Population genetics
57 flashcards covering Population genetics for the MCAT Biology & Biochemistry section.
Population genetics is the study of how genetic variations, such as allele frequencies, are distributed and change within groups of organisms over time. It explores factors like natural selection, genetic drift, mutation, and gene flow that drive evolution, helping us understand how populations adapt and diversify. This field bridges genetics and ecology, providing insights into real-world phenomena like disease spread and species conservation.
On the MCAT, population genetics appears in Biology & Biochemistry questions that test your ability to calculate allele and genotype frequencies, often using the Hardy-Weinberg equilibrium model. Common traps include overlooking assumptions like no mutation or random mating, which can lead to incorrect applications in passage-based scenarios. Focus on interpreting data graphs, recognizing evolutionary forces, and practicing problem-solving to handle these questions effectively.
A concrete tip: Always verify if a population meets Hardy-Weinberg conditions before performing calculations.
Terms (57)
- 01
Gene pool
The complete set of genetic information in a population, including all alleles for all genes.
- 02
Allele frequency
The proportion of a particular allele among all alleles for a gene in a population, often calculated as the number of that allele divided by the total number of alleles.
- 03
Genotype frequency
The proportion of individuals in a population that have a specific genotype for a gene.
- 04
Phenotype frequency
The proportion of individuals in a population that exhibit a specific observable characteristic resulting from their genotypes and environment.
- 05
Hardy-Weinberg equilibrium
A principle stating that allele and genotype frequencies in a population will remain constant from generation to generation if certain conditions are met, such as no mutation, no selection, and random mating.
- 06
Hardy-Weinberg equation
The formula p² + 2pq + q² = 1, where p is the frequency of one allele and q is the frequency of the other allele for a gene with two alleles, used to calculate genotype frequencies.
- 07
Conditions for Hardy-Weinberg equilibrium
The five requirements for a population to maintain constant allele frequencies: no mutations, no gene flow, large population size, random mating, and no natural selection.
- 08
Genetic drift
Random changes in allele frequencies due to chance events, especially in small populations, which can lead to loss of genetic variation over time.
- 09
Founder effect
A type of genetic drift where a new population is established by a small group of individuals, resulting in reduced genetic variation compared to the original population.
- 10
Bottleneck effect
A severe reduction in population size due to environmental events, leading to random changes in allele frequencies and decreased genetic diversity.
- 11
Gene flow
The transfer of alleles between populations through migration, which can increase genetic variation within a population.
- 12
Mutation
A change in DNA sequence that introduces new alleles into a population, serving as the ultimate source of genetic variation.
- 13
Natural selection
The process by which individuals with certain heritable traits that improve survival and reproduction are more likely to pass on those traits, changing allele frequencies over time.
- 14
Directional selection
A form of natural selection that favors individuals at one extreme of a trait's variation, shifting the population's trait distribution in that direction.
- 15
Stabilizing selection
Natural selection that favors individuals with intermediate traits, reducing variation and maintaining the population around an optimal phenotype.
- 16
Disruptive selection
Natural selection that favors individuals at both extremes of a trait's variation, potentially leading to speciation by increasing diversity.
- 17
Polymorphism
The presence of two or more distinct alleles for a gene in a population, where the least common allele has a frequency of at least 1%.
- 18
Balanced polymorphism
A stable polymorphism maintained by natural selection, often due to heterozygous advantage or frequency-dependent selection.
- 19
Heterozygote advantage
A situation where heterozygous individuals have higher fitness than either homozygous genotype, maintaining genetic variation in the population.
- 20
Inbreeding
Mating between closely related individuals, which increases homozygosity and can expose deleterious recessive alleles.
- 21
Inbreeding depression
The reduced fitness in a population due to increased homozygosity from inbreeding, often resulting in lower survival or fertility.
- 22
Assortative mating
Non-random mating where individuals with similar phenotypes or genotypes are more likely to mate, affecting genotype frequencies.
- 23
Random mating
A condition in population genetics where individuals pair by chance, without regard to genotype or phenotype.
- 24
Effective population size
The number of individuals in a population that contribute offspring to the next generation, often smaller than the actual population size due to factors like variance in reproductive success.
- 25
Fixation
The process by which an allele reaches a frequency of 100% in a population, eliminating all other alleles for that gene.
- 26
Genetic variation
The diversity of alleles and genotypes in a population, which is the raw material for evolution.
- 27
Selection coefficient
A measure of the strength of natural selection against a particular allele, denoted as s, where s = 1 minus the relative fitness of the allele.
- 28
Relative fitness
The proportional contribution of a genotype to the next generation compared to other genotypes, used to quantify the effects of selection.
- 29
Migration rate
The frequency at which individuals move between populations, influencing gene flow and allele frequencies.
- 30
Mutation rate
The probability per generation that a specific allele will mutate, contributing to new genetic variation.
- 31
Change in allele frequency due to selection
The formula Δp = spq / (1 - sq), where p and q are allele frequencies, and s is the selection coefficient, showing how selection alters frequencies.
- 32
Change in allele frequency due to mutation
The formula Δp = μq, where μ is the mutation rate from q to p, indicating how mutations introduce new alleles.
- 33
Change in allele frequency due to genetic drift
In small populations, allele frequencies change randomly each generation, modeled by the binomial sampling of gametes.
- 34
Chi-square test for Hardy-Weinberg
A statistical test used to determine if observed genotype frequencies in a population deviate significantly from those expected under Hardy-Weinberg equilibrium.
- 35
Neutral allele
An allele that does not affect fitness, so its frequency changes only due to genetic drift rather than natural selection.
- 36
Linkage disequilibrium
The non-random association of alleles at different loci, which can be broken down by recombination over generations.
- 37
Population stratification
The presence of subgroups within a population with different allele frequencies, which can confound genetic studies.
- 38
Evolutionary equilibrium
A state where allele frequencies do not change over time because the forces acting on them are balanced.
- 39
Frequency-dependent selection
Natural selection where the fitness of a genotype depends on its frequency in the population, often maintaining polymorphism.
- 40
Overdominance
A type of selection where heterozygotes have higher fitness than both homozygotes, leading to balanced polymorphism.
- 41
Underdominance
A situation where heterozygotes have lower fitness than homozygotes, which can lead to fixation of one allele.
- 42
Generational time
The average time between generations in a population, which affects the rate of evolutionary change.
- 43
Allelic substitution rate
The rate at which one allele replaces another in a population, often measured in substitutions per generation.
- 44
Wright's fixation index
A measure of inbreeding or population subdivision, calculated as F = 1 - (observed heterozygosity / expected heterozygosity).
- 45
Expected heterozygosity
The predicted proportion of heterozygous individuals in a population under Hardy-Weinberg equilibrium, calculated as 2pq.
- 46
Observed heterozygosity
The actual proportion of heterozygous individuals in a population, which can be compared to the expected value to assess deviations.
- 47
Coalescence
The process of tracing lineages backward in time to a common ancestor, used in population genetics to study allele histories.
- 48
Demographic stochasticity
Random variation in birth, death, and reproduction events that affects allele frequencies, especially in small populations.
- 49
Balancing selection
Any form of natural selection that maintains genetic variation, such as heterozygote advantage or frequency-dependent selection.
- 50
Purifying selection
Natural selection that removes deleterious alleles from a population, reducing genetic variation at affected loci.
- 51
Positive selection
Natural selection that increases the frequency of beneficial alleles, driving adaptation in populations.
- 52
Strategy for calculating Hardy-Weinberg
To calculate genotype frequencies, first determine allele frequencies from population data, then use the equation p² for homozygotes and 2pq for heterozygotes.
- 53
Common trap in genetic drift
Assuming genetic drift is negligible in large populations, but it can still occur, though at a slower rate, leading to mispredictions of evolutionary outcomes.
- 54
Example of directional selection
In a population of finches, birds with larger beaks survive better during droughts, increasing the frequency of the large-beak allele over generations.
Galapagos finches during dry periods show this, as observed by Darwin.
- 55
Worked example for Hardy-Weinberg
If allele A has frequency 0.7, then AA genotype frequency is (0.7)² = 0.49, Aa is 2(0.7)(0.3) = 0.42, and aa is (0.3)² = 0.09.
- 56
Impact of gene flow on variation
Gene flow increases genetic variation by introducing new alleles, potentially preventing populations from diverging into separate species.
- 57
Mutation-selection balance
The equilibrium where the rate of new deleterious mutations equals the rate of their removal by selection, maintaining a steady frequency of such alleles.