with having a penis without a penis bone, but rather that must be inflated with blood under pressure. Now, some animals have retractable penis bones. Why didn't "cheater" mutants just evolve a retractable penis bone? The implicit answer seems to be that that's difficult to do. And indeed some kinds of evolutionary outcomes must be more difficult (less likely, require more energy, etc.) than others. But you see the problem: without some testable account of what is difficult to evolve and what is less difficult to evolve, we find the adaptationist thesis is in danger of being used when it supports genocentrism, and set aside when it is inconvenient. When we attack group selection, we trot it out and just assert that the relevant kind of mutant is possible; when on the other hand some feature seems inconsistent with genocentrism, we cite constraints on what's evolutionarily possible. The methodological problem is: how do we verify when such moves are appropriate, and when they are cheating?

[revised 2/12/2012]