Mechanisms of Evolution 1 1 The Gene Pool

Mechanisms of Evolution 1 1

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 • • 2 2

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 • 3 3

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 • • 4 4

Speciation • 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 • • 5 5

Modern Evolutionary Thought

Modern Synthesis Theory • Combines Darwinian • • selection and Mendelian inheritance Population genetics study of genetic variation within a population Emphasis on quantitative characters (height, size …) 7 7

Modern Synthesis Theory • 1940 s – comprehensive theory of evolution • • theory of evolution (Modern Synthesis Theory) Introduced by Fisher & Wright Until then, many did not accept that Darwin’s theory of natural selection could drive evolution S. Wright A. Fisher 8 8

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 9 9

Microevolution • Changes occur in gene pools 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) 10 10

Hardy. Weinberg Principle 11

The Hardy-Weinberg Principle • Used to describe a non-evolving population. • Shuffling of alleles by meiosis and • random fertilization have no effect on the overall gene pool. Natural populations are NOT expected to actually be in Hardy. Weinberg equilibrium. 12 12

The Hardy-Weinberg Principle • Deviation from Hardy-Weinberg • equilibrium usually results in evolution Understanding a non-evolving population, helps us to understand how evolution occurs 13 13

5 Assumptions of the H-W Principle 1. Large population size - small populations have fluctuations in allele frequencies (e. g. , fire, storm). 2. No migration - immigrants can change the frequency of an allele by bringing in new alleles to a population. 3. No net mutations - if alleles change from one to another, this will change the frequency of those alleles 14 14

5 Assumptions of the H-W Principle 3. - 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. 4. - 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. 15 15

Traits Selected for Random Mating 16 16

The Hardy-Weinberg Principle The gene pool of a NON-EVOLVING population remains CONSTANT over multiple generations (allele frequency doesn’t change) The Hardy-Weinberg Equation: 1. 0 = p 2 + 2 pq + q 2 Where: p 2 = frequency of AA genotype 2 pq = frequency of Aa q 2 = frequency of aa genotype 17 17

The Hardy-Weinberg Principle Determining the Allele Frequency using Hardy-Weinberg: 1. 0 = p + q Where: p = frequency of A allele q = frequency of a allele 18 18

Allele Frequencies Define Gene Pools 500 flowering plants 480 red flowers 320 RR 160 Rr 20 white flowers 20 rr As there are 1000 copies of the genes for color, the allele frequencies are (in both males and females): 320 x (80%) 160 x (20%) 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0. 8 R 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0. 2 r 19 19

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Microevolution of Species

Causes of Microevolution • Genetic Drift - the change in the gene pool of a small population due to chance • Natural Selection success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance) - Cause ADAPTATION of Populations - • Gene Flow -is genetic exchange due to the migration of fertile individuals or gametes between populations 23 23

Causes of Microevolution • Mutation - a change in an organism’s DNA - Mutations can be transmitted in gametes to • Non-random mating offspring - Mates are chosen on the basis of the best traits 24 24

Genetic Drift 25

Factors that Cause Genetic Drift • Bottleneck Effect - a drastic reduction in population (volcanoes, earthquakes, landslides …) - Reduced genetic variation - Smaller population may not be able to adapt to new • Founder Effect selection pressures - occurs when a new colony is started by a few members of the original population - Reduced genetic variation - May lead to speciation 26 26

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Loss of Genetic Variation • Cheetahs have little genetic variation in their gene pool • This can probably be attributed to a population bottleneck they experienced around 10, 000 years ago, barely avoiding extinction at the end of the last ice age 28 28

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Founder’s Effect 30 30

Modes of Natural Selection

Modes of Natural Selection • Directional Selection - Favors individuals at one end of the phenotypic range Most common during times of environmental change or when moving to new habitats • Disruptive selection - Favors extreme over intermediate phenotypes - Occurs when environmental change favors an extreme phenotype 32 32

Directional Selection 33 33

Disruptive Selection 34 34

Modes of Natural Selection • Stabilizing Selection - Favors intermediate over extreme phenotypes Reduces variation and maintains the cureent average Example: Human birth weight 35 35

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Variations in Populations 37

Geographic Variations • Variation in a species • • due to climate or another geographical condition Populations live in different locations Example: Finches of Galapagos Islands & South America 38 38

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Heterozygote Advantage • Favors heterozygotes (Aa) • Maintains both alleles (A, a) instead of • removing less successful alleles from a population Sickle cell anemia > Homozygotes exhibit severe anemia, have abnormal blood cell shape, and usually die before reproductive age. > Heterozygotes are less susceptible to malaria 40 40

Sickle Cell and Malaria 41 41

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Other Sources of Variation • Mutations - In stable environments, mutations often result in - little or no benefit to an organism, or are often harmful Mutations are more beneficial (rare) in changing environments (Example: HIV resistance to antiviral drugs) • Genetic Recombination - source of most genetic differences between individuals in a population • Co-evolution -Often occurs between parasite & host and flowers & their pollinators 43 43

Coevolution 44 44

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