1 1 Population Genetics. 2 2 The Gene Pool Members of a species can interbreed & produce fertile...

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PopulatiPopulation on

GeneticsGenetics

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The Gene PoolThe 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

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The Gene PoolThe Gene Pool

•Different species do NOT exchange genes by interbreeding

•Different species that interbreed often produce sterile or less viable offspring e.g. Mule

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PopulationsPopulations

•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

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SpeciationSpeciation

•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

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Modern Modern Evolutionary Evolutionary ThoughtThought

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Modern Synthesis TheoryModern Synthesis Theory

•Combines Combines Darwinian selection Darwinian selection and and Mendelian Mendelian inheritanceinheritance

•Population genetics Population genetics - study of genetic - study of genetic variation within a variation within a populationpopulation

•Emphasis on Emphasis on quantitative quantitative characters (height, characters (height, size …)size …)

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Modern Synthesis TheoryModern Synthesis Theory

•1940s – 1940s – comprehensive comprehensive theory of evolution theory of evolution (Modern Synthesis (Modern Synthesis Theory)Theory)

•Introduced by Introduced by Fisher Fisher & Wright& Wright

•Until thenUntil then, many did , many did not accept that not accept that DarwinDarwin’’s theory of s theory of natural selection natural selection could drive evolutioncould drive evolution

S. Wright

A. Fisher

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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

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Modern Synthesis Theory

Darwin & Wallace (Natural Selection)+ Mendel (Genetics)+ Wright & Fisher (Population Genetics)+ Mayr & others (Biogeography, Paleontology) ____________________________________= Modern Evolutionary Synthesis• a more complete understanding of

how evolution proceeds in the wild

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Macro- vs. Micro- Evolution• Macroevolution - change that results

in the formation of new species, new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction.

• Microevolution: the generation-to- generation change in the frequency of alleles in a population– Evolution at its smallest scale– The core definition of evolution

Microevolutionary changes give rise to macroevolution

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MicroevolutionMicroevolution

•Changes occur in gene pools due to mutation, natural selection, genetic drift, etc.

•Gene pool changes cause more variation in individuals in the population

•Example: Bacteria becoming unaffected by antibiotics (resistant)

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MicroevolutioMicroevolution of Speciesn of Species

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Causes of MicroevolutionCauses 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

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Causes of MicroevolutionCauses of Microevolution•Gene Flow

-is genetic exchange due to the migration of fertile individuals or gametes between populations

•Mutation-a change in an organism’s DNA

-Mutations can be transmitted in gametes to offspring

•Non-random mating-Mate selection may be influenced by geographic proximity

-Individuals may select a mate with traits similar to their own

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Genetic DriftGenetic Drift

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Genetic DriftGenetic Drift• Changes in a population’s allele

frequencies due to chance• Drift can cause rapid change in

small populations• A problem when populations get

too small

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Factors that Cause Genetic Factors that Cause Genetic DriftDrift

•Bottleneck Effect

-a drastic reduction in population

- (volcanoes, earthquakes, landslides …)

-Reduced genetic variation

-Smaller population may not be able to adapt to new selection pressures

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Factors that Cause Genetic Factors that Cause Genetic DriftDrift

•Founder Effect

-occurs when a new colony is started by a few members of the original population

-Reduced genetic variation

-May lead to speciation

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FounderFounder’’s Effects Effect

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Loss of Genetic VariationLoss 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

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Natural Natural SelectionSelection

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Natural SelectionNatural Selection

•Natural selection – differential success in survival and reproduction

•N.S. leads to populations becoming better adapted to their environments (adaptation)

•N.S. works most powerfully in large populations (effect of drift is small, and changes due to N.S. are preserved)

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It IS possible to measure

Natural Selection in

the wild.

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Modes of Natural SelectionModes of Natural Selection

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Modes of Natural SelectionModes of Natural Selection

•Directional Selection

- Favors individuals at one end of the phenotypic range

- Most common during times of environmentalchange or when moving to new habitats

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Modes of Natural SelectionModes of Natural Selection

•Disruptive selection

-Favors extreme over intermediate phenotypes

-Occurs when environmental change favors an extreme phenotype

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Modes of Natural SelectionModes of Natural Selection

•Stabilizing Selection

- Favors intermediate over extreme phenotypes

- Reduces variation and maintains the current average

- Example:

- Human birth weight

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Modes of Natural SelectionModes of Natural Selection

•Sexual Selection

- Traits that enable an organism to reproduce, without necessarily helping them to survive are selected for

- Any inherited trait that improves the mating success of certain individuals will become more pronounced in succeeding generations.

Example: Bright plumage

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Gene FlowGene Flow

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Gene FlowGene Flow

•Migration of individuals can change the allele frequencies in a population

•Immigration – movement into a population

•Emigration – movement out of a population

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MutationMutation

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MutationsMutations

•Mutations are the origin of all differences between alleles

•But mutations are rare, so they must still spread by drift or selection if they are going to impact allele frequencies in a population

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Sickle cell anemia is the result of a single point mutation

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SpeciationSpeciation

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SpeciationSpeciation

•Speciation is the formation of a new species

•A Species is a population of organisms whose members can interbreed and produce fertile offspring.

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So, how does one species become two?

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Reproductive IsolationReproductive Isolation

•Reproductive Isolation occurs when a population splits into two groups and the two groups no longer interbreed.

•When populations become reproductively isolated, they can evolve into two separate species

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Mechanisms of IsolationMechanisms of Isolation

•Behavioral Isolation occurs when two populations that are capable of interbreeding develop differences in courtship rituals or other behaviors

•Ex: Eastern meadowlarks will not respond to western meadowlark songs, and vice versa.

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Mechanisms of IsolationMechanisms of Isolation

•Geographic Isolation occurs when two populations are separated by geographic barriers such as rivers, mountains or bodies of water

• Ex: The Kaibab squirrel is a subspecies of the Abert’s squirrel that formed when a small population became isolated on the north rim of the Grand Canyon.

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Mechanisms of IsolationMechanisms of Isolation

•Temporal Isolation occurs when two or more species reproduce at different times.

•Ex: Two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May

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