Non-Evolution: when evolution is NOT occurring At the turn of the last century, when Mendel’s experiments were rediscovered, it had become clear that the genetic material was particulate in nature. Most biologists accepted evolutionary theory as described by Darwin. Evolutionary biologists, namely, R. C. Punnett, were struggling to explain why dominant alleles do not tend to increase in frequency in a population. i.e. dominant alleles should become fixed in a population, and thus displace all recessive alleles.

Non-Evolution: when evolution is NOT occurring

Non-Evolution: when evolution is NOT occurring

Non-Evolution: when evolution is NOT occurring The Hardy-Weinberg equilibrium describes the constant frequency of alleles in a gene pool. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype.

Non-Evolution: when evolution is NOT occurring Remember, microevolution is a change in allele frequencies in a population over time. The Hardy-Weinberg theorem describes a population that is not evolving. P: 50% CR and 50% CW F1: 50% CR and 50% CW F2: 50% CR and 50% CW F3: 50% CR and 50% CW The population of flowering plant is not evolving.

Non-Evolution: when evolution is NOT occurring The Hardy-Weinberg theorem describes a population that is not evolving. This theorem states that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work. In a given population where gametes contribute to the next generation randomly, allele frequencies will not change.

Non-Evolution: when evolution is NOT occurring The five conditions for non-evolving populations are rarely met in nature: Extremely large population size (therefore, no genetic drift) No gene flow (no immigration or emigration) No mutations Random mating (no sexual selection) No natural selection The Hardy-Weinberg theorem describes a hypothetical population, but in real populations, allele and genotype frequencies do change over time.

Beginning Population: 96 Red flowered plants 48 Pink flowered plants 6 White flowered plants HW Theory predicts no transgenerational change in allele frequencies After four generations: 248 Red flowered plants 95 Pink flowered plants 57 White flowered plants At your Table: Test the hypothesis that the population has not evolved Test the hypothesis that the new population is in H-W equilibrium. 80% CR ( p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) CR (80%) Eggs 64% ( p2) CR CR 16% ( pq) CR CW 16% (qp) CR CW 4% (q2) CW CW CW (20%) 64% CR CR, 32% CR CW, and 4% CW CW Figure 23.7 The Hardy-Weinberg principle Gametes of this generation: 64% CR    +     16% CR    =   80% CR  = 0.8 = p 4% CW      +    16% CW    =  20% CW = 0.2 = q Genotypes in the next generation: 64% CR CR, 32% CR CW, and 4% CW CW plants

What alters allele frequencies? Three major factors alter allele frequencies and bring about most evolutionary change Genetic drift Gene flow Natural selection Natural Selection: Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions.

Gene Pools and Allele Frequencies When allele frequencies change in a population from one generation to the next, evolution has occurred. Determining the mechanism(s) for allele frequency change is often complicated. What mechanism(s) is/are involved in lactase persistence and nonpersistence? Sometimes the mechanisms are straightforward: industrial melanism and natural selection. Simulation of genetic drift of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to fixation is more rapid in the smaller population.