100th post: reversing evolution II: mimicry in snakes
This is a kilometerstone of sorts: my 100th post! Also, cumulative visits passed 10,000 this week. I know some blogs get more hits than that in only one day, but I used to spend hours preparing a lecture for 25 students, so I guess it's worthwhile to write a blog post for 10,000/100=100 readers. My readership trend over months seems to be slightly downward, however; I hope that's due to other blogs are getting better and readers having limited time, rather than my posts getting worse. Maybe I should be spending the time on my research or my Darwinian Agriculture book instead.
I recently wrote about mimicry in butterflies, then saw an interesting paper on how natural selection and migration affect mimicry in snakes. Selection and migration ("gene flow") are two of the four main processes responsible for evolutionary changes in the frequency of alternative genes in populations; the other two are the random ("drift") processes that can have a big effect in small populations but get smoothed out in large populations and, of course, mutation.
Selection and gene flow often act in opposite directions, because animals migrating into an area (or seeds or pollen blowing in) tend to be less well adapted to their new home, relative to animals or plants that have been evolving there. This general rule held up in this week's paper, as evident from the title: "Selection overrides gene flow to break down maladaptive mimicry", written by George Harper and David Pfenning and published in Nature.
The range of the nonpoisonous scarlet kingsnake overlaps with that of the poisonous coral snake. Snake-eating predators have learned or evolved avoidance of coral snakes. Where coral snakes are common, predators also tend to avoid king snakes, because they look similar.
But where there are no coral snakes, predators are not afraid to attack kingsnakes. Without natural selection for similarlity to coral snakes, kingsnake populations evolve, becoming less similar to coral snakes in appearance. In theory, this divergence could happen in either of two ways: natural selection (acting differently than where coral snakes are present) or random drift.
Consider random drift first. In a small population, one pregnant female might survive by chance when the remainder of a small local population is killed. Or she might migrate into an uninhabited valley and found a dynasty. Or a male snake might get lucky, for reasons unreleated to his genes, and father most of the baby snakes in some small area. Either way, that individual's genes will be strongly represented in that area, whether or not they would have been favored by natural (or sexual) selection.
Because drift operates in random directions, changes in the appearance of kingsnakes due to drift would occur differently in different parts of their coral-snake-free range. But that's not what Harper and Pfennig found. Instead, there was a consistent trend towards less black and more red, as distance from coral-snake country increased. So apparently we are seeing selection rather than random drift. But why more red? Wouldn't that make the snakes more visible to predators, rather than less? This is the opposite of what happened with the butterflies, which evolved to be less apparent when the distasteful species they mimic was absent. Even some poisonous snakes are well-camouflaged, like the one below I almost stepped on.
Could kingsnakes in areas free of coral snakes be mimicking some other distasteful or dangerous species? For example, could baby snakes benefit from mimicking the poisonous red eft salamander (also mimicked by some other salamanders!), with red color in adult snakes an unfortunate side-effect?
Photos from Cindy Tong and from Harper and Pfennig (2008).
If there's no selection to resemble coral snakes, where real coral snakes are absent, why hasn't natural selection (or perhaps drift) eliminated the resemblance altogether? Evolution operates across generations, but there have been a lot of snake generations since the glaciers retreated from the snake's territory. The authors hypothesize that gene flow could maintain the moderate resemblance to coral snakes. Gene flow would result from migration of snakes from coral-snake country into regions where coral snakes are absent, carrying in their DNA the instructions for making a body that looks somewhat like a coral snake.
If so, who is migrating, males or females? The authors looked at various genes to find out. The few genes in the genome of mitochondria (descendants of bacteria that moved into the cells of the common ancestor of all animals, a billion years or so ago, and now provide all animal cells with energy) are inherited only from mothers. They didn't find any evidence that these genes were moving, so apparently female snakes don't migrate much. Genes from the cell nucleus, inherited from both parents, did show evidence of migration. So male snakes are the travelers.
This reminds me of an earlier post, in which similar data provided evidence that male aurochs mated with domesticated cows, but not the other way around. Or at least, if any domesticated bulls mated with female aurochs, their descendants died out when the aurochs did.