Unnatural Selection

Wednesday, January 7, 2009

Everyone knows that over-hunting can decimate animal populations. One of the best-known examples is that of the passenger pigeon. This bird was once the most common bird in North America. It was not unheard of to see flocks containing an estimated billion birds flying overhead for days at a time, in a band a mile wide and 300 miles long. The flocks were so massive that they blocked the sun. At the time Europeans arrived in North America, it is estimated that there were up to five billion passenger pigeons inhabiting the continent. Unbelievably, due to massive over-hunting and a loss of habitat, the last known passenger pigeon, a bird in captivity named "Martha", died in 1914.

However, extreme over-hunting is not the only hunting threat to the survival of many species. It has been observed that hunting actually effects the evolution of species. In the upcoming Jan. 12 edition of Newsweek magazine, an interesting look is taken at the evolutionary effects wrought by the practices of poaching and "trophy hunting". In short, in some animal species, humans may be forcing a smaller-is-better scenario, and the ultimate outcome may be the extinction of some species even without over-hunting.

Big-game hunting is a rare and expensive proposition. Typically,  licenses for such hunting are either awarded on a lottery basis or auctioned for thousands of dollars.  So when a big-game hunter gets a possibly once-in-a-lifetime opportunity to hunt for a trophy to mount on his wall, he makes the most of it. He will be very selective, passing up smaller males in search of those with the biggest antlers, the largest tusks or the most beautiful manes, so the creature can be stuffed and displayed on a wall. Poachers, because they don't play by the rules, make an even larger detrimental evolutionary impact on the populations of species such as elephants.

This is where the evolutionary ramifications come into play. Characteristics like large tusks and antlers are the result of evolution. A specific elephant, for example, might have slightly larger tusks as a result of a random mutation. That elephant has a better chance of reproducing and propagating the big-tusk genes. That's because long tusks enable elephants to forage more efficiently, and defend themselves better against other males. Therefore, females seek them out as superior "alpha" mates. Unfortunately, the very characteristics that help improve a species are many times the ones that attract poachers and big-game hunters.

Therefore, poaching and big-game hunting have changed the evolutionary rules for many species. By hunting the largest and most magnificent of a species, the long lineage of mutation and natural selection that produce superior individuals is cut off. The average size of many species has gone down as a result - not just because the bigger animals are being hunted, but also because the prized parts of the species' gene pools are simply no longer there.

For example, biologist Marco Festa-Bianchet of the University of Sherbrooke in Quebec found a 25 percent decline in the size of horns on bighorn sheep over the past 30 years, and both male and female bodies are getting smaller. The logic is simple: male sheep with big horns tend to be larger and produce larger offspring. During the fall rut, or breeding season, these alpha rams mate more than any other males, by winning fights or thwarting other males' access to their ewes. Their success, however, is contingent upon their surviving the two-month hunting season just before the rut, and in a strange way, they're competing against their horns. Around the age of 4, their horn size makes them legal game—several years before their reproductive peak. That means smaller-horned males get far more opportunities to mate.

Bigger males with bigger attributes such as horns tend to father bigger offspring with similar attributes, causing the average size of a species to increase over time. With hunters targeting these trophies, smaller males are more successful at mating, so their genes are spread through the population more effectively and the average size shrinks. Females are forced to mate with males they would normally consider to be inferior.

Hunting (legal or illegal) of elephants for their ivory has also had a profound impact on tusks. Tuskless elephants, once a fluke that comprised about 2 percent of the population of African elephants, aren't sought as much by poachers (it's the ivory poachers usually want) so they breed more and are now 38 percent in one Zambian population, the Newsweek article states. Since one of the most important functions of elephant tusks is self-defense, this could have serious ramifications for the survival of African elephants as a species.

The same thing is happening in our oceans as well. Researchers have been aware for some time that the harvested fish of many species have been gradually decreasing in size. That's what happens when you take the genetically superior specimens out of a population.

Sources: Newsweek, LiveScience