Australian researchers have provided the first support for a 60-year-old evolution theory – that species with multiple colours in the same population evolve into new species faster than those with one.
The tendency of “colour polymorphic” species to evolve more quickly was predicted in the 1950s by the renowned English evolutionary biologist, Sir Julian Huxley, but has only now been confirmed by University of Melbourne scientists.
For their study, zoologists Devi Stuart-Fox and Andrew Hugall gathered many decades of natural history information and 25 years of genetic sequence information from around the world to build family trees for thousands of birds. Their findings are published in the journal Nature.
Dr Stuart-Fox, from the University’s Zoology Department, said that in three families of birds – the hawks and eagles, the owls, and the nightjars – the presence of multiple colour forms had led to rapid generation of new species.
“Well known examples of colour polymorphic species in these families include the Australian grey goshawk which has a grey and pure white form, the North American eastern screech owl and the Antillean nighthawk, each with grey and red forms,“ Dr Stuart-Fox said.
The pair focused on birds because although colour polymorphism occurs in many animals, such as fish, lizards, butterflies and snails, there is a wealth of information on colour variation in birds, as well as on species classification, partly thanks to birdwatchers, otherwise known as “twitchers”.
The pair looked at five bird families with a high proportion of colour polymorphism and compared their rates of evolution with those with only one colour form.
By modelling evolutionary rates using publicly available genetic information, they found that colour polymorphism speeds up the generation of new species.
Colour polymorphic species tend to evolve into species with only one colour form (monomorphic), explaining why existing species with different colour forms are relatively young and also rare.
The study found that colour polymorphic species were younger not only in the birds of prey but in the songbirds, which account for more than half of the world’s bird species.
Study co-author Dr Andrew Hugall noted that when Sir Huxley first proposed the theory, he did not have the sufficient data to support it.
“We were able to generate massive family trees, such as a tree of more than four thousand songbirds, needed to model rates of bird evolution in this study,” he said.
“Now that we’ve identified this pattern for the first time, our next step is to test some of the explanations proposed for why colour polymorphism leads to accelerated evolution.”
The Director and Research Leader of the Australian National Wildlife Collection, Leo Joseph, said that there was greater potential for natural selection to “see a species with more colours and therefore in some way to confer an advantage on that.
“It might be that one colour form is better camouflaged in a particular environment than another. It might be that one colour form is better at keeping heat in cool climates. Then there’s a reproductive advantage that can be seen and translated into a higher number of offspring.”
Dr Joseph, who was not involved in the study, added that “ultimately, it’s a mutation that causes the polymorphism in the first place. That provides the raw material. Then you have a range of ecological, geographic and genetic factors that accelerate reproduction and generation of new species.
“I think the big picture that they’ve accomplished in this paper is to look at this in the context of an evolutionary tree, so it’s not just going through a bird book and scoring the groups that have colour polymorphism and those that don’t. It’s doing that on the evolutionary tree, so that you know who’s related to who and you can plot on the tree where it happened. You can start to deduce something of the forces driving it.”