AP Biology Chapter 23 Notes
The Evolution of Populations
What you need to know…
 How ____________________ and ____________________ reproduction each produce genetic
__________________.
 The conditions for ______________________________________ equilibrium.
 How to use the Hardy-Weinberg equation to calculate allelic ________________________ and to test
whether a population is _______________________.
_______________________________ - change in allele frequencies over generations
Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible
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Two processes produce the variation in gene pools that contributes to differences among individuals
o ____________________________________
o ____________________________________ reproduction
_________________________ characters can be classified on an either-or basis
_________________________ characters vary along a continuum within a population
___________________________________________ measures the average percent of loci that are
heterozygous in a population
Most species exhibit _______________________________ variation, differences between gene pools of
separate populations or population subgroups
Some examples of geographic variation occur as a ____________________, which is a graded change in a
trait along a geographic axis
_______________________ are changes in the nucleotide sequence of DNA
o Mutations cause ____________ genes and alleles to arise
o Only mutations in cells that produce gametes can be ____________ to offspring
o A ______________ mutation is a change in one base in a gene
 The effects of point mutations can vary:
 in _____________________ regions of DNA are often harmless
 in a _____________ might not affect protein production because of redundancy in the
genetic code
 Mutations that result in a change in protein production are often _______________
 Mutations that result in a change in protein production can sometimes
_________________ the fitness between organism and environment
o Mutation _______________ are low in animals and plants
 The average is about one mutation in every 100,000 genes per generation
o Mutations __________________ are often lower in prokaryotes and higher in viruses
Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
 A ___________________________ is a localized group of individuals capable of interbreeding and
producing fertile offspring
 A _______________ pool consists of all the alleles for all loci in a population
 A _________________ is fixed if all individuals in a population are homozygous for the same allele
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The frequency of an allele in a population can be calculated
o For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2
o The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant
individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles
o If there are 2 alleles at a locus, p and q are used to represent their frequencies the frequency of all
alleles in a population will add up to 1
 For example______________________________________
The ___________________________________ principle describes a population that is not evolving
o If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that
the population is evolving
The ______________________________________ principle states that frequencies of alleles and
genotypes in a population remain constant from generation to generation
o _______________________________________
o where _________ and _________represent the frequencies of the homozygous genotypes and
________ represents the frequency of the heterozygous genotype
The five conditions for nonevolving populations are rarely met in nature:
o No ________________
o ____________________ mating
o No _____________________________________
o Extremely ________________ population size
o No _______________ flow
Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
 Three major factors alter allele frequencies and bring about most evolutionary change:
o ________________________________
o ________________________________
o ________________________________
 Genetic ___________________ describes how allele frequencies fluctuate unpredictably from one
generation to the next
o Genetic drift tends to ____________________ genetic variation through losses of alleles
o The _________________________ effect occurs when a few individuals become isolated from a
larger population
 Allele frequencies in the small founder population can be ______________________ from
those in the larger parent population
o The __________________________ effect is a sudden reduction in population size due to a change
in the environment
 The resulting gene pool may no longer be reflective of the original population’s gene pool
 If the population remains ________________, it may be further affected by genetic drift
o Effects of Genetic Drift: A Summary
 Genetic drift is significant in ____________________ populations
 Genetic drift causes ______________________ frequencies to change at
_________________
 Genetic drift can lead to a ____________________ of genetic variation within populations
 Genetic drift can cause ________________________ alleles to become fixed
 Gene ________________ consists of the movement of alleles among populations
o Alleles can be transferred through the movement of fertile individuals or gametes (for example,
pollen)
o Gene flow tends to _________________ differences between populations over time
o Gene flow is more likely than mutation to _______________ allele frequencies directly
o Gene flow can ____________________ the fitness of a population
Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution
 Only natural selection consistently results in _________________________ evolution
 Relative _______________________ is the contribution an individual makes to the gene pool of the next
generation, relative to the contributions of other individuals
 Selection favors certain _______________________ by acting on the _________________________ of
certain organisms
 Three modes of selection:
o __________________________ selection favors individuals at one end of the phenotypic range
o _________________________ selection favors individuals at both extremes of the phenotypic range
o ___________________ selection favors intermediate variants and acts against extreme phenotypes
 Label the picture
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____________________ selection is natural selection for mating success
o It can result in sexual ________________________, marked differences between the sexes in
secondary sexual characteristics
______________________ selection is competition among individuals of one sex (often males) for mates
of the opposite sex
______________________ selection, often called mate choice, occurs when individuals of one sex (usually
females) are choosy in selecting their mates
______________________ selection occurs when natural selection maintains stable frequencies of two or
more phenotypic forms in a population
_____________________________ advantage occurs when heterozygotes have a higher fitness than do
both homozygotes
o Natural selection will tend to ______________________ two or more alleles at that locus
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o The sickle-cell allele causes mutations in _____________________ but also confers malaria
resistance
In ________________________________ selection, the fitness of a phenotype declines if it becomes too
common in the population
o Selection can favor whichever phenotype is ___________ common in a population
___________________ variation is genetic variation that appears to confer no selective advantage or
disadvantage
o For example - Variation in ___________________ regions of DNA; Variation in proteins that have
little effect on protein function or reproductive fitness
Why Natural Selection Cannot Fashion Perfect Organisms
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Selection can _______ only on existing variations
Evolution is _________________ by historical constraints
Adaptations are often ____________________________
Chance, natural selection, and the environment _________________________
You should now be able to…
1. Explain why the majority of ______________ mutations are harmless
2. Explain how ________________ recombination generates genetic variability
3. Define the terms population, species, gene pool, relative fitness, and neutral variation
4. List the ______________ conditions of Hardy-Weinberg equilibrium
5. Apply the Hardy-Weinberg equation to a population genetics problem
6. Explain why natural selection is the ____________ mechanism that consistently produces adaptive change
7. Explain the ___________ of population size in genetic drift
8. Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual
and intersexual selection
9. List ____________ reasons why natural selection cannot produce perfect organisms