The Evolution of Populations Chapter 23 Biology – Campbell Reece.
Post on 30-Dec-2015
The Evolution of PopulationsChapter 23Biology Campbell Reece
PopulationWhat is a population?
Genetic Variation & EvolutionVariations 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 PopulationAllele frequency exampleRed 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 RrThe dominant allele (R) accounts for 800 or 80% of the total (1000) number of genesThe recessive allele (r) accounts for 200 or 20%
Hardy-Weinberg TheoremDescribes a nonevolving populationThe frequencies of alleles and genotypes in a populations gene pool remain constant over generationsChance of RR 0.8 x 0.8 = .64Chance of Rr 0.8 x 0.2 = .16 + .16 (for rR) = .32Chance of rr 0.2 x 0.2 = 0.04The allele frequency does not change
Hardy-Weinberg Equilibriump = one allele, q = other allelep + q = 1Frequency of RR = p2Frequency of Rr/rR = 2pqFrequency of rr = q2Hardy-Weinberg Equation:p2 + 2pq + q2 = 1
5 Conditions for H-W EquilibriumVery large population size.No migration.No net mutations.Random mating.No natural selection.
We do not really expect a natural population to be in H-W equilibrium
Altering Allele FrequenciesWhat might cause the allele frequencies to change?
Genetic DriftWhat is genetic drift?
What size population is most likely to be affected?Founder effect
Genetic Drift4 key points: Genetic driftis significant in small populationscan cause allele frequencies to change at randomcan lead to a loss of genetic variation within populationscan cause harmful alleles to become fixed
Gene FlowWhat is gene flow?
What results from gene flow?
No Perfect OrganismsSelection can act only on existing variationsEvolution is limited by historical constraintsAdaptations are often compromisesChance, natural selection, and the environment interact