Terms in this set (45)
Change in allele frequencies in a population over generations
3 Mechanisms that cause allele frequency change
chance events that alter allele frequencies
Transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes
Differences among individuals in the composition of their genes or other DNA segments
Only the genetically determined part of phenotypic variation can have evolutionary consequences.
Provides the raw material for evolutionary change: without genetic variation, evolution cannot occur.
Can be classified on an either-or basis
Determined by a single gene locus with different alleles that produce distinct phenotypes
Vary along a continuum within a population
Results from the influence of 2 or more genes on a single phenotypic character.
Study of how populations change over time
Total genetic compliment of and individual organism
All alleles at all gene loci in all individuals of the population
Consist of all copies of every type of allele at every locus in all members of the population
Changes in the gene pool of the population, not the genome of the individual
Group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
Gene pool of a population that is not evolving
The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work
p2 + 2pq + q2 = 1
p2: Expected frequency of genotype
q2: Expected frequency of genotype
Conditions for Hardy-Weinberg Equilibrium
No natural selection
Extremely large population size
No gene flow
Hardy-Weinberg: No Mutations
The gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated
Hardy-Weinberg: Random Mating
If individuals mate preferentially within a subset of the population, such as their close relatives (inbreeding), random mixing of gametes does not occur, and genotype frequencies change.
Hardy-Weinberg: No Natural Selection
Differences in the survival and reproductive success of individuals carrying different genotypes can alter allele frequencies
Hardy-Weinberg: Extremely Large Population
The smaller the population, the more likely it is that allele frequencies will fluctuate by chance from one generation to the next (a process called genetic drift).
Hardy-Weinberg: No Gene Flow
By moving alleles into or out of populations, gene flow can alter allele frequencies
Can populations have Hardy-Weinberg equilibrium for specific genes?
Yes - This apparent contradiction occurs because a population can be evolving at some loci, yet simultaneously be in Hardy-Weinberg equilibrium at other loci
Why is the Hardy-Weinberg equation used?
As an initial test of whether evolution is occurring in a population
Also has medical applications, such as estimating the percentage of a population carrying the allele for an inherited disease
Hardy-Weinberg Equation - PKU Ex
Condition 1: No new PKU mutations are being introduced into the population
Condition 2: People neither choose their mates on the basis of whether or not they carry this gene
Condition 3: Ignore any effects of differential survival and reproductive success among PKU genotypes
Condition 4: Assume that there are no effects of genetic drift
Condition 5: No effects of gene flow from other populations into the United States
Condition 6: US has a large population
Few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population
Accounts for the relatively high frequency of certain inherited disorders among isolated human populations
A sudden change in environment that may drastically reduce the size of a population (floods, fire)
Severe drop in population size
Alleles may be: Overrepresented, Underrepresented, Absent
Genetic Drift - Key Points
1. Significant in small populations
2. Cause allele frequencies to change at random
3. Lead to a loss of genetic variation within populations
4. Cause harmful alleles to become fixed
Genetic Drift: Significant in small populations
Chance events can cause an allele to be disproportionately over or underrepresented in the next generation. Although chance events occur in populations of all sizes, they tend to alter allele frequencies substantially only in small populations.
Genetic Drift: Allele frequencies to change at random
An allele may increase in frequency one year, then decrease the next; the change from year to year is not predictable. Thus, unlike natural selection, which in a given environment consistently favors some alleles over others, genetic drift causes allele frequencies to change at randoom over time.
Genetic Drift: Loss of genetic variation within populations
By causing allele frequencies to fluctuate randomly over time, genetic drift can eliminate alleles from a population. Because evolution depends on genetic variation, such losses can influence how effectively a population can adapt to a change in the environment.
Genetic Drift: Cause harmful alleles to become fixed
Alleles that are neither harmful nor beneficial can be lost or become fixed entirely by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occur's, the population's survival can be threatened.
Contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals
3 Modes of Selection
Directional, Disruptive, Stabilizing
Occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, shifting a population's frequency curve for the phenotypic character in one direction of the other.
Common when a population's environment changes or when a members of a population migrate to a new (and different) habitat.
Occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
Acts against both extreme phenotypes and favors intermediate variants.
Reduces variation and tends to maintain the status quo for a particular phenotypic character.
Form of selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates.
Difference between the 2 sexes in secondary sexual characteristics
Size, color, ornamentation, behavior
Selection within the same sex, individuals of one sex compete directly for mates of the opposite sex
Common is females
Individuals of one sex are choosy in selecting their mates from the other sex
Females choice depends on the showiness of the male's appearance or behavior
Preservation of Genetic Variation
Differences in DNA sequence that do not confer a selective advantage or disadvantage
Genetic variation hidden as recessive alleles in heterozygotes
Occurs when natural selection maintains 2 or more forms in a population
Individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes
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