Bio120 Lec 5: Natural Selection and Adaptation

Natural selection

Initial mechanism to explain evolution created by charles darwin but it isnt the only way to explain evolution

Fitness

The ability for an individual to survive and reproduce to produce fertile offspring compared to other individuals

Selective Advantage

• The amount by which individuals with a given genotype are better adapted to a given environment

• increases the likely hood that a species or individual will survive in its environment

Agent of selection

• environmental or external factor that influences the survival and reproduction of those that carry a specific genotype that allows them to have an advantage

• what is doing the selection

• different types of agents of selection

  • Abotic

  • Biotic

  • Anthropogenic

Abiotic

non living factors ex. climate

Biotic

living factors ex. predetation

Anthropogenic

influenced by human activity ex. artificial selection

Directional (positive) selection

• synonym for adaptation

• alleles increase in frequency

• will be fixated on a specific allele as the frequency of allele approaches 100%

• Dominant homozygote has high relative fitness

• recessive homozygote is not fit and has a 60% chance of surviving

Purifying (negative) selection

removal of deleterious mutations

Selection to maintain variation (balancing)

• prevents alleles from being loss

• maintains polymorphism and genetic variation

Polygenetic phenotypic traits

2 or more genes work together to express the phenotype

Modes of selection on quantitative traits (in continuous phenotypes)

• stabilizing selection

• directional selection

• disruptive selection

Stabilizing selection

• an average trait is selected for

• extreme values have low fitness

• shrinks variability

• ex. medium weight babies have a higher chance of survival compared to smaller or larger babies

Directional selection

• favours and extreme trait

• no change in variants since theres no variation due to fixation

• ex. giraffes with really tall necks

• ex. as seeds got harder in the habitat, stronger beaks were selected for

disruptive selection

• both extremes are selected for

• leads to trait divergence

• can lead to speciation: 2 extreme breed groups with the different phenotypes can stop breedin with each other and the groups become separate species (reproductive isolation)

• increases variants in population

• ex. larger and smaller beaks in birds are selected for due to available resources but medium sized beaks have a hard time getting nutrition

How to study adaptation

• Look for correlation between trait and environmental variable across space and time

• Compare the trends of alleles (traits) with the environment overtime which will identify genes that were shaped due to natural selection. Track shift in frequencies of different traits or alleles in relation to the climate or certain environmental factors

• Test for patterns in genes that were targeted by selection will show how specific variations of genes will increase an individuals chance of survival. 

• Conduct experiments

Determining agents of selection

• hard to find reasons for natural selection and link evolution to ecology

• hard to find the agent of selection for a specific trait

• there are meany measurements of selection that act at the same time

• biologist end up having different treatments that mimic agents of selection

Evolution due to pollution

• An agent of selection or a cause of the adaptation/ selection for a particular trait is pollution for species like the peppered moth and heavy metal tolerance in grass

• Studying evolution is the byproduct of pollution 

• Compare certain traits and evironements and then compare to present traits and environmental conditions in order to find a reason for this new selected trait

Peppered Moths

• Peppered moths used their ligh colouraion to blend into their environment so that birds would not be able to find them and eat them. However, due to the industrial revolution the trees became a darker colour, blackened by soot and smoke which allowed the darker moths to camoflaugve better and the white moths to be eaten causing the allele for black moths to increaese in frequency

• Due to the clean air act the frequency of the black moth declined 

• Agent ofselection for these moths are birds

Grass and heavy metal tolerance

• Grass was polluted by heavy metals due to mine waste and the grass with the genotype to tolerate metal increased in frequency while had a low frequency in uncontaminated areas 

• Mine tail: pile of rock which includes chemicals toxic to life

• Rare alleles of neighbouring populations colonize the mining area/ invade 

• Gene floe is bad 

• Tolerance of zinc metal in grass is very high near the mine but not as much further away

Stickleback fish and studying adaptation

• Occur in clear lands or black water lakes 

• In black water lakes its hard to see bottom of lake from the surface, visibility is low 

• They have allelic variation that affects how well they see

• If they have mutated allele it gives increased vision in black water 

• This mutated allele should eventually fixate and have lower variability 

• All fish in black water lake have homozygote for redshifted allele so no diversity

Hitchhiking

• pull along parts of genome that are physically linked to that gene 

• By association other genes connected to that selected genome will also rise in frequency and lack variation 

Experimental evolution

• Richard lenski conducted a ecxpeirament over many generations

• Genetically identical bacteria in dif flasks and he let them evolve 

• All flasks are evolved independently 

• He wanted to see how bacteria adapted in crappy situations

• Produced history of evolution 

• Put cultures in freezers and compare living bacteria with their ancestors 

• Take a percentage of bacteria and put it in new food 

• All adapted in similar ways even though all grew independently 

• In one flask their evolved an adaptation that allowed the bacteria to eat the citrate as well as the glucose in the food instead of just the glucose and fitness went off the charts 

• shows that evolution is repeatable

• When populations face the exact same selective pressures, natural selection predictably drives them toward similar optimal physical traits

• Even though the bacteria adapted similarly, the actual genetic mutations that occurred were random. This was dramatically proven around generation 31,500 in one specific flask showcasing that evolution also relies on chance