Wednesday, September 21, 2016

Not for the sake of the species

I frequently come across statements implying that a particular trait evolved because it increases the fitness of the species or that a behavior observed in some animal exists because it helps the species. I hear this from not only members of the general public, but also from biology students and even biologists whose work does not directly address evolutionary questions. Please be aware that this is almost entirely wrong.

In most circumstances, natural selection favors traits that increase the fitness of those individuals that have those traits. If a heritable traits is good for the species but bad for the fitness of those organisms that have it, then those that have it will tend to survive or reproduce less well than those that don't, such that in subsequent generations, the trait will be repeatedly rarer in the population. Even if it is good for the species but has no effect on the fitness of the individual, there is no strong reason to expect that it will increase in frequency.

In some special situations, selection can favor a trait that increases the frequency of a gene in the population, even if that gene causes the individuals that carry it to live less long or reproduce less well than individuals that don't have it. And there are indeed some cases where some biologists reasonably argue that selection occurs at the group level, with traits of the population or group determining which groups survive and which die out. But in almost any popular science context, if you imply that something is for the good of the species, you have gotten it wrong.

Note: this is something I started writing about a year ago, and never finished, until now. I had some particular example in mind, but don't know what it was.


Wendy in Vermont said...

Hi Dan,
I was recently reading a well accepted textbook on human development, and it mentioned the hypothesis that homosexuality in humans may have evolved because it reduced competition for opposite-sex mates and was therefore somehow good for the species at the societal level. I found myself wondering how homosexuality can be explained from an evolutionary perspective, since in my mind it would be an example of the kind of trait you mention here--a trait that would tend to reduce individual reproductive success. It occurred to me that perhaps there are some traits that cannot be explained through an evolutionary lens, but that are effects of other traits such as hormonal production in utero. Can you comment, please?


Dan Levitis said...

That hypothesis is one of many for the evolution of homosexuality. While it makes a certain sort of sense, there is no convincing evidence for it. The hypothesis is based in kin selection, the idea that behaviors can be selectively advantageous even if they decrease the lifetime reproductive success of the individual if they help the close kin of that individual even more. W.D. Hamilton, one of the leading evolutionary theorists of the 20th century, wrote that an individual should be willing to accept a fitness cost C to give a benefit of size B to a relative whose genetic relatedness to the individual is quantified as R if B*R>C. Homosexuality has been studied in that context, and some studies have suggested that there is a measurable B to relatives, but nobody I know of has found any case where B*R>C. I personally am skeptical of the kin selection hypothesis for the evolution of homosexuality, but note that it explains not in terms of "the good of the species" but in terms of the good of a gene that predisposes its carriers to homosexuality. The idea being that if I can give enough help to my family members who also carry the same gene I do to pass it on, I might pass on more copies of that gene by helping them than by reproducing myself. Kin selection and species-level selection have some similarities if you squint hard enough, but the underlying evolutionary processes and assumptions are very different.
The explanation of homosexuality that makes more sense to me is that it is not the result of selection for homosexuality, but rather of the fact that males and females share a huge portion of their genome, and development is an incredibly complex process subject to all kinds of environmental influences. There is enormous opportunity for the different developmental pathways that lead to male versus female humans to effectively mix, leading to a wide variety of phenotypes. In many of these, some traits that were selected for expression in males may express in females, and vice versa. This highlights another common misconception about evolution: that it is supposed to make things perfect. Evolution is a process that is founded upon happenstance and involves a tremendous amount of randomness. Evolutionary outcomes are rarely perfect in any sense, and there is no theoretical expectation that they should be. If one form of a gene leads to better reproduction or survival than others, there is a better than even chance it will increase in frequency in the population, but also some chance it will go extinct despite its advantage, and most potentially advantageous traits never arise at all, and therefore can't be selected for.
We know that homosexuality evolved. At some point in our evolutionary past (whether it be millions or hundreds of millions of years ago) it didn't exist, and now it does, and it has some biological basis. But we don't have any good evidence for, or against, the notion that it arose because it was selectively advantageous in some way.