Bio120 Lec 8: Population Structure - Genes and Phenotypes

Population

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

Migration

The movement of individuals from one population to another (may or may not lead to gene flow)

Gene flow

the movement of alleles from one population to another

Combined effects of selection, gene flow and genetic drift on population divergence

• imagine 2 populations with different environments and selection is acting different in these 2 populations

• expect that natural selection will make these population to be more different and diverge from each other

• genetic drift leads to the evolution of differences

• gene flow pulls a population together, alleles move back and forth between populations meaning the populations are sharing alleles

Geneflow role in divergence

does not diverge populations but brings them more alike to each other

natural selection and genetic drift role in divergence

diverges a population and makes them more different rom each other by selecting for separate traits in different environments

How to measure gene flow between populations?

• perform experiments

• use neutral genetic markers to look for evidence of recent gene flow

  • sequence natural regions of DNA that are “invisible to selection”

  • Because variations in silent mutations or introns do not impact an organism's fitness, they are invisible to natural selection. Their frequencies are governed exclusively by gene flow, genetic drift, and mutation rate

Potential gene flow (dispersal)

• physical movement of individuals away from their birthplace. 

  • gamete dispersal: haploid, due to wind, animal pollinators

  • individual dispersal: diploid, active locomotion

actual gene flow (interbreeding)

The successful mating of a dispersed individual (or pollen/seed) followed by the survival of fertile offspring.

Genetic differentiation of populations: gene flow

• Experimental populations separated from each other that have neutral genetic differences both populations only contain homozygotes but for different genes)

• Get heterozygosity in F1 generation you have evidence of gene flow since it means that allele from the other population migrated and mated with a homozygous individual of different homozygosity. 

• ex. wild sunflowers and gene flow 

  • Scientists first developed genetic modified crops 

  • Sunflowers are valuable for oil

  • Concerned that modified genes enter wild population and effect whats happening whats out their in nature 

  • Is their gene flow between wild sunflowers and commercial sun flower

  • Crop had homozygous trait  that’s not in wild 

  • Plotted crop sunflowers different distances from wild sunflowers 

  • Saw the level of gene flow  (closer more gene flow3 and further less gene flow)

Genetic differentiation of populations: genetic drift

• Random subsampling from allelic variation 

• Cannot predict exact outcome since its random
  
  

Stochastic processes resulting in loss of diversity

• Genetic drift

• Population bottle neck 

• Founder effect 

ex. humans originated in Africa, there was a loss of genetic variation with increasing distance from east Africa

Deterministic evolutionary force

• predictable or non random

• Natural slelection (made predictions if we know about the genetics and the selection agen

Stochastic changes in allele frequency

• due to random variation in fecundity & mortality (random w respect to phenotype of organism)

• Mutation (cannot predict what outcome will be) 

• Recombination (crossing over is random) 

• Genetic drift (cant predicts which alleles will be subsampled)

• Most important when populations are small since theres a more noticeable difference

Population Bottle neck

• A single sharp reduction in abundance, usually followed by rebound

• Causes a loss of diversity

• Massive reduction in abundance

• Subsampling effect of genetic diversity 

• Loss of allelic diversity

• Large population -> small size

Founder event

colonize new area, a lot of diversity in big sample to small sample with little diversity 

Human Genetic Variation Over Space

Human Genetic Variation Over Space

• Ex. individuals are circles and dif colour rep dif allele 

• Population cont to exist across populations and grow 

• Maintaining genetic diversity since its a large population

• Due to stochastic changes allele frequency can go up or down

• From first population some colozined 2 separate islands 

  • Genetic diversity is loss from founder populations 

  • After founder effect the smaller populations grow and get less genetic diversity 

low gene flow

• Imagine 2 population separated by mountain range 

• Not alot alleles passing mountain range

• Over time Expect orgnaims to diverge 

• Drift causing fixation of polymorphism from drift or selection

• Not a lot of gene flow

High gene flow

•  2 pop. experiencing gene flow 

• Light barrier (hills)

• Polymorphism shared even tho separated to some degree (a lot of gene flow)

• Gene flow homogonizes and makes populations more genetically similar

Low gene flow with increasing distance

• not just due to geographucal barriers

• Further away from ancestral home (africa) the more genetic differentiation you have 

• Most different between population separated from each other 

• Isolation by distance = genetic differentaaiation

  • Expected to have fixed genetic differences since gene flow takes a long time 

Phenotypic 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

• Testing for local adaptation and plasticity:

  • Reciprocal transplant studies

  • Genomic analyses

Phenotypic placisity

• Can Contribute to Populations Looking Different

• ability of a genotype to modify its phenotype in response to a

  particular environment

• changes phenotypes based on environement

ex. arrowhead, an aquatic plant common in Ontario wetlands

•  Terrestrial and aquatic phenotypes

• Leaves are different based on environments

• Genomes are identical

• Same genotype present dif phenotypes based on environment

• arrow head = terrestrial

• long leaves = aquatic

Reciprocal Transplant Studies

• studies phenotypic plasticity

• Individuals are cloned from og environment and moved to contrasting environment 

• Study local adaptation as well

Clausen-Keck-Hiese

• 3 populations of plants, wanted to set up reciprocal transplant to see if differences were genetic or plastic

• 3 plants were planted at 3 different elevations

• Plants are different and vary

• Isolate effects of plasticity from genetics since plants are genetically identical the only reason they would change is due to plasticity

• found that the plants were locally adaptive

local adaptation

• perform better in homeland environment

• ex. skin pigmentation

  • humans develop different skin pigmentation to better adapt to their local environment associated with UV radiation

Tradeoffs

• an evolutionary change that increases fitness in one context causes a decrease in fitness in another.

• ex. skin pigmentation

  • high UV: breaks down folate (critical in highly dividing tissue), dark skin acts as built in sunscreen

  • low UV: reduces vitamine D synthesis, reduced pigmentation so UV is very harmful