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.

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.

Years ago, after I met my future father-in-law for the first time, my grandmother asked me a curious question. Her exact words escape me, but the gist was: “So, is he a lot like you?” She figured that if my girlfriend liked me, I must remind her of her father. It’s a pervasive bit of folk wisdom: men go for women like their mothers, and women are attracted to men like their fathers. Right?

The evidence for such a pattern in humans is tenuous, but sexual imprinting – the process by which an individual’s mating preferences are influenced by its opposite-sex parent – is important in many species. It helps ensure that animals don’t mistakenly mate with the wrong species. And new research by Dr. Genevieve Kozak and colleagues suggests that sexual imprinting may also promote speciation – the process by which new species are born.

One way speciation can happen is if a single population splits into two, with each evolving unique “ecological traits,” such as dietary or habitat preferences. Biologists call this ecological speciation. Speciation isn’t complete until the two populations also evolve traits, such as species-specific mate preferences, that prevent them from interbreeding. This is actually more difficult than it sounds; since offspring inherit half their genes from each parent, mating preferences aren’t always inherited in tandem with ecological traits.

But what if a single trait affected a species’ ecology and its mating preferences? Such traits have been called magic traits. In a study published in Proceedings of the Royal Society, Kozak and her colleagues tested an intriguing idea: that sexual imprinting can transform an ordinary ecological trait into a magic trait. In theory, this was possible – if offspring imprint on an ecological trait that differs between species, that could create an automatic preference for own-species mates – but it had never been documented in nature.

Kozak and colleagues studied threespine sticklebacks (Gasterosteus sp.), fish that inhabit Canadian lakes created by retreating glaciers after the last ice age. In many lakes, sticklebacks have diverged into two forms: a benthic form that forages on lake bottoms, and a limnetic form that forages in open water. The two forms differ in several ecological traits, and they prefer to mate with partners of their own form. In other words, they seem to be two populations well on their way to speciation. For simplicity, let’s call them “species.” For decades, evolutionary biologists have studied sticklebacks to learn about speciation.

Male sticklebacks guard the fertilized eggs and newly hatched fry until they until they can fend for themselves. Could sexual imprinting explain the sticklebacks’ mating preferences? Kozak and colleagues tested this idea by matching stickleback eggs with “foster fathers” of either their own species or the other species. If sexual imprinting was occurring, the authors predicted that adult sticklebacks would prefer mates belonging to the same species as their foster father.

Sure enough, when tested later in life, female sticklebacks preferred males belonging to their foster father’s species, not their biological father’s species. Only females raised by a foster father of their own species chose own-species mates at a level greater than expected by chance. The mating preferences of male offspring were not affected by the species of the foster father.

Not every foster father showed the same parental diligence, so Kozak and colleagues asked whether specific parenting behaviors were related to the foster offspring’s mating preferences. They found that one parental behavior had the greatest impact on mating preferences: the amount of time the foster father spent depositing “nest glue” in the nest 4-5 days after the eggs were laid.

At this age, the embryos can smell but cannot yet see, suggesting that chemical cues (like the odors present in nest glue) guide their eventual mating preferences. These chemical cues depend partly on diet and habitat, so benthic and limnetic sticklebacks each have a distinct odor. Sexual imprinting creates an automatic association between a male’s odor and the mating preferences of his female offspring, transforming a simple ecological trait into a magic trait. In so doing, sexual imprinting promotes the evolution of populations that not only differ ecologically, but do not interbreed – in other words, species!

Sexual imprinting is widespread in animals, so how often does it play a role in ecological speciation? Is speciation more frequent, or more rapid, in species in which sexual imprinting occurs? These questions will require more data to answer properly. For now, we can say that for a couple of little fish that play a starring role in evolutionary biology, a girl’s attraction to guys like her father might just hold a key to the origin of species.

Citation:
Kozak, G., Head, M., & Boughman, J. (2011). Sexual imprinting on ecologically divergent traits leads to sexual isolation in sticklebacks Proceedings of the Royal Society B: Biological Sciences, 278 (1718), 2604-2610 DOI: 10.1098/rspb.2010.2466

Note: I wrote this post for a recent fellowship application, for which I had to provide an original writing sample. Since it was already written, and the application has been submitted, I figured it would be a shame not to share it here on the blog. Enjoy!