Bio1B Evolution 4 Last lecture More history Darwin Mendel the neodarwinian synthesis Mechanisms of evolution Evolution in populations population genetics Allele genotype and phenotype frequencies Predicting genotype freq s Hardy Castle Weinberg Equilibrium Application Null model for evolution Predicting heterozygote frequencies for recessive traits Today Evolutionary processes Sampling effects genetic drift Relevance in evolution loss of variation bottlenecks Mutation as the ultimate source of variation effects on fitness Migration gene flow spreading mutations clines Selection fitness Forms and consequences 1 Hardy Weinberg genotype frequencies as a function of allele frequencies at a locus with two alleles Futuyma 2nd Ed 2 Effect of small population size genetic drift Sampling gametes zygotes Small population have greater sampling error larger fluctuations in allele frequency reduced variation within populations Fig 23 8 3 Population bottlenecks Habitat loss or over harvesting Colonization of new areas eg islands humans Out of Africa Loss of genetic diversity Rapid change in allele frequencies divergence Fig 23 9 Fig 23 10 4 Mutations forms Changes in Coding sequence Gene regulation Gene copy number Chromosome number structure Regulation Fig 18 8 Coding Fig 17 22 HbbA HbbS 5 Exons mRNA aa Some key genes in melanin production pathway Agouti melanistic mutations Recessive regulatory coding X Mc1r Melanistic mutations Dominant structural So what Mc1r melanoma 6 Migration as gene flow Gene flow movement of genes among populations Arises from net movement birth reproduction or gamete dispersal zygotes Spreads new mutations maintains variation Opposes effects of genetic drift or local selection Spread of genes from GMO crops is a concern 7 Genetic fitness Selection acts through the phenotype Fitness Survival and reproduction relative to other phenotypes or genotypes in the population Relative fitness can be environment dependent Sorry Arnie 8 Forms of selection Fig 23 13 9 Example of Heterozygote advantage sickle cell anaemia Relative fitnesses Without malaria HbbAA HbbAS HbbSS anaemia With malaria HbbAS HbbAA HbbSS More resistant to malaria Note fitness of HbbAS depends on 10 environment malaria 11 Association between melanic phenotype and Mc1r allele in rock pocket mice from Pinacates lava flow Hopi Hoekstra in the field 12 Geographic cline Experimental evidence for rapid evolution due to selection Eg guppy color field experiments text pp460 Others microbial evolution etc etc Fig 22 13 13 Genomic signatures of recent selection Variable site New mutation arises that increases fitness Under directional selection increases to p 1 drags linked sites with it Results in a region of low variation relative to others Storz 2005 Variation high high low 14 high Genomic signatures of selection localized reductions in diversity What s with my crazy dog Sutter et al 2007 Science 316 112 15
View Full Document
Unlocking...