Microevolution: How Does a Population Evolve?

1. Microevolution: How Does a Population Evolve? Chapter 16

2. The Evolution of Evolution. Blending inheritance Inheritance of Acquired Characteristics Genetics alone causes evolution Modern synthesis – evolution is due to natural selection working on inherited traits

3. Population genetics • Microevolution – changes in the frequency of the alleles of genes in a population. • Industrial melanism • Macroevolution - the process by which species of organisms originate, change and go extinct.

5. What is the source of variation within a population? • Either point mutations of genes or chromosomal mutations • If there is only one allele for a gene, the population is homozygous for that gene. • If there are two or more alleles for a gene, the population is polymorphic for that gene. • If the members of a population come in two or more forms, the population is polymorphic.

6. Most human traits are polygenic – controlled by many genes • These traits vary smoothly and continuously within a population. • The graph of these traits is a bell curve.

9. In a changing environment, highly variable populations evolve more rapidly than less variable populations. • The factors that determine the genetic variability of a population are: • The rate at which mutations accumulate in the DNA • The rate at which changes spread through a population • The rate at which deleterious mutations are eliminated from a population by natural selection.

10. How much genetic variation is there? • In humans about 25% of all proteins have an alternate form which is present in at least 5% of the population. • In humans about 7 % of our genes are heterozygous. • Invertebrates -13% • Plants -17% • Drosophila – 25%

11. Remember: Natural selection works only on the Phenotype which is an interaction of the geneotype and the influences of the environment. • Genetic variation is fuel for evolution • Yet, natural selection favors those traits best suited to the environment and weeds out the rest.

12. All of the genes of all the individuals in a population is called the gene pool. • Hardy-Weinberg principle: sexual reproduction by itself does not change the frequencies of alleles within a population. Genotype frequencies stay the same from generation to generation as long as certain conditions are met. • Hardy-Weinberg equilibrium: • p + q = 1 and p2 + 2pq + q2 = 1

13. Conditions: • There was random mating • There is a large population size • There are no mutations • There is no breeding with other populations • There is no selection, either natural or artificial

14. In reality, these conditions are hardly ever met, but it gives us a standard against which to measure evolution. • Of all the conditions mentioned, only natural selection leads to adaptive change. • The rest cause changes in gene frequency which may or may not be adaptive.

15. Random mating • Only practiced by organism which release gametes on the wind or in the water. • Assortative mating – based on choice • May be without regard to one’s phenotype • Positive assortative mating – choose individuals like ourselves • Inbreeding – increases the incidence of recessive disorders, leading to a less healthy population • Negative assortative mating - outbreeding

17. Large population size • Random drift or genetic drift is a change in the allele frequency due to random events. This is more likely in a small pop. • Founder effect –a small subset of a population founds a new population. • Bottleneck effect – the population is reduced to a few individuals by some random disaster or harsh selection pressure (such as over hunting). • Causes new mutations to spread or be removed.

19. No interbreeding between populations • Gene flow occurs as the result of interbreeding between two populations. • Individuals immigrate and bring new alleles into the population. • It increases the variation within a population. • It makes adjacent populations more alike.

21. No selection • Natural selection • Harmful genes are selected against • Useful genes accumulate

22. Types of Natural Selection • Directional selection – selects for one end of the bell curve

23. Types of Natural Selection • Stabilizing selection – the extremes of a population are selected against and the average is favored.

24. Types of Natural Selection • Disruptive selection – selects for extremes and against the average.

25. Disruptive selection Taste good Taste bad

27. Sexual selection • Male competition • Male competes against other males for territory, or access to females • Anything that gives him an advantage makes him more likely to pass on his genes • Female selection ( or male selection) • Leads to sexual dimorphism • Male must prove he is genetically good enough • Plumage, gifts, nesting site or mating rituals

28. Natural selection can also encourage genetic variation when different alleles of a gene are equally useful. • Different local environment • One allele is better at a certain time of year • Balanced polymorphism • Sometimes the superiority of the heterozygote may maintain a high incidence of an allele which is harmful to the homozygote • Sickle cell anemia and malaria

29. Blue = malaria Red = sickle cell anemia Purple = overlap