Post on 16-Mar-2016
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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!