The Evolution of Populations and Speciation

1. Reference: Campbell 7th Ed. Chapters 23 & 24 The Evolution of Populations and Speciation

2. Evolution by natural selection gains wide acceptance Early 1900’s birth of genetics field Questions resurface about evolution and natural selection “Population Genetics”: study of evolution from genetic point of view Involves gradual changes in genetic material over generations, in groups of organisms Variation of Traits in a Population

3. A Population is the smallest unit in which evolution occurs (“microevolution”) • Individuals may vary in observable traits • Studying variation in a single trait – use a large sample • Quantitative traits in a population (height, weight) show variation in a bell-shaped “normal” curve • Ex. Body length in a population of fish • X axis: fish length (cm) • Y axis: # of fish Variation of Traits in a Population

4. What causes variation in traits? • Environmental factors & Hereditary can account for different phenotypes within a single family • Genotypes (alleles) come from same parents but in different combinations can account for variations in successive offspring due to formation of gametes & how they fuse (Segregation of Alleles) • Ex: Rr x Rr = ? Variation of Traits in a Population

5. Mutation: flawed copies of individual genes • Recombination: reassociation of genes in diploid individual (occurs during meiosis) • Segregation of alleles • Independent assortment (nonhomologous) • Crossing over (homologous) Causes of Variation

6. Random fusion of gametes: chance game played by gametes • Millions of sperm in mating • “Chosen One” fertilizes egg • Ensures variation in offspring • No exact copies of parents, or other offspring likely Causes of Variation Try this game: The Great Sperm Race

7. “Gene pool”: total genetic information available in a population • “Allele frequency”: percentage of allele in gene pool (expressed as a decimal) • Ex: If there are ten individuals in a population and at a given locus there are two possible alleles, A and a, then if the genotypes of the individuals are: • Population 1: AA, Aa, AA, aa, Aa, AA, AA, Aa, Aa, and AA • Then the allele frequencies of alleleA and allelea are: • pA = (2+1+2+0+1+2+2+1+1+2)/20 = 0.7 • pa = (0+1+0+2+1+0+0+1+1+0)/20 = 0.3 • *remember, gametes are haploid, and carry only one form of allele Allele Frequencies and Gene Pool

8. Phenotypes are controlled by which alleles are inherited (genotypes) Phenotype frequency: ratio stating number of times a specific phenotype occurs in a population in a single generation Predicting Phenotype example, F2: red 1/6 = 0.17 pink = 3/6 = 0.50 white = 2/6 = 0.33

9. British mathematician Godfrey Hardy • German physician Wilhelm Weinberg • Independently showed that allele frequencies in a population “tend to remain the same from generation to generation unless acted on by outside influences”when populations are in “genetic equilibrium”. • Hardy-Weinberg Equilibrium • Based on set of assumptions about ideal hypothetical population that is not evolving Hardy-Weinberg

10. 1) No mutations occur • Allele frequencies do not change overall • 2) Individuals don’t migrate • 3) Population is large • 4) Individuals mate randomly • 5) Natural selection does not occur Hardy-Weinberg conditions:

11. Equation:     p2+2pq+q2=1.0 p2 = homozygous dominant condition;  AAq2 = homozygous recessive;  aa2pq = heterozygous ; Aa Hardy-Weinberg Equation:

12. Theoretical state in which allele frequencies remain the same over generations (P = F1 = F2 = F3, etc) Showed what forces disrupt genetic equilibrium and led to evolutionary change Real populations usually violate HW conditions, causing gene frequencies to fluctuate Hardy-Weinberg

13. Evolution results from the change of population’s allele frequencies (genetics) over generations • Any violation of 5 conditions of Hardy-Weinberg Equilibrium results in evolution • Mutagens can cause increase/decrease in allele frequency • Spontaneous mutations occur constantly • Mutations can produce new alleles for trait • Affect genetic equilibrium Mutation

14. Spontaneously introduces new allele variants into a population • Natural selection is often slow to eliminate harmful recessive mutations • Natural selection operates only when genes are expressed (phenotypes); often not when “carried” • Beneficial mutations are vital to evolution in long run Mutation

15. Mutations: beneficial or not?

16. Gene flow: process of genes moving from one population to another ex: Baboons Migration Gene frequency changes Immigration: movement of individuals into a population Emigration: movement of individuals out of a population

17. Genetic Drift: allele frequencies in a population change as result of random events or chance. • Example: • Small population can be affected by single organism’s ability to reproduce low or high • Small populations are much more susceptible. Why? • Abrupt changes in alleles shows high genetic drift • Large population • Retain stable allele frequencies; low genetic drift Genetic Drift

18. Small population loses genetic variability and becomes vulnerable to extinction “Bottlenecking” a population Northern Elephant Seals Cheetahs = very little genetic variability left Genetic Drift

19. genetic drift population bottleneck Founder effect

20. Most species do not mate randomly • Geographic proximity is a factor • Matings of related individuals can amplify traits & result offspring with disorders • Similar recessive genes (carried, masked) often present in genomes of related individuals Nonrandom Mating

21. Physical Characteristics (similar genes) • Assortative Mating: selection of mate based on similarity of characteristics • Nonrandom mating can affect genotypes (combination of alleles) of population • May not affect on overall allele frequencies Nonrandom Mating Blue and white snow geese

22. Ongoing process in populations • Single most significant factor that disrupts genetic equilibrium • Individuals reproduce more successfully as result of natural selection • Contribution of genes to next generation • Stabilizing, Directional, Disruptive and Sexual all cause evolution in a population (microevolution) Natural Selection

23. Stabilizing Selection: average form of trait causes organism to have an advantage in reproduction; high fitness • Lizard size • Small lizard runs too slow • Large lizard easily spotted and captured • Selection reduces size range • Most common type of selection Stabilizing Selection

24. Directional Selection: individuals that display more extreme form of trait have higher fitness than individuals with average Directional Selection

25. Disruptive Selection : individuals with either extreme variation of trait have higher fitness than average form of trait • Limpets • Shell color • Pure white to dark tan • White on rocks with goose barnacles • Dark tan on bare rocks blend in • Intermediate color at disadvantage Disruptive Selection

26. Selection Charts

27. Sexual selection: preferential choice of a mate based on specific phenotypic trait Females tend to choose males they mate with due to certain traits male expresses Genes of successful reproducers rather than of merely successful survivors are amplified through natural selection Sexual Selection The Tale of the Peacock

28. Total # of species today is inaccurate due to numerous undiscovered species • Currently, scientists have named and successfully classified over 1.5 million species. It is estimated that there are as little as 2 million to as many as 50 million more species that have not yet been found and/or have been incorrectly classified. • Remote locations: Rainforests and Oceans • New species discovered while others become extinct at fast rate • One species can become two through process of speciation • Speciation results in many related populations Concept of Species

29. Concept of Species

30. Morphological Species Concept: Morphology: study of internal and external structure and form of an organism Using the MSC, species are defined by structure and appearance Aka “Phenetic” species concept: a species is a set of organisms that are phenotypically similar and that look different from other sets of organisms.

31. Phenotypic differences may exist among individuals in one population. Limitations to MSC Adult & juvenile herring gulls Mallards (Anas platyrhynchos) American Black duck (Anasrubripes)

32. Organisms may appear different enough to belong to different species. How different do they have to be to be considered a unique species? • Biological Species Concept:A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions. • Defines a species as those organisms that can produce viable offspring together. Same chromosome # • Issues: • What about hybrids? • What about plants, etc that reproduce asexually? • What about extinct species? Biological Species Concept

33. Geographic Isolation: physical separation of members of population • Allopatric Speciation • Populations physically isolated by an extrinsic barrier • Gene flow between them stops • Natural selection and genetic drift cause divergence • Individuals of two populations can no longer interbreed Geographic Isolation

34. Reproductive Isolation: results from barriers to successful breeding between population groups in same area • Parapatric Speciation • Two or more separate gene pools form, and eventually these diverge into different species • Two broad types • Prezygotic: before fertilization • Postzygotic: after fertilization Reproductive Isolation

35. Types of postzygotic isolation • Offspring of interbreeding species are underdeveloped, die early, or are not fertile • If death or infertility occurs parents have wasted gametes from evolution standpoint • Prezygotic • Incompatible behavior • Reduce chance of hybrid formation • Mating times, calls • Frogs, birds Reproductive Isolation

37. Gradualism –vs- Punctuated Speciation • Speciation usually takes millions of years, but some species form more rapidly • “Gradualism” - Fossil record indicates many species existed without change for long periods • Fossil evidence seems to indicate that “instant” changes can occurred within few thousand years (Hox genes) • Punctuated Equilibrium: theory that speciation occurs during brief periods of rapid genetic change, interspersed with long equilibrium periods • In 1972 paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing this idea. Rates of Speciation