Chapter 23 - The Evolution of Populations Flashcards
The three main mechanisms that can cause changes in allele frequencies:
natural selection, genetic drift, and gene flow
Microevolution -
evolution on its smallest scale, shown in changes in allele frequencies in a population over generations
Genetic drift -
chance events that alter allele frequencies
Gene flow -
the transfer of alleles between populations
Natural selection/adaptation -
consistently improves the degree to which organisms are well suited for life in their environment
Genetic variation -
differences among individuals in the composition of their genes or other DNA sequences
Gene variability -
genetic variation at the whole-gene level can be quantified as the average percentage of loci that are heterozygous
Nucleotide variability -
genetic variation can be measured at the molecular level of DNA
Phenotype -
the product of an inherited genotype and many environmental influences
Genetic variation originates from
mutation, gene duplication, or other processes that produce new alleles and new genes
Mutation -
a change in the nucleotide sequence of an organism’s DNA
“heterozygote protection” -
recessive genes that can maintain a pool of alleles that may be harmful under present conditions, but that could be beneficial if the environment changes
Neutral variation -
differences in DNA sequence that do not confer a selective advantage or disadvantage
Sexual reproduction then shuffles existing alleles and deals them at random to produce individual genotypes.
Three mechanisms contribute to this shuffling:
crossing over, independent assortment of chromosomes, and fertilization
A population -
a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
The characterized genetic makeup of a population -
gene pool, consisting of all copies of every type of allele at every locus in all members of the population
“fixed in the gene pool”
If only one allele exists for a particular locus in a population, that allele is said to be _________ and all individuals are homozygous for that allele.
Hardy-Weinberg equilibrium -
a population that is not evolving (allele and genotype frequencies will remain constant from generation to generation) is said to be in
Condition 1 for Hardy-Weinberg equilibrium -
No mutations
Consequence if Condition Does Not Hold: The gene pool is modified if mutations occur or if entire genes are deleted or duplicated.
Condition 2 for Hardy-Weinberg equilibrium -
Random mating
Consequence if Condition Does Not Hold: If individuals mate within a subset of the population, such as near neighbors or close relatives (inbreeding), random mixing of gametes does not occur and genotype frequencies change.
Condition 3 for Hardy-Weinberg equilibrium -
No natural selection
Consequence if Condition Does Not Hold: Allele frequencies change when individuals with different genotypes show consistent differences in their survival or reproductive success.
Condition 4 for Hardy-Weinberg equilibrium -
Extremely large population size
Consequence if Condition Does Not Hold: In small populations, allele frequencies fluctuate by chance over time (genetic drift).
Condition 5 for Hardy-Weinberg equilibrium -
No gene flow
Consequence if Condition Does Not Hold: By moving alleles into or out of populations, gene flow can alter allele frequencies.
Hardy-Weinberg equation: q^2
frequency of homozygotes
Hardy-Weinberg equation: q
frequency of the recessive allele
New mutations (_____________) can alter allele frequencies, but because mutations are rare, the change from one generation to the next is likely to be very small.
violation of condition 1 (No mutations)
Nonrandom mating (____________) can affect the frequencies of homozygous and heterozygous genotypes but by itself has no effect on allele frequencies in the gene pool.
violation of condition 2 (Random mating)
The concept of natural selection is based on differential success in survival and reproduction:
Individuals in a population have different heritable traits so those with traits better suited to their environment tend to produce more offspring
Natural selection in genetic terms:
selection results in alleles being passed to the next generation in proportions that differ from the present generation
Adaptive evolution -
in which traits that enhance survival or reproduction tend to increase in frequency over time, typically caused by some alleles favored over others
Genetic drift
chance events causing allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations
The founder effect
When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population
The bottleneck effect
where a population passes through a “bottleneck” that greatly reduces its size
Genetic drift is 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 can cause allele frequencies to change at random:
Because of genetic drift, 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 random over time.
Genetic drift can lead to a 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 can cause harmful alleles to become fixed:
Alleles that are neither harmful nor beneficial can be lost or become fixed (reach a frequency of 100%) by chance through genetic drift. In very small populations, genetic drift can also cause alleles that are slightly harmful to become fixed. When this occurs, the population’s survival can be threatened (as in greater prairie chickens).
Gene flow -
the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes. Can also affect how well populations are adapted to local environmental conditions
Relative fitness -
the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals
Directional selection -
occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting a population’s frequency curve for the phenotypic character in one direction or the other
Disruptive selection -
occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes
Stabilizing selection -
acts against both extreme phenotypes and favors intermediate variants
Sexual selection -
a process in which individuals with certain inherited characteristics are more likely than other individuals of the same sex to obtain mates
Sexual dimorphism -
a difference in secondary sexual characteristics between males and females of the same species
Intrasexual selection -
meaning selection within the same sex, individuals of one sex compete directly for mates of the opposite sex. In many species, intrasexual selection occurs among males.
Intersexual selection (mate choice) -
where individuals of one sex (usually females) are choosy in selecting their mates from the other sex
Balancing selection -
this type of selection includes frequency-dependent selection and heterozygote advantage
Frequency-dependent selection -
the fitness of a phenotype depends on how common it is in the population
Heterozygote advantage
If individuals who are heterozygous at a particular locus have greater fitness than both kinds of homozygotes
