1 1 Population Genetics

2 2 The Gene Pool Members of a species can interbreed & produce fertile offspring Species have a shared gene pool Gene pool – all of the alleles of all individuals in a population

3 3 The Gene Pool Different species do NOT exchange genes by interbreeding Different species that interbreed often produce sterile or less viable offspring e.g. Mule

4 4 Populations A group of the same species living in an area No two individuals are exactly alike (variations) More Fit individuals survive & pass on their traits

5 5 Speciation A Species is a group of populations whose individuals have the ability to breed and produce fertile offspring. Formation of new species One species may split into 2 or more species A species may evolve into a new species Requires very long periods of time

6 6 Modern Synthesis Theory Today’s theory on evolution Recognizes that GENES are responsible for the inheritance of characteristics Recognizes that POPULATIONS, not individuals, evolve due to natural selection & genetic drift Recognizes that SPECIATION usually is due to the gradual accumulation of small genetic changes

7 7 Microevolution Changes occur in gene pools ( gene pool is the total of all genes in the population at any one time) due to mutation, natural selection, genetic drift, etc. Gene pool changes cause more VARIATION in individuals in the population This process is called MICROEVOLUTION Example: Bacteria becoming unaffected by antibiotics (resistant)

8 8 Assumptions of the Hardy-Weinberg Theory : - Large population size : small populations can have chance fluctuations in allele frequencies (e.g., fire, storm). - No migration : immigrants can change the frequency of an allele by bringing in new alleles to a population. - No net mutations : if alleles change from one to another, this will change the frequency of those alleles. - Random mating : if certain traits are more desirable, then individuals with those traits will be selected and this will not allow for random mixing of alleles. - No natural selection : if some individuals survive and reproduce at a higher rate than others, then their offspring will carry those genes and the frequency will change for the next generation.

9 9 The Hardy-Weinberg Purpose Used to describe a non-evolving population. Understanding a non-evolving population, helps us to understand how evolution occurs. Meiosis and random fertilization have no effect on the overall gene pool. Natural populations are not expected to actually be in Hardy-Weinberg equilibrium. Deviation from H-W equilibrium usually results in evolution.

10 Hardy-Weinberg Equilibrium The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time. The Hardy-Weinberg Equation: 1.0 = p 2 + 2pq + q 2 where p 2 = frequency of AA genotype; 2pq = frequency of Aa plus aA genotype; q 2 = frequency of aa genotype

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13 But we know that evolution does occur within populations Evolution within a species/population = microevolution. Microevolution refers to changes in allele frequencies in a gene pool from generation to generation. Represents a gradual change in a population. Causes of microevolution: 1) Genetic drift 2) Natural selection (1 & 2 are most important) 3) Gene flow 4) Mutation

14 migration mutation natural selection genetic drift non-random mating cause changes in allele frequencies How does genetic structure

15 1) Genetic drift Genetic drift = the alteration of the gene pool of a small population due to chance.

16 2) Natural selection -Natural selection is success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance). The only agent that results in adaptation to environment. 3) Gene flow -is genetic exchange due to the migration of fertile individuals or gametes between populations.

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18 4) Mutation Mutation is a change in an organism’s DNA and is represented by changing alleles. Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool. The original source of variation.

19 Genetic Variation, the cause of Natural Selection Genetic (heritable) variation within and between populations: exists both as what we can see (e.g., eye color) and what we cannot see (e.g., blood type). However, Natural selection can only act on phenotypes!!!! (major point) Not all variation is heritable. Environment also can alter an individual’s phenotype butterflies - color changes are due to seasonal difference in hormones. Cloned cats have different coat patterns chameleons change colors with there surroundings.

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21 Variation within populations Most variations occur as quantitative characters (e.g., height); i.e., variation along a continuum, usually indicating polygenic inheritance. Few variations are discrete (e.g., red vs. white flower color).

22 Variation between populations Geographic variations are differences between gene pools due to differences in environmental factors. Natural selection may contribute to geographic variation. It often occurs when populations are located in different areas, but may also occur in populations with isolated individuals.

23 Geographic variation between isolated populations of house mice. Normally house mice are 2n = 40. However, chromosomes fused in the mice in the example, so that the diploid number has gone down.

24 Example: Body size of North American birds tends to increase with increasing latitude. Can you think of a reason for the birds to evolve differently? A genetic variation that results in no competitive advantage to any individual. - Example: human fingerprints

25 Example: Height variation in yarrow along an altitudinal gradient. Can you think of a reason for the plants to evolve differently?

26 Kinds of Selection

27 a. Directional selection favors individuals at one end of the phenotypic range. Most common during times of environmental change or when moving to new habitats.

28 Divergent selection favors extremes over intermediate phenotypes. - Occurs when environmental change favors both extreme phenotype. -Example = Galapagos lizards when explorers introduced dogs into the ecosystem. They only ate the medium sized lizards. Stabilizing selection favors intermediate over extreme phenotypes. - Reduces variation and maintains the current average. - Example = human birth weights.

29 Divergent selection

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31 Sexual selection leads to differences between sexes a. Sexual dimorphism is the difference in appearance between males and females of a species. -Intrasexual selection is the direct competition between members of the same sex for mates of the opposite sex. -This gives rise to males most often having secondary sexual equipment such as antlers that are used in competing for females. -In intersexual selection (mate choice), one sex is choosy when selecting a mate of the opposite sex. -This gives rise to often amazingly sophisticated secondary sexual characteristics; e.g., peacock feathers.

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34 Natural selection does not produce perfect organisms a. Adaptations are compromises. (Humans are athletic due to flexible limbs, which often dislocate or suffer torn ligaments.) b. Not all evolution is adaptive. Chance probably plays a huge role in evolution and not all changes are for the best. c. Selection edits existing variations. New alleles cannot arise as needed, but most develop from what already is present.

35 Five Agents of Evolutionary Change Selection pressures: avoiding predators matching climatic condition pesticide resistance