# the evolution of populations

Post on 13-Jan-2016

22 views

Category:

## Documents

Embed Size (px)

DESCRIPTION

The Evolution of Populations. Chapter 23 Biology Campbell Reece. What is a population? Species Gene pool. Population. Variations within a population AND Geographic variation How does variation occur? What is the ultimate source of new alleles? - PowerPoint PPT Presentation

TRANSCRIPT

• The Evolution of PopulationsChapter 23Biology Campbell Reece

• PopulationWhat is a population?

Species

Gene pool

• 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 Theorem

• Hardy-Weinberg Theorem

• 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

Bottleneck effect

• Genetic Drift

• Bottleneck Effect

• Bottleneck 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?

• Directional Selection

• Disruptive Selection

• Stabilizing Selection

• No Perfect OrganismsSelection can act only on existing variationsEvolution is limited by historical constraintsAdaptations are often compromisesChance, natural selection, and the environment interact

*