Saturday, March 28, 2009
Bighorn Sheep and Unnatural Selection
On page 254, Sean Carroll talks about the bighorn ram. Female bighorn sheep desire male bighorn sheep with the largest horns; bighorn sheep grow their horns usually between the ages of two and four years old. However, “rams at on locale in the Canadian Rockies have shown a marked downward trend in their “breeding value”” (255). Why is this happening? How would this be a selective advantage to the bighorn ram? Are there any other examples in the book or otherwise that is similar to this (where an outside factor/group affects a species)?
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The Colorado bighorn suffers from a plague of many wild animals that have horns: the redneck hunter. The hunter actually judges his kill on the basis of the antlers, or the horns, the best looking of them get killed off to become a plaque above the fire place wall. However, with the bighorn, the meat is also apparently very tasty, also contributing to its hunting. The Colorado bighorn, besides being killed off and stuffed, has had to deal with environmental factors- in 81 and 82, there was an outbreak of pinkeye among the animal, causing deaths among the animals due to them simply just walking off the cliffs.
ReplyDeleteAnother thing that helps thing the bighorn ranks is the separation of male and female during the summer- males usually move up to the summits, while females hang out on treacherous pathways or in caves. It’s easier to find something, as well as kill it and retrieve, if its on a flat, relatively open surface, compared to hiding in a cave or on the side of the mountain, as the ram would fall down the mountain if shot or run forward if disturbed.
It’s not a selective advantage at all- those rams with the biggest horns are often the toughest, and thus to be forced to pass on the genes of a weaker animal would eventually cause a species to be come weaker and weaker- eventually unable to fight off a predator, such as a coyote or a wolf, and perhaps severe concussions to the rams could occur during mating season- over time, skull bone where the horns are located, would decrease in size, and when the rams butt head, as is mating custom, in order to find a mate, concussions could cause severe damage in the animals.
This type of overhunting, unnatural selection, unfortunately is something that we see, and almost always are responsible for one way or another. Deer, for instance, are frequently hunted and their packs are constantly being thinned of some of the better males suited for reproduction- a hunter bases the points of their kill, as the phrase it, on the number of antler tips on the said buck- if the deer has 10 tips on its antlers, it is probably not only bigger, but able to wrestle better with it’s foes. Unfortunately, it most likely will never meet mating season- instead, it will fall prey to a buck shot and live the rest of its existence out above the fireplace, mounted grotesquely for all to see.
Another victim of overhunting is fish, such as the ones that show up on Bouvet Island- at the end of the book, Carroll mentions that the icefish mackerel is being overfished, due to a lack of big fish left. A possible solution would be farming the fish to eat- that way, the natural population isn’t cut so thin that we wind up cutting out several levels of the food web by overfishing. It isn’t reproductive overfishing- catching so much of the animal it no longer has the numbers to reproduce, but it definitely is ecosystem overfishing- thinning the levels so much that fish end up going and eating other fish to compensate the lack of the level that was cut out, but having to deal with new competitors. We never overfish a population to extinction, but it gets close, than there is simply a new target- a practice that can not afford to go on.
http://www.enature.com/flashcard/show_flash_card.asp?recordNumber=MA0042
http://www.nps.gov/yell/naturescience/bighorn.htm
Since both hunters and female bighorns both desire rams with larger horns, there have been some “critical unintended consequences of the selective hunting” (254). The unintended consequence that Carroll is talking about is evolution in the opposite direction. What this means is because of hunting, rams have started to select against the faster growing horns and started to favor shorter horns. This is very strange because they are starting to evolve in the opposite direction away from their naturally selected optima that the females prefer. Another example of unnatural selection can be seen in the wild goose.
ReplyDeleteDue to hunting and fishing, the consequence is that large and rapid changes occur in certain characteristics in order for the species to survive. This is natural selection working in the oppistie way. What seemed to be the best trait and the trait with the most benefits, has turned deadly and in return is killing species with those traits. So now the species must evolve back to the unfavorable traits.
http://www.nature.com/nature/journal/v457/n7231/full/457803a.html
http://www.fieldandstream.com/blogs/fshuntress/2009/01/unnatural-selection-2
http://people.ucsc.edu/~cwilmers/publications/Stenseth2009%20human%20predators.pdf
Since both hunters and female bighorns both desire rams with larger horns, there have been some “critical unintended consequences of the selective hunting” (254). The unintended consequence that Carroll is talking about is evolution in the opposite direction. What this means is because of hunting, rams have started to select against the faster growing horns and started to favor shorter horns. This is very strange because they are starting to evolve in the opposite direction away from their naturally selected optima that the females prefer.
ReplyDeleteAnother example of unnatural selection can be seen in the wild goose. This is also seen in elephants. Since tusk less elephants are not as favorable by hunters, their population has increased from 2% to 38%.Due to hunting and fishing, the consequence is that large and rapid changes occur in certain characteristics in order for the species to survive. This is natural selection working in the opposite way. What seemed to be the best trait and the trait with the most benefits, has turned deadly and in return is killing species with those traits.
The average size of many species has gone down as a result to hunting and unnatural selection and because the prized parts of the species' gene pools are simply no longer there. There are other scientists besides Carroll who have studied the decline in size of the rams horns. 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. Since it is around the age of 4 that their horn size makes them legal game and that is several years before their reproductive peak, smaller-horned males get far more opportunities to mate. And since the big horned males are usually killed before they can reproduce, the smaller rams mate and therefore create more small horned rams.
http://www.nature.com/nature/journal/v457/n7231/full/457803a.html
http://www.fieldandstream.com/blogs/fshuntress/2009/01/unnatural-selection-2
http://gumbythecat.blogspot.com/2009/01/unnatural-selection.html