Lecture 20 Flashcards
Population
A group of individuals of a single species
occupying a given area at the same time
Migration
The movement of individuals from one
population to another
Gene flow
The movement of genes from one population
to another
How to Measure Gene Flow?
Difficult to observe and measure
– Potential (dispersal) vs. actual (interbreeding)
– Gamete vs. individual
* Use experimental approaches
* Use neutral genetic markers
– Polymorphic genetic variants that aren’t direct
targets of selection
– Let us infer non-selective processes affecting
genetic diversity of populations
Gene Flow Between
Crop and Weed Sunflowers
Most gene flow occurs over a short distance,
but a small amount occurs as far as 1 km
What Does Random Mean in Evolution?
Stochastic (unpredictable or random)
evolutionary forces:
– Mutation
– Recombination
– Genetic drift
* Deterministic (predictable or non-random)
evolutionary force:
– Natural selection
Stochastic Processes Resulting
in a Loss of Diversity
Genetic Drift:
* Stochastic changes in allele frequency due to random variation in
fecundity & mortality
* Most important when populations are small
* Population Bottlenecks:
* A single sharp reduction in abundance, usually followed by rebound
* Causes a loss of diversity
* Founder Events:
* Colonization by a few individuals that start a new population
* Colonizing group contains only limited diversity compared to the source
population
Random Fluctuations in Allele Frequencies
in Populations of Different Size
Genetic drift is more pronounced in small populations
* More drastic fluctuations each generation
* More rapid loss of genetic diversity
* Faster time to allele fixation (or loss)
* Less consistency across replicate populations
Human Genetic Variation Over Space
Humans show a loss of genetic variation with
increasing distance from East Africa
* Reflects serial founder events as humans
migrated from source population
Human Population Differentiation:
One Gene With Multiple Alleles
Across many genes: 93 to 95% of genetic variation is
observable within populations
* Human populations experienced recent origins and
reasonably high gene flow
Gene Flow Between Humans and Neanderthals
Human populations out of Africa:
* Genomes have short stretches of Neanderthal-derived DNA
* Averages about 2% of the genome
* Consistent with ancient interbreeding
Differences Between Populations
Within a Species
Genetic differentiation among populations
is often observed across a geographic range
– Different allele frequencies in different populations
* Phenotypic differentiation may be:
– Adaptive (“local adaptation”) OR
– Due to genetic drift OR
– Phenotypic plasticity
* Evidence for local adaptation:
– Reciprocal transplant studies
– Genomic analyses
Phenotypic Plasticity:
The ability of a genotype to modify its phenotype
in response to changes in the environment
Occurs through modifications to growth and
development and behavior
– Under genetic control
* Common in sedentary organisms e.g. plants, corals
– Also in animal behavior
* Phenotypic plasticity often is an adaptation to
unpredictable environments
– But not all phenotypic plasticity results from adaption
Reciprocal Transplant Studies
Growth of equivalent genotypes in contrasting
environments and comparisons of their relative
performance
- Can separate phenotypic variation into
genetic and environmental components
* Enables measurement of selection against
non-local genotypes
* Provides evidence of local adaptation
Clausen-Keck-Hiesey
Transplant Conclusions
Differences between populations due to
BOTH plasticity and genetics
* Evidence for widespread local adaptation
– Local populations had highest fitness
