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Lecture 15 - Speciation and the genetics thereof

Lecture 15 - Speciation and the genetics thereof - 1 III...

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1 III. Genomic signature of natural selection (in humans; con't from last time) A. Comparative data: increased rate of amino-acid substitution at protein coding loci relative to synonymous substitution (Ka/Ks) - at neutral locus, Ka=Ks; if Ka/Ks < 1, protein evolution is constrained (purifying selection); if Ka/Ks >1, protein has experienced elevated rate of replacement substitutions, signature of positive sel'n B. Intraspecific data (< 4N e gens): - Reduction in genetic variation (signature of a sweep) - Local (i.e., region of the genome) increase in linkage disequilibrium - Skew in allele freq. distribution toward more rare alleles - High frequency of derived allele (requires comparison e.g. between human-chimp) - Anomolously high among-population variance (i.e., Fst) i) Incomplete sweeps (alleles in the middle of a sweep): (figure) - e.g., lactase in European (and African pastoral) pops - G6PD; malaria resistance - skin color genes out of Africa Note: Many genes show evidence of selection only in one population (or group) ~ 1800 (1.6%) of human genome shows evidence of recent positive selection - Role of local recombination rate: greatly biased toward finding genes in regions of low recomb. ii) Negative selection against disease alleles - in certain cases, disease alleles at relatively high frequency; possible causes include balancing sel'n (e.g., alpha-globin [sickle-cell anemia], G6PD), and recent change in direction of sel'n (e.g., "thrifty genotype" & susceptibility to diabetes); recent hitchhiking during a selective sweep - Comparison of genes associated w/ Mendelian disease, complex disease and no known disease show difference in level of evolutionary constraint (by measure of human-chimp-macaque divergence) show that Mendelian genes under greater constraint than complex genes, and that complex genes subject to weaker purifying seln' than either Mendelian or non-disease genes. Lecture 15 - Speciation and the genetics thereof "Origin of Species" is an old problem. Or really two problems.
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2 1. What is a species? 2. How do species come to be? I. What is a species? - Any intuitive definition includes a criterion of reproduction, along the lines of: members of the same species can reproduce with other members of the same species but not with members of other species. - Biological Species Concept (Ernst Mayr): "Actually or potentially interbreeding set of organisms that exists apart from all other such groups." The trouble is, what does "potentially" include? - Many "good" species, that anybody with any common sense and many biologists would recognize as species, can "interbreed" if given the opportunity. For a familiar example, dogs, coyotes, wolves occasionally hybridize ("hybridize" means inter-species cross; the offspring are "hybrids"). Many birds, fish, etc. hybridize. Plants hybridize rampantly. -
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Lecture 15 - Speciation and the genetics thereof - 1 III...

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