Evolution 4802

Lecture 12, Chapter 8 and beginning of Chapter 9


Topics for today

    1. How variation arises by mutation?
    2. What impact does it have on organisms?
    3. Macromutation - Polyploidy
    4. Variation through recombination


Products from gene duplications strongly influence fitness in nature

1.         Example - duplication in vertebrate chemosensory receptors

2.         Six multigene families

·         OR – olfactory receptor

·         TAAR – trace amine receptor (neural transmitters)

·         Vomeronasal receptors (1 and 2) (pheromone detection)

·         Taste receptors (1 and 2)

Does additional DNA represent a greater evolutionary opportunity?

1.      Genome size is not related to organismal complexity

2.      But is it all “junk DNA?”

Fig. 20.2 (19.7 old), Fig.8.6 (8.5 old)

In general, should synonymous or nonsynonymous changes accumulate faster?

1.      Synonymous – because they have no effect on phenotype

2.      If nonsynonymous changes are accumulating at a relatively fast rate, assume that the changes must be under selection

3.      Ratio of nonsynonymous to synonymous changes used to identify candidate genes with great functional significance

Fig. 8.9 (new and old)

Rates of spontaneous can be estimated by visible polymorphisms

Table 8.2 old

How fast do mutations accumulate?

1.      Candidate genes

·        Rare mutation in human FOXP2 gene causes severe speech and language disorders

·        Candidate gene invovled in the evolution of language?

·        Looked at the ratio of nonsynonymous:synoymous mutations

·        Even ratio in humans

Table 8.1 (8.2 old)

Table 8.2 (8.3 old)

Estimates of mutation rates in functional parts of the genome

1.      Mutation rates are slow

2.      BUT must extrapolate across the whole genome

3.      Lots of raw material for change over 1,000’s of generations

Fig. 8.9 (new and old)

Mutation rates are sensitive to the environment

1.      Do mutation rates of repetitive elements increase in baby mice when their parents are raised in different conditions?

Fig. 8.11(new and old)

How does mutation accumulation influence genetic variance and viability?

1.      1.7 million flies examined to estimate effects on viability of mutations on chromosome 2

2.      Balanced by wild type chromosome

3.      Represent 1/3 of genome

Fig. 8.10 (new and old)

Do mutations have beneficial effects?

1.      Single genetically identical line at generation zero

2.      Samples of founders frozen

3.      Mutations accumulate spontaneously each generation

4.      Compared population growth rate of evolving population to the founding population

5.      E. coli

Fig. 8.18 (8.17 old)

Beneficial effects of mutations shown through experimental evolution

1.      Elegant example of potential evolution in response to climate change

Limits of mutation

1.      Mutation only alter preexisting traits

2.      Many mutations may have the same phenotypic effects

a.      Retinitis pigmentosa can be caused by mutations of genes on 8 of 23 of human chromosomes

3.      Not all mutations are equally likely to occur

a.      AT and GC pairings work

b.      Purine-purine and pyrimidine-pyrimidine pairings don’t work

c.      Experimental evolution in phage strains

d.      Most mutations occur repeatedly at a small number of sites

e.      Mutations match those found in natural populations

f.        Limited number of pathways to adaptation

Fig. 8.20 (old 8.18)

Polyploidy is a macromutation that doubles genome size

Does this provide greater evolutionary potential?

1.      More gene products

2.      Greater genetic diversity

3.      Opportunity for duplicated genes to diverge in function

4.      More gene interactions

Do polyploids evolve faster in response to climate change?

Etterson research: Artificial selection on drought tolerance on two different ploidy levels

Meiosis rapidly produces genetic variance by two mechanisms

  1. Independent segregation of bivalents

The number of combinations equals 2n

Example n = 36

# possibilities = 6.8710

Example n = 82

# possibilities = 4.8424

Meiosis rapidly produces genetic variance by two mechanisms

  1. Recombination

Genetic variance increases due to independent segregation & recombination

VP = VG + VE

Fig. 8.12 old

Generation of genetic diversity

Mutation creates different alleles