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Choosy mothers may choose wisely

Two papers on sexual selection in birds this week:
Adaptive Plasticity in Female Mate Choice Dampens Sexual Selection on Male Ornaments in the Lark Bunting , published in Science by Alexis Chaine and Bruce Lyon, and Natural and sexual selection against hybrid flycatchers, published in Proceedings of the Royal Society by Nina Svedin and colleagues.

Evolution of the peacock’s extravagant tail presumably required many generations of selection, over which peahens must have been reasonably consistent in their preference for a longer tail. But are female mate preferences always so consistent?

Chaine and Lyon found that, in lark buntings, various male traits influenced their reproductive success, both in winning a “social mate? and in number of offspring (including those with another male’s social mate). However, patterns that were statistically significant in one year were sometimes reversed in other years. For example, males with larger wing patches were more likely to win a mate in 2000, but less likely to win a mate in 2002, due to changing preferences of individual females. These changing preferences were apparently not random, as “there were significantly more matches than expected by chance between the traits of males chosen by females and the male traits associated with nesting success within years.?

This is a remarkable result: apparently female buntings had some ability to tell what kind of male would increase their reproductive success in a given year, and then tended to choose those males. I hope someone will follow up on this to see whether this result can be confirmed and, if so, how it works. Why, for example, would having a bigger wing patch be better in one year and worse in another? And how do the females tell, in advance, whether this is will be a big- or small-patch year? Even if changing female preferences turn out to be more arbitrary, they have important consequences. For example, the authors noted that fluctuating patterns of sexual selection are analogous to fluctuating patterns of natural selection by climate. Darwin’s finches are a famous example: Peter and Rosemary Grant found that wet years cause rapid evolution of beak shape, but long-term trends are slowed by selection in the opposite direction during dry years.

The second paper looked at the reproductive success of hybrids between two flycatcher species. Hybrids are often assumed to have intrinsically lower fertility. Female hybrid flycatchers are sterile. Male hybrids tend to have lower reproductive success, but is that due to defective sperm, say, or is it because nobody wants to mate with a hybrid?

To find out, Svedin and colleagues compared the reproductive success of hybrid males to similar-looking purebreds. The males that looked like they could be hybrids were less successful at mating. They concluded that “sexual selection? against male hybrids accounts for approximately 75% of the reduction in their fitness.? As in the other study, females that could, in theory, make “foolish choices? tended to chose males with whom they were more likely to have healthy chicks.

Comments

Mate selection is a clear example of a situation where a species can indulge in active goal-directed evolution. The males in these situations act as focal points where females collect. Ignore the chauvinist view that they must be admiring the male. What they have before them is an array of females with their varied numbers of daughters. A neat histogram of those characteristics that are currently resulting in reproductive success. This allows the hen birds to actively manage their own evolution, which is far more effective than relying on chance and consequently leads to a greater proportion of active evolution.

If you have difficulty with the idea of species actively evolving note that there are only three conditions required for active evolution: 1) at the time of active evolution for the species performing the evolution there must be a trigger stimulus in the environment. 2) at this time, for this species there must be a response behaviour that moves the genome in a favourable direction and 3) the combination of circumstances must occur suficiently frequently for the advantageous combination of stimulus and response to be naturally selected. These conditions are met for large environmental changes where some species are disadvantaged more than others. A disadvanted species has an example of a better genome for the environment for observation and any behaviour that results in acquisition of genes from the successful species will tend to move its genome toward a succesful genome with an associated increase in survival.

Would you like to rephrase your ideas as a scientific (i.e., falsifiable) hypothesis? If so, please explain what experimental results would disprove your hypothesis. No? I didn't think so.

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