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.
The key finding: only female sticklebacks raised by a conspecific (same-species) father preferred own-species mates at a level greater than chance. Own-species preference scores are above the dotted line.
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.
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!