Chapter 23 Study Guide
Vocabulary:
Word Roots:
inter- = between
intra- = within
micro- = small
muta- = change
poly- = many
morph- = form
Key Terms:
Average heterozygosity: The percent, on average, of a population’s loci that are heterozygous in
members of the population.
Bottleneck effect : Genetic drift resulting from the reduction of a population, typically by a natural
disaster, such that the surviving population is no longer genetically representative of the original
population.
Cline: A graded variation in a trait that parallels a gradient in the environment.
Founder effect: Genetic drift that occurs when a few individuals become isolated from a larger
population, with the result that the new population’s gene pool is not reflective of the original
population.
Gene flow: Genetic additions to or subtractions from a population resulting from the movement of
fertile individuals or gametes.
Gene pool: The total aggregate of genes in a population at any one time.
Gene variability: The percentage of loci in a population that are heterozygotes
Genetic drift: Unpredictable fluctuations in allele frequencies from one generation to the next
because of a population’s finite size.
Geographic variation: Differences between the gene pools of separate populations or population
subgroups.
Hardy-Weinberg equilibrium: The condition describing a non-evolving population (one that is in
genetic equilibrium).
Intersexual selection: Selection whereby individuals of one sex (usually females) are choosy in
selecting their mates from individuals of the other sex; also called mate choice.
Intrasexual selection: A direct competition among individuals of one sex (usually the males in
vertebrates) for mates of the opposite sex.
Microevolution: Evolutionary change below the species level; change in the genetic makeup of a
population from generation to generation.
Mutation: A rare change in the DNA of a gene, ultimately creating genetic diversity.
Nucleotide variability: The amount of differences in a population of all nucleotides possibilities at
given gene loci.
Population: Localized group of individuals who are capable of interbreeding and producing fertile
offspring.
Relative fitness: The contribution of one genotype to the next generation compared to that of
alternative genotypes for the same locus.
Sexual dimorphism: A special case of differences between individuals based on the distinction
between the secondary sex characteristics of males and females.
Chapter 23 Important Points:
 Microevolution is the change in actual allele frequency in populations over generations
o Looks at only individual allele changes, not in overall speciation patterns
 Microevolution can occur by 3 mechanisms
1. Natural selection
2. Genetic drift
3. Gene flow
 Evolution requires populations have genetic variation which can be measured by:
o Gene variability of how many loci in a given population are heterozygotes
o Nucleotide variability which measures the overall difference in nucleotide
sequences between individuals in a population
o If an allele becomes fixed, there is only one version for that gene (i.e. all
homozygotes for that loci)
 Causes of genetic variation
o Geographic: Populations can differ because of geographic separation which is a
cause of geographic variation in genetic material
 Clines are gradual changes in geography that can cause gradual changes in
genetic differences of the individuals along the cline
o Mutation of single base pairs or larger segments of DNA repeats or chromosomal
organization
o Events in meiosis such as crossing over and independent assortment
o Random fertilization
 The Hardy Weinberg Equation allows us to determine if a population is evolving
o The theory assumes there are 2 alleles for a given loci so that a population in Hardy
Weinberg Equilibrium can be represented by the equation:
 p2+ 2pq + q2 = 1 with the frequency of p and q representing the allele
frequencies with p2 = the frequency of homozygotes of one allele, q2 is the
frequency of the other homozygotes and 2pq is frequency of the
heterozygotes
o For a population to be in Hardy Weinberg Equilibrium, no evolution is occurring so
this means:
 No mutations
 Random mating
 No natural selection
 Extremely large population size
 No gene flow
 Natural selection will cause adaptive changes in allele frequencies and thus evolution
 Genetic drift is the change in gene frequency caused by random chance events that have no
adaptive value in increasing or decreasing certain allele’s frequency
Genetic drift can often lead to a loss of genetic variation in a population as certain
alleles become fixed even if they do not have an adaptive advantage
o Two example of genetic drift caused by small populations
 Founder Effect: Relatively small group of individuals that break off from
original larger populations and may have a non-representative allele
frequency
 Bottleneck Effect: Dramatic and quick reduction in population size in which
certain alleles may be present that don’t represent the original allele
frequency
Gene flow can also alter allele frequencies in populations but allowing interchange of alleles
between populations
o Generally, gene flow reduces the genetic variation between populations
Relative fitness refers to how well an individual is able to contribute to the gene pool
compared to other individuals—with the assumption they will contribute MORE if are better
adapted
Sexual selection is a specific type of selection that favors mating success
o Could drive differences between the sexes called sexual dimorphism
o Intrasexual selection is competition within the same sex such as 2 males fighting for
dominance
o Intersexual selection involves mate choice when one gender selects his or her mate
based on certain characteristics
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