Evolution of PopulationsChapter 23.3-23.4AP Biology

23.1-23.2Recap

Two Types of Evolution

Macroevolution

• Evolution that takes place over LONG periods of time• Change on a grand

scale• Mass extinctions

Microevolution

• Change that occurs within a species• Change in gene

frequencies• Focus of Darwin’s

theory of evolution via natural selection

How can we study microevolution?• Study population genetics to

understand changes that occur within a species• Hardy-Weinberg

There are 3 mechanisms for evolution

• Natural Selection• Leads to an accumulation of

favorable adaptations in a population

• Genetic Drift• Chance events that alter allele

frequencies• Gene Flow• The transfer of alleles between

populations

23.3

Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Review: Natural Selection• This is a change in gene frequencies due to differential reproductive

success. • Example: antibiotic resistance• Variation already present• Introduction of antibiotic selects for those with higher resistance• Frequencies for the resistance gene increase over time

Genetic Drift

• Changes in the gene pool of a small population due to chance (random)• Founder Effect or Bottleneck Effect

Genetic Drift: Founder Effect• A few (random) individuals from a population start a new

population with a different allele frequency than the original population

Genetic Drift: Bottleneck Effect• The population experiences a huge decrease in size • Result: severe reduction in diversity of the original gene pool

because a small percentage of chance survivors remain• Endangered species can experience this

Gene Flow• Migration of fertile individuals, or the transfer of gametes,

between populations• Populations may gain or lose alleles• Reduces differences between neighboring populations

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23.4

Natural selection is the only mechanism that consistently causes adaptive radiation

Natural Selection• Natural selection results in development of features which

increase an organism’s likelihood for survival and reproduction—adaptations

• Therefore, Darwin’s explanation of evolution goes like this: • Adaptation + natural selection change within species• Leads to adaptive evolution

Modes of Natural Selection A. Directional Selection – favors variants of one extremeB. Disruptive Selection – Opposite phenotypic extremes

favored over intermediateC. Stabilizing Selection – favors intermediate variants by

selecting against extreme phenotypes

Maintaining Variation• Natural selection is the main mechanism of evolution• For this to work, variation must be maintained• Variation is good (why?)

Conserving Variation: Polymorphism• The presence of two or more

distinct phenotypes in a population• Example: sexual dimorphism

• Leads to the enigma of sexual reproduction• Evolutionary advantage outweighs

cost of sex• Costs: energy, resources,

competition• Advantage: pass on genes, variation

of genes (2 parents vs. 1 parent, meiosis)

Conserving Variation: Diploidy• The presence of homologous chromosomes (one from mom, one

from dad) maintains variety in the population• A dominant allele can cover up a harmful recessive one• Example: cystic fibrosis

Conserving Variation: Heterozygote Advantage• Heterozygotes more likely to survive than homozygotes• Example: sickle-cell anemia and malaria resistance

Natural Selection isn’t perfect1. Nature can only select

for what traits are present.

2. Ancestry is a legacy that is modified; nothing is built from scratch.

3. Adaptations are often compromises.

4. Chance, natural selection and the environment interact.