New mechanism of evolution uncovered

Natural selection is the hard core of evolutionary biology. Yet it's not the only mechanism that can result in evolutionary change within a population.

Australian scientists have now outlined another mechanism that can drive evolutionary change, which they call 'spatial selection.'

The finding has implications on how we understand evolution to operate, providing another tool in the evolutionary toolkit for explaining how populations change not only through time but also over space.

It is generally accepted that there are two mechanisms that can account for evolution: natural selection and genetic drift.

The former is where some organisms have greater lifetime reproductive success than others, and thus yield a greater number of offspring that themselves survive to reproductive age.

They then pass on heritable traits to the next generation, and these traits increase in frequency within the population compared to traits held by organisms with lesser lifetime reproductive success.

Genetic drift is a stochastic – or 'random' – variation in traits that occurs over time, particularly in traits that are neutral in respect to survival value.

Now, the researchers from the University of Sydney and James Cook University have outlined another mechanism by which species can evolve based on their movements through the environment.

Taking cane toads as a paradigm example, they describe how the toads at the frontier of the population – i.e. the toads that have migrated the farthest from their place of birth – tend to possess traits that aid their dispersal, such as longer legs and greater endurance.

Given these toads at the frontiers mate with other toads in the same region, they tend to share such dispersal-enhancing traits. When they mate, additive genetic variation will tend to compound these traits in subsequent generations.

As a result, the offspring of toads at the frontier are even more likely to travel farther than the previous generation, thus compounding the effect when they, in turn, breed.

“The toad’s Australian invasion can be thought of as a race, with only the fastest toads at the frontline," said lead author, Professor Rick Shine, from the University of Sydney’s School of Biological Sciences.

"These athletic toads inevitably breed with each other, because all the slow toads have been left behind. Some of their offspring inherit 'speedy' genes from both parents and thus the rate of invasion increases every year.”

Such spatial selection isn't accounted for by natural selection or by genetic drift, and the authors argue it represents another vector of evolution, and should be considered as a separate process from natural selection.

The study was published today in the journal, Proceedings of the National Academy of Sciences and is available free online.