Download Ch 23 Evolution of Populations

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The Evolution of Populations
Ch 23
Modern Synthesis=
Natural Selection + Genetics
• Population- a group of the same species that will mate and make
viable offspring
• Gene pool- all the genes in a population
• MICROEVOLUTION= change in allele frequencies in a population
• Measured using Hardy- Weinberg Theorem
Hardy Weinberg Equilibrium
• How many alleles does an individual have per gene?
• What are the possibilities?
• HW is a way to measure if the allele frequency is evolving (changing)
or in equilibrium(stable).
To observe equilibrium these must be met:
• Large population
• No gene flow (migration)
• No mutations
• Random mating
• No natural selection
Mutations and Sexual Reproduction produce
• Mutations may be random or induced by the environment. The ONLY
source of new genes and NEW alleles.
• Deletions, duplications or rearrangements of many loci are usually harmful.
• Point mutations may or may not change an amino acid/protein.
• Duplications within ONE gene provide a large variation for selection to work.
• Rates
• Rare in plants/animals (about 1/100,000 genes per generation)
• Highest in bacteria and viruses
Sexual Reproduction
• Variation arise from
• meiosis (crossing over, independent assortment)
• Random fertilization
• Conjugation- sexual reproduction in bacteria (not common)
***Horizontal gene transfer can occur between DIFFERENT closely related
Genetic Drift
• Small populations can see changes in a phenotype frequency by
chance alone
• Bottleneck Effect- a disaster may greatly reduce population size
• Founder effect- when a small group establishes a new population
Gene Flow
• Results from movement into or out of a population.
• Tends to reduce the differences between populations
Genetic Variation
• Not ALL variation is heritable. Some results from environmental
• Phenotypic Polymorphism- if there are two+ distinct morphs
Types of Selection
• Directional- a shift in a phenotype towards one of the extremes
• Disruptive- when BOTH extremes are favored
• Stabilizing- when the average individual is favored
Heterozygous Advantage
Heterozygote advantage- if the hetero. has a greater fitness, then
selection favors it
EX: Sickle cell heteros. are protected against malaria but if sickle
cell(recessive) will probably not survive
• Neutral variations have no selective advantage
• Low fitness, genes decline…
• Fixed alleles are when all members of a population are homozygous
Sexual Selection
• Selection for mating success can result in dimorphism (differences
between the sexes)
• Intrasexual- same sex competition
• Intersexual- usually the female is picky when selecting a partner