Outcomes of Natural Selection (Chapter 19 and part of 20) Small Steps or Great Leaps? See Fig. 20.20 in text punctuated equilibrium site • Microevolution (IB) • Macroevolution (IB) – The Effects of Selection on Populations (Fig. 19-3 in text) 1. Stabilizing selection 2. Directional Selection 3.Disruptive Selection Animation of Selections Random Changes in Frequencies • a change in allele frequency in a population due simply to chance is called genetic drift • Processes leading to genetic drift: 1. population bottleneck – 2. founder effect – • Afrikaaners of South Africa in text – In the 1680s Ariaantje and Gerrit Jansz emigrated from Holland to South Africa, one of them bringing along an allele for the mild metabolic disease porphyria. Today more than 30000 South Africans carry this allele and, in every case examined, can trace it back to this couple — a remarkable example of the founder effect. • Amish in Pennsylvania – Eastern Pennsylvania is home to beautiful farmlands and countryside, but it's also a gold mine of information for geneticists, who have studied the region's Amish culture for decades. Because of their closed population stemming from a small number of German immigrants -- about 200 individuals - - the Amish carry unusual concentrations of gene mutations that cause a number of otherwise rare inherited disorders, including forms of dwarfism and plolydactyly.
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