Population Genetics Chapter 18, 19

1. Population GeneticsChapter 18, 19 Developed by Adam F. Sprague OCC Biology 114

2. Populations: Members of a sexually-reproducing species are able to interbreed, produce fertile offspring, and have a shared gene pool Gene pool refers to the collective group of alleles of all the individuals in a population

3. Populations Different species do not exchange genes with each other by interbreeding A population is a group of organisms of the same species occupying a certain area

4. Populations Members of a population vary from one another Variation is the raw material for evolutionary change Features that make an organism suited for its environment so it can survive, reproduce, & pass its alleles onto its offspring are called adaptations

5. Populations Speciation is the splitting of one species into two or more species or the transformation of one species into a new species over time Speciation is the final result of changes in gene pool allelic and genotypic frequencies

6. Micro- & Macro- Evolution: Macroevolution refers to large scale evolutionary changes such as the formation of new groups above the species level

7. Micro- & Macro- Evolution: Microevolution refers to smaller scale changes such as changes within species

8. Micro- & Macro- Evolution: In studying evolution at the population level, geneticists focus on the gene pool When the relative frequency of alleles in a population changes over a number of generations, it is called microevolution

9. Causes of Change in Gene Pools: Mutations Mutations result in the introduction of new genes (new genetic information) into a gene pool Mutations can be changes in genes (DNA sequences) or changes to chromosomes (additions, deletions, substitutions, translocations)

10. Causes of Change in Gene Pools: Gene mutations provide new alleles, and therefore are the ultimate source of variation A gene mutation is an alteration in the DNA (deoxyribonucleic acid) nucleotide sequence of an allele

11. Causes of Change in Gene Pools: Mutations occur at random Mutations can be beneficial, neutral, or harmful Some chromosomal mutations are alterations in the number of chromosomes inherited Others are alterations in arrangement of alleles on chromosomes due to inversions and translocations

12. Gene Flow Gene flow is the movement of genes into (immigration) & out of a population (emigration) Migration of breeding individuals moves alleles among populations through interbreeding Gene flow may be agent of microevolution (e.g. isolated populations with limited gene flow result in genetic distinctions among groups living in different locations) Continued gene flow tends to decrease the diversity among populations, causing gene pools to become similar

13. Genetic Drift Genetic drift refers to changes in allele frequencies of a gene pool due to chance (random) events

14. Genetic Drift Genetic drift occurs in both large and small populations Larger populations suffers less sampling error Genetic drift causes gene pools of two isolated populations to become dissimilar as some alleles are lost and other are fixed Genetic drift occurs when founders start a new population, or after a genetic bottleneck with interbreeding Founder effect is a case of genetic drift in which rare alleles, or combinations of alleles, occur in higher frequency in a population isolated from the general population (e.g. dwarfism is much higher in a Pennsylvania Amish community due to a few German founders) Bottleneck effect is genetic drift in which a severe reduction in population size due to natural disaster, predation, or habitat reduction, causes severe reduction in total genetic diversity of the original gene pool (e.g. Intense interbreeding in cheetahs has prevented most genotypes from being passed to the next generation)

15. Non-random Mating Inbreeding (mating with close neighbors instead of more distant members of a population) can effect the frequency of some genotypes Causes a reduction in heterozygous genotypes & an increase in homozygous genotypes

16. Non-evolving Populations: Gene pool of a non-evolving population remains constant over the generations The shuffling of genes that accompanies sexual reproduction does not alter the genetic makeup of the population (i.e. sexual reproduction alone does not lead to microevolution) Hardy-Weinberg equilibrium = frequency of each allele in the gene pool tends to remain constant unless affected by other agents

17. Hardy-Weinberg equation :

18. Hardy- Weinberg Thus the genotypic frequencies will be:WW = p2 = 0.64Ww = 2pq = 2(0.8)(0.2) = 0.32ww = q2 = 0.2 The allele frequencies will be p = 0.8, and q = 0.2; they remain unchanged from the previous generation so this population has not evolved

19. Five conditions are required for Hardy-Weinberg equilibrium: Hardy-Weinberg equilibrium says that something other than sexual reproduction is required to alter the gene frequencies in a population from one generation to the next The Hardy-Weinberg equilibrium provides basis for comparing idealized, non-evolving populations with actual ones in which gene pools are changing For a population to be at Hardy-Weinberg equilibrium, it must satisfy 5 main conditions Population must be very large (no genetic drift) Population must be isolated (no migration or gene flow) No mutation (Rate of mutation does not alter gene pool) Random mating All individuals are equal in reproductive success (i.e. natural selection does not occur) These 5 conditions are rarely met

20. Natural Selection: Populations must adapt to their environment Natural selection produces adaptive evolution

21. Natural Selection: Natural selection requires variation (heritable genetic differences) in the members of a population Some differences affect how well an organism is adapted to its environment & make them more fit or more likely to reproduce Fitness is the extent to which an individual contributes fertile offspring to the next generation & is measured against the reproductive success of other genotypes in the same environment

22. Natural Selection There are three natural outcomes of natural selection --- Stabilizing selection, Directional selection, & Diversifying selection Stabilizing selection (most common) favors intermediate variations in a population & eliminates the extremes (e.g. majority of human birth weights within 6.4-9 lb range)

23. Directional selection Directional selection (common during periods of environmental change or during migration to new habitat with different environmental conditions) favors one extreme (e.g. antibiotic resistance or a shift of dark-colored peppered moths from light-colored correlated with increasing pollution)

24. Diversifying selection (occurs when environmental conditions are varied in a way that favors individuals at both extremes of phenotypic range) selects against intermediate Beaks

25. Isolation Factors for Species: A biological species is a category whose members are reproductively isolated from all other such groups Reproductive isolation occurs when members of one species can only breed successfully with each other Modern biochemical genetics uses DNA hybridization techniques to determine the relatedness of organisms Reproductive isolating mechanisms are any structural, functional, or behavioral characteristic that prevents successful reproduction from occurring

26. Habitat isolation Habitat isolation occurs when two species occupy different habitats, even within the same geographic range, so that they are less likely to meet and to attempt to reproduce

27. Temporal isolation Temporal isolation occurs when two species live in the same location, but each reproduces at a different time of year, and so they do not attempt to mate

28. Behavioral isolation Behavioral isolation results from differences in mating behavior between two species

29. Mechanical isolation Mechanical isolation is the result of differences between two species in reproductive structures or other body parts, so that mating is prevented

30. Gamete isolation Gamete isolation is physical or chemical incompatibility of gametes of two different species so that they cannot fuse to form a zygote; an egg may have receptors only for the sperm of its own species Zygote mortality is when hybrids (offspring of parents of two different species) do not live to reproduce