Genes Within Populations Chapter 20

Genetic Variation and Evolution Evolution: change in a species through time Species accumulate differences Descendants differ from their ancestors New species arise from existing ones

Mechanism of evolutionary change Lamarck’s theory of evolution Inheritance of acquired characteristics: Individuals passed on physical and behavioral changes to their offspring Variation by experience…not genetic

Lamarck’s theory of how giraffes’ long necks evolved

Darwin’s Theory of Evolution Observed land & organisms everywhere they went Began to notice connections between species

Charles Darwin Served as naturalist on 5 year mapping expedition(HMS BEAGLE) around coastal South America. Used many observations to develop his ideas Proposed that evolution occurs by natural selection

Voyage of the Beagle

http://etext.library.adelaide.edu.au/d/darwin/charles/

Charles Darwin evolution: modification of a species over generations -“descent with modification” natural selection: individuals with superior characteristics are more likely to survive and reproduce than those without such characteristics

Tortoises on different islands… “normal” tortoise Saddleback allows tortoise to reach higher leaves on drier islands

Darwin’s Evidence Similarity of related species - Darwin noticed variations in related species living in different locations

Most famous for his observations of Galapagos finches Some islands much drier than others Different islands had their own, slightly different varieties of animals Darwin hypothesized that new species could gradually appear, much like animal breeders can artificially develop new varieties through selective breeding

Explaining his observations: natural selection Variation: individuals in a population differ from one another Heritability: variations are inherited from parents Overproduction: organisms produce more offspring than can survive (survival of the fittest) Reproductive advantage: some variations allow the organism that possesses them to have more offspring – those variations become more common, and the population changes over time

Darwin’s theory for how long necks evolved in giraffes

Post-Darwin Evolution Evidence Fossil record Mechanisms of heredity Comparative anatomy Molecular evidence

Why does a peacock have a large tail? Why are albinos rare? Why does a peacock have a large tail?

Hardy-Weinberg Principle -Genetic equilibrium: Allele (and genotype) frequencies in a population will remain constant from generation to generation -if equilibrium is upset  evolution (punctuated equilibrium chp 22)

Hardy-Weinberg Principle Requirements to maintain genetic equilibrium: No mutation No genes are transferred to or from other sources Random mating Very large population No selection

Hardy-Weinberg Principle Calculate genotype frequencies (p+q)2 = p2 + 2pq + q2 p = frequency of the 1st allele q = frequency of the 2nd allele p2 = individuals homozygous for 1st allele 2pq = heterozygous individuals q2 = individuals homozygous for 2nd allele because there are only two alleles: p plus q must always equal 1

Hardy-Weinberg Principle

Hardy-Weinberg Principle Using Hardy-Weinberg equation to predict frequencies in subsequent generations

A population not in Hardy-Weinberg equilibrium indicates an agent of evolutionary change is operating in a population (one or more of the 5 conditions are not being met)

Agents of Evolutionary Change Mutation: A change in a cell’s DNA Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. Ultimate source of genetic variation

Agents of Evolutionary Change Gene flow: A movement of alleles from one population to another Powerful agent of change Tends to homogenize allele frequencies between populations

Agents of Evolutionary Change Nonrandom Mating: mating with specific genotypes E.G. Sexual Selection – “Peacocks” Shifts genotype frequencies

3a. Assortative Mating: mates that are phenotypically similar does not change frequency of individual alleles Disruptive selection: forms at both ends of the range of variation are favored over intermediate forms

3b. Disassortative Mating: phenotypically different individuals mate Stabilizing selection: intermediate (heterozygous) forms are favored and extremes are eliminated

Genetic Drift Small populations Genetic drift: Random fluctuation in allele frequencies over time by chance important in small populations founder effect – When a few individuals start a population (small allelic pool) Amish bottleneck effect - drastic reduction in population, and gene pool size

Founder Effect

Genetic Drift: A bottleneck effect

Selection Natural selection: environmental conditions determine which individuals in a population produce the most offspring This is the only agent that produces adaptive evolutionary change (selects individuals that are more fit)

Selection

http://www.pbs.org/wgbh/evolution/library/01/1/l_011_03.html Evolution of the eye: http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html

Fitness and Its Measurement Fitness: A phenotype with greater fitness usually increases in frequency Fitness is a combination of: Survival: how long does an organism live Mating success: how often it mates Number of offspring per mating that survive

Body size and egg-laying in water striders

Oscillating selection: selection favors one phenotype at one time, and a different phenotype at another time Galápagos Islands ground finches Wet conditions favor big bills (abundant seeds) Dry conditions favor small bills

Maintenance of Variation Heterozygotes may exhibit greater fitness than homozygotes Heterozygote advantage: keep deleterious alleles in a population

Maintenance of Variation Example: Sickle cell anemia Homozygous recessive phenotype: exhibit severe anemia Homozygous dominant phenotype: no anemia; susceptible to malaria Heterozygous phenotype: no anemia; less susceptible to malaria

Maintenance of Variation Frequency of sickle cell allele

Maintenance of Variation Disruptive selection acts to eliminate intermediate types

Maintenance of Variation Disruptive selection for large and small beaks in black-bellied seedcracker finch of west Africa

Maintenance of Variation Directional selection: acts to eliminate one extreme from an array of phenotypes

Maintenance of Variation Stabilizing selection: acts to eliminate both extremes

Maintenance of Variation Stabilizing selection for birth weight in humans