Friday, December 15, 2006

Arms Races and Manipulation, Part 2

If evolution can be viewed as a sort of “arms race,” one often assumes that if an animal manipulates another, the victim will, via random mutation and natural selection, develop counterstrategies to escalate the arms race rather than capitulate. However, Dawkins shows several examples of how this is not necessarily so.

For example, the foster parents of the cuckoo continue to feed their changeling “child” despite the obvious absurdity of a tiny Garden Warbler straining to feed a cuckoo several times its size. Why doesn’t the Warbler recognize this incongruity, when apparently some host parents of cuckoos can indeed recognize flaws in the cuckoo’s egg mimicry? Why does the host parent recognize the parasitic egg but not the more obvious parasitic changeling fledgling bird?

Dawkins’s answer is that natural selection does not act uniformly at all times in any animal’s life. Selective pressure may be stronger at some points in the life cycle than others, or natural selection may have no effect on evolution even if a beneficial mutation were to arise. As an example of the first case, Dawkins points out that recognition of a cuckoo egg in one’s nest gives the host parent the chance to gain an entire breeding cycle, whereas recognition of the incongruous fledgling would buy at most a few days, and that probably too late for the host parent to breed again. Moreover, the actions of the cuckoo, its exaggeratedly gaping mouth, its size, could indeed act as a “drug” on the foster parent, no less than the song of a male nightingale acts as a drug on the female reproductive cycle (and, incidentally, upon the poet’s imagination).

However, a fascinating example of how an animal’s victimhood can be perpetuated is exemplified by slave-making ants. Some species of ants spend a great deal of their time raiding the nests of others and carrying off the larvae and pupae, which subsequently hatch in the new nest and begin to labor for their “masters.” This is a disturbing and puzzling development. Why don’t the enslaved worker ant colonies develop a resistance to the strange environment, filled with others not their genetic sisters—for example, by evolving a genetic disposition to cease work (to go on “strike”) when in a strange queen’s lair?

Remember that worker ants do not reproduce. Therefore, any beneficial mutation that arose in the enslaved ants would not be passed on to the rest of their home nest. At any rate, the raids do not happen often enough to destroy the victims utterly, who are under little selective pressure to evolve complex adaptive countermeasures against slave-making behavior on the part of others that, while aggressive, does not threaten the existence of the nest. An uneven battle ensues, in which the slave-making ants can be said to win the war.

Dawkins indicates that this situation is not unlike the phenomenon of a certain species of hybrid frog, which has one set of chromosomes that is jettisoned in meiosis and one set that is passed on to its offspring. The set of chromosomes (dead-end replicators) that is jettisoned in the hybrid frog is perpetuated in the pure bred species that carries two sets of these chromosomes (which become germ-line, not dead-end, replicators in this species). Thus, any beneficial mutation in the dead-end replicator line will be passed on in the pure bred species that contains two sets of these chromosomes (because in this species these sets of chromosomes are not dead-end replicators), but will not be passed on in the hybrid species. The situation of the enslaved ants is like that of the hybrid species of frog: their genes have phenotypic effects and can even be selected, but they will not be transmitted in the hybrid species, and thus are irrelevant to that particular species' evolution.

9 comments:

JanieBelle said...

My my you've been busy!

JanieBelle said...

Hey! It got through! I guess Blogger got tired of hearing people bitching about their crappy pressure tactics and turned our commenting privileges back on in the new Beta.

I don't buy the "accidental bug" excuse for one minute.

I'm exhausted, so I'll be back to make a substantive comment in the AM.

Kisses.

dogscratcher said...

"Why doesn’t the Warbler recognize this incongruity, when apparently some host parents of cuckoos can indeed recognize flaws in the cuckoo’s egg mimicry?"

Confirmation bias?

dogscratcher said...

"Why doesn’t the Warbler recognize this incongruity, when apparently some host parents of cuckoos can indeed recognize flaws in the cuckoo’s egg mimicry?"

Confirmation bias?

dogscratcher said...

Sorry about the double, my bad.

Kristine said...

My apologies for not getting back sooner. (Don't worry about the double post. I do it all the time.)

It's been a while so I need to ease back into the book to see what the heck I was talking about when I originally wrote these posts (at Amused Muse).

I think that Dawkins' answer (I assume you're not talking about Dawkins' confirmation bias) was that birds that could recognize the egg and roll it out of the nest had an evolutionary advantage, whereas once the bird hatches it's really too late to manufacture more eggs in the current season, so natural selection doesn't act upon the bird's recognition or not of the changeling chick. The chances of having a changeling chick with great egg mimicry is so slight (though it happens) that it represents no significant detriment to the host parents' genetic legacy for natural selection to arm the parents against such a selective blindness to live changeling chicks.

In other words, "fitness" (I hate that word) in one area can lead to disadvantages in another, but less crucial, area. It's always a trade-off. Adaptations always have costs, and this is one of them.

Greg Laden said...

For example, the foster parents of the cuckoo continue to feed their changeling “child” despite the obvious absurdity of a tiny Garden Warbler straining to feed a cuckoo several times its size. Why doesn’t the Warbler recognize this incongruity, when apparently some host parents of cuckoos can indeed recognize flaws in the cuckoo’s egg mimicry?

But they do, over evolutionary time.

Chicks have increasingly specific markings and parents increasingly specific recognition abilities. The ability of a cuckoo to parasitize a particular species depends on this. Cuckoos generally parasitize a particular species, but they occasionally try out alternatives. So there is variation to work on on both sides of the equation.

A really cool test of this idea is the following: Try to see how easy it is to parastizie a given bird species with a particular egg. Birds species with highly specific markings (i.e. on the inside of the chick's gaping mouth) are hard to fool, those with less specific markings are easier to fool.

Species that build closed nests (like hanging nests of weavers) are very easy to fool, while those with open nests are generally harder to fool. Parasitic birds can't really parasitize the hanging-nest birds because there is no way to get their egg in there.

These comments don't necessarily speak o the main point, but since the bird example is basic to the discussion, I felt moved to toss on this particular wet blanket....

Kristine said...

Thank you, Greg. Naturally, evolution is always happening and nature's "balance" is a moving equilibrium between self-tweaking strategies.

I find it curious that the closed nests belong to the birds that are easier to fool, since I don't consider the nest structure to be based upon an attempt to repel cuckoos (or am I wrong?).

Greg Laden said...

Regarding the closed nests.

That is a very interesting question. It could be that closed nests are built that way for some reason that has nothing to do with cuckoldry. But if cuckoldry is in play, you would expect that in those species males are building the nests. (After all, it is their potential evolutionary loss ... and a very scary one from a guy-bird's point of view). This is in fact what happens in (many?) weaver birds ... the males build the nests.

But even in species where closed nests are constructed for a reason unrelated to cuckoldry, we should expect those parent birds to be easily fooled (by an ornithologist with a thesis to write, anyway) because the fact that their nest is normally closed means they have never been under selection to have offspring recognition cues.

If memory serves, it may have been suggested that at least some birds build closed nests to avoid cuckoldry. I'm remembering an article in Scientific American from possibly the 1980, which may have made this assertion. But alas, that is pre-PDF, so even Googling will not help us now...