Animal Behavior/Population Genetics
Population Genetics and Behavior
Population genetics described the statistical distribution of genes in a particular breeding population, such as a breed of dog, and examines the role of various biological effects on gene distribution. While ordinary genetics predicts phenotypic makeups of the next generation, population genetics concerns itself with genetics predictions for a breeding group several generations into the future.
A population is the collection of organisms of a particular species within a given geographic area. When all individuals in a population are identical with regard to a particular phenotypic trait they are known as monomorphic. When the individuals show several variants of a particular trait they are polymorphic. The Gene pool for a species or population is the complete set of unique alleles present in it. While large gene pools indicate robust populations, extensive genetic diversity, and a good chance to survive bouts of intense selection, low genetic diversity is associated with reduced biological fitness and an increased chance of extinction.
The Hardy-Weinberg Equilibrium posits that allele frequencies will remain constant across generations if the population is large and individuals are mating randomly. Moreover, we will be able to accurately predict genotype frequencies from gene frequencies (e.g., in a two-allele system - a2+2aA+A2). It produces a null model against which genotype frequencies of a real population are compared. Even when a disequilibrium exists, a HW equilibrium will reestablish 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 may derive from a variety of sources.
Mutations are alterations in genetic code ranging from changes to individual nucleotide bases along the DNA to largescale rearrangements of chromosomes. Mutations are rare and even at maximum rates allele frequencies will only change from p=1 to p=0.9993 per generation.
With small effective population size, random events can take on increasingly prominent roles. This focuses on changes in the genetics of the population as a result of chance alone.
- Founder Effect
Examples include the relatively high rate of an autosomal recessive skeletal dysplasia (Ellis-van Creveld Syndrome) in the Amish population of Lancaster, Pa. Resulting in short-limbed disproportionate dwarfism, its incidence within the population is approximately 5 per 1000 live births and 2 per 1000 living persons. It can be traced to the initial immigrants Samuel King and his wife.
Selection: Strong selection for traits with high heritability can change proportions in excess of 10% per generation
Non-random Mating: Mate Choice with assortative mating (e.g., blue and white form of snow geese), Positive assortative mating: mating among individuals who share particular genes or phenotypes. Negative assortative mating: preference for mating among individuals who are unlike each other with respect to particular genes or phenotypes. Inbreeding and Outbreeding. 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 diseases; 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
Example: Matrilineal Kin Clusters and Rates 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 disproportionately high expression of different genotypes - rapid rate of evolution in mammals