Chapter18
- 1. Process of Evolution Chapter 18
- 2. Evolution in a Genetic Context (Population Genetics) Microevolution Change in gene frequency in a population over time Gene pool = ALL alleles at ALL gene loci in ALL individuals of the population. Allelic frequency = # specific alleles total alleles in population Shown as f A or f a (f followed by subscript) Indicates probability of allele
- 3. Microevolution What could cause CHANGES in allele frequencies?
- 4. Causes of Microevolution Genetic Mutations Mutated alleles (or combinations of alleles) may be more adaptive Gene Flow Movement of alleles between populations by migration of breeding individuals. Continual gene flow reduces variability (differences) between populations.
- 5. Gene Flow
- 6. Causes of Microevolution Nonrandom Mating Individuals do not choose mates randomly. Assortative mating - Individuals tend to mate with those with the same phenotype. Sexual selection - Males compete for the right to reproduce and females choose to mate with males possessing a particular phenotype.
- 7. Causes of Microevolution Genetic Drift Changes in allele frequencies of a gene pool due to CHANCE. EX: natural disaster, weather change Larger effect in small populations.
- 8. Genetic Drift Bottleneck Effect Event prevents majority of genotypes from entering the next generation
- 9. Genetic Drift Founder Effect Subgroup starts new population Alleles carried by population founders are dictated by chance Founding population does NOT reflect original population
- 10. Founder effect Sample of original population Descendants Founding population B Founding population A
- 11. Natural Selection Natural Selection results in adaptation of a population to the environment Adaptation is result of new allele frequencies
- 12. Hardy-Weinberg Principle Hardy-Weinberg equilibrium = NO microevolution ** Allele frequencies of gene pool will stay the SAME (equilibrium) IF
- 13. Hardy-Weinberg Principle ** Allele frequencies of gene pool will stay the same (equilibrium) IF What could cause CHANGES in allele frequencies?
- 14. Hardy-Weinberg Principle ** Allele frequencies of gene pool will stay the same (equilibrium) IF No Mutations No Gene Flow Random Mating No Genetic Drift No Selection
- 15. Hardy-Weinberg Under real conditions, these conditions are rarely, if ever, met, and allele frequencies in the gene pool of a population change between generations. Evolution has occurred.
- 16. Hardy-Weinberg Math Consider a trait with 2 possible alleles… p = frequency of dominant allele The probability that an allele chosen at random is dominant q = frequency of recessive allele The probability that an allele chosen at random is recessive p + q = 1; Why?
- 17. Hardy-Weinberg Math p = frequency of dominant allele The probability that an allele chosen at random is dominant q = frequency of recessive allele The probability that an allele chosen at random is recessive p + q = 1; Why? If there are only two allele possibilities, the sum of their frequencies must be 1
- 18. Hardy-Weinberg Math What is the probability of an individual being homozygous dominant (AA) – the frequency of the AA genotype? What is the probability of an individual being homozygous recessive (aa)?
- 19. Hardy-Weinberg Math What is the probability of an individual being homozygous dominant (AA) – the frequency of the AA genotype? f AA = p x p = p 2 What is the probability of an individual being homozygous recessive (aa)? f aa = q x q = q 2
- 20. Hardy-Weinberg Math What is the probability of an individual being heterozygous (Aa) – the frequency of the Aa genotype?
- 21. Hardy-Weinberg Math What is the probability of an individual being heterozygous ( Aa ) – the frequency of the Aa genotype? f Aa = (p x q) + (q x p) = 2pq There are 2 possible combinations: allele 1 can be A and allele 2 can be a OR allele 1 can be a and allele 2 can be A
- 22. Hardy Weinberg Math Remember: p + q = 1 AND (p + q) 2 = p 2 + 2pq + q 2 = 1 q 2 is usually known! (What does q 2 refer to?)
- 23. H-W Practice: Practice Problems 18.1 1. In a certain population, 21% are homozygous dominant, 49% heterozygous, and 30% homozygous recessive. What percentage of the next generation is predicted to be homozygous dominant, assuming a Hardy-Weinberg equilibrium?
- 24. H-W Practice: Practice Problems 18.1 1. In a certain population, 21% are homozygous dominant, 49% heterozygous, and 30% homozygous recessive. What percentage of the next generation is predicted to be homozygous dominant, assuming a Hardy-Weinberg equilibrium? If it is in Hardy-Weinberg equilibrium, we would expect the same genotypic frequencies in the next generation. 21% homozygous dominant Let’s do the math!
- 25. H-W Practice: Practice Problems 18.1 1. In a certain population, 21% are homozygous dominant, 49% heterozygous, and 30% homozygous recessive. What percentage of the next generation is predicted to be homozygous dominant, assuming a Hardy-Weinberg equilibrium? q 2 = .30 so q = √.30 = 0.55 p + q = 1, so p = 1 – q = 1 – 0.55 = 0.45 p 2 = (0.45) 2 = 0.2025 Cheap example!
- 26. H-W Practice: Practice Problems 18.1 2. Of the members of a population of pea plants, 9% are short (recessive). What are the frequencies of the recessive allele t and the dominant allele T ? What are the genotypic frequencies of the population?
- 27. H-W Practice: Practice Problems 18.1 2. Of the members of a population of pea plants, 9% are short (recessive). What are the frequencies of the recessive allele t and the dominant allele T ? What are the genotypic frequencies of the population? q 2 = .09 so q = √.09 = 0.30 f t = 0.3 p + q = 1, so p = 1 – q = 1 – 0.30 = 0.70 f T = 0.7
- 28. H-W Practice: Practice Problems 18.1 2. Of the members of a population of pea plants, 9% are short (recessive). What are the frequencies of the recessive allele t and the dominant allele T ? What are the genotypic frequencies of the population? f t = 0.3 f T = 0.7 f TT = p 2 = (0.7) 2 = 0.49 f Tt = 2pq = (2 x 0.3 x 0.7) = 0.42 f tt = q 2 = (0.3) 2 = 0.09 f TT = 0.49 f Tt = 0.42 f TT = 0.09
- 29. Types of Selection Directional Selection An extreme phenotype is favored and the distribution curve shifts in that direction. Can occur when a population is adapting to a changing environment.
- 31. Types of Selection Stabilizing Selection Occurs when an intermediate phenotype is favored. Can improve adaptation of the population to constant conditions.
- 33. Types of Selection Disruptive Selection Two or more extreme phenotypes are favored over any intermediate phenotype. Two distinctly different phenotypes are found in the population.
- 35. Maintenance of Variations Maintenance of variation is beneficial because populations with limited variation may not be able to adapt to new conditions. Only exposed alleles are subject to natural selection. Sickle-Cell Disease Homozygote remains in equilibrium in some regions of Africa because the heterozygote is protected from sickle-cell and malaria. http://www.pbs.org/wgbh/evolution/library/01/2/l_012_02.html
- 36. Speciation Speciation is the splitting of one species into two or more species, or the transformation of one species into a new species over time. Species Definition Morphological Biological Reproductive Isolation Phylogenetic
- 37. Reproductive Isolating Mechanisms Prezygotic Isolating Mechanisms Prevent reproduction attempts, and make it unlikely fertilization will be successful. Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gamete Isolation
- 38. Reproductive Isolating Mechanisms Postzygotic Isolating Mechanisms Prevent hybrid offspring from developing or breeding. Zygote Mortality Hybrid Sterility F 2 Fitness
- 39. Modes of Speciation Allopatric Speciation Occurs when one population is geographically isolated from other populations. Sympatric Speciation A population develops into two or more reproductively isolated groups without prior geographic isolation. Common in plants – polyploidy
- 40. Adaptive Radiation Adaptive Radiation is an example of allopatric speciation. Many new species evolve from a single ancestral species when members of the species become adapted to different environments.