Variation and Natural Selection (Chap. 18). Two basic points of variation. Variation can occur in a wide array of traits (physical features, color, size....) We may not be aware of the variation (enzymes, disease susceptibility, blood type…). Variation comes from:. Sexual reproduction
·genetic recombination in meiosis continually produces variation by shuffling existing genes and alleles
·fertilization allows the alleles from two diff. individuals to be brought together in one new individual
·provides a steady input of new genes and alleles
Sickle Cell Video clip
a. The distribution of the falciparum malaria (Plasmodium falciparum) in the Old World before extensive eradication of mosquitoes begun in the '20s. b. The distribution of the sickle-cell allele within the same area. Intensity of color refers to increasing allele frequencies. Note the correlation between the two distributions.
The answer turned out to be yes. As the population of G. fortis recovered after the rains returned, the average body size and beak depth of their offspring was greater than before (an increase of 4–5% for beak depth). The bell-shaped curve had been shifted to the right — directional selection.