Transcript Ch. 23 The Evolution of Populations. Rauch 2007-2008

Chapter 23: The Evolution of
Populations
MAIN IDEAS OF CHAPTER 23
 WHAT
IS POPULATION GENETICS
 CAUSES
OF MICROEVOLUTION
 HOW
GENETIC VARIATION IN A POPULATION
ARISES
 THE
IMPACT OF SELECTIVE FORCES ON
POPULATIONS
Population genetics
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Population: a localized group of
individuals belonging to the same species
Species: a group of populations whose
individuals have the potential to interbreed
and produce fertile offspring
Gene pool: the total aggregate of genes
in a population at any one time
Population genetics: the study of genetic
changes in populations

“Individuals are selected, but populations
evolve.”
Microevolution:
 Evolution on the small scale
 A change in the allele frequencies in a
population.

Fig. 23.1
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Hardy-Weinberg Theorem

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Serves as a model for the genetic structure of a
nonevolving population (equilibrium)
5 conditions:
1- Very large population size;
2- No migration;
3- No net mutations;
4- Random mating;
5- No natural selection
Hardy-Weinberg Equation

p = frequency of one allele (A);
q = frequency of the other allele (a);

p+q = 1.0
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(p=1-q
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P2=frequency of AA genotype;
2pq=frequency of Aa plus aA genotype;
q2=frequency of aa genotype;

&
q=1-p)
p2 + 2pq + q2 = 1.0
Microevolution
A
change in the gene pool of a
population over a succession
of generations
How do changes in the gene pool
occur?
 1.
genetic drift
 The Bottleneck Effect

 2.
The Founder effect
gene flow
 3. mutation
 4. natural selection
1. Genetic drift:


Changes in the gene
pool of a small
population due to
chance .
(usually reduces
genetic variability)
Genetic Drift

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The Bottleneck Effect:
genetic drift resulting
from a reduction of a
population (natural
disaster).
such that the surviving
population is no longer
genetically representative
of the original population

Bottlenecking is an important concept in
conservation biology of endangered species.
 Populations
that have suffered bottleneck incidents
have lost at least some alleles from the gene pool.
 This reduces individual variation and adaptability.
 For example, the genetic variation
in the three small surviving wild
populations of cheetahs is very low
when compared to other mammals.
 Their genetic uniformity is
similar to highly inbred
lab mice!
Fig. 23.5x
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
Understanding the bottleneck effect

Can increase understanding of how human
activity affects other species
(b) Similarly, bottlenecking a population
of organisms tends to reduce genetic
variation, as in these northern
elephant seals in California that were
once hunted nearly to extinction.
Figure 23.8 B
Genetic Drift

Founder effect
 the
new population
could have a much
different genetic ratio
than the original one.

Founder Effect:
genetic drift
attributable to
colonization by a
limited number of
individuals from a
parent population
Microevolution: 2. Gene Flow



Genetic exchange due to
the migration of fertile
individuals or gametes
between populations.
(reduces differences
between populations)
Pollen carried on the wind
Microevolution: 3. Mutations

a change in an organism’s
DNA.

Mutation in gamete can
immediately change gene
pool.

original source of genetic
variation (raw material for
natural selection)
Microevolution: 4- Natural Selection
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Differential success in
reproduction.
The only form of
microevolution that
adapts a population
to its environment
Genetic Variation within and
between populations:

Polymorphism:
coexistence of 2 or more
distinct forms of
individuals (morphs)
within the same
population
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Geographical
variation: differences in
genetic structure between
populations (cline)
Variation preservation
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Prevention of natural
selection’s reduction of
variation
Diploidy
2nd set of chromosomes
hides variation in the
heterozygote
Balanced polymorphism
1- heterozygote advantage
(hybrid vigor; i.e.,
malaria/sickle-cell anemia);
2- frequency dependent
selection (survival &
reproduction of any 1 morph
declines if it becomes too
common; i.e., parasite/host)
The effect of selection on a varying
characteristic: Selection Trends .
3 types

1. Directional Selection:
 favors
variants at one extreme

2. Diversifying Selection
 favors
variants of opposite extremes

3. Stabilizing Selection
 Acts
against extreme phenotypes
 Favors more common intermediate
 Reduces variation & maintains status quo.
Sexual selection-may lead to pronounced
differences between the sexes.

Sexual dimorphism:

secondary sex characteristic
distinction
A product of sexual selection
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Sexual selection:
A) intrasexual selection
(within a sex)
Rams butting horns
B) intersexual selection

Mate choice
Natural selection cannot fashion
perfect organisms
1. Evolution is limited by historical constraints.
 Evolution
does not scrap ancestral anatomy
and build from scratch.
 Evolution co-opts existing structures and
adapts them to new situations.
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2. Adaptations are often compromises.
 Organisms
are often faced with conflicting situations that
prevent an organism from perfecting any one feature for
a particular situation.


For example, because the flippers of a seal must not only allow it
to walk on land, but also swim efficiently, their design is a
compromise between these environments.
Similarly, human limbs are flexible and allow versatile
movements, but at the cost of injuries, such as sprains, torn
ligaments, and dislocations.

Better structural reinforcement would compromise agility.
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3. Not all evolution is adaptive.
 Chance
affects the genetic structure of
populations to a greater extent than was once
believed.

For example, founders of new populations may not
necessarily be the best individuals, but rather those
individuals that are carried into the open habitat by
chance.
4. Selection can only edit existing variations.
 Selection
favors only the fittest variations from
those phenotypes that are available.
 New alleles do not arise on demand.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Natural Selection maintains sexual
reproduction
3. Natural selection maintains
sexual reproduction


Sex is an evolutionary enigma.
It is far inferior to asexual reproduction as
measured by reproductive output.
 If
a population consisted of half sexual females and half
asexual females, the asexual condition would increase.


All offspring of asexual females would be reproductive
daughters.
Only half of the offspring of sexual females would be daughters;
the other half would be necessary males.
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
Theoretically, sex has a “two-fold
disadvantage.”
 A female
producing two offspring per generation
would generate a population of eight females
after four generations if reproducing asexually,
but only one female if reproducing sexually.
Fig. 23.15
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
Sex must confer some selective advantage
to compensate for the costs of diminished
reproductive output.
 Otherwise,
a migration of asexual individuals or
mutation permitting asexual reproduction would
outcompete sexual individuals and the alleles
favoring sex.

In fact, most eukaryotes maintain sex, even
in those species that can also reproduce
asexually.
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
The “textbook” explanation for the
maintenance of sex is that the process of
meiosis and fertilization generate genetic
variation on which natural selection can act.
 However,
this hypothesis that sex is maintained
in spite of its disadvantages because it produces
future adaptation in a variable world is difficult to
defend.
 Natural selection acts on the present, favoring
individuals here and now that best fit the current,
local environment.

A stronger hypothesis would present
advantages to sex that place a value on
genetic variation on a generation-togeneration time scale.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings