Evolution: Illustrating Natural Selection Using Artificial Selection
In Charles Darwins arsenal of concepts, from which grew his idea of Evolution by Natural Selection, as a knowledge of pigeon breeding. When humans develop new types of domestic animals, plants, or fungi by selectively breeding for particular traits, it is called artificial selection. Because an example of artificial selection may be easier for a student to understand, well work from the production of new "types" by humans to the production of new species by nature.
Say you want to produce a breed of hypoallergenic cat. What makes a good hypoallergenic cat is that allergic folks won't be allergic to the cat dander from your cat breed. Very short hair is a known way of reducing cat dander, but you are going to try to develop a cat that is particularly hypoallergenic. A hypoallergenic will fit your requirements. In a wild environment, the requirement is to have features that keep you alive and breeding; we select "successful" cats, nature "selects" survivors and good breeders.
Best start with a short-hair breed of cat; this will start you out closer to your goal. Nature also "works with what's there": individuals (or whole groups) that are not well-suited to conditions have less chance to survive.
But you don't start with a single cat, or even a pair; to do this right, you'll need to develop a population of breeders. You won't wind up with a single, perfect hypoallergenic cat, you'll get a breeding group of cats that all are hypoallergenic. You will have to make sure that your cats only breed with selected cats; in the wild, isolation of breeding groups is an important factor in generating new types as well. Natural isolation can work in many different ways.
So you take a decent-sized group of cats (we'll say 100), and test them with allergic humans, selecting maybe the 40 least-allergy-generating individuals to breed. The 60 most sneeze-inducing cats don't get to make kittens. This increases the odds that your first group of kittens will be fairly hypoallergenic, and reduces the odds that they'll be making noses run. This also gets you a breeding group diverse enough that you won't soon be breeding close relatives together (there are genertic reasons why that's a very bad idea). In Nature, conditions weed out breeders the same way, except the individuals not as well-suited to the environment tend to die before they make babies; the next generation will have a bigger fraction of individuals with traits that fit well into conditions.
If all you do is take your kittens, test them on allergic people, select the 40 best to breed, and repeat, how many generations will it take to satisfy your requirements and produce a starting population of properly hypoallergenic kitties? They may not look anything like what you expected - maybe you'll get long-haired cats whose dander doesn't set off an allergic response. Maybe, because of the genetics of the dander reduction, other genes may be selected as well; they'll perhaps have a different coat color, or different temperaments, than the original cats. Since you've been selecting for an ability that is reachable several ways, your cats may have several combined ways of preventing a reaction which keep combining in their babies. They'll never be perfectly hypoallergenic, probably, and some will be more than others, but they'll do.
The same thing happens in
Nature. Under a set of conditions, there are many trait variations
that can help toward survival and/or reproductive success, that get combined
in offspring and increase their chances of success. Over time, the
basic description of the "average individual" may change enough to establish
a new species, or genus, and with enough distance, bigger groups. How
many generations does it take? There's no set answer to that -
significant changes in populations have been seen over just a few
generations when conditions change dramatically (dramatic changes push
dramatic evolution). We do see similar distinctions in artificial
selection, but although chihuahuas and huskies, if they were wild, would be
considered different species, we just call them "dogs" because we know where
Copyright 2009, 2016, Michael McDarby.
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