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Evolution of Populations

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I.                     Microevolution

A.     Change in a gene pool on the smallest scale.

B.     To maintain Hardy Weinberg, five criteria must be met

1. Very large population

2. Isolation from other populations

3. No net mutations

4. Random mating

5. No natural selection

C.     Five potential agents for microevolution

1. Genetic drift

2. Gene flow

3. Mutation

4. Nonrandom mating

5. Natural selection

II.                   Genetic Drift

A.     Changes in the gene pool of a small population due to chance.

B.     The Bottleneck effect

1. Disasters may reduce a population size unselectively.

2. Results in a small surviving population that is not representative of the original genetic makeup.

3. Some alleles are over represented in the survivors.

4. Some may be completely eliminated.

5. Genetic drift may continue until the population is large enough for errors to be insignificant.

6. Reduces genetic variability

C.     The Founder effect

1. Genetic drift is likely in a small population.

2. The smaller the population, the less genetic makeup available.

3. May be responsible for relative high frequency of certain inherited disorder.

III.                  Gene Flow

A.     A population may gain or lose alleles due to the migration of fertile individuals or gametes between populations.

B.     Reduces differences between populations that have accumulated because of natural selection or genetic drift.

IV.                Mutations

A.     A new mutation that is transmitted in gametes changes the gene pool by replacing one allele for another.

V.                  Nonrandom mating

A.     Individuals usually mate with close neighbors.

B.     Referred to as inbreeding.

C.     Most extreme case is self-fertilization.

D.     Causes relative frequencies of genotypes to deviate.

E.     Assortative mating is where individuals select partners that are like themselves in phenotype.

F.      Both tend to increase the number of homozygotes.

VI.                Natural selection

A.     Genetic variation

1. Polymorphism

a.       When tow or more forms of a character are represented, the contrasting forms are called morphs.

b.       A population is said to be polymorphic if tow or more distinct morphs are represented in high enough frequencies to be noticeable.

2. Geographical variation

a.       Differences in genetic structure between populations.

b.       Cause by environmental factors or genetic drift.

c.       A cline is a graded change in some trait along a geographic axis.  Can be caused by overlap of neighboring populations inbreeding or an environmental variable.

3. Mutation

a.       New alleles originate by mutation.

b.       Accident

c.       Rare and random

d.       Only mutations in gametes are inherited.

e.       Most point mutations are relatively harmless.

f.         Sometime mutations may fit in better than the original and enhance reproductive success.

g.       Chromosomal mutations affect many genes and disrupt the development of the organi8sm.

h.       Duplications are almost always harmful.

4. Recombination

a.       Members of a population owe nearly all their genetic difference to recombinations.

B.     Preserving genetic variation

1. Dipolidy

a.       Diploid nature hides some variation.

b.       Heterozygotes cause recessive alleles to persist.

c.       Heterozygotes protection maintains a huge pool of alleles that may not be suitable in the present but may help with a change in the environment.

2. Balanced polymorphism

a.       The ability of natural selection to maintain diversity.

1.)     Heterozygote advantage is when Heterozygotes have a greater survivorship and reproductive success.

2.)     Hybrid vigor is caused by crossbreeding between two different inbred varieties that produces a more vigorous plant.

b.       Also results from geographical boundaries.

c.       Frequency dependent selection is reproductive success when any one morph declines in the population if that phenotype becomes too common.

C.     Genetic variation and success

1. Neutral variation has not select advantage for some individuals over others.

2. Theory of neutral evolution says that many variant alleles have no selective advantage or disadvantage.

3. Not affected by natural selection.

VII.               Adaptive evolution

A.     Fitness

1. Darwinian fitness is the relative contribution an individual makes to the gene pool for the next generation.

2. Relative fitness is the contribution of a genotype to the next generation compared to the contributions of alternative genotypes.

3. Survival does not guarantee reproductive success.

4. Survival is a prerequisite to reproduction.

5. Longevity increases fitness.

B.     Selection Acts on?

1. Phenotypes cause selection to indirectly adapt to its environment.

2. The ability of genes to influence many phenotypic characters is called pleiotropy.

C.     Modes

1. Natural selection can affect the frequency of a heritable trait in a population.

a.       Stabilizing selection acts against extreme phenotypes and favors the more common intermediate variants.

1.)     Reduces variation

2.)     Maintains status quo

b.       Directional selection

1.)     Common during periods of change in the environment or migration.

2.)     Sifts the frequency in one direction or another by favoring what is usually a relatively rare phenotype.

c.       Diversifying selection

1.)     Rare

2.)     Favors individuals on both extremes over the intermediate.

3.)     Can result in balanced polymorphism.

D.     Sexual selection

1. Sexual dimorphism is the distinction between secondary sex characteristics of males and females.

a.       Expressed in size

b.       Color

c.       Manes

d.       Antlers

e.       Usually the males are showier sex.

f.         Those most impressive attract females.

2. Sexual selection is a process leading to sexual dimorphism.

3. Is not adaptive and may cause more harm.