Biol 4802 Evolution

Lecture 16, Clean up bits from Chapter 9 and Chapter 10


Topics for today

    1. Coalescence
    2. Neutral theory of molecular evolution
    3. Gene flow


Fig. 10.3(new and old)

Genetic drift causes coalescence

Fig. 10.(new and old)

Time to coalencence differs according to population size

Expected relationship?

Fig. 10.18 (Fig. 10.15 old)

How much evolution is random?

Basic ideas underlying the neutral theory of molecular evolution

Effects of mutation

•         Minority are advantageous and are fixed by natural selection

•         Many are disadvantageous and eliminated by natural selection

•         Majority are effectively neutral with respect to fitness

Causes of variation

•         Most genetic variation at the sequence level is neutral

•         Most phenotypic variation (morphology, physiology, behavior) is due to natural selection

Basic principals

Overall mutation rate:                        uT per gamete per generation

Fraction that are neutral:                    fo

Neutral mutation rate:                       uo = fouT

uo differs among genes

What is the most important factor that determines the neutral mutation rate, uo?

Function determines uo for any given gene

b        polypeptide chain of hemoglobin

All sites except those numbered are variable

uo depends upon functional constraint

What kinds of genes or gene regions should show the least constraint?

Fig. 10.14 (Fig. 10.12 old)

Synonymous substitutions more common

14 nuclear genes

Rates differ among lineages

Synonymous rate low in primates

Fig. 10.16 (Fig. 10.13 old)

Evolution of neutral variation

Evolution of neutral variation

      = 2Neuo * 1/2Ne

      = uo

The rate of fixation for any given gene is theoretically constant and equals the neutral mutation rate

How to estimate the rate of neutral evolution?

Important qualification

Fig. 10.12 (Fig. 10.10 old)

Neutral theory of evolution describes constant approach to fixation

·         Mutation

·         Certain kinds of natural selection (more on this topic in future lectures)

·         Gene flow

Summary from simulations

·         Populations diverge from each other due to genetic drift (and selection but that wasn’t modeled).

·         Genetic drift has a strong effect in small populations

·         Migration counters the effects of genetic drift and causes:

·         fixation of alleles

·         loss of genetic diversity

·         Migration is especially important for maintaining genetic variation in small populations

·         Populations that are already diverged are homogonized by migration

Migration reduces divergence

Fig. 9.30 (Fig. 9.28 old)

How do we know the extent of gene flow?

Measure gene flow with genetic markers

Infer gene flow from natural patterns of diversity

How much genetic divergence among populations?

Infer gene flow from natural patterns of diversity