Sunday, October 14, 2012

Human-induced evolution

ResearchBlogging.orgHuman activities have influenced that evolution of many species and not just through artificial selection. Our impacts on ecosystems, use of drugs and pesticides and our harvesting of wild populations is all having an effect on the rate and direction of evolution in many organisms. In fact, many of the frequently cited examples of 'evolution in action' are also examples of human-induced evolution, such as mosquito resistance to DDT and drug resistant bacteria.

The detailed studies on the peppered moth, Biston betularia, provide a classic illustration of evolution in action. The peppered moth is nocturnal, resting during the day on light coloured trees, where it is reasonably well camouflaged. However, during the industrial revolution, trees in forests between London and Manchester became covered in soot and dark coloured morphs increased in frequency from 0.01% of the population to 98% due to increased bird predation on the less camouflaged light coloured morph.

The light (top) and dark (bottom) coloured morphs of the peppered moth, Biston betularia (images Wikipedia)
Although we don't often think of ourselves as predators, hunting and fishing are essentially the same thing. Like predation on peppered moths by birds, they can produce evolutionary change in the target populations. For instance, trophy hunting of bighorn sheep, Ovis canadensis, results in sheep with smaller horns and lighter body weight over time. A couple of recent studies show that behavioural traits are selected too.

Bighorn sheep in Montana (image Wikipedia)
Using GPS devices, Ciuti et al. tracked 122 (77 females and 45 males) elk, Cervus elaphas, to monitor their movements over the course of a year. The males that were the most likely to fall victim to hunters were those that moved more often, traveled the furthest and made greater use of open areas. The pattern was similar, but less pronounced, in females. Older females tended to move less and use of open areas less than younger females, suggesting that they may learn to avoid hunters. They could not assess learning with age in males as all the tracked males were of the same age.

A male elk (image Wikipedia)
Ciuti et al. suggest that the bolder behaviour of the elk that were harvested may provide them with protection from other predators, like wolves and bears. Moving long distances and using open areas may make it easier for elk to avoid natural predators, but it favours harvesting by humans with high-powered rifles. It's a neat hypothesis and they say that they intend to test it in future experiments.

A second study in rainbow trout, Oncorhynchus mykiss, looked at growth rate, a trait closely correlated with activity rate. To fuel a fast growth rate, it's thought that fish must spend more time actively searching for food, which is supported in the literature. Biro stocked four fishless lakes in Canada with trout that were slow-growing, intermediately-growing and fast-growing. By stocking the lakes, Biro knew the numbers of fish present in each lake and in each experimental group. He then randomly sampled the four lakes using a sampling method that wasn't size-selective.

Rainbow trout (image US Fisheries and Wildlife Service)
There was substantial variation in the proportion of each of the experimental groups that was caught in each lake. However, faster growing trout were consistently more likely to be caught than intermediate or slow-growing trout. Overall, fast-growing trout were nearly twice as likely to be caught than the two groups with slower growth rates. Importantly, size did not matter; small, slow-growing fish were still less likely to be caught than small, fast-growing fish.

Biro's study has the issue that it did not directly assess behaviour, but relied on growth rate as a proxy measure. However, it is consistent with other studies that show fish personalities influence the probability that they are caught by different collection techniques. Bluegill sunfish, for instance, are more likely to be caught in the wild by angling when they're less active. Intriguingly, there is also an interaction between habitat and capture method as less active bluegill sunfish are also less likely to be caught by angling in the open areas of artificial ponds.  

I'm troubled by the correlation between growth rate and supposed personality traits. It suggests that what is being measured as personality might actually be a by-product of physiology and not a separate trait. But, other studies I looked at showed that in some situations less active fish grow faster than more active fish, which suggests that they are independent traits.

In any case, the Cuiti et al. and Biro studies show quite nicely that humans are probably influencing the direction of evolution in the populations that we harvest by hunting and fishing. Their work adds to a growing body of research that humans are influencing the evolution of many species. Indeed, Stephen Palumbi has argued that humans are currently the World's greatest evolutionary force.

Biro PA (2012). Are most samples of animals systematically biased? Consistent individual trait differences bias samples despite random sampling. Oecologia PMID: 22885993  

Ciuti, S, Muhly, T B, Paton, D G, McDevitt, A D, Musiani, M, & Boyce, M S (2012). Human selection of elk behavioural traits in a landscape of fear Proceedings of the Royal Society: B, 279 (1746), 4407-4416 DOI: 10.1098/rspb.2012.1483  

Palumbi, S R (2001). Humans as the World's greatest evolutionary force Science, 293 (5536), 1786-1790 DOI: 10.1126/science.293.5536.1786  

Wilson, A D M, Binder, T R, McGrath, K P, Cooke, S J, & Godin, J J (2011). Capture technique and fish personality: angling targets timid bluegill sunfish, Lepomis macrochirus Canadian Journal of Fisheries and Aquatic Sciences, 68 (5), 749-757 DOI: 10.1139/f2011-019

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