2011-15_Sel2 - EVOLUTION(11:704-486 SELECTION 2 SMOUSE...

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EVOLUTION (11:704-486) SELECTION - 2 SMOUSE – SPRING 2011 1 M ORE ON S ELECTION Biston betularia : We can illustrate Cases I & II with a nice example, the moth Biston betularia Figure 12.23, Page 328, Futuyma Camouflage – The moth has two color morphs, one that is mottled, and looks like a lichen. It is coded for by the T-allele ( typica wildtype). In beech woodlands in the UK, these moths light on lichen-covered tree trunks. They are superbly camouflaged. That is important, because the native thrushes just love to eat these moths, and will, if they can spot them. There is a rare mutant allele, called the C-allele ( carbonaria sooty black). There are three genotypes, namely TT, TC and CC. The carbonaria allele is dominant, so both heterozygotes and carbonaria homozygotes are dark: TT TC CC mottled black Unfortunately for the carbonaria color morph, they stick out like a sore thumb against the lichen-covered bark of the beech trees, and the thrushes have a nice lunch at their expense. Their fitness is seriously reduced. Just to illustrate, let me put some numbers in: TT TC CC W TT = 1.00 W TC = 0.60 W CC = 0.60 ** What would you expect to happen? ** It is not an accident that carbonaria moths are rare in pristine woodlands. They are really exposed to the bird predators, and they have seriously reduced fitness.
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EVOLUTION (11:704-486) SELECTION - 2 SMOUSE – SPRING 2011 2 In the early 1800’s, as industrial factories began to crop up all over the UK, the carbonaria morph began to increase in frequency. By the mid 1950s, huge areas had carbonaceous soot covering tree trunks, killing the lichens, and turning the tree trunks very dark. Against this altered backdrop, the carbonaria moths had become almost invisible to the thrushes, but the mottled ( typica ) moths were so obvious as to be seriously vulnerable. The thrushes were cueing in on what they could see, and what they could see was the TT form. TT TC CC W TT = 0.30 W TC = 1.00 W CC = 1.00 ** Now, what would you expect to happen? ** The point is that selection in Biston betularia can go either way, just depending on the state of air pollution and the resulting fitness values. More recently, with a reduction in air pollution, at least the upland populations are seeing an increase in the typica morphs. Balanced Polymorphism: We also need to talk about the case where the heterozygote has a selective advantage over both homozygotes. To illustrate this one, let’s return to sickle cell anemia in humans, where there are three genotypes for the -globin locus, labeled AA AS SS wildtype carrier sickle cell Most people in the US are of genotype AA ( p 2 A ), a modest number are of the genotype AS (2 p A q S ), and a small fraction are of genotype SS ( q 2 S ). Under prevailing US conditions, AS heterozygotes and AA wildtype homozygotes have the same relative fitness values (W AA = 1.00 = W AS ), but sickle homozygotes suffer loss of red cells and a whole host of ensuing health
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This note was uploaded on 08/18/2011 for the course ECOLOGY 301 taught by Professor Staff during the Spring '11 term at Rutgers.

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2011-15_Sel2 - EVOLUTION(11:704-486 SELECTION 2 SMOUSE...

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