Population Genetics Flashcards
How do you describe the allele frequencies in a population that is not evolving?
Allele frequencies are stable
What is it called when a population is not evolving?
Hardy-Weinberg Equilibrium
What is Hardy-Weinberg Equilibrium used as when studying actual populations?
A null hypothesis
What are the characteristics of a population not evolving (a population in Hardy-Weinberg Equilibrium)?
- No natural selection
- Extremely large population size (no genetic drift)
- No mutations
- No gene flow
- Random mating
If you can’t observe a population from generation to generation, how can you determine if the population is in Hardy-Weinberg Equilibrium?
Use allele/genotype frequencies within a generation and probability to determine if the population is likely evolving
What is a gene pool?
All copies of every (or a specific) gene in a population
What are the equations for Hardy-Weinberg Equilibrium?
p + q = 1
p^2 + 2pq + q^2 = 1
In the Hardy-Weinberg Equilibrium equation, what does each letter represent?
p is the frequency of one allele (typically the dominant allele), and q is the frequency of the other allele (typically the recessive allele)
The probabilities of p and q can be applied to what?
Random mating
i.e. there is a p% chance that each sperm and egg has allele 1 and a q% chance that each sperm and egg has allele 2
What do population Punnett Squares do?
Predict the genotype frequencies based on population allele frequencies for a population that is not evolving
Find the probability that sperm and egg will have allele 1 or allele 2
What are the 5 properties of natural selection?
- There needs to be variation in the population for a given phenotypic characteristic
- There needs to be a selective/environmental pressure
- There needs to be differential survival/reproduction of phenotypes in response to a selective pressure
- There needs to be heritability of the phenotypic characteristic
- Alleles for the adaptive phenotype will be more common in the next generation
What are the 5 types of selection?
- Directional selection
- Stabilizing selection
- Disruptive selection
- Balancing selection
- Sexual selection
What is the biological definition of fitness?
The average reproductive output of all organisms bearing a specific genotype
Individuals with a higher fitness do what?
Pass their genes on at a greater rate than individuals with lower fitness
If individuals with fitness differences are due to differences in alleles, what is expected?
Alleles associated with higher fitness will be more frequent in the next generation
What are the 5 components of differential reproductive success that having a specific allele might affect? (Does having this allele impact…)
- Viability
- Fecundity
- Gamete Viability
- Mating Success
- Fertilization Success
What is viability?
The probability that an individual bearing a genotype will survive
What is fecundity?
Number of gametes per individual
What is gamete viability?
Alleles that impact longevity or quality of sperm/egg
What is mating success?
Number and quality of mates
What is fertilization success?
Alleles impact the probability that fertilization will occur
What is directional selection?
Changes in the average phenotype in the population in one direction
What does directional selection do to a population’s genetic diversity?
Reduces it
What is stabilizing selection?
Reduces genetic variation in a trait, but does not change average value of a trait over time by reducing both extremes in a population
What is disruptive selection?
When intermediate phenotypes are selected against and extreme phenotypes are favored
What does disruptive selection do to a trait’s diversity within a population?
Increases the trait’s variation
Disruptive selection may be a factor in what?
Speciation- the formation of new species
Disruptive selection leads to the presence of what?
Extremes
What is balancing selection?
When no single allele has distinct advantage and there is a balance among several alleles in terms of their fitness and frequency
What type of individuals have the higher fitness in a population experiencing balancing selection?
Heterozygous individuals
What happens to genetic variation in a population experiencing balancing selection?
Genetic variation is maintained
Can be a consequence because selection will not happen quickly
With small populations, genetic drift can have what affects
- Random, non-adaptive changes in allele frequency between generations
- Founder effects
- Bottleneck effects
With respect to fitness, genetic drift is what?
Random
With genetic drift, changes in allele frequency are ________.
Not adaptive
Genetic drift is most pronounced in what kind of populations?
Small populations
What two things are more likely to occur with genetic drift?
- Random fixation
- Loss of alleles
What is a founder event?
When a new, smaller group of individuals establishes a new population in a new area
When a founder event occurs, how can the allele frequencies of the new population be described?
Likely different than the source population if the new population is small enough
What is the founder effect?
The change of allele frequencies due to a founder event
Occurs only if the new population has lower genetic diversity than the original population
Where and when do founder events/effects usually occur?
In the colonization of isolated habitats
What is a population bottleneck?
A sudden decrease in population size in a large population that results in the reduction of the number of alleles in a population
What occurs during a genetic bottleneck?
Genetic drift which causes a change in allele frequencies
Mechanisms like natural selection and genetic drift may do what to genetic diversity?
Decrease it over time
What do mutations do to genetic diversity?
Restore it by creating new alleles
What is a point mutation?
A change in a single base pair in DNA
What is a chromosome-level mutation?
Increases the number of chromosomes; polyploidy
What is lateral gene transfer?
Transfer of genes from one species to another
What are mutations with respect to the fitness of an individual?
Random
In what way do mutations not occur?
In a way that specifically tends to increase or decrease fitness. Can be neutral, beneficial, or deleterious
What type of organisms are mutations significant in?
Organisms with very short generation times
Mutations can an be an evolutionary process, but are what compared to selection, genetic drift, and gene flow?
Very slow
What is horizontal gene transfer?
Movement of genes between organisms without reproduction- typically seen in bacteria
What type of cell does a mutation need to occur in in order to be passed down?
Gamete
What is gene flow?
Introduction or removal of alleles from a population
When does gene flow occur?
When individuals leave one population, join another, and breed
Gene flow does what to allele frequencies?
Homogenizes them between the source and recipient populations
What can gene flow do to fitness?
Increase or decrease fitness depending on the situation
What are the three results of gene flow in respect to genetic diversity?
- Tends to reduce genetic differences among populations
- Can replenish lost alleles
- Increase genetic diversity of a population
What is asexual reproduction?
Formation of offspring without the fusion of egg and sperm
How are offspring and parent related when reproduction occurs asexually?
Offspring is genetically identical to the parent in most cases
What is sexual reproduction?
The formation of offspring by fusion of a female gamete (egg) with a male gamete (sperm)
How are offspring and parents related when reproduction occurs sexually?
Offspring inherits features from both parents, identical to neither
What is budding?
Offspring develop as an outgrowth of the parent
What is fission?
Parent splits into two organisms of roughly equal size
What is parthenogenesis?
When unfertilized eggs develop into offspring
What is sexual selection?
When individuals with certain inherited characteristics are more likely than others to gain access to mates and therefore pass on their alleles at a greater frequency than individuals without those phenotypic characteristics
Sexual selection is a form of what?
Natural selection
What is the operational sex ratio (OSR)?
The ratio of males ready to mate with females in that population that are ready to mate
What is differential parental investment (PI)?
Measured as each parent’s contribution to raising offspring
What are females’ gamete situation and PI in a sexually reproducing population?
Females have limited gametes and have a high PI
What are males’ gamete situation and PI in a sexually reproducing population?
Males do not have limited gametes and have a low PI
What is the average reproductive output between gender of a species?
Will be the same between genders- even though males will try to mate more, reproductive output is limited by females
Which gender will have greater variation in reproductive output among individuals of that gender?
Males
How can you describe female reproductive success in a population?
Relatively uniform
Why are females’ reproductive success relatively uniform?
They are not limited by mating opportunities, all are limited by gamete resources (once their egg is fertilized they can’t be fertilized until they give birth), and are all limited by resources to raise young
How can you describe male reproductive success in a population?
Variable
Why are males’ reproductive success relatively uniform?
They are limited by mating opportunities (females prefer to mate with high quality males only), males that demonstrate quality are preferred by females
What are the 4 things selection acts on?
- Variation, which is stronger in males because of greater variation in reproductive success
- Characteristics that provide greater access to mates
- Characteristics that allow defense of territories/resources and thus greater opportunities to mate
- Characteristics that result in greater female choice
What is intrasexual selection?
A type of sexual selection resulting from competition between members of one sex for access/opportunity to mate with members of the other sex
Intrasexual selection usually happens between what gender?
Males
Intrasexual selection is usually a competition for what two things?
- Access to females
- A resource that females need
What is intersexual selection?
When mate selection is based on phenotype of behavior and/or body structure correlated with positive survival attributes such as health and strength, thereby enhancing offspring’s chances of survival (fitness)- called the Good Genes Hypothesis
Which gender usually chooses the other in intersexual selection?
Females usually choose the males
What does sexual selection often result in?
Sexual dimorphism
What is sexual dimorphism?
The difference in appearance of males and females of the same species.
This can be seen through weaponry, ornamentation and behavior in courtship, and body size
