While teaching biology to college freshman, I noticed that some of my male students had shaved legs. As the years went by, more students showed up with hairless limbs, but I never really paid attention until one of my female friends told me that she “couldn’t stand men with hairy legs.” Since then, I’ve discovered that more and more American men are removing their body hair and that more and more American women prefer hairless men. Not surprisingly, women from many countries find shaved legs on men odd.
Cultural variation in women’s preferences for male characteristics is a fun discussion topic for dinner parties, but in nature, variation in these preferences among different populations of animals may lead to the birth of new species.
Speciation—the process by which new species arise can occur in many ways. Biologists think that a common avenue for one species to become several species is through changes in female mating preferences for male traits. A great example of this type of speciation may have occurred in a group of cricket species in the genus Laupala. This group of crickets has diversified into many species among the Hawaiian Islands.
Four of the 227 native species of crickets discovered by Daniel Otte. From left to right: Leptogryllus kauaiensis (a sword-tail cricket), Trigonidium crepitans and Lampula nigra (both tree crickets), and Caconemobius sandwichensis (a ground cricket). Illustrations are by Daniel Otte.
Male crickets from all islands sing songs to attract females. Females don’t sing, but depending on the island, prefer males that sing songs with different pulse rates. Scientists believe that before these crickets diversified into unique species, an original population of crickets had females that preferred males that sang songs at a specific pulse rate. When this original population got separated onto different islands, male pulse rates and female preference for specific pulse rates diverged among populations. Females on some islands chose males with faster pulse rates and on others chose males with slower pulse rates. Even if different populations from various islands came back into contact now, pulse rates and preferences are so different among populations, that crickets from different islands may no longer recognize one another as mates.
For these crickets to have diverged as described above, selection on male pulse rate needed to influence female preference for that trait in the same direction and vice versa. This would only work if the genes for male pulse rate and for female preference were inherited as a package. The problem is that when our bodies create eggs or sperm, a process called recombination shuffles the copy of genes we inherited from each parent to makes a new single copy of our genes that’s a mix from both parents. This shuffled copy of our genome is what we pass on during reproduction. Recombination reduces the possibility that the genes for preferences and pulse rate are inherited together.
One way for several genes to be inherited as a package is for the different genes to be physically located near to one another on the same chromosome. That way, recombination is less likely to break them apart.
Dr. Kerry Shaw and colleagues study speciation using Hawaiian crickets. They have pinpointed regions on certain chromosomes that contain the genes for male pulse rate. If female preference is controlled by genes that are physically linked to the genes for male pulse rate that might explain how these crickets diverged into unique species with different pulse rates and corresponding preferences for those pulse rates.
By selectively breeding crickets from slow- and fast-pulse rate cricket species, researchers moved regions of the chromosome controlling pulse rate from a ‘slow-pulse-rate’ cricket species into the genome of a ‘fast-pulse-rate’ cricket species, replacing the chromosome region known to contain genes for fast pulse rate with the chromosome region of the other species known to contain genes for slow pulse rate genes. The researchers then tested these females’ preference for male pulse rate.
Amazingly, ‘fast-pulse-rate’ females, now with ‘slow-pulse-rate’ genes, preferred males with slow pulse rates. This result suggests that the genes for female preference are located near the genes for song pulse rate on these cricket’s chromosomes. By giving ‘fast-pulse-rate’ females the genes for slow pulse rate, researchers also gave them the genes for preferring slow pulse rate.
Women’s preference (or lack thereof) for shaved men is not going to cause speciation in humans any time soon. But, the fact that the genes for female preference and male traits are inherited as a single unit in Laupala crickets helps explain how this group of insects has diversified into such an astonishing array of unique species. The physical linkage of genes for preference and male traits may be a common avenue for speciation in many other organisms.
Wiley, C., C. K. Ellison, et al. (2011). “Widespread genetic linkage of mating signals and preferences in the Hawaiian cricket Laupala.” Proc Biol Sc
Note: Similar to Neil’s post I few days ago, I wrote this post for a recent fellowship application.
