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A new study seems to indicate that evolution is mainly the result of time and luck rather than being dependent primarily on environmental adaptation.
One reason scientists are skeptical is that Hedges’ clocklike pattern conflicts with the traditional picture of how evolution unfolds. “The classic view of evolution is that it happens in fits and starts,” Benton said. A change in the environment, such as a rise in temperatures after an ice age, might spark a burst of speciation as organisms adapt to their new surroundings. Alternatively, a single remarkable adaptation such as flight in the ancestors of birds or hair in mammals might trigger a massive expansion of animals with those characteristics.
Hedges argues that while such bursts do occur, the vast majority of speciation is more prosaic and evenly timed. To start, two populations become separated, driven apart by geography or other factors. New species emerge every 2 million years, on average, in a metronomic rhythm tapped out by the random nature of genetic mutations. He likens the process to radioactive decay. It’s impossible to predict when an individual radioactive nucleus will decay, but a clump of many atoms will decay at a highly predictable rate known as the material’s half-life. Similarly, mutations strike the genome randomly, but over a long enough time the accumulation of mutations follows a pattern. “There is a kind of speciation clock ticking along,” Hedges said.
Consider Hawaii’s honeycreepers. The speciation clock started once the birds migrated to a new island and began to accumulate random mutations. The vast majority of these mutations were neutral, having no effect on the birds’ appearance or behavior. Occasionally a beneficial mutation appeared, such as one that made the beak longer and its bearer a more efficient hunter. According to the traditional model of speciation, the adaptations eventually made the two populations too different to interbreed even if they were to come back into contact. In this scenario, adaptations drive the creation of a new species.
But Hedges contends that speciation and adaptation are two distinct processes, each proceeding along its own path. (A team led by Mark Pagel, an evolutionary biologist at the University of Reading in England, has made a similar proposal, though for different reasons.) According to Hedges’ model, after about 2 million years the two groups of birds accrued so many random genetic differences that they became incompatible. It wasn’t adaptive mutations that made it impossible for the birds to intermingle, but rather the accumulation of enough mutations overall, most of them neutral ones. Geographic isolation provided the necessary spark for speciation, but simple time drove the process to its conclusion.