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Animal Behavior - Biology 4200/5430
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Bowling Green State University, Fall 2009
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Hardy-Weinberg Equilibrium and Population Genetics in Behavior
"Nature abhors perpetual self-fertilization" -- Charles Darwin in "The Effects of Cross- and Self-Fertilization in the Vegetable Kingdom" (1876)
Population Genetics
- Hardy-Weinberg Equilibrium: Allele frequencies will remain constant across generations if the population is large and individuals are mating randomly. Moreover, we will be able to acurately predict genotype frequencies from gene frequencies (e.g., in a two-allele system - a2+2aA+A2). HW produces the null hypothesis against which genotype frequencies are compared. HW equilibrium will occur after one generation provided that the same gene frequencies occur in both sexes. If disequilibrium occurs, equilibrium will be re-established after one generation of random mating. What are the assumptions? What will produce deviations from the HW equilibrium? What might produce an excess of heterozygotes (e.g. HLA-1, MHC)? What might produce an excess of homozygotes?
- Deviations from HW equilibrium:
- Mutations are rare (e.g. Maximum mutation rate can change allele frequency only from p=1 to p=0.9993 per generation)
- Selection: Strong selection for traits with high heritability can change proportions in excess of 10% per generation
- Migration:
- Random events: Population size, Genetic drift, and Founder effects
- Mate Choice: assortative mating (e.g., blue and white form of snow geese)
Non-random Mating, Inbreeding and Outbreeding
- Positive assortative mating: mating among individuals who share particular genes or phenotypes
- Negative assortative mating: preferrence for mating among individuals who are unlike each other with respect to particular genes or phenotypes
- Inbreeding: refers to (preferential) mating between biological relatives. As relatives (i.e., ancestors of the first individuals are shared with those of the second individual), they carry genes which are "identical by descent"; extreme inbreeding: mating between sibs, half-sibs, parent-offspring
- Individual inbreeding coefficient (i.e., Pedigree inbreeding): F represents the probability that the offspring is homozygous due to identity by descent (ibd) at a randomly chosen autosomal locus, ranges in value from 0 (no locus ibd) to 1 (all loci ibd). Significant factors for pedigfree inbreeding are
- Factors which determine the probability that various kinds of individuals will come into contact, e.g., population demography (sex ratio, birth or death rates), individual and population dispersal patterns, spatial distributions
- Behavioral preferences for or against certain classes of individuals as potential mates. Behavioral preferences influence the acceptance or rejection of different types of individuals as mating partners
- Population inbreeding coefficient: f measured by deviations from Hardy-Weinberg Equilibrium (a2+2aA+A2), examine for an over-representation of homozygotes. f ranges from -1 (inbreeding avoidance) to 0 (random mating) to 1 (inbreeding)
- Outbreeding: (preferential) mating between non-relatives
- Inbreeding depression: overrepresentation of homozygotes; e.g. matings between cousin are rare (0.05% in U.S.) but account for 20% of albinos; deficiency in immune systems; recessive, deleterious alleles occur more likely in homozygous condition; humans have on average 8 recessive, deleterious alleles. 1% increase in inbreeding produces 11% increase in the incidence of genetic deseases; Even small amounts of pedigree inbreeding in a random mating population greatly increase the incidence of some types of genetic disease. This is of particular concern in groups with small effective population sizes - Amish, Ashkenzi Jews
- Inbreeding advantages: functional association of gene complexes shaped by local adaptations
- Outbreeding depression: Extreme case, hybridization across genetically differentiated populations
- Hybrid vigor
- Population genetics and the Rate of Evolution: Ground squirrels have matrilineal kin clusters and estrous is synchronized. Thus the same male is likely to mate with several closely-related females - genetic diversity of populations is primarily contained between kin clusters, high local levels of inbreeding assure the dispropotionately high expression of different genotypes - rapid rate of evolution in mammals
Reading Assignment
Links of Interest
last modified: 11/12/03
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