Population Genetics Population Genetics

Populations A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits

Modern Synthesis Theory Combines Darwinian selection and Mendelian inheritance Population genetics - study of genetic variation within a population Emphasis on quantitative characters

Modern Synthesis Theory Today’s theory on evolution Recognizes that GENES are responsible for the inheritance of characteristics Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

Microevolution Changes occur in gene pools due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more VARIATION in individuals in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant)

The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 20 white flowers 320 RR 160 Rr 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R 160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

Gene Pools A population’s gene pool is the total of all genes in the population at any one time.  Each allele occurs with a certain frequency (.01 – 1).

The Hardy-Weinberg Theorem Used to describe a non-evolving population. Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool.   Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium.

The Hardy-Weinberg Theorem Deviation from Hardy-Weinberg equilibrium usually results in evolution Understanding a non-evolving population, helps us to understand how evolution occurs                         .

Sources of genetic variation (Disruption of H-W law) Mutations - if alleles change from one to another, this will change the frequency of those alleles 2. Genetic recombination - crossing over; independent assortment 3. Migration - immigrants can change the frequency of an allele by bringing in new alleles to a population. - emigrants can change allele frequencies by taking alleles out of the population

Sources of genetic variation (Disruption of H-W law) 4. Genetic Drift - small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). - bottleneck; founder effect 5. Natural selection - if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

Hardy-Weinberg Equilibrium                                     The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time.   The Hardy-Weinberg Equation:                                     1.0 = p2 + 2pq + q2                                                 where p2 = frequency of AA genotype; 2pq = frequency of Aa plus aA genotype; q2 = frequency of aa genotype