Population Genetics - the foundation of evolution Microevolution: change in the genetic makeup of a population from generation to generation; individuals are selected and populations evolve Grass growing on toxic heavy metals only ones that can tolerate the metals survive

Chapter 23 The Evolution of Populations Mrs. MacWilliams AP Biology

Darwin and Mendel Darwin: evolution by natural selection but how do organisms get heritable variations and how do they transmit them to their offspring? Blending Hypothesis??? No…this eliminates differences Mendel: Particulate Theory of Inheritance; pass on discrete units (genes) that retain identity

Variation Within a Population Discrete characters can be classified on an either-or basis  Example: EITHER red OR white flowers Quantitative characters vary along a continuum within a population  Example: Variation in Beak size

Gene Pools and Allele Frequencies Population: a localized group of individuals that are capable of interbreeding and producing fertile offspring; some are isolated, some are not Gene Pool: the total aggregate of genes in a population at any one time

Hardy-Weinberg Theorem Hardy-Weinberg equation Predict allele frequencies within a population. Its prediction of genetic equilibrium or no frequency change under a specific set of conditions also serves as strong evidence for microevolution. As long as a population does not meet all the prerequisite conditions for genetic equilibrium, allele frequencies must change over time, and this is the primary process of evolution.

Hardy-Weinberg Theorem p 2 + 2pq + q 2 = 1 AA + Aa + aa = 100% p + q = 1 A + a = 100%

HARDY-WEINBERG & EVOLUTION Biologists can determine whether an agent of evolution is acting on a population by comparing the population ’ s genotype frequencies with Hardy– Weinberg equilibrium frequencies. If there is no change in frequencies, there is no evolution Conversely, if there have been changes in the frequencies, then evolution has occurred. Evolution is change of allelic frequencies

H-W Conditions 1. Extremely large population size …small populations have greater chances of fluctuations 2. No Gene Flow …no transfer of genes between populations 3. No Mutations …introducing, reducing, modifying genes can change the gene pool 4. Random Mating …if individuals preferentially choose mates then no random mixing of gametes 5. No Natural Selection …differential survival and reproductive success will alter allele frequency

Mutation & Sexual Recombination = variation Only mutations in gametes can be passed to offspring Point mutations, Deletion… Sexual Recombination Shuffling of genes…

Genetic Drift, Gene Flow = variation Genetic Drift : small population = greater chance of deviation

Genetic Drift, Gene Flow = variation Genetic Drift --> ex: Bottle Neck Effect

Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000— but their genes still carry the marks of this bottleneck: they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

Genetic Drift, Gene Flow = variation Genetic Drift --> ex: Founder Effect The establishment of a new population by a few original founders (in an extreme case, by a single fertilized female) which caarry only a small fraction of the total genetic variation of the parental population. The result is that a given allele, gene, chromosome or part of a chromosome found in members of the population can be traced back to the one ancestral individual(s).

Genetic Drift, Gene Flow = variation Genetic Drift --> ex: The Founder Effect

An Example of Founder Affect: The Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

Genetic Drift, Gene Flow = variation Gene Flow: immigration/emigration ; add/subtract genes

Gene Flow in Humans Gene flow has been observed in humans. For example, in the United States, gene flow was observed between a white European population and a black West African population, which were recently brought together. In West Africa, where malaria is prevalent, the Duffy antigen provides some resistance to the disease, and this allele is thus present in nearly all of the West African population. In contrast, Europeans have either the allele Fya or Fyb, because malaria is almost non-existent. By measuring the frequencies of the West African and European groups, scientists found that the allele frequencies became mixed in each population because of movement of individuals. It was also found that this gene flow between European and West African groups is much greater in the Northern U.S. than in the South.

Gene Flow

Natural Selection = adaptive evolution Of all the factors that can change a gene pool, only natural selection is likely to adapt a population to its environment Natural selection accumulated & maintains favorable genotypes in a population Variations that are heritable are the raw material for natural selection

Variation Within a Population Polymorphism: 2 or more distinct morphs (different types) represented in a population Average Heterozygosity-measures the average percent of loci that are heterozygous in a population Geographic Variation

Modes of Selection

Preserving Genetic Variation Diploidy- maintains genetic variation in the form of hidden recessive alleles

Preserving Genetic Variation Balancing Selection- natural selection maintains stable frequencies of two or more phenotypic forms in a population  Heterozygote Advantage-heterozygotes have a higher fitness than do both homozygotes Ex. Sickle Cell and Malaria Ex. Cystic Fibrosis and cholera  Frequency Dependent Selection- fitness of a phenotype declines if it becomes too common in the population Neutral Variation- genetic variation that appears to confer no selective advantage or disadvantage

Preserving Genetic Variation Sexual Selection Intrasexual selection- competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection-mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates sexual dimorphism- marked differences between the sexes in secondary sexual characteristics

Reproductive “ handicap ” of sex Sexually reproducing organisms don ’ t leave as many offspring BUT offspring varies which may make them more ‘ fit ’

Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations