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Evolution Pre-conditions for natural selection Heritability. Differential rates of survival and reproduction in response to environmental and genetic factors. Phenotypic variation. THIS IS DEFINITELY GOING TO BE ON THE TEST. Study of adaptation and natural selection is really understanding these pre-conditions. There are three modes of natural selection. Imagine a normally distributed range of phenotypes - May have1. Stabilising selection - acting against extremes, or just on the centre of the distrubution. Example is human birth weight. Infant mortality high in small and large infants (don't come out of womb, mother may die).
2. Directional selection distribution is moved to one side or the other. For example, prickly pear differ in number of spines. If predated, number of spines shift towards positive, if not then fewer spines.
3. Disruptive selection selection against the mean value. For example, salmon may be different phenotypes - hook-nosed male and jack male. The jack male is small and does not present much of a threat. The way these interactions work is that big males beat up smaller males and get most of the mating. So good thing to be a big male. Being a middle male- lousy. Being a small male is NOT lousy - jack males are 'sneaker' males. They act like females, not a threat to males, and as they are broadcast spawners they release their sperm into the water, and gets lots of reproduction, so small males are just as successful as large males. So good to be big, good to be small, but not good to be in the middle - enough of a threat to the large males that you cannot sneak past them and not big enough to fight off larger males. Doesn't work well in fish that are not broadcast spawners. Why are polar bears white?
Possible theories - camouflage? Allows them to sneak up on prey?
Polar bears do not have any predators so could be camouflage from prey. This is testable. Polar bears are hunters that eat seals - they can do it in three different ways -
1. Jump and crush
2. Sneak and pounce
3. Sit and wait by airholes If white hair was for camouflage, then they would probably have a preference for a certain strategy - sneak and pounce. Stirling observed many cases of polar bear predation. What he found was that the 'sneak and pounce' method ONLY HAPPENED ONCE - NOT TYPICAL HUNTING. They mostly sit and wait. Doesn't necessarily disprove camouflage is wrong but it doesn't correlate with evidence. It is thought that the hairs are more to do with heat absorption as they are transparent.
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