The Evolution of Populations Chapter 23.1

Weaknesses  He didn’t know how heritable traits pass from one generation to the next  Although variation in heritable traits was central to Darwin’s theory, he had no idea how that variation appeared Making it Stronger  Finding Mendel’s work  Genes control heritable traits  Changes in genes produce heritable variation on which natural selection can operate  Discovering the structure of DNA  Helped understand the molecular nature of mutation and genetic variation

Causes of Genetic Variation in a Population  Mutations ▪ Point mutations ▪ Gene duplications ▪ Chromosomal translocations  Sexual reproduction ▪ Causes gene shuffling ▪ Crossing-over ▪ All gametes are genetically different  Genetic variety allows evolution to occur. If all individuals of a population were clones, and mutations didn’t occur nor sexual reproduction, then evolution would never occur. All genotype frequencies would remain constant.

 Population – a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.  Gene pool – the sum of ALL the alleles in all the individuals of the population.  Allele frequency – the # of times an allele occurs in a gene pool compared to the # of times other alleles for the same gene occur (typically written as a percentage)

 A wildflower population contains 2 alleles, R (red) and r (white). RR and Rr flowers are red and rr flowers are white. If all of the alleles were dumped into a bucket (the gene pool) the frequency of each allele would be: # of the particular allele Total # of alleles Freq = Freq. of R = 16/20 = 0.8 or 80% Freq. of r = 4/20 = 0.2 or 20%

 Can be used to determine if evolution is occurring in a population  Principle states that the frequency of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.  If this is true for a population, then the gene pool is in Hardy-Weinberg equilibrium

 Calculating allele frequency: p + q = 1 p = frequency of one allele (usually the dominant one) q = frequency of the other allele (usually the recessive one)  Calculating genotype frequency: p 2 + 2pq + q 2 = 1 p 2 = frequency of homozygous dominant genotype q 2 = frequency of homozygous recessive genotype 2pq = frequency of heterozygous genotype

Hardy-Weinberg equilibrium is only true if: 1. There are no mutations 2. Mating is random 3. Natural selection is not occurring 4. Population size is large 5. No gene flow  This is never going to happen! But the equation is useful to prove that evolution IS happening

In a population of 500 wildflowers, calculate how many flowers represent the following genotypes: RR, Rr, and rr. Freq. of R = 16/20 = 0.8 or 80% Freq. of r = 4/20 = 0.2 or 20%

Altering Allele Frequencies Chapter 23.3

 Alters frequencies by favoring certain alleles over others, which will get passed on to offspring  Ex) Development of the insecticide DDT selected for DDT resistant mosquitoes to survive, thus causing the DDT resistant allele frequency to increase in the population

 When chance events cause allele frequencies to fluctuate unpredictably from one generation to the next  Can be disastrous for small populations  Founder Effect – when a few members of a population become isolated from the rest  Bottleneck effect – when the population size is reduced drastically possibly causing certain alleles to be lost forever; reduces genetic variation  Summary on page 478 of text

 The transfer of alleles into or out of a population due to the movement of individuals  Can reduce genetic differences between 2 different populations of the same species  Ex) human travel – the sharing of genes between populations that were once fairly isolated

Natural Selection and Adaptive Evolution Chapter 23.4

 Natural selection is the only mechanism which allows for adaptive evolution – not genetic drift or gene flow.  Can alter the frequency distribution of heritable traits in 3 ways, depending on which phenotypes are selected for 1.Directional selection 2.Disruptive selection 3.Stabilizing selection

1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact Refer to page 484 in your text