Population genetics

Evolution in more detail

Populations• Group of individuals of the same species in a given

area• All of the genes in a population = their gene pool

– All alleles at all loci

Outline• 1. Modern Evolutionary Synthesis• 2. Individuals live or die, but

POPULATIONS evolve• 3. Mechanisms of Evolution

Modern Evolutionary Synthesis

• In Darwin’s time, only one person had figured out how inheritance works

• Mendel’s work was not widely known• Darwin knew less about the mechanisms of genetics than you

do!• This gap caused difficulties for understanding Evolution by

N.S.• In particular, if you assume blending inheritance, N.S. seems

not to work.

? !!!

Blending Inheritance ExampleLarge population of 20mph bunnies

Lesson:• If inheritance happens by blending,

rare variants will get diluted over time - useful variation cannot be preserved for long.

Particulate (Mendelian) inheritance

30mph bunny 20mph bunny

x

What happens if rare variant is due to a new allele?Common allele: b20Rare allele: b30 (assume it’s dominant for this example)

Lesson• 30 mph bunnies can become more common

if they survive or reproduce better (without losing speed through blending).

• Mendelian inheritance allows N.S. to work.

Now you’re talking!

Modern Evolutionary Synthesisor “Neodarwinism”

Darwin gathered extensive evidence on geographical patterns of biodiversity, on variation in domestic and wild animals, and on artificial selection. But it’s a big planet out there.

In the 50 years after the publication of the Origin of Species, many paleontologists, biogeographers, population geneticists and others made more intensive studies. Those studies, together with the rediscovery of Mendel’s papers on genetics, led to a new, more complete understanding of evolution

Modern Evolutionary Synthesis

or “Neodarwinism,” summarized

Darwin, Wallace (Natural Selection)+ Mendel (Genetics)+ Wright, Fisher (Population Genetics)+ Mayr, others (Biogeography, Paleontology)

= Modern Evolutionary Synthesis - a more complete understanding of how evolution proceeds in the wild.

Macro- vs. Micro- Evolution• Macroevolution - change that results in the

formation of new species, new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction.

• Microevolution: the generation-to-generation change in the frequency of alleles in a population– Evolution at its smallest scale– The core definition of evolution

Microevolutionary changes give rise to macroevolution

Mechanisms of Evolution: How do allele frequencies

change?

‘I am convinced that Natural Selection has been the most

important, but not the exclusive, means of modification’- Charles Darwin, On the Origin of Species

Mechanisms of Evolution• Why does the allele frequency in a

population change?– Genetic drift– Natural selection– Gene flow– Mutation

Mech. of Evol. (1) Genetic Drift• Changes in a population’s allele frequencies

due to chance– Is genetic drift a form of evolution?

• Drift can cause rapid change in small populations

• A problem when populations get too small • Related issues with small populations

– Bottleneck effect– Founder effect

Bottleneck effect• Survivors after large reductions in populations do not

represent the gene pool as well as before the reduction

• Reduced genetic variability + drift lead to changes in allele frequencies that are not necessarily adaptive

Fig. 23.5

Cheetah

Founder effect• When a few individuals colonize a new area,

they represent the entire gene pool• Reduced genetic variability + drift lead to

changes in allele frequencies that are not necessarily adaptive

Polydactyly due to Ellis-van Creveld syndrome

Mech. Of Evol. (2) Natural Selection

• Natural selection: differential success in survival and reproduction

• N.S. leads to populations becoming better adapted to their environments (adaptation)

• N.S. works most powerfully in large populations (effect of drift is small, and changes due to N.S. are preserved)

Fitness• No, not the body-builder kind• Darwinian fitness: an individual’s

contribution to the gene pool of the next generation compared to the contribution by others

• This is what biologists really mean when they talk about ‘survival of the fittest’

Modes of selection• Three ways that natural selection

affects the frequency of heritable traits– Directional selection– Diversifying (disruptive) selection– Stabilizing selection

Modes of selection (2)

Fig. 23.12

It is Possible to Measure Natural Selection in the Wild.

Sexual selectiona form of Natural Selection

• Darwin’s term• Why is there sexual

dimorphism?

If natural selection is so powerful, how come we’re not perfect

already?• Evolution is limited by history or

ancestry• Adaptations are often

compromises• Not all evolution is adaptive• Selection can only edit existing

variation

Mechanisms of Evol. (3): Mutation

• Mutations are the origin of all differences between alleles

• But mutations are rare• So mutations must still

spread by drift or selection if they are going to impact allele frequencies in a population

Sickle cell anemia is the result of a single point mutation

Mech. Of Evol. (4): Gene Flow

• Migration of individuals can change allele frequencies in a population

Mechanisms of Evolution:Summary

• Drift and Natural Selection seem to be the most powerful forces in natural populations

• Drift is important in very small populations

• Natural Selection is important in all other populations, and is especially powerful over long time periods.

Hardy-Weinberg theorem• What would the gene pool (and thus the gene

frequency) of a NON-evolving population look like?

• In 1908, Hardy and Weinberg independently developed this idea

• The frequencies of alleles in gene pool remain constant over generations unless they are acted upon by things other than segregation in meiosis and random fertilization– In a state of Hardy-Weinberg equilibrium

Allele frequency in a gene pool• Evolution is measured as changes in

gene frequency

Fig. 23.3a

Allele frequency in a non-evolving gene pool

• Using the same example as before, what do the offspring look like?

Fig. 23.3b

Hardy-Weinberg Equation• p2 + 2pq + q2 = 1• p = frequency of allele 1• q = frequency of allele 2• Using this equation, you

can calculate frequencies of alleles in a non-evolving gene pool

Hardy-Weinberg assumptions

• Very large population size• No migration• No mutations• Random mating• No natural selection

• How realistic are these conditions?

Assumptions of HWE• 1. Large population size• 2. No migration• 3. No mutation• 4. Random mating• 5. No selection• Deviation from HW assumptions usually means

allele frequencies are changing between generations - Evolution is happening

• HWE is more interesting when absent!