evolution of populations chapter 23.3-23.4 ap biology
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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.