1. The Evolution of Populations Chapter 23

2. Populations and Gene Pools A population is a group of individuals in a certain time and place that belong to the same species – that is: they can interbreed and produce viable offspring A Gene Pool is the total number of genes within a population at a particular time and place.

3. Alleles and Genetic Variation Germline Mutations in an original gene will give rise to new alleles in a population. Alleles will create the variations in a population that Darwin talked about Some alleles will be beneficial to an organism, others will be detrimental The alleles that contribute to the survival of a species in a particular environment, will be passed on to subsequent generations. Non beneficial alleles will perish with the individuals.

4. What else (other than mutations) creates and maintains Genetic Variation? Sexual Recombination- sex reshuffles alleles – that is better for a population Diploidy – two alleles better than one Balanced Polymorphism – The ability of natural selection to maintain diversity in a population - Heterozygote Advantage – heterzygotes are naturally selected Neutral Variation – a trivial variation like human fingerprints

5. Counting Alleles The Hardy Weinberg Theorem allows a population geneticist to count the alleles of a particular gene in a population –How many dominant alleles? –How many recessive? –How many homozygous or heterozygous individuals? But one can ONLY count alleles in a non- evolving, fixed-size population – one with a closed gene pool.

6. Get Out! The Pool is Closed! The Amish for example – the only variations in the population arise from independent assortment and crossing over during meiosis and nonrandom fertilization (mating within the same community) No gene flow! (immigration, for example) My cousin’s a hottie!

7. Nonrandom mating / fertilization Nonrandom Mating – “mating in the same neighborhood” or inbreeding –Also known as “selfing” in plants Gene Flow – the flow of new genes into a population from neighboring populations – in other words - migration

8. Gene Flow Populaitons used to be isolated from each other. But migration of people has increased gene flow between populations. Gene flow homogenizes the gene pools of a population and reducing geographic variation in appearance.

9. Problems with Inbreeding Leads to an excess of homozygotes Many diseases are recessively inherited and one must be homozygous recessive to have the disease Closely related “healthy” individuals could be heterozygous since the allele is already in the family They are not be affected by the disease, but will pass it on to offspring if they mate

10. Non-Evolving Populations The Hardy-Weinberg theorem applies to non-evolving populations – a population in equilibrium. Hardy-Weinberg Rules: 1. Mutations must not occur 2. Natural selection must not be occurring 3. The population must be infinitely large 4. All members of the population must breed 5. All mating must be totally random 6. Everyone must produce the same number of offspring 7. NO GENE FLOW - there must be no migration in or out of the population

11. The formula is : p 2 + 2pq + q 2 = 1 Where: p = the dominant allele q = the recessive allele p 2 = homozygous dominant q 2 = homozygous recessive 2pq = heterozygous

12. Sample Problem 1 in 1700 US Caucasian newborns have cystic fibrous. C for normal is dominant over c for cystic fibrous. ALLELE FREQUENCY CALCULATIONS: cc = cystic fibrosis. Therefore q 2 = 1/1700, q 2 = 0.00059 or 0.059% (with both recessive alleles) If q 2 = 0.00059, then q = square root of 0.00059 = 0.024 or 2.4% for cystic fibrosis allele if p = 1-q, then 1- 0.024 = 0.976 or 97.6% for normal, dominant allele Now that you know that p =.976 and q =.024. The following genotypes can be found. (f)CC- Normal homozygous dominant = p2 = ? (f)Cc -carriers of cystic fibrous = 2pq = ? p 2 = (0.976)2 = 0.953 or 95.3% frequency for both normal alleles (CC) 2pq = 2(0.976 x 0.024) = 0.0468 or 4.68% heterozygous or carriers (Cc) So remember the formula: p2 + 2pq + q2 = 1 Plug in the numbers – do they work?

13. Practice Problem 1 1.In a population with two alleles for a particular locus, B and b, the allele frequency of B is 0.7. What would be the frequency of heterozygotes if the population is in Hardy-Weinberg equilibrium?

14. Practice Problem 2 2.In a population that is in Hardy-Weinberg equilibrium, 16% of the individuals show the recessive trait. What is the frequency of the dominant allele in the population?

15. Microevolution When the actual values deviate from the expected values of the Hardy-Weinberg equilibrium, it means the population is evolving.When the actual values deviate from the expected values of the Hardy-Weinberg equilibrium, it means the population is evolving. Usually, evolution on a grand scale is not visible – we did not see any of the current species evolve.Usually, evolution on a grand scale is not visible – we did not see any of the current species evolve. But small generation-to-generation changes in one population’s alleles and genotypes CAN be observable (mixing of races from colonization, for example)But small generation-to-generation changes in one population’s alleles and genotypes CAN be observable (mixing of races from colonization, for example) This is evolution on a very small scale - MicroevolutionThis is evolution on a very small scale - Microevolution

16. Causes of Microevolution Genetic Drift – changes in the gene pool of a population due to chance. Two situations can lead to genetic drift: 1.Bottleneck Effect - natural disasters/hunting may kill many, leaving a few whose genetic makeup does not reflect that of the original population 2.The Founder Effect – refers to the loss of genetic variation when a new colony is established by a very small number of individuals from a larger population.

17. Bottleneck Effect

18. Founder Effect The Amish population in PA. stemmed from a small number of German immigrants -- about 200 individuals. The Amish carry unusual concentrations of gene mutations that cause a number of otherwise rare inherited disorders – such as dwarfism. One form of dwarfism, Ellis-van Creveld syndrome, involves short stature, polydactyly (extra fingers or toes), abnormalities of the nails and teeth, and, in about half of individuals, a hole between the two upper chambers of the heart.

19. Geographical Variation Geographic variation results from differences in phenotypes or genotypes –Variation Within a Population – Subtle difference between members of the same race that live in the same geographic area (between 2 Caucasians) –Variation Between Populations - between populations or between subgroups of a single population that inhabit different areas (human races) Geographic variation in the form of graded change in a trait along a geographic axis is called a cline. For example, as you travel towards the equator, people have more pigment in their skin (they’re darker)

20. Variations within Populations Cline – a graded change in a particular trait along a geographic axis.

21. Types of Natural Selection

22. Directional Selection When conditions favor one extreme phenotype, so the allele frequency is shifted to one side. The best example for this the phenomenon of peppered moths and industrial melanism

23. Stabilizing Selection Removes extreme phenotypes or variants from a population. So population looks more homogenous. In this case, the darker and lighter snail probably got eaten, so the “middle” colored shells survived.

24. Disruptive or Diversifying Selection Phenotypes at the opposite ends of the spectrum are naturally selected. Intermediate phenotypes are lost.

25. Disruptive Selection, cont’d. Darwin observed disruptive beak sizes in certain seed- eating finch species: This variation appeared to be adaptively related to the seed size available on the respective islands - big beaks for big seeds - small beaks for small seeds. Medium beaks had difficulty retrieving small seeds and were also not tough enough for the bigger seeds. So medium beaks were lost as a phenotype.

26. Sexual Selection A form of natural selection where individuals with certain inherited traits are more likely to finds mates. Sexual selection causes sexual dimorphism, where there are clear differences between the two sexes (Like in peacocks and peahens)

27. Intersexual Selection – A form of sexual selection in which members of one sex are differentially attracted to members of the opposite sex, and so competition is for the attention of the opposite sex. Birds Intrasexual Selection – A form of sexual selection in which members of one sex compete with each other to mate with the opposite sex. Males will frequently fight each other for a female or for a desirable territory to attract a female. Mammals

28. END OF CHAPTER 23