Manes, trains and antlers explained
August 21, 2008For Charles Darwin, the problem of the peacock's tail, in light of his theory of natural selection, was vexing in the extreme.
Indeed, in 1860, writing to Asa Gray, his most ardent American champion, Darwin confessed: "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!"
In his struggle to explain why such extravagant and seemingly burdensome features existed, the great English naturalist struck upon the idea of sexual selection -- that showy traits such as the Peacock's ornamentation were an advantage in the mating game that outweighed other disadvantages.
A team of Wisconsin scientists has turned from the question of why such male traits exist to precisely how they evolved. They have worked out the molecular details of how a simple genetic switch controls decorative traits in male fruit flies and how that switch evolved. By extension, the work explains the mechanics of how the male lion got his mane, how the bull moose acquired such an impressive set of antlers and, yes, how the peacock got its magnificent tail.
Writing in the latest edition (Aug. 22, 2008) of the journal Cell, a team led by University of Wisconsin-Madison molecular biologist Sean Carroll describes the regulation and evolution of a genetic circuit in fruit flies that permits the male to decorate its abdomen. The work also shows how the regulation of the same genetic circuit in females represses such ornamentation.
"This study is about the how, not the why," says Carroll, a Howard Hughes Medical Institute investigator and one of the world's noted evolutionary biologists. "How can this trait be made in one gender and not the other?"
The question of the origins of secondary sexual characteristics -- traits other than reproductive organs that are peculiar to one gender or another -- is one that dominates modern evolutionary biology, says Thomas Williams, a UW-Madison postdoctoral fellow who helped lead the study. "Males and females basically have the same set of genes, so how do you specifically modify the activity of a male's genes but not a female's genes?"
The answer, according to the new Cell report, resides in the genetic repression of a protein in the male fruit fly that permits it to color the tail end of its abdomen.
"The flies did not need new genes to make a new pattern," Carroll says. "They just changed how males and females use a common set of genes."
The genetic switch that governs expression of the protein, Carroll notes, is ancient and originally evolved for an entirely different purpose, but over time mutations accumulated, perhaps in response to sexual selection, that drove the evolution of male flies with more colorful derrieres.
"The switch existed for tens of millions of years because it had a different job," says Carroll. "But it got remodeled. Evolution is a cumulative process. You have this machinery and it's easy to add a bell or a whistle. With this particular trait, it evolved by exploiting (genetic) information that was already there to make male bodies different from female bodies."
According to Williams and Carroll, the study provided no evidence that the ornamentation process ever occurred in females and was subsequently repressed. "We have enough evidence to believe this evolved in a male-specific way," says Carroll.
The same process, Carroll and Williams argue, is at play in animals from humans and elephant seals to fish and beetles. There is a world of exaggerated traits in animals and evolutionary biologists today, like Darwin 150 years ago, are engaged by the question of what advantages they confer.
"These are the most rapidly evolving traits in evolution," Carroll explains. "If female tastes change, these traits go away. There is no reinforcement.
"It's a tradeoff," Carroll concludes. "As long as the gain outweighs the cost, the feature will survive. The fruit fly's color pattern is a paradigm for understanding how to use the same sets of genes in different sexes to come up with different features."
Source: University of Wisconsin-Madison
Apr 20, 2006 |
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variation in the extent it's put into effect? In other words why is the peacock's tail so extravagant?
Males who are more noticeable to females may tend to mate more, that's the basic idea of sexual selection.
The limiting factor is that males who look/sound/smell/behave too differently from other males may end up mating less. It's a fine balance: "just enough" phenotypic change to get the genotype propagated.
With this in mind, developing extravagant features are a natural outcome - for example, having a bigger or brighter tail than the other males, is a great example of "just enough" change.
The crucial link, not often brought up, is that sexual selection also has a natural selection benefit.
In particular, males who are more noticeable to females are also more noticeable to predators, they are more noticeable to their prey, and they need to expend more energy to support themselves
As a consequence, with more visibility there comes a distinct advantage to being smarter, faster and/or stronger.
Then to bring it around full circle, females who prefer more noticeable males will, as a side effect, be selecting males who are smarter, faster and stronger. These genes are the "payload".
Sexual selection is really just a form of natural selection. Instead of predator-prey interaction, it's male-female-predator-prey interaction.
If all the other genes of the superior male peacock along with the genes for the most attractive tail were passed on then I would feel more comfortable - but as far as I understand it this won't be so. That would mean that a proportion of the peahen's male chicks will suffer a disadvantage thus reducing her capacity to spread her own genes. Lumbering your own progeny with such a ridiculous burden to their survival chances hardly seems like motherly devotion or a successful stategy to me.
There must be some kind of balance between sexual success and survival success. Males that aren't so attractive won't have much success at mating but males that are dead have even less.
Sexual selection and natural selection seem to be pulling in opposite directions and in the case of the peacock bizarrely so.
Does the peacock inhabit a niche where it can afford to spend so many resources on its tail? Or has a snowball effect taken hold of the peahen's fussiness to the detriment of the species?
I think that if Darwin felt sick at the sight of the peacock's tail we should at least feel queasy.
If a peacock's tail is hard to accept, image starting with a male who looks like a peahen, and just working from there. It's very unlikely that a mutation would generate a complete peacock tail from nothing, but incrementally, you can get some males with maybe a slightly longer tail, others with a slightly iridescent feathers, others with stronger tail muscles to display better. These are minor mutations, and in wild populations, you'll see offspring with these kinds of variations.
After enough generations and combinations, eventually you get some with all three mutations: slight longer tails, slightly iridescent feathers, slightly better displays. These birds might look considerably different from the original peahen-like male. Maybe it turns out that so much ornamentation is a disadvantage, and these triply endowed guys don't do so well, or maybe they do better. As you mentioned, it's a balancing act.
I think of sexual selection vs. natural selection pulling more sideways than in the opposite direction. They both get to the same outcome, but different paths. There are other cases where the relationships are very complex and evolution doesn't follow a simple path: mimicry and parasitism being two big examples.
If a lion has a bigger mane than his opponent then he gains an advantage (even though it'a only a bluff). This is a kind of sexual selection I can accept because the lion expends trivial amount of resources developing the mane. He has fur on his neck anyway - he just grows it longer. Large gain for small outlay. Profitable!
The moose's antlers require much more in the way of resources - but this is offset by the advantage it has for the moose's survival. Predators will be deterred from targeting the bigger looking moose and will select females, calves, immature males etc first. So the moose's antlers make sense - large outlay, large gain - but still profitable.
But what advantage does the peacock's tail have other than impressing peahens? None. Worse than that it actually handicaps the peacock. Large outlay, low or even negative returns, resulting by my accounting in a Loss. In this case the pressures due to sexual selection and to that of natural selection seem to be out of balance and are pulling in different directions.
Possibly it can be explained by considering that the peahen occupies a niche whereby she is spoilt for choice. If one male could mate with ten peahens, say, losing 90% of males to predators etc wouldn't adversely affect the viability of the species. The peahen can thereby quite arbitrarily indulge her preferences, resulting in a runaway gratuitous selections. This, however, wouldn't explain why other birds haven't followed the same path.