Kim asks: > I am having an interesing discussion at the moment about Natural > selection. The context is a single population of individuals that, due > to changes in the environment, are now maladapted and the population is > reducing in size. Based on the often used definition of differential > reproduction, when there is not much to differentiate with, there is no > longer differential selection, and as such, no natural selection. > However, they are maladapted, so unfit to survive. Any opinions about > this nice contradiction?
No population is monomorphic. There is always some variation, and so long as that variation exists, some individuals will be favored over others and thus will differentially reproduce. One of my favorite stories in that regard concerns an artificial shift in environment from the 1970's. A blood-feeding barn fly was a serious pest of sheep in Western Australia. The people at the CSIRO developed a very potent, long-lasting pesticide that could be sprayed within the upper walls and ceilings of barns, where the flies rested. Because flies "taste" with pads on their tarsi, uptake of the poison was easily administered and safely out of reach for almost everything else. As predicted, the barn fly population of Western Australia plummeted and the ranchers were finally free of the plague, but only for a few years. Then the flies came back in numbers as large as ever. The question was: how did the insects develop resistence to such a powerful toxin so quickly? The answer was: they didn't. When the new flies were brought back to the lab and a drop of the insecticide was placed on them, they died as quickly as ever. Rather, it was determined what the flies did do was rapidly shift their populational phenotypic characteristics. A very small percentage of the original population had tarsal hairs slightly longer than others. In these individuals, the tarsal pads never touched the surface. Essentially, they were walking on their tiptoes. Ordinarily, this characteristic might have been described as a mild genetic defect, but now, in the new highly toxified environment, it was an extraordinary benefit. Individuals with these longer tarsal hairs prospered and reproduced all out of proportion to their peers, abd as a consequence, the longer tarsal hairs rapidly become the common trait of the population. All evolution is like this. If the population isn't completely eliminated by the initial enviornmental catastrophe, it will begin to rapidly move to a more optimally appropriate populational phenotype, deriving whatever benefit it can from whatever variation existed in the population at the time of the catastrophe. Wirt Atmar
