The Evolution of Populations
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Transcript of The Evolution of Populations
The Evolution of The Evolution of PopulationsPopulations
Chapter 23Biology – Campbell • Reece
PopulationPopulation
What is a population?
Species
Gene pool
Genetic Variation & Genetic Variation & EvolutionEvolution
Variations within a population ANDGeographic variation
How does variation occur?What is the ultimate source of new
alleles?Where must mutations occur in order
to be passed to the next generation?
Alleles in a PopulationAlleles in a Population
Allele frequency example…◦Red flower (R) is dominant over white flower (r)
◦In a population of 500, 20 have white flowers (rr)
◦The other 480 have red flowers (RR or Rr) 320 are RR, 160 are Rr
◦The dominant allele (R) accounts for 800 or 80% of the total (1000) number of genes
◦The recessive allele (r) accounts for 200 or 20%
Hardy-Weinberg TheoremHardy-Weinberg Theorem
Describes a nonevolving populationThe frequencies of alleles and
genotypes in a population’s gene pool remain constant over generations◦Chance of RR – 0.8 x 0.8 = .64◦Chance of Rr – 0.8 x 0.2 = .16 + .16 (for rR) = .32
◦Chance of rr – 0.2 x 0.2 = 0.04◦The allele frequency does not change
Hardy-Weinberg TheoremHardy-Weinberg Theorem
Hardy-Weinberg TheoremHardy-Weinberg Theorem
Hardy-Weinberg Hardy-Weinberg EquilibriumEquilibrium
p = one allele, q = other allelep + q = 1Frequency of RR = p2
Frequency of Rr/rR = 2pqFrequency of rr = q2
Hardy-Weinberg Equation:◦p2 + 2pq + q2 = 1
5 Conditions for H-W 5 Conditions for H-W EquilibriumEquilibrium
1. Very large population size.2. No migration.3. No net mutations.4. Random mating.5. No natural selection.
We do not really expect a natural population to be in H-W equilibrium
Altering Allele FrequenciesAltering Allele Frequencies
What might cause the allele frequencies to change?
Genetic DriftGenetic Drift
What is genetic drift?
◦What size population is most likely to be affected?
◦Founder effect
◦Bottleneck effect
Genetic DriftGenetic Drift
Bottleneck EffectBottleneck Effect
Bottleneck EffectBottleneck Effect
Genetic DriftGenetic Drift
4 key points: Genetic drift…1. is significant in small populations2. can cause allele frequencies to change
at random3. can lead to a loss of genetic variation
within populations4. can cause harmful alleles to become
fixed
Gene FlowGene Flow
What is gene flow?
What results from gene flow?
Directional SelectionDirectional Selection
Disruptive SelectionDisruptive Selection
Stabilizing SelectionStabilizing Selection
No ‘Perfect’ OrganismsNo ‘Perfect’ Organisms
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromisesChance, natural selection, and the
environment interact