Evolutions Of Populations (Part 2) Flashcards
Evolution
A change in allele frequencies in the gene pool of a population
Four mechanisms of evolution in populations
- Natural Selection
- Genetic Drift
- Gene Flow
- Mutation
Mechanism of Genetic Drift
Random increase or decrease in allele frequencies due to chance events.
Genetic drift effect on genetic variation
Tends to decrease genetic variation.
Genetic drift effect on average fitness
Changes are random with respect to fitness. Usually decreases fitness.
What is genetic drift?
Genetic drift is any change to allele frequencies in a population that is due to chance. It causes allele frequencies to drift up and down randomly over time due to sampling error.
genetic drift results in what?
- Genetic drift can occur by any process or event that involves sampling error where observed results do not fit expected results based on probabilities.
What can result in random changes in allele frequencies in a population?
- Random fertilization in every generation in every population of species that reproduce sexually.
- Accidents that remove individuals at random occur in every population in every generation.
- A small population can be separated from a parent population.
3 important points illustrated here about genetic drift due sampling in sexual reproduction:
- Genetic drift changes are random with respect to fitness as they are cause by chance.
- Over time, genetic drift can reduce genetic variation, cause harmful alleles to be fixed, cause beneficial alleles to be lost.
- The effect of genetic drift is more pronounced in small populations. The smaller the population, the larger the sampling error.
Random samples of populations leads to genetic drift
Two examples:
1. Founder Events
2. Population Bottlenecks
Both of these take a small random (with respect to fitness) sample from a larger population and can lead to genetic drift.
The small new populations they create are
Subject to continued genetic drift because of their small size.
What is a founder event
Founder Event: group of individuals from a large population emigrates to a new geographic area and establishes a new population.
Founder effect
Founder Effect: change in allele frequencies that occurs when this new population is established.
Immigrants establish new population: New population is likely to have different allele frequencies than the source population, by chance
Once breeding takes place (founder effect)
- Drift continues to act via sampling errors that occur during fertilization.
- Small population more sensitive to effects.
Population Bottleneck
sudden reduction in size of a large population due to disease outbreaks, natural catastrophes such as floods or fires or storms, or other sudden destructive events.
Genetic bottleneck
- sudden reduction in the number of alleles in a population. Usually follow a population bottlenecks.
- Bottlenecked population is likely to have different allele frequencies than original population, by chance
- high mortality strikes individuals at random
Once breeding takes place (bottleneck)
Drift continues to act via sampling errors that occur during fertilization. Small population more sensitive to effects.
Gene Flow Mechanism
Addition or loss of alleles by movement of individuals in or out of the population.
Gene flow Effect on Genetic Variation
May increase or decrease genetic variation.
Gene flow effect on average fitness
Changes are random with respect to fitness. May increase or decrease average fitness.
Gene Flow mechanism
- Gene flow is the movement of alleles from one population to another.
- It occurs when individuals leave one population, join another, and breed.
Gene flow consequences
- It can increase or decrease genetic variation in a population.
- Reciprocal gene flow equalizes allele frequencies between populations.
Time 1 and Time 2 (gene flow)
At time 1, populations differ in allele frequencies
At time 2, Gene flow causes allele frequencies in the two populations to be more alike
Reducing genetic variation
- Many mechanisms in the evolution of populations tend to reduce genetic variation.
- Population with reduced genetic variation may not be able to adapt if the environment changes rapidly and run the risk of extinction.
Restoring Genetic Variation
- Most forms of natural selection favour certain alleles and lead to a decrease in overall genetic variation.
- Genetic drift tends to decrease genetic variation over time, as alleles are randomly lost or fixed.
- Gene flow may increases genetic variation in a recipient population if new alleles arrive with immigrating individuals. But gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals.
Mutation mechanism
Production on new alleles
Mutation effect on genetic variation
Increases genetic variation.
Mutation effect on average fitness
Random with respect to fitness. May increase or decrease average fitness.
Consequences of mutations
Small-scale mutations in gene sequences can result in:
A. Deleterious alleles
B. Neutral alleles
C. Beneficial alleles
Deleterious alleles
These alleles decrease fitness and can be eliminated by natural selection (called purifying selection).
Neutral alleles
These alleles have no effect on fitness and are under no selection pressure.
Beneficial alleles
These alleles are rare but will increase fitness and be favoured by natural selection.
Does mutation occur often enough to make it an important mechanism of evolution?
No. Mutation is not an important mechanism by itself.
As an evolutionary mechanism
In eukaryotes, mutation is slow compared with natural selection, genetic drift, and gene flow.
Based on the highest mutation rates recorded at individual genes in humans, it would take how long for mutation to produce a change in allele frequency of 1%!
4000 years
Mutation can be a significant force in
bacteria and archaea, which have short generation times.
Take home messages on mutation
- Mutation is the ultimate source of genetic variation. Crossing over and independent assortment shuffle existing alleles into new combinations, but only mutation creates new alleles.
- If mutation did not occur, evolution would eventually stop. Many evolutionary processes tend to eliminate alleles. Without mutation, there would be no variation in alleles, no variation in traits to fuel evolution.
- Mutation alone is usually rare and inconsequential in changing allele frequencies at a particular gene. However, when our many genes across the genome are considered and combined with natural selection, it becomes an important evolutionary mechanism.
Genetic drift is a concern for conservation biology
- Many populations are being reduced in size by habitat destruction/fragmentation and other human activities.
- Small population size subject to continued genetic drift.
- Populations become less fit and are subject to extinction.
