Chapter 6The Ways of Change: Drift and Selection Flashcards
what is the term for the study of the distribution of alleles in populations and causes of allele frequency changes?
Population genetics
__________: location of a specific gene or sequence of DNA on a chromosome
__________: individual carries two copies of the same allele
__________: individuals carries different alleles
Genetic locus
Homozygous
Heterozygous
Diploid individuals carry two alleles at every locus
__________: alleles are the same
__________: alleles are different
Evolution: __________________
Homozygous
Heterozygous
Evolution: change in allele frequencies from one generation to the next
Hardy-Weinberg equilibrium
Population allele frequencies do not change if:
5 things on population, genotypes, mutation, mating, and migration
Population is infinitely large
Genotypes do not differ in fitness
There is no mutation
Mating is random
There is no migration
Allele frequencies predict genotype frequencies
write down the Hardy-Weinberg equation
p2 + 2pq + q2 = 1
what does the Hardy-Weinberg theorem prove?
Mechanisms of evolution are forces that do what?
proves that that allele frequencies do not change in the absence of drift, selection, mutation, and migration
Mechanisms of evolution are forces that change allele frequencies
Populations Evolve Through a Variety of Mechanisms
analyze and review figure on powerpoint slide pg. 12
Key Concept
Hardy-Weinberg serves as the fundamental?
null model in population genetics
what does Bottlenecks reduce?
genetic variation
Key Concepts
____ ____ is the random, non-representative sampling of alleles from a population during breeding
________ are lost more ____ in ____ populations
Genetic drift is the random, non-representative sampling of alleles from a population during breeding
Alleles are lost more rapidly in small populations
Key Concept
Even brief bottlenecks can lead to a drastic reduction in genetic diversity that can persist for generations
The Concept of Fitness
Fitness: ________________
Components of fitness:
*
*
*
____ fitness: fitness of a genotype standardized by comparison to other genotypes
Fitness: the reproductive success of an individual with a particular phenotype
Components of fitness:
Survival to reproductive age
Mating success
Fecundity
Relative fitness: fitness of a genotype standardized by comparison to other genotypes
Contribution of Alleles to Fitness
_____ ____ _: difference between average fitness of individuals with allele vs. those without
average excess fitness
Natural selection more powerful in large populations
Drift is ____ in large populations
____ advantages in fitness can lead to ____ changes over the long term
Drift is weaker in large populations
Small advantages in fitness can lead to large changes over the long term
Pleiotropy May Constrain Evolution
Pleiotropy: ________________
Can be antagonistic
Net effect on fitness determines outcome of selection
mutation in a single gene affects many phenotypic traits
Natural Selection in Action
Alleles that lower fitness experience ______ ______
Alleles that increase fitness experience ______ ______
negative selection
positive selection
Dominance: Allele versus Allele
Relationships Among Alleles at a Locus
All alleles can be either ________ or ________ in terms of their effects on fitness
________: allele yields twice the phenotypic effect when two copies present
It can be good, or twice as good!
It can be bad, or twice as bad!
________: dominant allele masks presence of recessive in heterozygote
Recessive alleles can “________”
- negative or positive
- Additive
- Dominance
- “hide”
. Alleles with additive effects on phenotypes are always exposed to selection, so they will increase steadily from the moment they arise due to mutation until they are fixed in the population.
Recessive alleles are not exposed to selection initially, because they are likely to occur only in heterozygous genotypes. They may linger for thousands of generations until drift either removes them or increases their frequency. Eventually, if drift increases their frequency sufficiently, homozygous recessive individuals will begin to appear in the population. As soon as this happens, selection will begin to increase the frequency of the allele and swiftly carry it to fixation.
Dominant alleles are exposed to selection immediately and their frequency will increase rapidly. However, as dominant alleles become increasingly common, the alternative alleles (by definition, recessive) become increasingly rare. As we’ve just seen, rare recessive alleles are invisible to the action of selection because they are carried in a heterozygous state. Thus, selection alone is unlikely to fix a completely dominant allele.
Selection occurs when genotypes
differ in fitness
Outcome of selection depends on
frequency of allele and effects on fitness
Population size influences power of drift and selection
- Drift more powerful in _______
- Selection more powerful in ________
small population
large population
Alleles may have pleiotropic effects
- When fitness effects oppose each other ________ determines direction of selection
Laboratory evolution studies reveal how alleles rise and spread through populations
Rare alleles almost always carried in a heterozygous state
- Recessive alleles invisible to selection
- Selection cannot drive dominant to fixation
- ## environment
_______ are the source of new genetic variation in populations
- Can be many in a large population
mutations
Balancing selection maintains multiple alleles in populations
Negative frequency-dependent
Heterozygote advantage
Inbreeding increases percentage of loci that are homozygous for alleles identical by descent
Genetic bottlenecks often go hand in hand with inbreeding and selection
- Recessive alleles exposed to selection
The extent of population subdivision depends on features of the landscape as well as how readily individuals move between locations.
A: When landscapes are homogeneous and/or individuals move readily, populations have little or no subdivision.
B: At the other extreme, subpopulations may be completely isolated from each other, with almost no movement between them.
C: Many populations fall somewhere in between, such that movement between locations is less frequent than movement within but still occurs regularly enough to shuttle alleles from place to place.
______ ________ increases faster when subpopulations are small.
Genetic distance
Populations with extreme subdivision often show heterogeneity in allele frequencies from place to place called _____ reflecting divergence due to drift
genetic structure
_____ _____ enhances the effects of genetic drift
Divergence in allele frequencies
Population subdivision
Gene flow counteractions subdivision by
homogenizing allele frequencies
Δp = p x (aA1/ϖ)
list the formula pieces for what they represent
Δp = the change in allele frequency due to selection
p = the frequency of the A1 allele
ϖ = the average fitness of the population
aA1 =the average excess of fitness for the A1 allele
Equilibrium frequency reached through tug-of-war between negative selection and new mutation
Explains persistence of rare deleterious mutations in populations
Selection occurs when genotypes differ in fitness
Outcome of selection depends on frequency of allele and effects on fitness
Population size influences power of drift and selection
Drift more powerful in small population
Selection more powerful in large population
Thanks to genetic drift, isolated populations grow more distant from each other over time, as alleles are randomly fixed or lost from place to place
Rate changes dependent on size of population
Populations with extreme subdivision often show heterogeneity in allele frequencies from place to place, called genetic structure, reflecting divergence due to drift. Selection also can drive subpopulations apart. However, since most genetic markers are selectively neutral, spatial patterns in their relative frequencies reflect drift rather than selection.
Over the same landscape, species may differ in the extent of population subdivision due to their habitat preferences and behavior. Lynx, for example, move widely; as a result, their populations show almost no genetic divergence from place to place. Populations of Bighorn Sheep, in contrast, show much more extensive genetic divergence across the same distances.
