Evolution Biol 4802

Lecture 14, Chapter 9


Topics for today

1.   Genetic variability at:

         Two loci

         Many loci

2.   Causes of variation in polygenic traits

3.   Distribution of variation across geographic range


Polymorphism within a species caused by segregation at two loci

1.      Trait that distinguish higher taxonomic division are still segregating in this species

2.      Alleles are segregating independently - called linkage equilibrium

3.      Why would alleles at different loci ever segregate together?

Why would alleles be in linkage disequilibium?

1.      Linked – physically proximal on a chromosome

a.      Reduced rate of recombination

2.      Selection may favor particular allelic combinations

a.      Intermediates have low fitness

3.      Without constraints, linkage equilibrium is restored

Fig. 9.19 (9.16 old)

Linkage equilibrium is restored

  1. Frequency of A1A1B1B1?
    1. pA1B12
    2. 0.5*0.5 = 0.25
  2. Frequency of A2A2B2B2?
    1. qA2B22
    2. 0.5*0.5 = 0.25
  3. Frequency of A1A2B1B2?
    1. 2pA1B1qA2B2
    2. 2(0.5)(0.5) = 0.5
  4. If the alleles were segregated into separate populations because of selection, what will happen to the fitness of the population with genetic mixing?

Fig. 9.17 (9.15 old)

Fitness consequences of interbreeding with individuals that differ in genetic distance

Most traits are not discrete

  1. Name a discrete trait that is polymorphic in humans
    1. Gender
    2. Eye color
    3. Ear lobe attachment
    4. Handedness
    5. Sickle cell anemia
    6. Ability to taste bitterness
  2. Run out of ideas pretty quickly… Why?
    1. Most traits are polygenic
    2. Due to variation at several or many loci

What factors contribute to polygenetic traits?

  1. Genetic factors
    1. Number of loci
    2. Variability at those loci
    3. Allele frequencies at those loci

Fig. 9.15 (9.13 old)

  1. Categories smoothed by:
    1. Developmental noise
    2. Environmental variation

Fig. 9.19 (old only, see below) and 9.20 (old 9.17)























Other environmental effects

  1. Environmental factors
    1. Overall effect on the mean
    2. Genotype-environment interactions

Genotype by environmental interactions shown in norm of reaction plot

1.   Crossing reaction norms show genotype by environment interactions

  1. Genotypes may differ in fitness in different environments

Fig. 9.18 only only


How can we understand polygenic traits?

1.   Think in terms of variances

2.   Two descriptors:

a.      Mean

b.      Variance

  1. Phenotype is a function of genetic and environmental sources of variance
  2. Heritability describes the proportion of phenotypic variance that is from genetic sources

Box 9C (old and new)

How do we separate VG from VE?

  1. Two ways
    1. Resemblance among relatives

                                                              i.      More closely related, the greater the expected resemblance

                                                            ii.      Comparison is often between parents and offspring

                                                          iii.      Slope is the heritability

                                                           iv.      Two hypothetical extreme cases

Fig. 9.23 (9.20 old)

    1. Artificial selection

Fig. 9.24 (9.21 old)

A more realistic example

  1. Heritability of height of students in an Evolution course at Pacific Northwest

Genetic variation is segregated across the geographic range of species

  1. Interbreeding at a hybrid zone

Fig. 9.23 (9.25 old)


  1. Discordant patterns of molecular and morphological patterns of variation in humans
    1. Genetic similarity based on numerous enzymes and blood groups
    2. Different traits show different patterns (abrupt change shown)

Fig. 9.27 (9.32 old)

  1. Clinal variation in alleles of alcohol dehydrogenase
    1. Trend corresponds to rainfall patterns

Fig. 9.28 (9.25 old)

  1. Moasic pattern of invasive red rosette disease of roses that is spread by a mite
  2. Variation among human populations

Fig. 9.34 new only