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