Evolution Of Populations (Part 1) Flashcards
The unit of evolution is
The population
The fuel for evolution is
variation in traits based on genetic variation.
What produces genetic variation in populations
- Mutations
- Sexual reproduction
- Horizontal Gene transfer
Mutations
- permanent changed to DNA
- Creates new alleles (and in some cases new genes or even genomes) in populations.
Sexual reproduction
Creates different combinations of pre-existing alleles in populations.
Horizontal gene transfer
- Allows new alleles (or new genes) to be introduced into populations.
- Important in unicellular species.
Importance of mutation in genetic variation and evolution
- Can introduce new alleles in populations.
- Is one mechanism of evolution; although not usually an important force on its own.
Although mutation is rare
- it is the ultimate source of all genetic variation in sexually and asexually reproducing populations.
- If mutation did not occur, evolution would eventually stop.
- primary source of genetic variation in asexual populations.
Importance of sexual reproduction in genetic variation and evolution
- Sexual reproduction can shuffle existing alleles into new combinations.
- Not a mechanism of evolution but can act as a supporting factor.
- In organisms that reproduce sexually, shuffling of alleles is more important than mutation in producing genetic variation in each generation
How is evolution Measured
Evolution of a population is detected by measuring allele frequency changes in the gene pool.
What is the Gene Pool?
- all of the alleles of all the genes in a certain population.
No evolution
No change in allele frequencies
Evolution
Change in allele frequencies
Some extreme fates of alleles
During the evolution of populations, alleles in a can become:
A. Fixed: reach a frequency of 1
B. Lost: reach a frequency of 0
Genetic equilibrium
A population is in genetic equilibrium (for a given gene) if the allele and genotype frequencies for that gene do not change generation after generation.
Genetic equilibrium occurs when
A. Evolution is not occurring: allele frequencies are constant
B. Mating is Random: genotype frequencies are constant
Predicting genotype frequencies in genetic equilibrium
Can predict the genotypic frequencies that should exist in the current population if the population is in genetic equilibrium by doing a population-wide Punnett square.
Measuring evolutions: The Hardy-Weinberg principle
- Most of the time, we don’t have allele frequencies from the previous generation to compare to today’s population to see if a population is evolving.
- The H-W Genetic Equilibrium Principle acts as a null hypothesis when researchers want to test whether evolution (and/or non-random mating) is occurring in a population with respect to a particular gene.
From H-W Equations:
Expected genotype frequencies (if no mechanisms of evolution are at play and mating is random) can be compared to from observation: observed genotype frequencies
Results from the H-W principle
Possibility 1: expected Genotype frequencies= observed genotype frequencies
Conclusion:
1. No mechanisms of evolution are at play
AND
2. Mating is random
Possibility 2: expected Genotype frequencies does not equal observed genotype frequencies
Conclusions:
1. One or more mechanisms of evolution are at play
AND/OR
2. Mating is NOT random
The Hardy-Weinberg Principle Steps
- Determine genotype frequencies (if available).
- Determine the allele frequencies (using genotype frequencies).
- Determine what the genotype frequencies should be if the previous generation:
A. had the same allele frequencies (no evolution)
B. was mating randomly - Compare the genotype frequencies predicted by the H-W principle with the actual genotypic frequencies.
- Draw conclusions.
If they are the same
Evolution is not occurring and mating is random. In H-W (Genetic) Equilibrium.
If they are different
Evolution is occurring AND/OR mating is not random.
Non-random Mating
- Random mating is a requirement for genetic equilibrium.
- In nature, matings between individuals in a population may NOT be random with respect to the gene (and phenotype) in question.
Sexual selection
- A type of non-random mating that does cause evolution (allele frequencies change)
Other types of non-random mating include:
- Inbreeding and Outbreeding
- Assortative Mating
These types of non-random mating do NOT directly change allele frequencies (so do not cause evolution) but can influence genotype frequencies in a population and can play an supporting role in evolution.
Interbreeding
Mating between relatives more common than by chance. The most extreme form of inbreeding is selfing.
Outbreeding
Mating between non-relatives more common than by chance.
The basis for assortative mating
- Is not common ancestry but mating based on phenotypic similarity or dissimilarity.
Four point about interbreeding (consequences)
- Inbreeding does not cause evolution. Allele frequencies do not change in the population. Inbreeding and other forms of nonrandom mating change genotype frequencies—not allele frequencies. Can be a supporting force however.
- Inbreeding increases homozygosity. In essence, inbreeding takes alleles from heterozygotes and puts them into homozygotes.
- Inbreeding can lead to inbreeding depression.
- Inbreeding can help purifying selection.
Only heterozygotes produce
Heterozygote offspring, but only 50% of the time
Do A1 and A2 allele frequencies change in each generation when inbreeding occurs?
- Does not change allele frequencies (no evolution) but increases the frequencies of homozygotes.
Inbreeding depression
a decline in average fitness that takes place when homozygosity increases and heterozygosity decreases in a population.
How does inbreeding depression occur
Has two main causes:
1. Many recessive alleles represent loss-of- function mutations.
2. Many genes are under intense selection for heterozygote advantage.
Inbreeding helps “purge”
- recessive deleterious alleles
- Even though inbreeding does not cause evolution directly— because it does not change allele frequencies—it can speed the rate of evolutionary change.
- More specifically, it increases the rate at which purifying selection eliminates recessive deleterious alleles from a population.
When no inbreeding is occurring
- the recessive deleterious allele found in heterozygotes cannot be eliminated.
When inbreeding is occurring
more recessive deleterious alleles are found in homozygotes and are quickly eliminated.
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
Consequences of evolution
Each mechanism can change allele frequencies and cause evolution in a population.
It will affect the:
1. Genetic Variation of the population
2. Fitness of the population
- Natural selection
Me chanism: Certain alleles are favoured.
Effect on Genetic Variation: Increases, maintains, or decreases genetic variation.
Effect on Average Fitness: Increases fitness by producing adaptations.
Modes of natural selection
3 patterns of natural selection exist for quantitative characters in populations:
A. Directional Selection
B. Stabilizing Selection
C. Disruptive Selection
A. Directional Selection
Changes the average value of a trait
Mechanism of directional selection
Selection favours one extreme and greatly reduces the other in the range of phenotypes.
Consequences of directional selection
• Results in a directional change in the average phenotype
• Trait/Genetic variation in populations can be reduced.
• If directional selection continues, the beneficial alleles become fixed in a population while harmful alleles become lost via purifying selection
Directional selection is usually limited by
an opposing directional selection from fitness trade-offs.
B. Stabilizing Selection
Reduces the amount of variation in a trait
Mechanism of stabilizing selection
Selection favours intermediate phenotypes and reduces both extremes in a population.
Consequence of stabilizing selection
- No change in the average phenotype over time.
- Trait/Genetic variation in the population is reduced.
- not all evolution is directional!
Disruptive selection
Increases the amount of variation in a trait.
Mechanism of disruptive selection
Selection favours extreme phenotypes and reduces intermediate phenotypes.
Consequences of disruptive selection
- Two average phenotypes develop over time (bimodal distribution).
- Trait/Genetic variation in the population is increased.
Disruptive selection is important
Because it often plays a part in speciation.
Sexual selection
- A special case of natural selection where non-random mating causes evolution of populations.
- usually a mechanism of evolution in males of a sexually reproducing population.
Sexual selection favours
individuals with heritable traits that enhance their fitness by increasing their chance to attract mates.
The fundamental asymmetry of sex
- Males and females have different roles in the reproduction process.
- The roles produce distinct criteria that increases the fitness of a specific sex.
What increases fitness in females?
- Reproduction is energetically expensive in females.
- Strategy: Reproduce a few times and do it well.
Traits that increase a females fitness allow her to
A. Support the development of offspring.
B. Choose males with “good alleles” to pass on.
C. Choose males that will provide resources and care for offspring.
What is fitness not limited by
the ability to find a mate.
What increases fitness in males?
- Reproduction is energetically cheap.
- Strategy: Reproduce as often with as many females as possible.
- Leads to COMPETITION among males to attract mates.
Traits that increase a male’s fitness:
allow him to outcompete other males in attracting mates by:
A. Force
B. Being chosen by females
Two types of sexual selection
- female choice (intersexual selection)
Example: Female zebra finches prefer males with more colorful beaks and feathers (indicating better health). - male-male competition (intrasexual selection)
Example: Male elephant seals establish territories, areas that they defend and can use exclusively for breeding.
What are the consequences of sexual selection?
- Results in sexual dimorphism.
- Traits that differ between males and females.
Males tend to have:
many more traits that function in courtship or male-male competition as sexual selection is more intense in males.
