EVOLUTION MAKING SENCE OF LIFE
Population Genetics
• The study of allele distributions and frequencies alleles is population, and
• How and why allele frequencies change.
Some Vocabulary Population: a group of interacting and potentially interbreeding individuals of the same species. Genetic Locus: location of a gene or piece of DNA sequence on a chromosome.
Homozygous: individual carries two copies of the same allele at a locus.
Heterozygous: individual carries different alleles at a locus.
Diploid individuals carry two alleles at every locus
- Homozygous alleles are the same
- Heterozygous alleles are different Evotution change in allele frequencies from one generation to the next.
Hardy–Weinberg equilibrium assumptions Allele frequencies of a population will not change if: - Population is infinitely large
- Genotypes do not confer differences in fitness %3D
- There is no mutation
- Mating is random
- There is no migration
Population in HWE Meet 5 Criteria
1. Large population
2. No mutations
3. No migration
4. Random mating
5. No natural selection
Hardy–Weinberg Equilibrium Assumptions İn Populations
Are these assumptions reasonable?
- Population is infinitely large: populations are always finite, but some are large enough to function nearly as though they are infinite
- Genotypes do not confer differences in fitness: natural selection imposes differential survival and reproduction
- There is no mutation: mutation rates have been studied and are known
- Mating is random. mating is assumed to be random at specific loci of interest
- There is no migration this may occasionally be true but not often
- Hardy-Weinberg theorem predicts that allele frequencies will not change in the absence of drift, selection, mutation, and migration
- Mechanisms of evolution are forces that change allele frequencies
Populations Evolve Througha Variety Of Mechanis
Allele frequencies can be used to predict genotype frequencies p2 + 2.p.q + q2 = 1 Where p = frequency of one allele at a locus, and q = frequency of the other allele at the same locus. Hence, p + q = 1.
Hardy-Weinberg serves as the fundamental null model in population genetics • Hardy-Weinberg is useful because it provides mathematical proof that evolution will not occur in the absence of selection., drift, migration, or mutation
Genetic drift: is a powerful mechanism of evolution, especially in small populations.
The Concept Of Fitness
• Fitness: the survival and reproductive success of an individual with a particular phenotype • Components of fitness:
- Survival to reproductive age
- Mating success
- Fecundity
• Relative fitness(w): contribution of individuals with one genotype compared with the average contribution of all individuals in the population.
The Concept Of Fitness
• Fitness: the survival and reproductive success of an individual with a particular phenotype • Components of fitness:
- Survival to reproductive age
- Mating success
- Fecundity
• Relative fitness(w): contribution of individuals with one genotype compared with the average contribution of all individuals in the population.
An Example Of Antagonistic Pleiotropy Ester
raised the fitness of carriers in coastal areas because of insecticide use, but carrying this allele farther inland proved detrimental in escaping predation.
• Alleles that lower fitness experience
- negative selection
• Alleles that increase fitness experience
- positive selection
Even after 60.000 generations, these populations are still evolving in response to selection pressure.
Population genomes were sequenced from every 500 generations
Some mutations increased and became fixed. while others disappeared from the population.
• Selection occurs when genotypes differ in fitness
• Outcome of selection depends on frequency of allele and its effects on fitness
• Population size influences power of drift and selection
- Drift more powerful in small populations
- Selection more powerful in large populations
Relationships among alleles at a locus
• Additive alleles: homozygous condition yields twice the phenotypic effect for the gene as compared with heterozygotes.
• Dominance: dominant allele masks presence of recessive in heterozygote
Effects of positive selection on different types of alleles
Predicted changes in allele frequencies with a selection coefficient of 0.05
• Mutation rates for any given gene are low
• Per genome and population, many new mutations arise each generation
- Estimate in humans 79 billion new mutations per year
• Source of variation upon which selection and drift act. Mutation-selection balance
• Equilibrium frequency reached through "tug-of-war" between negative selection on deleterious alleles and new mutations
• Explains persistence of deleterious mutations in populations
Balancing selection
• Some forms of selection that maintain diversity in populations:
- Negative frequency dependent selection: common phenotypes are selected against, and rare phenotypes are favored
- Heterozygote advantage: heterozygosity confers greater fitness than homozygotes
