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1. How Populations Evolve: The Modern SynthesisLecture 13, Part 2 http://www.tpwd.state.tx.us

2. Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

3. Outline • Darwin, Mendel, and the modern synthesis • Evolution of populations • Population genetics • Phenylketonuria • Sources of genetic variation • Mechanism of microevolution • Retinitis pigmentosa • A closer look at natural selection • Species extinctions • Words and terms to know • Possible test items

4. Our Planet at Night Composite image http://www.theodoregray.com Many human population centers are apparent.

5. Darwin and Mendel • Natural selection involves hereditary processes that Darwin was unable to explain. • Although Darwin and Mendel worked at about the same time, Mendel’s discoveries went unnoticed for many years. • Mendel defined the hereditary processes for natural selection in his experiments. • The works of Darwin and Mendel were consolidated in the mid-1900s, many years after their discoveries. • This fusion of evolutionary biology with genetics is known as the ‘modern synthesis.’

6. Populations • A population is defined as a group of individuals from the same species living in the same area at the same time. • Natural barriers can form over time to separate populations of animals, plants, or other organisms. • A population, for example, may be isolated from another by mountain range or river.

7. Some Natural Barriers River http://www.100thpenn.com Mountain range http://www.michigandnr.com http://proprphotos.com

8. Gene Exchange • The genes in isolated populations are infrequently exchanged unless there are opportunities to interbreed. • Populations, however, are usually not completely isolated and rarely have sharp boundaries. • One population center may blur into another where members of both populations are present. • Individuals are more concentrated in population centers, and are most likely to breed with other local members.

9. Evolution of Populations • To a biologist, a population is the smallest biological unit that can evolve. • A common misconception is that individual organisms evolve during their lifetimes. • Natural selection does act on individuals—inherited characteristics affect their survival and reproductive success. • The evolutionary impact of the natural selection process is only apparent when population changes are tracked over generations. • For DDT resistance in mosquitoes, evolution is measured by the change in the percentage of resistant individuals over the span of several gener-ations.

10. Unnatural Selection http://caspar.bgsu.edu Far Side cartoon by Gary Larson

11. Population Genetics • The focus on populations as the basic evolutionary unit is known as pop-ulation genetics. • Biologists examine genetic variation within populations and track genetic changes over time. http://images.perasoned-ema.com

12. Faces in Groups http://mdhsimage.mdhs.org http://www.worth100.com http://debate.uvm.edu

13. Genetic Variation in Populations • We generally have no trouble in recognizing the faces of our friends and acquaintances in a crowd. • Each person has a unique genome as reflected in individual variations in her or his physical appearance (phenotype) and temperament. • Individual variation exists in the populations of all species that reproduce sexually.

14. Factors in Genetic Variation • Most populations have substantial variation that can only be detected through biochemical means (such as DNA analysis). • Not all of the variation in a population is heritable—a phenotype can result from a combination of genotype and environmental influences. • Strength-training, for example, can build-up muscle mass beyond what would normally be expected for a person’s genotype—this would not be passed to offspring.

15. Polygenic Inheritance • Many of the variable traits in a population result from combined effects of several genes. • Polygenic inheritance produces traits that can vary continuously, such as skin color and human height. http://bp2.blogger.com

16. Polymorphism • Other features such as ABO blood groups result from a single gene with different alleles that produce distinct phenotypes. • In-between blood types do not exist. • The contrasting phenotypes (blood types A, B, AB, and O) are known as ‘morphs.’ • A population is said to be polymorphic if two or more morphs are present in substantial numbers (such as what usually happens with blood types).

17. Sources of Genetic Variation • Genetic variation is produced by mutations or sexual recombination of alleles—both are random processes. • A mutation in the nucleotide sequence of a gene can lead to harmless, harmful, or beneficial effects. • On rare occasions, a mutant allele may enhance reproductive success especially if the environment has changed—for example, the mosquito populations sprayed with DDT.

18. Generations and Life Spans • In bacteria, which have short life spans and can evolve rapidly, the only form of genetic variation is mutation since they reproduce asexually by a process known as budding. • Natural selection can increase the frequency of a beneficial mutation in a few days or even a few hours because bacteria can multiply quickly. • The outcome of natural selection for most animals and plants is not as apparent because of the longer length of time between generations.

19. Gene Pool • The gene pool consists of all alleles—alternative forms of genes—in all individuals making-up a population. • The gene pool is the reservoir from which the next generation draws its genes. • Genes, as we have discussed, consist of DNA nucleotide sequences (A, T, G, and C). http://www.ruf.rice.edu

20. Gene Pool Analysis • Consider a wildflower population of only two varieties (morphs) for flower color. • The allele for red flowers (R) is dominant to the allele (r) for white flowers (so far, just like Mendel’s pea plant experiments). • Let’s say the R allele represents 80 percent of genes in the gene pool, or p = 0.8. • Since there are only two possibilities, we can infer the r allele represents the other 20 percent, or q = 0.2. • Using just this information, the frequencies of genotypes in the gene pool can be calculated.

21. Calculation of Genotype Frequencies Allele frequencies: p = 0.8 (R) and q = 0.2 (r) R R p = 0.8 p = 0.8 Sperm Eggs RR p2 = 0.64 r r q = 0.2 q = 0.2 rR p * q = 0.16 Rr p * q = 0.16 rr q2 = 0.04 Hardy-Weinberg formula: p2 + 2pq + q2 = 1.00 Genotype frequencies: p2 = 0.64 (RR), 2pq = 0.32 (Rr), and q2 = 0.04 (rr) RR = 64 percent, Rr = 32 percent, and rr = 4 percent

22. Sometimes It May Seem Like This… http://www.sciencecartoonplus.com Cartoon by Sid Harris

23. Phenylketonuria • The Hardy-Weinberg formula can be used to calculate the percentage of a population that carries the allele for an inherited disorder. • Phenylketonuria (PKU) is an inherited disorder of the inability of the body to breakdown the amino acid, phenylalanine. • PKU, the result of a recessive allele from both parents, affects about one in 10,000 newborn in the United States. • The disorder can cause severe mental retardation if left untreated early in life.

24. Genetic and Biochemical Bases http://upload.wikimedia.org http://newbornscreening.org

25. Frequency of PKU Carriers • Newborn are routinely tested for the disorder—the symptoms can be pre-vented by following a strict diet. • The frequency of U.S. newborn with PKU corresponds to q2 in the Hardy-Weinberg formula. • If q2 = 0.0001, q = .01, the frequency of homozygous recessive carriers in the general population.

26. Microevolution • Microevolution is evolution at its smallest scale, such in found in Darwin’s finches or a population of wildflowers. • Four mechanisms can change the gene pool of a population: genetic drift, genetic flow, mutations, and natural selection. Darwin’s finches http://www.d113.lake.k12.il.us

27. Small Sample Sizes • If we toss a coin 1000 times, and it comes up heads 700 times, we might conclude something is wrong. • If we tossed the coin ten times, and it comes up heads seven times, we may think nothing of it. • The smaller the sample size, the greater potential deviation from the expected result (50/50) for a random event with only two possibilities. http://www.moderncoinmart.com

28. Sampling Error • The deviation from the expected result is known as sampling error—it is often a problem with small sample sizes. • Sampling error applies in population biology in what is known as genetic drift.

29. A Wildflower Population • The coin toss logic can be applied to a population’s gene pool where we substitute the population size for the sample size of the number of coin tosses. • For instance, say we have a population of wildflowers that had drawn its alleles at random from the previous generation. • The larger the population of the previous generation, the more likely the new generation (offspring) will represent the previous generation’s gene pool.

30. Genetic Drift • While a large gene pool tends to maintain its status quo, a small gene pool may not be accurately represented in the next generation due to sampling error. • A change in the gene pool of a small population due to chance is known as genetic drift. • The bottleneck effect and founder effect can shrink a population to a size where genetic drift can occur.

31. Wildfires Portugal Wildfires are not limited just to California and other western states. Both images from http://upload.wikimedia.org

32. Bottleneck Effect • Large-scale disasters—including wildfires, floods, earthquakes, and droughts—can drastically reduce the size of a population. • The surviving members of the now much smaller population may not represent the original popula-tion’s gene pool. • By chance, some alleles might be overrepresented among the survivors while others may be underrep-resented. • Other alleles may have been eliminated altogether from the surviving population. • This bottleneck effect reduces genetic variability in the population since some alleles are lost from the gene pool. http://www.fao.org

33. Cheetahs • Cheetahs, the fastest of all running animals, were once widespread in Africa and Asia. • Their populations fell drastically during the last ice age about 100,000 years ago. • The species suffered a severe bottleneck—it then experienced a second bottleneck in the 19th century when they were hunted to near-extinction. Painting http://www.onlineartdemos.co.uk

34. Perils of Low Genetic Variability • Genetic variability is low in the few small cheetah populations existing in the wild. • With so little genetic variability, the population has a reduced capacity to adapt to environment changes including loss of habitat and the spread of disease. • The reduced genetic variability makes its future precarious in its natural state.

35. Founder Effect • Genetic drift can also occur when a few individuals—including animals, plants, and bacteria—colonize a new habitat such as an island or lake. • The smaller the colony, the less its genetic makeup will represent the gene pool of the larger population from where the colonists originally came. • Genetic drift in a newly-established colony is known as the founder effect. • The effect explains the relatively high frequency of certain inherited disorders among human populations established by a few colonists.

36. Tristan da Cunha http://volcano.und.edu Tristan da Cunha http://www.pmel.noaa.gov Tristan da Cunha is one of the most remote places on Earth. The volcanic island is astride the mid-Atlantic Ridge between two tectonic plates. The surrounding ocean is rich in crawfish, a major source of revenue for the ~300 islanders. A second major revenue source is the production and sale of postage stamps to collectors. http://www.visitandlearn.co.uk

37. Retinitis Pigmentosa • Fifteen people established a colony on the island of Tristan de Cunha in 1814. • One of the colonists happened to carry the recessive allele for a rare and progressive form of blindness known as retinitis pigmentosa. • Among the 240 residents on the island in the 1960s, four had the genetic disorder and nine were known to be heterozygous carriers. • The incidence on Tristan de Cunha is much higher than in Great Britain, the homeland of the original colonists.

38. Gene Flow • Most populations are not completed isolated—a population may gain or lose alleles by gene flow, that is, the exchange of genetic material with other populations. • Gene flow occurs when fertile individuals or gametes migrate between populations. • Consider two similar species of wildflowers in adjacent woodland areas. • Wind may carry pollen from one flower population to the other enabling the populations to interbreed.

39. Reduction in Genetic Differences • Gene flow tends to reduce the genetic differences among populations. • If extensive, gene flow can result in the combination of populations into a single population with a common gene pool. • Gene flow is not always possible due to reproductive barriers between species as we will discuss next week.

40. Human Migration and Gene Flow The migration of people throughout the world is transferring alleles between populations that were once isolated. http://courses.washington.edu

41. Genetic Mutations • A mutation results from a change in the DNA nucleotide sequence, the genetic language of life. • A new mutation can immediately change the gene pool of a population by substituting one allele for another. • Although a mutation on any one gene is rare, the cumulative impact of mutations can be substantial since each individual in a population has thousands of genes. • Over many generations, mutations can be important to the evolutionary process. http://berkeley.evolution.edu

42. A Closer Look at Natural Selection • We return to natural selection for a closer look at its basic mechanisms. • Genetic drift, gene flow, and mutation can result in microevolution, but they usually do not lead to adaptation since they are largely based on chance. • Of all of the forms of microevolution, only natural selection is considered adaptive.

43. Adaptation • On the average, those individuals best suited to the environment leave the most offspring and have a disproportionate impact on the gene pool. • When fields were sprayed with DDT, the resistant mosquitoes survived and increased the frequency of the pesticide-resistant allele in the gene pool. • Natural selection is adaptive because it enables some members of a species to survive and reproduce in a changed or new physical environ-ment.

44. Darwinian Fitness • The ‘struggle for existence’ and ‘survival of the fittest’ are sometimes interpreted by the ‘Social Darwinists’ as competitive contests between individuals. • While the male members in some animal species do battle for mating privilege, reproductive success is generally more subtle and passive. • A frog, for example, may produce more eggs than others because she is more efficient at catching insects for food. • Individuals in a wildflower population may differ in reproductive success because some are better able to attract pollinators due to subtle differ-ences in their color, shape, or fragrance. • Darwinian fitness is the contribution a individual makes to the gene pool of the next generation relative to the contributions of other individuals.

45. Attracting Pollinators Ruby-throated hummingbird and honeybee http://www.fs.fed.us Darwinian fitness of flowering plants depends in part on a competitive advantage in attracting pollinators.

46. Directional Selection • Three modes of natural selection are directional selection, diversifying selection, and stabilizing selection. • Directional selection shifts a population to an extreme phenotype—it is most common when the local environment changes or when organisms migrate to a new environment.

47. Diversifying and Stabilizing Selection • Diversifying selection can lead to a balance between two or more morphs, or phenotype forms. • Stabilizing selection maintains the variation of a phenotype within a narrow range—it typically occurs in a stable environment in which populations are already well adapted.

48. Survival of a Species • When challenged with a new set of environmental problems, a population either adapts through natural selection or can become extinct in that locale. • Some human activities can be so sudden or overwhelming that a species is unable to survive.

49. http://upload.wikimedia.org The dodo bird lived on Mauritius Island in the Indian Ocean. Related to the pigeon and dove, it was about three-feet tall. The dodo was flightless, unafraid, and lived on fruit. In the 1700s, it was one of the first species to be extinguished due to indiscriminate hunting. Dodo Bird

50. Passenger Pigeon http://www.stanford.edu Over five billion passenger pigeons were thought to have resided in the Eastern United States in the 1800s. Migratory flocks were reported as long as 100 miles, a mile wide, and several birds deep. Within 20-to-30 years the passenger pigeon was largely extinct due to indiscriminate hunting and other human activities. Female (top branch) feeding a squab http://vrcoll.fa.pitt.edu