Lecture 4 modern evolutionary theory


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Lecture 4 - Modern Evolutionary Theory

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Lecture 4 modern evolutionary theory

  1. 1. Anthropology 101: Human Biological Evolution Lecture 4: Modern Evolutionary Theory Dr. Leanna Wolfe LAWolfe@aol.com Office AHS Drop In Hours: Thursday 5:00-6:30 PM AHS 303
  2. 2. All this variation! Where does it come from? • Mutation • Meiosis  Recombination of alleles into unique gametes  increases genetic variation at a faster rate than mutation • Sexual Reproduction  Recombination of alleles from unique gametes into unique offspring  New phenotypes for NS to act upon • Complex genetic inheritance  Polygenic traits  Environment interacts with genotype
  3. 3. Hidden Variation allows species change • Normal sized wolves carry some alleles for small body size (some – alleles, and many + alleles) • As big wolves die (or people prefer small ones), frequency of – alleles increases • Variation is shuffled, some new combinations arise • As – alleles become more common, more – alleles likely to be combined in a single individual • New combinations with more – alleles will be outside initial range of variation
  4. 4. What forces produce and redistribute variation in a population? • Mutation introduces variation at low rates • Recombination allows different combinations of traits and alleles to appear (meiosis, independent assortment, sexual reproduction) • Natural selection works with existing variation to change its representation in a population • Gene flow introduces variation from other populations • Genetic drift can eliminate variation in small populations  Founder Effect • Sexual selection favors traits that enhance success in reproductive competition • Nonrandom Mating is bias in how we choose our mates
  5. 5. Gene Flow • Exchange of genes between populations • Can be one-way or two-way • Not exactly the same as migration  Amish: Migration without gene flow  Vietnam War: Gene flow without migration • Maintains variation within populations • Reduces variation between populations
  6. 6. Genetic Drift • In small populations, random processes affect variation • Frequencies in small samples show small random differences from large source population  5 red:5 black  6 red:4 black  3 red:7 black  7 red:3 black
  7. 7. Population Bottlenecking • Population crash • Sampling error • Reduces genetic variability
  8. 8. Population Genetics allows us to track the changes in allele frequencies in a population • Understand how traits in a population can change over time = Evolution • Demonstrate how alleles stay hidden in the genome & available for natural selection to act upon
  9. 9. Evolution is the change in the genetic composition of a population over time • Natural selection acts on phenotypes (genotypes affected) • Next generation has different traits (and genes) that are common • When allele frequencies change from one generation to the next, evolution is occurring • Equilibrium = no change 25% Aa 70% AA 5% aa
  10. 10. Let’s follow a Mendelian trait… • Sickle-cell anemia • Two alleles  A = normal hemoglobin  S = sickling hemoglobin • Three genotypes  AA normal blood cells  SS sickled blood cells  AS slightly impaired blood cells, but greater defense against malaria
  11. 11. Selection for AS genotype (and S allele) • AS people have an advantage • S allele favored as part of the heterozygotes despite SS disease & death (before reproduction) F0 generation F1 generation F2 generation AAAA AA AS AS AS SS SS SS
  12. 12. Natural selection is not guaranteed to produce “perfect” adaptations • NS can only act on existing traits • Traits arise via mutation • Natural selection does not have “foresight” • natural selection is limited by variation that is currently available
  13. 13. Pleiotropy • In Darwin’s finches, beak traits are correlated  Deeper & wider  Shallow & narrow • one gene affects multiple traits = correlated traits
  14. 14. • If environment changes, population is not perfectly suited  Playing catch up as NS favors different traits  Finches on Galapagos at beginning and end of drought In Humans!! • Certain kinds of nutrients (fat, salt, sugar) were rare in the past • Selection created strong preferences for them that persist even though they are no longer rare Disequilibrium
  15. 15. Laws of Physics & Chemistry • Some designs may be physically impossible • Why can’t elephants fly?  Heavy animals need strong bones  Strong bones are heavy • Why can’t male mammals lactate?  Hormones that facilitate milk production inhibit testosterone production
  16. 16. Species are easy to recognize but hard to define Biological species concept • Fertile interbreeding (gene flow) (gets stopped by) Reproductive isolation Ecological species concept • Exploit a single ecological niche • Natural selection keeps individuals in a species similar (same useful traits)
  17. 17. Biological Species Concept: Reproductive isolation prevents gene flow • Isolated populations can’t exchange genes  Geographical barriers (pre-mating)  Behavioral differences (pre-mating)  Viability of offspring (post-mating)
  18. 18. Premating isolating mechanisms prevent accidental matings between species • Mechanical incompatibility • Habitat isolation • Temporal isolation • Behavioral isolation
  19. 19. Postmating isolating mechanisms reduce the fitness of accidental matings between species • Sperm-egg incompatibility • Zygote failure • Embryonic failure • Offspring death • Offspring sterility
  20. 20. Problems: • Gene flow can occur between “good” species • Medium finches mate with large ground finches 10% of time • Why aren’t they one species? Medium ground finch Large ground finch Biological Species Concept is not perfect
  21. 21. Ecological species concept is based on selection • Selection favors certain phenotypes in a particular niche • Hybrids do poorly (post-mating reproductive isolating mechanism!) food available to birds
  22. 22. Ecological species concept is based on selection • Selection maintains differences between species even if gene flow occurs  Darwin’s finches • Selection maintains similarity between species even if no gene flow occurs  Checkerspot butterfly Checkerspot butterfly
  23. 23. How does a new species form? • Speciation - process by which a new species evolves from an earlier species = macroevolution  Anagenesis  Cladogenesis • Adaptive Radiation  Competition pushes some individuals to exploit new habitats  Diversify and separate from original species • Adaptive potential (species & traits)  Generalized  Specialized
  24. 24. Generalized vs. Specialized Traits: Hands vs. Feet
  25. 25. Generalized vs. Specialized Species: Norwegian Rat vs. Naked Mole Rat
  26. 26. Three kinds of speciation • Allopatric speciation • Parapatric speciation • Sympatric speciation
  27. 27. Allopatric Speciation • Closely linked to biological species concept • Gene flow interrupted • Isolated population adapts (allopatric species) • If the two populations reunited (sympatric species now) • No longer able to interbreed
  28. 28. food available to birds Sympatric species kept distinct via Reproductive Isolating Mechanisms • Hybrids do poorly • Selection for pre-mating barriers
  29. 29. The tempo of evolutionary change • Gradualism - Microevolution leads to the accumulation of small changes over time  Fossil record one long series of small changes from one generation to the next  Small differences eventually add up to a new species
  30. 30. The tempo of evolutionary change • Punctuated Equilibrium - evolutionary change proceeds through long periods of stasis punctuated by rapid periods of change  Transition to new species is rapid  Evolution leads to new species (rather than gradual changes in same species)