Last day… talked about 5 causes of evolution - ended up on topic of natural selection…

1. Last day… talked about 5 causes of evolution - ended up on topic of natural selection…

2. Selection acts on pops. in 3 different ways: Stabilizing selection – average individuals favored, extremes selected against

3. - tends to maintain status quo, may reduce variation - most common form of natural selection, maintains stability of spp.

4. 2) Directional Selection – favors individuals at one • extreme, acts against those at other extreme, • shifts curve • often when environment changes • - probably most important for • actually producing evolution

6. 3) Disruptive Selection – favors individuals at both extremes, selects against intermediates • probably rare – unusual to have 2 phenotypes favored, • and hard to maintain 2 forms when inter-breeding - e.g. Black-bellied Seedcracker

7. A very different form of selection... • Skeptics asked Darwin how Nat. Select. could account • for bright colors, beautiful songs, etc. • Darwin came up with another • form of selection: • Sexual Selection – process • which favors individuals that • have advantage in • competition for mating • opportunities Temminck’s Tragopan

8. Two main forms: • Mate choice – favors traits that are preferred by members of opposite sex when choosing mates • - e.g. song, bright colors of male birds Blue Bird-of-Paradise

9. 2) Intra-sexual competition – favors traits that give • advantage in contests between members of same • sex (usually males) • - e.g. large size, horns Moose

10. Darwin used sexual selection to explain sexual dimorphism (differences in form between males & females), but sex. select. can occur in both sexes Red-necked Phalaropes

11. Selection & Variation Genetic variation is essential for evolution, but natural selection tends to remove it – why is there still any? Variation is present – e.g. 30% of Drosophila loci show variation (in electrophoresis)

12. In 2007, diploid human genome sequence for one individual • (J. Craig Venter) published • comparing genes on 2 chromosomes revealed 44% of • genes were different in some way (heterozygous)

13. Hidden Recessives– in diploid spp. recessive alleles often not expressed (esp. if rare) because in heterozygotes, thus immune to selection

14. Heterozygote advantage • – sickle-cell anemia, • resistance to malaria • - other examples scarce?

15. Frequency-dependent selection – rare allele is more fit • (until it becomes common!) • - scale-eating cichlids • - still pretty rare? Perissodotus microlepis

16. Variation in time & space – if selection favors different • alleles at diff. times or places, may at least slow • down selection • - e.g. Medium Ground Finch (Galapagos) - prob. common, but unclear if this is enough to maintain variation Maintenance of variation still something of a mystery...

17. Macroevolution Evolution above the species level • Implies longer time scale, not studied as directly as • microevolution Important source of data: fossil record Fossils provide not only evidence for evolution, but data to test ideas about processes Archaeopteryx

18. How Fast is Macroevolution? Darwin emphasized gradual change over long periods of time, eventually producing major change Some paleontologists claim pattern is usually different – species remain same (stasis) for very long time, then abrupt appearance of new sp. Punctuated Equilibrium (Gould & Eldredge 1972) vs. traditional Gradualism Stephen Jay Gould Niles Eldredge

19. Incomplete fossil record can produce apparent punctuations because no fossils preserved of intermediates • But, examples exist that appear to show clear cases of • ‘Punk Eek’ & Gradualism • - e.g. some trilobites: gradual

20. but some bryozoans: P.E. …

21. Two ‘theories’ represent extremes; actual patterns • may fall in between sharp punctuations or • complete gradualism • What appears abrupt may be gradual, but relatively fast • (10’s or 100s of thousands of years for change)