Transcript Population Evolution

Chapter 23
 Natural selection does act on individuals.
 Each individual’s combo of traits affects survival
and reproductive success relative to

other individuals (competition)
 What was missing from Darwin’s explanation was
an understanding of inheritance that could explain
how chance variations arise
 Gregor Mendel proposed a model of inheritance
that supported Darwin’s theory.
 Mendel’s particulate hypothesis of inheritance
stated that parents pass on discrete heritable units
(genes) that retain their identities in offspring.
At the beginning, many geneticists believed that
Mendel’s laws of inheritance conflicted with
Darwin’s theory of natural selection.
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Darwin emphasized quantitative characters that
vary along a continuum.
Mendel and later geneticists investigated discrete
“either-or” traits.
It was not obvious that there was a genetic basis to
quantitative characters.
Geneticists determined that quantitative characters
are influenced by multiple genetic loci and that the
alleles at each locus follow Mendelian laws of
inheritance.
This led to the birth of population genetics, the
study of how populations change genetically over
time.
One definition of a species is a group of natural
populations whose individuals have the potential
to interbreed and produce fertile offspring.
Populations of a species may be isolated from each
other and rarely exchange genetic material.
Members of a population are more likely to breed
with members of the same population
Individuals near the population’s center are more
closely related to one another
The total aggregate of genes in a population at any
one time is called the population’s gene pool.
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It consists of all alleles at all gene loci in all
individuals of a population.
If only one allele exists at a particular locus in a
population, that allele is said to be fixed in the
gene pool, and all individuals will be homozygous
for that gene.
If there are two or more alleles for a particular
locus, then individuals can be either homozygous
or heterozygous for that gene.
Each allele has a frequency in the population’s
gene pool.
When there are two alleles at a locus, the
convention is to use p to represent the frequency of
one allele and q to represent the frequency of the
other.
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The Hardy-Weinberg
Theorem
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 The Hardy-Weinberg Theorem describes the gene
pool of a nonevolving population
 This theorem states that the frequencies of alleles
and genotypes in a population’s gene pool will
remain constant over generations unless acted upon
by agents other than Mendelian segregation and
recombination of alleles.
 The shuffling of alleles by meiosis and random
fertilization has no effect on the overall gene pool of
a population.
Hardy-Weinberg
equation/ f ormula
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The genotype frequencies must add up to 1.0
p2 + 2pq + q2 = 1.0
We can calculate frequencies of alleles in a gene
pool if we know the frequency of genotypes
Or we can calculate the frequency of genotypes if
we know the frequencies of alleles
The Hardy-Weinberg theorem describes a
hypothetic population that is not evolving.
Real populations evolve, and their allele and
genotype frequencies do change over time
Five conditions must be met for
Hardy-Weinberg equilibrium.
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1. Extremely large population size. In small
populations, chance fluctuations in the gene
pool can cause random changes, called genetic
drift.
2. No gene flow. Gene flow, the transfer of alleles
due to the migration of individuals or gametes
between populations, changes allele proportions
3. No mutations. Introduction, loss, or
modification of genes will alter the gene pool.
Five conditions, con’t
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4. Random mating. If individuals pick mates
with certain genotypes, or if inbreeding is
common, the mixing of gametes will not be
random.
5. No natural selection. Differential survival
or reproductive success among genotypes
will alter their frequencies.
New genes /alleles originate
only by mutation.

A mutation is a change in the nucleotide sequence
of an organism’s DNA.
Mutations in somatic (body) cells are lost when
the individual dies.
Only mutations in gametes (sex cells) can be
passed on to offspring (only a small fraction
spread through populations)
A new mutation transmitted to an offspring can
immediately change the gene pool by introducing
a new allele.
A point mutation is a change of a single base in a
gene.
Point mutations can have a significant impact on
phenotype, as in the case of sickle-cell disease;
most point mutations are harmless.
Much of the DNA in eukaryotic genomes does not
code for protein products.
However, some do; so changes in these regulatory
regions of DNA can have profound effects.
Genetic code is redundant, so some point
mutations that code for proteins may not alter the
protein’s composition of amino acids.
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On a generation-to-generation timescale, sexual
recombination is far more important than
mutation in producing the genetic differences
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that make adaptation possible.
Sexual reproduction rearranges allele
combinations every generation.
Bacteria and viruses can also undergo
recombination, but less often than animals and
plants.
Bacterial and viral recombination may cross
species barriers.
Genetic drift
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Genetic drift occurs when there are changes in
gene frequencies from one generation to another
In a large population, allele frequencies will not
change from generation to generation by chance
alone.
Genetic drift at small population sizes may occur
as a result of two situations: the bottleneck effect or
the founder effect.
Bottleneck Effect
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 When a large pop. is drastically reduced by a disaster
 By chance, some alleles may be overrepresented and
others underrepresented among survivors; some
eliminated
 Genetic drift will continue to change the gene pool until
the population is large enough to eliminate chance
fluctuations or reach equilibrium
 The bottleneck effect is an important concept in
conservation biology (i.e. endangered species)
 Bottleneck incidents reduces genetic variation and may
reduce adaptation
Founder Effect
When a new population
is started by only a few
individuals not representative of the gene pool of
the larger source
Could be started by a single pregnant female or
single seed with only a fraction of the genetic
variation
Genetic drift would continue from generation to
generation until the population grew large enough
for sampling errors to be minimal.
Have been demonstrated in humans; started from
colonists
Natural selection can alter the frequency
distribution of heritable traits in three ways,
depending on which phenotypes in a population
are favored.
The three modes of selection are called
directional, disruptive, and stabilizing selection.
Directional selection is most common during
periods of environmental change or when
members of a population migrate to a new habitat
with different environmental conditions.
Directional selection shifts the frequency curve
for a phenotypic character in one direction by
favoring individuals who deviate from the
average.
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Disruptive selection occurs when environmental
conditions favor individuals at both extremes of
the phenotypic range over those with intermediate
phenotypes.
For example, two distinct bill types are present in
Cameroon’s black-bellied seedcrackers. Largerbilled birds are more efficient in feeding on hard
seeds and smaller-billed birds are more efficient in
feeding on soft seeds.
Disruptive selection can be important in the early
stages of speciation.
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Stabilizing selection favors intermediate
variants and acts against extreme
phenotypes.
Stabilizing selection reduces variation and
maintains the status quo for a trait.
Human birth weight is subject to stabilizing
selection.
Babies much larger or smaller than 3–4 kg
have higher infant mortality than averagesized babies.
This is being altered with C-sections.
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Natural selection cannot fashion perfect
organisms.
Four reasons natural selection cannot produce
perfection.
1. Evolution is limited by historical constraints.
Evolution adapts existing features to new
situations.
2. Adaptations are often compromises.
Each organism must do many different
things.
Better structural reinforcement would
compromise agility.
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3. Chance and natural selection interact.
Chance events affect the subsequent
evolutionary history of populations.
Founders of new populations may not
necessarily be best suited
4. Selection can only edit existing variations.
Natural selection favors only the fittest
variations from those phenotypes that are
available.
New alleles do not arise on demand.
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Speciation
Speciation—the origin of new species—is the
source of biological diversity.
Microevolution - study of adaptive change in a
population.
Macroevolution - evolutionary changes above
the species level; deals with questions such as

the appearance of evolutionary novelties
(e.g., feathers and flight in birds) used to
define higher taxa (genus, family, etc).
Speciation addresses the question of how new
species originate and develop through the
divergence of gene pools.
The fossil record chronicles two patterns of
speciation: anagenesis and cladogenesis.
Anagenesis, phyletic evolution, is the
accumulation of changes associated with the
gradual transformation of one species into
another.
Cladogenesis, branching evolution, is the
budding of one or more new species from a
parent species.
Only cladogenesis promotes biological
diversity by increasing the number of
species.
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A species is defined as a population or
group of populations whose members have
the potential to breed with each other in
nature to produce viable, fertile offspring,
but who cannot produce viable, fertile
offspring with members of other species.
A biological species is the largest set of
populations in which genetic exchange is
possible and that is genetically isolated from
other populations.
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Species are based on interfertility, not physical
similarity.
Reproductive barriers:
prezygotic / postzygotic
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 Prezygotic barriers impede mating or hinder ova fertilization
 Habitat isolation. Two organisms use different habitats (even
in the same geographic area) are unlikely to encounter each
other to even attempt mating.
 Behavioral isolation. Many species use elaborate courtship
behaviors unique to the species to attract mates.
 Temporal isolation. Two species that breed during different
times of day, seasons, or years cannot mix gametes.
 Mechanical isolation. Closely related species may fail because
they are anatomically incompatible and transfer of sperm is
not possible.
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 Gametic isolation. The gametes are not fertilized
because of incompatibilities.
 In species with internal fertilization, the
environment of the female reproductive tract may
not be conducive to the survival of sperm from
other species.
 For species with external fertilization, gamete
recognition may rely on the presence of specific
molecules on the egg’s coat, which adhere only to
specific molecules on sperm cells of the same
species.
 Postzygotic barriers may prevent the hybrid zygote
from developing into a viable, fertile adult.
 Reduced hybrid viability. Genetic incompatibility
between the two species may abort the development of
the hybrid at some embryonic stage or produce frail
offspring.
 Reduced hybrid fertility. The hybrids may be infertile,
and the hybrid cannot backbreed
 This infertility may be due to differences in
chromosome number or structure.
 Hybrid breakdown. In some cases, first generation
hybrids are viable and fertile.
 However, when they mate with either parent species
or with each other, the next generation is feeble or
sterile. (Too many recessive alleles may be present)
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