Evolution of Populations

1
Evolution of Populations

• Chapter 23

2
Populations Evolve

• individuals do not evolve, populations do
• a population is a localized group of individuals
  that are capable of interbreeding producing
  fertile offspring
• microevolution a change in the genetic makeup
  of a population from generation to generation
• population genetics the study of how
  populations genetically change over time

3
Gene Pool

• the total collection of genes in a population at
  any one time consists of all the alleles in all
  the individuals of that population
• each allele has a frequency in the population
• for a trait controlled by two alleles, p q are
  used to represent the frequency of the alleles
• usually
• p represents the frequency of the dominant allele
• q represents the frequency of the recessive allele

4
Calculating Allele Frequencies

• EXAMPLE the incomplete dominant trait for
  flower color is controlled by two alleles
• R red
• W white
• in a population of 500 flowers
• 320 are red (RR), 160 are pink (RW), 20 are
  white (WW)
• because each flower has two alleles for the
  trait, there are 1000 alleles in the population
• ? p (the frequency of R) 800/1000 0.8
• total R RR R from RW (320 x 2) 160 800
• ? q (the frequency of W) 200/1000 0.2
• total W W from RW WW 160 (20 x 2) 200
• notice that p q 1

5
Hardy-Weinberg Theorem

• the proportion of genotypes (AA, Aa, aa) in a
  population will remain constant from generation
  to generation if
• the population is very large
• there is no gene flow (immigration or emigration)
• there are no mutations
• random mating is occurring
• there is no natural selection
• because under these conditions, the frequency
  of the alleles (A a) does not change
• populations exhibiting these conditions are not
  evolving are said to be in Hardy-Weinberg
  equilibrium

6
Hardy-Weinberg Equation

• p2 2pq q2 1 where
• p the frequency of the dominant allele (A)
• q the frequency of the recessive allele (a)
• p2 the frequency of the homozygous dominant
  genotype (AA)
• 2pq the frequency of the heterozygous genotype
  (Aa)
• q2 the frequency of the homozygous recessive
  genotype (aa)
• uses
• to calculate the frequencies of alleles
  genotypes in a population
• to determine if a population is in Hardy-Weinberg
  EQ
• to estimate what of a population is carrying
  the allele for a recessively inherited disease

7
Example 1

• What are the allele genotypic frequencies for a
  population of 500 mice in which 245 are black
  (BB), 210 are brown (Bb), 45 are white (bb)?
• ? (BB) 245/500 0.49
• ? (Bb) 210/500 0.42
• ? (bb) 45/500 0.09
• ? (B) ?p2 ?0.49 0.7
• ? (b) ?q2 ?0.09 0.3

8
Example 2

• What are the allele genotypic frequencies for a
  population of 1000 pea plants in which 750 have
  purple flowers (PP) 250 have white flowers
  (pp)?
• ? (pp) 250/1000 0.25
• ? (p) ?q2 ?0.25 0.5
• recall p q 1, ? ? (P) 1 q 1 0.5 0.5
• ? (PP) p2 (0.5)2 0.25
• ? (Pp) 2pq 2(0.5)(0.5) 0.5

9
Example 3

• What of the US population carries the allele
  for PKU, a homozygous recessive disease? (NOTE 1
  in 10,000 babies born in the US have PKU)
• ? (aa) 1/10,000 0.0001
• ? (a) ?q2 ?0.0001 0.01
• ? ? (P) 1 q 1 0.01 0.99
• ? (Aa) 2pq 2(0.01)(0.99) 0.0198
• ? percent carriers in US population is 2

10
Factors that disrupt HW-EQ

• natural selection
• results in alleles being passed to the next
  generation in different proportions
• genetic drift changes in allele frequencies due
  to chance
• bottleneck effect occurs when a population is
  drastically reduced in size due to an
  environmental disaster (ie fire, flood) the
  gene pool of the survivors no longer represents
  that of the original population
• founder effect occurs when a small group of
  individuals is isolated from the larger
  population the gene pool of this splinter
  population does not reflect the source population
• gene flow the loss/gain of alleles due to
  emigration/immigration

11
Sources of Variation

• mutations
• point mutations, chromosome rearrangements,
  duplication
• rate if low in animals plants
• rate is higher in viruses bacteria b/c they
  have short life spans
• sexual recombination
• reshuffling of alleles can lead to phenotypic
  variations

12
3 Modes of Natural Selection

• directional selection favors variants at one
  extreme
• disruptive selection favors variants at both
  extremes
• stabilizing selection favors intermediate
  variants

13
Final Question

• Why doesnt natural selection result in the
  culling of all unfavorable genotypes?
• recessive alleles are carried (hidden) in
  heterozygotes
• heterozygote advantage (ex sickle-cell trait)
• frequency-dependent selection
• when the fitness of any one variation declines if
  it becomes too common
• neutral variation
• genetic variation that has no impact on
  reproductive success