Could you explain what you mean, Tim? Natural selection always applies, as far as I am aware. It isn't constant, however, so maybe that's what you are thinking of?
Natural selection isn't just one thing, like predation, or mate choice, or growth rate. I think a lot of people get confused because it encompasses a wide variety of factors, it is probabilistic, it varies over time, and often times has components that may even work against other components. Let's take a classic example, sickle cell anemia. Sickle cell anemia is a condition in which some percentage of the red blood cells in your body are deformed and can't carry sufficient oxygen to keep your body in shape. This is a heritable trait and, obviously, if you suffer from sickle cell anemia you are more likely going to die before having children than if you do not suffer from it. There is strong selective pressure against sickle cell anemia and the allele (one of the inherited copies of dna that make up your genes) is recessive. This means that you have to have two copies of the allele that changes the blood cell shape in order to have sickle cell anemia. Yet in spite of the fact that having sickle cell anemia is strongly selected against and the fact that the trait is recessive, it still persists in the human population. Why is that? And why is the disease almost exclusively found in people of African descent? The reason is that there turns out to be an advantage to having a single copy of the sickle cell allele and a single copy of the normal allele (what we call heterozygous or hybrid, one copy of each). People who are heterozygous turn out to have a stronger ability to resist malaria. Malaria, as I'm sure you know, is a great killer of young people in the tropics, especially in Africa. So the increased likelihood of reproduction in people who are heterozygous helps counteract the decreased likelihood of reproduction in people who have two copies of the mutant allele and, thus, the mutant allele hangs around in the population at large, staying (statistically) within an expected mathematical range of presence based on statistical likelihood of the pre-reproductive lethalness of the different scenarios. Now you move on to Europe and North America and you'll find that there is a lot less incidence of malaria. And, sure enough, you find much lower incidence of the sickle cell allele in populations that have been mating in those non-malaria regions for a long time because now the selective pressures are different. There is now a much lower advantage to having a single copy of the allele and still roughly the same disadvantage to having two copies of the allele (and developing sickle cell anemia). But the mutant allele still hangs around to some extent because there does not seem to be great selective pressure against having a single copy, it does not seem to significantly decrease reproduction opportunities. So the allele persists in the population but at a, predictably, much lower rate...because the selective pressures are different. Now look to the future and maybe we are able to wipe out malaria in sub-Saharan Africa. What will happen to the selective pressures then? How will the distribution of the allele in the population change? Natural selection is always in play but it can be a very subtle and complex thing indeed. The sickle cell example is one of the most well known and most straight forward, yet even that is complex because you have multiple forces working in different directions and you end up with a dynamic balance that changes over both time and geography. Hope that helps some. Cheers, Judah On Fri, Jan 3, 2014 at 12:30 PM, Timothy Heald <[email protected]>wrote: > > That's some of why is theory. > > We know that natural selection/survival of the fittest isn't always true > now. And that mutations occur from environmental factors and other factors > we don't understand. > On Jan 3, 2014 2:26 PM, "Scott Stroz" <[email protected]> wrote: > > > > > Not every instance of every organism would have the same mutations. > > > > Not all mutations would dictate that those without them would perish. > > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| Order the Adobe Coldfusion Anthology now! http://www.amazon.com/Adobe-Coldfusion-Anthology/dp/1430272155/?tag=houseoffusion Archive: http://www.houseoffusion.com/groups/cf-community/message.cfm/messageid:369111 Subscription: http://www.houseoffusion.com/groups/cf-community/subscribe.cfm Unsubscribe: http://www.houseoffusion.com/groups/cf-community/unsubscribe.cfm
