Lecture 20 Flashcards

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1
Q

Population

A

A group of individuals of a single species
occupying a given area at the same time

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2
Q

Migration

A

The movement of individuals from one
population to another

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3
Q

Gene flow

A

The movement of genes from one population
to another

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4
Q

How to Measure Gene Flow?

A

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

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5
Q

Gene Flow Between
Crop and Weed Sunflowers

A

Most gene flow occurs over a short distance,
but a small amount occurs as far as 1 km

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6
Q

What Does Random Mean in Evolution?

A

Stochastic (unpredictable or random)
evolutionary forces:
– Mutation
– Recombination
– Genetic drift
* Deterministic (predictable or non-random)
evolutionary force:
– Natural selection

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7
Q

Stochastic Processes Resulting
in a Loss of Diversity

A

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

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8
Q

Random Fluctuations in Allele Frequencies
in Populations of Different Size

A

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

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9
Q

Human Genetic Variation Over Space

A

Humans show a loss of genetic variation with
increasing distance from East Africa
* Reflects serial founder events as humans
migrated from source population

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10
Q

Human Population Differentiation:
One Gene With Multiple Alleles

A

Across many genes: 93 to 95% of genetic variation is
observable within populations
* Human populations experienced recent origins and
reasonably high gene flow

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11
Q

Gene Flow Between Humans and Neanderthals

A

Human populations out of Africa:
* Genomes have short stretches of Neanderthal-derived DNA
* Averages about 2% of the genome
* Consistent with ancient interbreeding

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12
Q

Differences Between Populations
Within a Species

A

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

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13
Q

Phenotypic Plasticity:

A

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

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14
Q

Reciprocal Transplant Studies

A

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

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15
Q

Clausen-Keck-Hiesey
Transplant Conclusions

A

Differences between populations due to
BOTH plasticity and genetics
* Evidence for widespread local adaptation
– Local populations had highest fitness

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16
Q

Tradeoffs Associated with Skin Pigmentation

A

High UV radiation:
– Interferes with folate
– May have selected for increased pigmentation
* Low UV radiation:
– Reduced vitamin D synthesis
– May have selected for reduced pigmentation
* No single “best” phenotype across the globe
due to trade-offs

17
Q

Was There a History of Local Adaptation
on Skin Pigmentation?

A

Numerous genes known to affect skin pigmentation
* Alleles of these genes show rapid allele frequency
change over time using ancient genomes (indicates
natural selection)
* Alleles of these genes show higher between-
population differentiation than most other genes
– Evidence supporting a history of local adaptation

18
Q
A