Download Chapter 23AP Biology

The Evolution of Populations
The Smallest Unit of Evolution
 Microevolution – the smallest unit of evolutionary
change in populations. The changes in allele
frequencies in a population over generations.
 There are three main mechanisms that can cause allele
frequency change:
 Natural selection
 Gene flow
 Genetic drift
Genetic Variation
 Darwin provided evidence on how life changed over
time. He knew that natural selection was the primary
mechanism for change in a species population.
 Darwin could not explain how organisms pass
heritable traits to their offspring.
 Gregor Mendel – proposed that organisms transmit
discrete heritable units (genes) to their offspring.
 Worked with pea plants to show inheritance of genes.
 Genetic differences are produced by:
 Mutation
 Sexual reproduction
Variation Within a Population
 Characters that vary within a population may be
discrete or quantitative.
 Discrete characters – can be classified on an eitheror basis, and are determined by a single gene locus
with different alleles that produce distinct phenotypes.
 Quantitative characters – vary along a continuum
within a population. Usually results from the
influence of two or more genes on a single phenotypic
character.
Discrete and Quantitative
Characters
Variation within a Population
 Average heterozygosity – the average percent of loci
that are heterozygous.
 Average heterozygosity is usually estimated by
surveying the protein products of genes using gel
electrophoresis.
Variation Between Populations
 Besides observing variation in a population, species
also exhibit geographic variation.
 Geographic variation- the differences in the genetic
composition of separate populations.
 When isolated from one another, populations can
evolve independently from the other population
resulting in genetic variations from chance events
(drift) rather than natural selection.
Geographic Variation
Variation Between Populations
 Cline – a graded change in a character along a
geographic axis.
 Some clines are produced by a gradation in an
environmental variable, as illustrated by the impact of
temperature on the frequency of cold-adaptive allele in
mummichog fish.
 Clines usually result from natural selection.
A Cline Determined by
Temperature
Mutation
 Mutations are the ultimate source of new alleles.
 Mutation – a change in the nucleotide sequence of an
organism’s DNA.
 It cannot be predicted which segments of DNA will be
altered.
 In multicellular organisms, only mutations in cell lines that
produce gametes can be passed to offspring.
 Most mutations occur in somatic cells and are lost when the
organism dies.
 Point mutations – a change of as little as one base in a
gene. Can have significant impact on phenotypes.
 Sickle-cell disease
Mutations that alter Gene Number
or Sequence
 Chromosomal changes that delete, disrupt, or
rearrange many loci at once are almost certain to be
harmful.
 When these large-scale mutations leave genes intact,
their effects on organisms may be neutral.
 Sometimes these mutations can be beneficial.
 A translocation of part of one chromosome to a
different chromosome could link DNA segments in a
way that results in a positive effect.
Mutation Rates
 Mutation rates are really low in plants and animals and
even lower in prokaryotes.
 One every 100,000 genes per generation.
 However, prokaryotes have short generation spans, so
mutations can quickly generate genetic variation in a
population.
Sexual Reproduction
 Sexual reproduction produces the most genetic
variation in a population that reproduces sexually.
 Sexual reproduction shuffles the existing alleles and
deals them at random to determine individual
genotypes.
 Shuffling is due to three mechanisms:
 Crossing over
 Independent assortment of chromosomes
 Fertilization
Hardy-Weinberg and Evolving
Populations
 The individuals in a population must differ genetically
for evolution to occur.
 Population – a group of individuals of the same
species that live in the same area and interbreed,
producing fertile offspring.
 Gene pool – a populations genetic makeup that
consists of all the alleles for all the loci in all
individuals of the population.
 If there are two or more alleles for a particular locus in
a population, individuals may be either homozygous or
heterozygous.
Hardy-Weinberg Principle
 The gene pool of a population that is not evolving can
be described by the Hardy-Weinberg principle.
 This principle state that 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.
 This type of gene pool is in Hardy-Weinberg
equilibrium.
Hardy-Weinberg Equilibrium
Hardy-Weinberg Equation
Conditions for Hardy-Weinberg
Equilibrium
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
Natural selection
 Natural selection can alter allele
frequencies in a population.
 Natural selection can cause adaptive
evolution (evolution that results in a
better match between organisms and
their environment.
Genetic Drift
 Genetic drift can alter allele frequencies in a
population.
 Genetic drift - Chance events that cause allele
frequencies to fluctuate unpredictable from one
generation to the next, especially in small populations.
Genetic Drift
The Founder Effect
 The founder effect can also cause a fluctuation in the
allele frequencies of a population.
 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 Founder Effect probably accounts for the relatively
high frequency of certain inherited disorders among
isolated human populations.
The Bottleneck Effect
 A certain change in the environment, such as a fire or
flood, may drastically reduce the size of a population.
 A severe drop in population size can cause the
bottleneck effect.
 The bottleneck effect is named so because the
population has passed through a restrictive
“bottleneck” in size.
 Some alleles may be overrepresented and some will be
underrepresented. Some populations may even cease
to exist.
Bottleneck
Effects of Genetic Drift: a summary
1. Genetic drift is significant in small
populations
2. Genetic drift can cause allele frequencies to
change at random.
3. Genetic drift can lead to a loss of genetic
variation within a population.
4. Genetic drift can cause harmful alleles to
become fixed.
Gene Flow
 Gene flow can also cause the alleles
frequencies in a population to fluctuate.
 Gene flow – the transfer of alleles into or
out of a population due to the movement of
fertile individuals or their gametes.
 Because alleles are exchanged among
populations, gene flow tends to reduce the
genetic differences between populations.
Gene flow and human evolution
A closer look at natural selection
 Relative fitness – the contribution an
individual makes to the gene pool of the
next generation, relative to the
contributions of other individuals.
 The terms “survival of the fittest” and
“struggle for existence” are sometimes
misleading if taken to mean direct
competition.
Directional, Disruptive, and
Stabilizing Selection
 Natural selection can alter the frequency
distribution of heritable traits in three ways:
 Directional Selection
 Disruptive Selection
 Stabilizing Selection
Directional Selection
 Directional Selection – occurs when conditions
favor individuals exhibiting one extreme of a
phenotypic range, thereby shifting the frequency
curve for the phenotypic character in one direction
or the other.
 Directional selection is common when a
population’s environment changes or when
members of a population migrate to a new habitat.
Disruptive Selection
 Disruptive selection – occurs when conditions favor
individuals at both extremes of a phenotypic range
over individuals with intermediate phenotypes.
 Ex. Members of a population of birds whose members
display different beak sizes.
Stabilizing Selection
 Stabilizing Selection – acts against both extreme
phenotypes and favors intermediate variants.
 This mode of selection reduces variation and tends to
maintain the status quo for a particular phenotypic
character.
The Role of Natural Selection in
Adaptive Radiation
 Sexual Selection – a form of natural selection in
which individuals with certain inherited
characteristics are more likely than other individuals
to obtain mates.
 Sexual selection can result in sexual dimorphism.
 Sexual dimorphism – marked differences between
the two sexes in secondary sexual characteristics which
are not directly associated with reproduction or
survival.
Sexual Dimorphism
How does Sexual Selection Work?
 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 – also called mate choice,
individuals of one sex (usually the females) are
choosy in selecting their mates from the other sex.
 In many cases, the female’s choice depends on
the showiness of the male’s appearance or
behavior.
Intersexual Selection
The Preservation of Genetic
Variation
 Because most eukaryotes are diploid, a considerable
amount of genetic variation is hidden from selection
in the form of recessive alleles.
 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 – exhibited when
individuals who are heterozygous at a particular locus
have greater fitness than do both kinds of
homozygotes.
Heterozygous Advantage
Preservation of Genetic Variation
cont…
 Frequency-Dependent Selection – the fitness of a
phenotype declines if it becomes too common in the
population.
 Neutral Variation –It happens when, in humans,
many of the nucleotide differences in noncoding
sequences appear to confer no selective advantage or
disadvantage.
Frequency-dependent selection in
scale eating fish.