Human Interference Causes Evolutionary Changes

Today, I read a story in the latest issue of New Scientist that discussed how human activities are driving evolution of animals in dramatic and often unexpected ways. In effect, we have turned earth into a large uncontrolled evolutionary laboratory.

Biologists are struggling to understand what is happening although there is no shortage of species that are evolving in response to human interference. For example, chinook salmon in the Snake River are growing smaller and smaller, possibly as the result of dam construction. Additionally, the fish are apparently putting off migration out to sea so they can overwinter in the reservoirs behind the dams before making the trip the following year. If the dams were removed, it is unclear whether these slower growing salmon would even survive.

Hybridization between domestic and wild populations also is occurring. For example, second-generation hybrids of wild and farmed Atlantic salmon show differences in gene expression that are greater than those seen in either pure farmed salmon or pure wild salmon. This observation suggests that the effects of hybridization will not be diluted out quickly.

Another astonishing example of unnatural selection reveals two diverging populations of Darwin's Finches are collapsing back into one species because people in nearby settlements are putting out birdfeeders for them. As a result, the finches do not need different beak sizes as they adapt to different diets.

These examples also raise the issue of the hidden effects of captive breeding. Currently, nearly 250 species of animals are kept in captive breeding situations, waiting for the day when they can be reintroduced into the wild. However, the more generations that animals are kept in captivity, the more they adapt to confinement -- which is deleterious to living in the wild.

Another effect of human interference is seen when a particular ecosystem is "re-wilded" by introducing long-lost species or by substituting a lost species with others that seemingly perform the same function. For example, red squirrels were a black spruce cone predator in Newfoundland, but they had been absent for 9000 years. As a result, the spruce trees lost their defenses against the squirrels and instead developed defenses against red crossbills by evolving thicker scales as the birds developed larger beaks.

But after the red squirrel was re-introduced into Newfoundland in 1963, the red crossbills were outcompeted for the spruce seeds and went into a dramatic decline. The birds' current population is only approximately 500 individuals.

These data are even more alarming in view of global climate change, which we are quite ignorant about. Researchers have documented changes in numerous species that are linked to increasing temperature, but it is not known if these constitute shifts in behavior in response to environmental changes, known as "phenotypic plasticity", or if it is the result of evolution.

Loren Reiseberg from the University of British Columbia in Vancouver, Canada, has been studying variations in traits including body size and the timing of reproduction in 126 plant and animal species living mainly in northern temperate regions. He has found that the rate of adaptation needed to keep pace with global warming exceeds the theoretical maximum for species with generation times longer than a couple years.

"These are crude, back-of-the-envelope calculations," Reiseberg stresses. indeed, some biologists argue that we don't know how to calculate a theoretical maximum for the rate of adaptive change.

However, biologists agree that efforts must be made to preserve hotspots of adaptation and genetic diversity while allowing species to migrate into neighboring habitats are conditions change. So far, California is the only state that is testing evolution-based conservation.

Last November, California state's voters approved Proposition 84, a plan to raise $5.4 billion to invest in environmental improvements. A team led by Craig Moritz of the UCBerkeley was asked to advise the state on how to preserve the state's evolutionary heritage.

To do this, Moritz and his colleagues are producing a series of maps that document hotspots of recent evolutionary change for most groups of vertebrates and some plant species. This should help protect the genetic diversity that species need to respond to global warming. Additionally, Moritz suggests that future reserves be connected by corridors that species can use to move to higher elevations as temperatures rise.

Cited Story: Humans Take Control of Evolution by Peter Aldhous. New Scientist 193(2591):6-7 (17 February 2007).

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