Calendar Updates and disruptive selection

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Examples for each mode of Natural Selection:

Disruptive (bimodal) selection:
Darwin observed that the species of finches were similar enough to ostensibly have been descended from a single species. However, they exhibited disruptive variation in beak size. This variation appeared to be adaptively related to the seed size available on the respective islands (big beaks for big seeds, small beaks for small seeds). Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds, and were hence maladaptive.

Directional Selection:

Directional selection favors those individuals who have extreme variations in traits within a population. A useful example can be found in the breeding of the greyhound dog. Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 64km/h (40mph).

Stabilizing Selection:
A good classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious A good classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious disease more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. disease more easily, whereas babies of large body weight are more difficult to deliver through the pelvis.

Quiz 23.1 Key Points:

Darwin
Natural Selection
5 conditions of H-W: mutation, natural selection, genetic drift, gene flow, mating
H-W calculations

Quiz 23.2 Key Points:
Modes of Natural selection