Since Charles Darwin wrote "The Origin of Species" more than 150 years ago, it's been known that nature's selection creates some species and ends others.
But researchers at the University of Maryland, Baltimore County suggest that another actor is responsible for driving wedges in animal populations to create new species — mothers themselves.
A study published last month in the journal Ecology Letters suggests that female creatures' sexual preferences may launch an evolutionary process that can lead to the creation of new species. Whom they choose for mates trickles down through generations in such a way that, combined with other factors, may make a population distinct from others with which it once intermingled.
It's a theory that is gaining acceptance relative to the idea that adaptation to differing environments — Darwinian "natural selection" — is mainly, if not solely, what drives evolution of new species.
"The sexual environments of two populations don't have to be different," said Tamra Mendelson, an associate professor of biological sciences at UMBC. "There's just different ways of being sexy, or being attractive, or being competitive in a sexual context."
The traditional Darwinian view holds that over time, species adapt to their environments in such a way that protects them from predators, gives them an advantage over prey and helps them compete for food — "survival of the fittest." Environmental differences thus encourage species to diverge, as groups adapt to their particular circumstances.
Mendelson's research doesn't discredit that idea but suggests that environmental differences aren't required for new species to branch off from existing ones.
She began testing the concept after finding no apparent differences in the environments of different species of darter fish, small perch-like swimmers found in freshwater streams in 200 varieties. Despite living in water with similar depth and speed, for example, males of differing species can vary widely in their coloration, a form of "sexual ornamentation" to compete for mates, Mendelson said. Females are often nearly indistinguishable from one species to the next.
So, Mendelson wondered, what happens when a new coloration arises through mutation — that is, just by genetic chance — and immediately is irresistible to female darters? She enlisted the help of Samuel Flaxman, an assistant professor of ecology and evolutionary biology at the University of Colorado, Boulder, to use computer simulations to answer the question.
Models accounting for darters' genetic makeup and expected success in mating, as well as different factors of female preference or male-to-male competition, produced expectations of the genes and traits of future generations. The models showed that over time, the preferences and competition could inflate differences between populations, making one trait proliferate in one group and a different trait come to dominate another group, because of a combination of sexual preferences and competition, and genetic chance.
Mendelson suggested that similar groups of female darters might have the same predisposition toward particular preferences in a mate, but which traits become popular — and thus, how species diverge — depends on what mutations occur. She likens "attractive" darters to actors like Pierce Brosnan and Daniel Craig — both are handsome versions of British secret agent character James Bond, though in different ways. But they're still the same species, and so might be the darters.
While the simulations don't prove that sexual selection can lead to speciation, or the creation of new species, Flaxman said they suggest that sexual selection can catalyze the process.
One widely used standard to determine whether a new species has formed will be used to test Mendelson's research further — the ability for different species to interbreed. She plans to test whether different types of darters are willing to mate, and whether it's biologically possible for them to successfully reproduce.
In the meantime, researchers said Mendelson's paper could spur discussion and more study of the factors influencing speciation. Most research in the field has focused on speciation from a more traditional perspective, exploring the effects of differing environmental factors, said Brian Langerhans, an assistant professor of biological sciences at North Carolina State University, who wasn't involved in the study.
"Maybe we need to rethink some of the cases where we've previously concluded that selection wasn't that important in speciation," he said.
Though the concept of sexual selection's influence on species evolution has been gaining credence over time, there has been little research testing it. Mendelson's paper could change that, and bring more clarity.
"Certainly the idea that sexual selection can do the same thing is not a new idea," Flaxman said. "Its importance in the process of forming a species remains a bit more up in the air."