Darwinian shift: survival of the smallest
(Page 2 of 2)
The reason: Trophy hunters focused on taking the largest rams with the largest horns. These rams typically were shot before they reached their peak reproductive years. So, with many of those animals gone, the gene pool narrowed to favor the smaller rams.Skip to next paragraph
Subscribe Today to the Monitor
The same trends and mechanisms also appear to be affecting two other wild sheep populations subject to the same management regime, says David Coltman, a University of Sheffield evolutionary biologist who led the team conducting the study.
Several factors have led to a deeper appreciation of the role contemporary evolution can play in the wild.
By the mid-1990s, an increasing number of researchers were finding examples of contemporary evolution - ranging from Darwin's finches in the Galapagos Islands and guppies in Trinidad to bacteria that quickly developed resistance to antibiotics. Moreover, researchers had new tools - from increasingly sophisticated statistical models to DNA sequencing. This allowed them to ask questions and test ideas in ways that hadn't been possible before, Dr. Coltman says.
Finally, "wildlife managers and people interested in ecology came from different schools from those of us who worked in evolutionary biology. We traditionally worked on different questions," he says. These days, an increased emphasis on interdisciplinary science has brought these groups of researchers together, he adds.
In some respects, several conservation approaches already account for evolutionary effects - if inadvertently - in the populations they seek to protect.
Coltman says that in Europe, for example, the size of the animals that hunters are allowed to bag depends on the experience level of the hunter. The biggest, wiliest game are reserved for those who have been hunting the longest.
Thus, human predation on game is spread more randomly throughout the game animal's population, and hunters are allowed to take females. The regime more nearly mimics predation found in the wild, and so doesn't put undue evolutionary pressure on the animals that can provide the most robust breeding stock.
Such techniques are more problematic in fisheries, says David Conover, marine ecologist at the State University of New York, Stony Brook. Historically, management regimes have specifically protected smaller fish while allowing the largest to be caught, he notes. "Rather than use a harvest strategy that mimics nature, fishing reverses the tables 100 percent."
Dr. Conover has studied the evolutionary effects on laboratory populations, harvesting only the heftiest of the fish known as Atlantic silverside. He says that for fishing techniques such as trawling, net design would make it difficult to toss back the biggest. Yet gill-net and long-line fishing, in which more fish are handled by humans as they are brought in, could be regulated in ways that reduce their evolutionary effect on fisheries, he adds.
Another approach, already being applied along coastal stretches of the United States, is to establish protected areas where fishing is banned and stocks are allowed to rebuild with as much of their natural genetic variation as possible, he adds.
For endangered species, the implications of managing for as natural an evolutionary future as possible holds its own challenges. For example, selection pressures on a species can vary greatly along the length of a river and within its tributaries, says Michael Kinnison, a biologist at the University of Maine in Orono who has studied the intersection of ecology and contemporary evolution.
Each population adapts to its local conditions. Thus, when it comes to reintroducing species to portions of a river from which it vanished, one can't always pluck salmon from one tributary and use them to populate another.