lecture 6 - micro evolution

7/31/2019 Lecture 6 - Micro Evolution

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Chapter 23

The Evolution of Populations


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Population Genetics

Combines Darwinian selection and Mendelian inheritance

Population genetics - study of genetic variation within apopulation.

Emphasis on quantitative characters.

1940s comprehensive theory of evolution (the modernsynthesis).

Until then, many did not accept that Darwins theory of

natural selection could drive evolution.


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The modernsynthesis combined discoveries from

paleontology, taxonomy, biogeography, and population genetics.

It emphasizes the importance of populations as unitsof evolution,

natural selection as the most important mechanism of

evolution,

and gradualism.


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Allele frequencies define gene pools

As there are 1000 copies of the genes for color,the allele frequencies are (in both males and females):

320 x 2 (RR) + 160 x 1 (Rr) = 800 R; 800/1000 = 0.8 (80%) R160 x 1 (Rr) + 20 x 2 (rr) = 200 r; 200/1000 = 0.2 (20%) r

500 flowering plants

480 red flowers 20 white flowers

320 RR 160 Rr 20 rr


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Population - a localized group of individuals of the samespecies.

Species - a group of populations whose individuals havethe ability to breed and produce fertile offspring.

Individuals near a population center are, on average, moreclosely related to one another than to members of other

populations.


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A populations gene pool is the total of all genes in thepopulation at any one time.

If all members of a population are homozygous for aparticular allele, then the allele is fixed in the gene pool.


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The Hardy-Weinberg Theorem

Used to describe a non-evolving population.

Shuffling of alleles by meiosis and random fertilization haveno 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.

Understanding a non-evolving population, helps us tounderstand how evolution occurs.


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Assumptions of the H-W Theorem:

- Large population size: small populations can have chancefluctuations in allele frequencies (e.g., fire, storm).

- No migration: immigrants can change the frequency of anallele 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, thenindividuals with those traits will be selected and this will notallow for random mixing of alleles.

- No natural selection: if some individuals survive andreproduce at a higher rate than others, then their offspringwill carry those genes and the frequency will change for thenext generation.


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Hardy-Weinberg Equilibrium

The gene pool of a non-evolving population remains constant

over multiple generations; i.e., the allele frequency does notchange over generations of time.

The Hardy-Weinberg Equation:

1.0 = p2 + 2pq+ q2

where p2= frequency of AA genotype; 2pq= frequency of Aaplus aA genotype; q2= frequency of aa genotype


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But we know that evolution does occur within populations.

Evolution within a species/population = microevolution.

Microevolution refers to changes in allele frequencies in agene pool from generation to generation. Represents agradual change in a population.

Causes of microevolution:

1) Genetic drift

2) Natural selection (1 & 2 are most important)

3) Gene flow

4) Mutation


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1) Genetic drift

Genetic drift = the alteration of the gene pool of a small

population due to chance.

Two factors may cause genetic drift:

a) Bottleneck effect may lead to reduced genetic variabilityfollowing some large disturbance that removes a largeportion of the population. The surviving population oftendoes not represent the allele frequency in the originalpopulation.

b) Founder effect may lead to reduced variability when a fewindividuals from a large population colonize an isolatedhabitat.


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*Yes, I realize that this is not really a cheetah.


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2) Natural selection

As previously stated, differential success in reproductionbased on heritable traits results in selected alleles beingpassed 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 individualsor gametes between populations.


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4) Mutation

Mutation is a change in an organisms DNA and isrepresented by changing alleles.

Mutations can be transmitted in gametes to offspring, andimmediately affect the composition of the gene pool.

The original source of variation.


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Genetic Variation, the Substrate for Natural Selection

Genetic (heritable) variation within and between populations:exists both as what we can see (e.g., eye color) and what wecannot see (e.g., blood type).

Not all variation is heritable.

Environmentalso can alter an individuals phenotype [e.g.,the hydrangea we saw before, and

Map butterflies (color changes are due to seasonaldifference in hormones)].


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Variation within populations

Most variations occur as quantitative characters (e.g.,height); i.e., variation along a continuum, usually indicatingpolygenic inheritance.

Few variations are discrete (e.g., red vs. white flowercolor).

Polymorphism is the existence of two or more forms of a

character, in high frequencies, within a population. Appliesonly to discrete characters.


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Variation between populations

Geographic variations are differences between gene poolsdue 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 isolatedindividuals.


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Geographic variationbetween isolated

populations of housemice.

Normally house mice are2n = 40. However,

chromosomes fused in themice in the example, sothat the diploid numberhas gone down.


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Cline, a type of geographic variation, is a graded variation inindividuals that correspond to gradual changes in theenvironment.

Example: Body size of North American birds tends toincrease with increasing latitude. Can you think of a reasonfor the birds to evolve differently?

Example: Height variation in yarrow along an altitudinalgradient. Can you think of a reason for the plants to evolvedifferently?


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Mutation and sexual recombination generate genetic variation

a. New alleles originate only by mutations (heritable only ingametes; many kinds of mutations; mutations in functional geneproducts most important).

- In stable environments, mutations often result in little or nobenefit to an organism, or are often harmful.

- Mutations are more beneficial (rare) in changingenvironments. (Example: HIV resistance to antiviral drugs.)

b. Sexual recombination is the source of most geneticdifferences between individuals in a population.

- Vast numbers of recombination possibilities result in varyinggenetic make-up.


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Diploidy and balanced polymorphism preserve variation

a. Diploidy often hides genetic variation from selection in the

form of recessive alleles.

Dominant alleles hide recessive alleles in heterozygotes.

b. Balanced polymorphism is the ability of natural selection tomaintain stable frequencies of at least two phenotypes.

Heterozygote advantage is one example of a balancedpolymorphism, where the heterozygote has greater survivaland reproductive success than either homozygote (Example:Sickle cell anemia where heterozygotes are resistant tomalaria).


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Frequency-dependent selection = survival of one phenotypedeclines if that form becomes too common.

(Example: Parasite-Host relationship. Co-evolution occurs,so that if the host becomes resistant, the parasite changes toinfect the new host. Over the time, the resistant phenotypedeclines and a new resistant phenotype emerges.)


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Neutral variation is genetic variation that results in nocompetitive advantage to any individual.

- Example: human fingerprints.


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A Closer Look: Natural Selection as the Mechanism ofAdaptive Evolution

Evolutionary fitness - Not direct competition, but instead thedifference in reproductive success that is due to manyvariables.

Natural Selection can be defined in two ways:

a. Darwinian fitness- Contribution of an individual to the genepool, relative to the contributions of other individuals.

And,


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b. Relative fitness

- Contribution of a genotype to the next generation, comparedto the contributions of alternative genotypes for the same

locus.

- Survival doesnt necessarily increase relative fitness;relative fitness is zero (0) for a sterile plant or animal.

Three ways (modes of selection) in which natural selectioncan affect the contribution that a genotype makes to the nextgeneration.

a. Directional selection favors individuals at one end of thephenotypic range. Most common during times ofenvironmental change or when moving to new habitats.


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Directional selection


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Diversifying selection favors extreme over intermediatephenotypes.

- Occurs when environmental change favors an extremephenotype.

Stabilizing selection favors intermediate over extremephenotypes.

- Reduces variation and maintains the current average.

- Example = human birth weights.

Diversifying selection


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Diversifying selection


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Natural selection maintains sexual reproduction

-Sex generates genetic variation during meiosis and

fertilization.

-Generation-to-generation variation may be of greatestimportance to the continuation of sexual reproduction.

-Disadvantages to using sexual reproduction: Asexualreproduction produces many more offspring.

-The variation produced during meiosis greatly outweighs this

disadvantage, so sexual reproduction is here to stay.


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All asexual individuals are female (blue). With sex,offspring = half female/half male. Because males dont

reproduce, the overall output is lower for sexual

reproduction.


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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 betweenmembers of the same sex for mates of the opposite sex.

-This gives rise to males most often having secondarysexual equipment such as antlers that are used incompeting 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 secondarysexual characteristics; e.g., peacock feathers.


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N t l l ti d t d f t i


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Natural selection does not produce perfect organisms

a. Evolution is limited by historical constraints (e.g., humans

have back problems because our ancestors were 4-legged).

b. Adaptations are compromises. (Humans are athletic due toflexible limbs, which often dislocate or suffer torn ligaments.)

c. Not all evolution is adaptive. Chance probably plays a hugerole in evolution and not all changes are for the best.

d. Selection edits existing variations. New alleles cannot arise

as needed, but most develop from what already is present.

lecture 6 - micro evolution

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