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Ch. 21: The Evolution of Populations
The Smallest Unit of Evolution
- Natural selection acts on ___________________________
- Each organism’s trait affects its _________________________and ____________________
success compared with that of other individuals
- Evolutionary impact of natural selection is only apparent in the changes in a ______________________
of organisms through time
- _____________________________ evolve…. NOT its individual members
- _______________________________
- Evolution on a _________ scale is a change in allele frequencies in a population over generations
- Three main mechanisms that can cause ______________________ frequency to change
- ____________________________
- ____________________________: change events that alter allele frequencies
- ____________________________: the transfer of alleles between populations
21.1 Genetic variation makes evolution possible
- Darwin observed that individuals differ in their _________________________ traits and that selection
acts on such differences, leading to evolutionary change
- Genetic variation at the whole gene level can be quantified as the average _______________________
of loci that are heterozygous
- Some phenotypic variation is not ______________________________
- Only the genetically determined part of phenotypic variation can have ____________________
consequences
- Genetic variation provides the raw material for evolutionary ____________________: Without genetic
variations, evolution cannot occur
Sources of Genetic Variation
- Originates when mutation, gene duplication, or other processes produce new alleles and new genes
- Can be produced in short periods of time in organisms that reproduce __________________
- Sexual reproduction can also result in variation as existing genes are arranged in new ways
Formation of New Alleles
- New alleles can arise by ____________________________________
- In most animals, the majority of mutations occur in somatic cells and are lost when the individual dies
- Point mutation, a change in one base in a gene, can have a significant impact on phenotype
- Organisms reflect many __________________________ of past selection so their phenotypes
Ch. 21: The Evolution of Populations
Genetic Variation
- Individuals within a species ___________ in their specific ____________________________
- Phenotypic variations often reflect genetic variations
- _________________________ variation: differences among individuals in the composition of
their genes or other DNA sequences
- Some heritable phenotypic differences occur on an ‘either-or’ basis
- Typically determined by a __________________ gene locus with different alleles
producing distinct phenotypes
- Some phenotypic differences vary in gradations along a continuum
- Typically results from the influence of two or more genes on a single phenotypic
character
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tend to be well matched to their environment
- Unlikely that a new mutation that alters phenotype will __________________organism
- Most mutations are at least slightly __________________________
- Point mutations in non-coding regions are generally harmless
- Redundancy in the genetic code
- May not change protein __________________ and _________________________
Altering Gene Number or Position
- Chromosomal changes that ______________, disrupt, or rearrange many loci at once are usually harmful
- Such large scale changes that leave the gene intact may not affect the organisms’ phenotype
- A key potential source of variation is the duplication of genes due to errors in __________________,
slippage during DNA replication, or the activities of ______________________________ elements
- Gene duplications that do not have severe effects can persist over generations, allowing mutations to
______________________________
- Result: an expanded genome with new genes that may take on new ____________________________
Sexual Reproduction
- Most of the genetic variation in a population results from the unique ___________________________
of alleles that each individual receives from its parents
- Existing alleles are __________________________and dealt at random to produce individual genotypes
- Three mechanisms contribute to this shuffling:
1. ______________________________________
2. _____________________________________________ of chromosomes
3. ______________________________________
- The combined effects of these 3 mechanisms ensure that sexual reproduction rearranges existing alleles
into fresh combinations each generation, providing much of the genetic variation that makes
________________________________ possible
Gene Pools and Allele Frequencies
- ____________________________: a group of individuals of the same species that live in the same area
and interbreed, producing fertile offspring
- Members of a population typically breed with one another and thus on average are more
______________________related to each other than to members of other populations
- Population's genetic makeup can be described by its ____________________________________
- Gene pool: all the copies of every type of allele at every locus in all members of the population
- If only one allele exists for a particular locus in a population.. the allele is ____________
- If there are two or more alleles for a particular locus in a population, individuals may be
either _______________________________ or ________________________________
- Each allele has a ______________________________in the population
- When studying a locus with two alleles ___ & ___ are used to represent the frequency of each allele
The Hardy-Weinberg Principle
- Assess whether natural selection or other factors are causing evolution at a particular locus by
determining what the genetic makeup of a population would be if it were not evolving at that locus
- Compare the scenario with the ______________________ data from the population
- No differences: population is _________ evolving
- _______________________: population is evolving
Ch. 21: The Evolution of Populations
21.2 The Hardy-Weinberg equation can be used to test whether a population is evolving
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The Hardy-Weinberg Equilibrium
- The gene pool of a population that is ______ evolving can be described by the Hardy-Weinberg Principle
- States: Frequencies of alleles and genotypes in a population will remain ____________________
from generation to generation, provided that only Mendelian segregation and recombination of
alleles are at work
- This is the Hardy-Weinberg _______________________________
- Principle considers the combination of alleles in all ______________________ in a population
- Reproduction occurs be selecting alleles at ____________________
- Viewing as a process of randomly selecting and combining alleles from , we are in effect
assuming that mating occurs at random (all male-female matings are equally likely)
- Genotype frequencies in the next generation must add up to ___ (100%)
- A locus with two alleles, the three genotypes will appear in the following proportions
p2 + 2pq + q2 = 1
- Sum of the frequencies of the three genotypes must _____________ 1 (100%) in any population
regardless of whether the population is in HWE
- Population is in HWE only if the genotype frequencies are such that the actual
frequency of one homozygous is p2, the actual frequency of the other homozygote is q2, and the
actual frequency of heterozygous is 2pq
Conditions for Hardy-Weinberg Equilibrium
- Describes a hypothetical population that is not evolving
- Real populations do change over time when at least one of the following five conditions of the
Hardy-Weinberg equilibrium is not met:
1. _______________________________________________________
2. _______________________________________________________
3. _______________________________________________________
4. _______________________________________________________
5. _______________________________________________________
- Departure from these conditions usually results in ___________________________ change
- Can be common for natural populations to be in Hardy-Weinberg equilibrium for specific genes
- Populations can be evolving at some loci, yet simultaneously in HWE at other loci
21.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
- A _____________________________from any of the 5 HWE conditions is a potential cause of evolution
- Three mechanisms that alter allele frequencies directly and cause most evolutionary change:
______________________________________________________________________________
Natural Selection
- Concept based on differential ____________________ in survival and reproduction
- Individuals in a population exhibit variations in their ___________________________ traits
- Those with traits that are better suited to their environment tend to produce more
____________________________ than those with traits that are not as well suited
- _______________________ results in alleles being passed to the next generation in proportions that
differ from those in the present generation
Ch. 21: The Evolution of Populations
Applying the Hardy-Weinberg Principle
- Hardy-Weinberg equation is often used as an initial __________ of whether evolution is occurring in a
population
- Yields an _____________________________________; real numbers may differ
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- By consistently favoring some alleles over others, natural selection can cause adaptive evolution
Genetic Drift
- Chance events can cause allele frequencies to fluctuate ____________________________ from one
generation to the next, especially in a small population
- Unpredictable changes in allele frequencies can be caused by chance events associated with
_____________________________and _____________________________________
- Allele frequencies can also be affected by chance events that occur during __________________
- Two examples are the ____________________________ and the ________________________________
The Founder Effect
- A few individuals become _________________ from a larger population, this smaller group may
establish a new population whose gene pool _________________ from the source population
The Bottleneck Effect
- A _________________ change in the environment that may drastically reduce the size of the population
- By chance alone, certain alleles may be overrepresented, and some may be absent altogether
- Ongoing genetic drift is likely to have ______________________ effects on the gene pool until the
population becomes large enough that chance events have less impact
- Even when a population has passed through the bottleneck and recovers in size, it may have ________
levels of genetic variation for a long period of time - a ___________of the genetic drift that occurred
when the population was small
Gene Flow
- Allele frequencies can change by ___________________________
- Gene flow: the __________________ of alleles into or out of a population due to the movement
of fertile individuals or their ________________________
- Alleles are transferred between _________________________therefore gene flow
tends to reduce the genetic differences between populations
- If extensive enough, gene flow can result in ______ populations __________________
into a single population with a common gene pool
- Alleles transferred by gene flow can also affect how well populations are ___________________ to local
environmental conditions
- Can transfer alleles that _______________ the ability of populations to adapt to local conditions
- Gene flow has become an increasingly important agent of evolutionary change in human populations
- __________________ move much more freely about the world today then they did in the past
- ______________________ is more common between members of populations that
previously had very little contact, leading to an exchange of alleles and few genetic
differences between those populations
21.4 Natural selection is the only mechanism that consistently causes adaptive evolution
- Natural selection consistently _____________________ the frequencies of alleles that provide
reproductive advantage and thus leads to adaptive evolution
Ch. 21: The Evolution of Populations
Effects of Genetic Drift: A Summary
1. Genetic drift is significant in ____________________populations
2. Genetic drift can cause allele frequencies to change at _______________________
3. Genetic drift can lead to a _____________________of genetic variation within populations
4. Genetic drift can cause ____________________ alleles to become ____________________
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Natural Selection: A Closer Look
Relative Fitness
- Relative ____________________: the contribution an individual makes to the gene pool of the next
generation relative to the contributions of other _____________________________
- Selection acts more ________________ on the phenotype than on the genotype; its acts on the
genotype _______________________, via how the genotype affects the phenotype
Directional, Disruptive, and Stabilizing Selection
- Natural selection can occur in three ways
- ______________________________________________________________________________
- Directional selection: occurs when conditions favor individuals at one extreme of a phenotypic range,
thereby shifting a population’s frequency curve for the phenotype character in one direction or the other
- Common when a population’s environment ____________________ or when members of a
population _________________________ to a different habitat
- _______________________ selection: occurs when conditions favor individuals at both extremes of a
phenotypic range over individuals with _________________________ phenotypes
- Stabilizing selection: acts _______________ both extreme phenotypes and favors intermediate variants
- This selection reduces variation and tends to maintain the status quo for a particular phenotypic
Sexual Selection
- A form of natural selection in which individuals with certain _______________________ characteristics
are more likely than other individuals to obtain ___________________
- Can result in sexual __________________________: a difference in secondary sexual
characteristics between males and females of the same _______________
- Distinctions include differences in size, color, ornamentation, and behavior
- Intrasexual selection: selection within the same sex, individuals of one sex __________________ directly
for mates of the opposite sex
- Ex: Males fighting other males for females
- Intersexual selection: (mate choice) individuals of one sex are _________________ in selecting their
mates from the other sex
- Ex: Females choose males based off of characteristics
Ch. 21: The Evolution of Populations
The Key Role of Natural Selection in Adaptive Evolution
- Adaptations can arise ______________ over time as natural selection ___________________ the
frequencies of alleles that enhance survival and reproduction
- As the proportion of individuals that have favorable traits ______________________, the match
between a species and its environment improves; that is, __________________ evolution occurs
- Physical and biological components of an organism’s environment may change over time
therefore what constitutes a ‘__________________________’ between organism and
environment is constantly changing making adaptive evolution a continuous, dynamic process
- Genetic drift and gene flow can ________________________ the frequencies of alleles that improve the
match between organisms and their environment, but neither does so consistently
- Genetic drift can cause the frequency of a slightly beneficial allele to __________________, but
can also cause it to ___________________________
- Similarly, it can _________________advantageous or ones that are disadvantageous
- Natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution
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The Preservation of Genetic Variation
- Some of the genetic variation in populations represents ________________________ variation
- Neutral variation: differences in DNA sequence that do not confer a __________________
advantage or disadvantage
- Tendency for directional and stabilizing selection to ______________ variation is countered by ]
mechanisms that _________________ or restore it, such as diploidy and balancing selection
Diploidy
- Diploid organisms have a considerable amount of genetic variation hidden from selection in the form of
_______________________ alleles
- Recessive alleles that are less favorable than their dominant counterparts or even harmful in the
current environment can persist by propagation in heterozygous individuals
- Variation is exposed to natural selection only when ____________ parents carry the same
recessive allele and two copies end up in the same zygote
- Happens only rarely if the frequency of the recessive allele is __________
- Heterozygous protection maintains a huge pool of _____________ that might not be
favored under present conditions, but which could bring new benefits if the
environment changes
Balancing Selection
- Occurs when natural selection maintains two or more forms in a population
- This type of selection includes heterozygote advantage and frequency-dependent selection
Heterozygote Advantage
- Individuals who are heterozygous at a particular locus that have a greater fitness than both kinds of
___________________________________
- Advantage defined in terms of genotype, not phenotype
- Whether heterozygote advantage represents stabilizing or directional selection depends on the
relationship between the ___________________ and the phenotype
Why Natural Selection Cannot Fashion Perfect Organisms
- Though natural selection leads to __________________, nature abounds with examples of organisms
that are less than ideally suited for their lifestyles
1. Selection can act only on _________________________ variations
- New advantageous alleles do not arise on demand
2. Evolution is limited by historical constraints
- Evolution operates on the traits an organism already has
3. _______________________________________________________________________
4. Chance, natural selection, and the environment interact
- No t all alleles present in the founding population’s gene pool are better suited to the
new environment than the alleles that are ‘left-behind’
- The environment at a particular location may change unpredictably from year to
year, again limiting the extent to which adaptive evolution results in a close match
between the organism and current environmental conditions
- With these four constraints, evolution does not tend to craft perfect organisms
- ___________________________________operates on a ‘better than’ basis
Ch. 21: The Evolution of Populations
Frequency-Dependent Selection
- The _________________________ of a phenotype depends on how common it is in the population
Ex: Left mouth/Right mouth fish
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