Mechanisms of Evolution

1. Mechanisms of Evolution Why does genetic change over time occur?

2. Introductory Concepts and Vocab • Populations, NOT individuals evolve • gene pool:all of a population’s genes • allele frequency: % of any allele in gene pool • genetic equilibrium:frequency of alleles remains the same over generations Question: If a population is in Genetic Equilibrium, is it evolving? Answer: NO!

3. Mechanisms of Evolution • Evolution occurs whenever genetic equilibrium is disrupted! • In other words, anytime allele frequencies change… • Change can be caused by the following: • 1. mutation: • 2. genetic drift: • 3. gene flow: • 4. natural selection • 5. non-random mating

4. CAUSES OF CHANGE • Mutation: Any change in the DNA sequence of an organism • caused by radiation, chemicals, chance etc. (any mutagen) • This is how new traits could arise…

5. Genetic Drift • Alteration of allele frequencies by RANDOM, CHANCE events • In GENETIC DRIFT, survival has nothing to do with the fitness of an individual- it’s LUCK! • can greatly affect SMALL populations

6. Genetic Drift Examples • Bottleneck • Drastic reduction in population • Ex. Natural Disaster, …

7. Bottleneck Examples • Northern Elephant Seal • 1892- 50-100 seals • Today- 160,000 seal • Cheetah • Very low genetic diversity may be due to a bottleneck

8. Genetic Drift Examples • Founder effect • Part of the population founds a new colony • HD allele: • Dutch couple migrated to South Africa • Today- 30,000+ have gene that causes Huntington’s disease & can be traced back to the couple • Polydactyly • Amish population in Lancaster, PA

9. Gene Flow • * migrating individuals entering and leaving a population • Immigration • Emigration Bu-Bye! Hello!

10. non-random mating- • choosing mates based on preferred characteristics. Ex. peacocks

11. Natural Selection • Selection of variations in organisms which enhance their survival • There are 3 types of Natural Selection that act on variation: • Stabilizing • Directional • Disruptive

12. Thought Questions on Natural Selection • Why does natural selection act on the phenotype rather than the genotype of an organism? • Why is genetic variation in a species valuable? • Ex. Potato famine • If natural selection is happening, why do some lethal alleles continue in populations? Shouldn’t diseases like sickle-cell anemia, cystic fibrosis and Tay-Sachs disappear? • See “A Mutation Story” • See “HIV Immunity” and “Double Immunity” to discuss the value of mutations and how natural selection affects which mutations persist. • http://www.pbs.org/cgi-registry/2wgbh/evolution/library/search.cgi

13. Macroevolution and Speciation • Speciation Definition: the evolution of new species (REPRODUCTIVE ISOLATION = THE KEY!!) • Species: a group of organisms that can interbreed and produce fertile offspring in nature… • Reproductive Isolation: • Occurs when formerly interbreeding organisms can no longer mate & produce fertile offspring

14. Causes of Reproductive isolation: a.geographic isolation: physical barrier divides a population (ex. Islands, mountains, rivers, canyons etc.) b. seasonal isolation: breed at different times c. mechanical isolation physical incompatibility d. behavioral isolation: different courtship behaviors/signals e. changes in chromosome number (see next slide) Example: mountain range or body of water dividing a population

15. Reproductive Isolation,cont’d: • Change in chromosome numbers • example: donkey (64) and horse (62) create mule (63) which is sterile • Polyploidy:having multiple sets of chromosomes (common in plants) • example: crop plants like wheat mule

16. Assignment • Draw a comic strip showing how geographic isolation, seasonal isolation, and behavioral isolation could result in speciation (reproductive isolation) over time.

17. Patterns of Evolution • Divergent Evolution:a pattern of evolution in which species that were once similar to an ancestral species, become increasingly distinctor “diverge” • example: Adaptive Radiation evolution of species into an array of species to fit a number of diverse habitats.

18. * Convergent Evolution • pattern of evolution in which distantly related organisms evolve similar traits usually because of similar environments

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20. Hardy-Weinberg Equilibrium • Equilibrium = no evolution, no genetic change! • Conditions for Hardy-Weinberg Equilibrium = exactly the opposite of conditions for evolution • 1. NO mutation: • 2. NO genetic drift (Very Large Population size) : • 3. NO gene flow: • 4. NO natural selection • 5. random mating Discussion: Why are the conditions for Hardy-Weinberg Equilibrium unlikely to occur in nature?

21. Hardy-Weinberg Equation • p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p) • p2 =frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2 =frequency of aa genotype; • Hardy Weinberg Equation: p2 + 2pq + q2= 1.0 • Can be used to calculate allele and genotype frequencies if a population is in Hardy-Weinberg Equilibrium

22. Practice Problem • Calculate (predict) the frequency of each possible genotype in the following population of dogs. • 25 % (0.25) of the dogs have blue eyes • 75% (0.75) of the dogs have brown eyes. • Answer: homozygous recessive = 0.25 (25%), Homozygous dominant = 0.25 (25%), Heterozygous= 0.5 (50%).