Download Lecture 3 - Ecology and Evolution

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Ecology and Evolution
Outcomes
1. Apply the Hardy-Weinberg principle to test for
changes in allele frequency.
2. Apply logical deduction using the conditions of the
H-W principle to determine the mechanism of
evolutio n.
3. Distinguish between adaptive and non-adaptive
mechanisms of evolution.
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Ecology and Evolution
Why is evolution important to the
discipline of ecology?
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Ecology and Evolution
 The oldfield mouse (Peromyscus polionotus) is
widely distributed in the southeastern U.S. It is
preyed upon by a variety of visually hunting
predators such as hawks and owls.
 The mouse displays considerable variation in coat
color both within and between populations across its
range.
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Ecology and Evolution
Figure 3.5 Coat color variation in mice.
Two color variants of Peromyscus polionotus: (A) the darker inland form, and (B) the lighter beach-dwelling
form.
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Ecology and Evolution
Figure 3.6 Variation in coat color and genotypes at the Mc1R locus.
Peromyscus polionotus exhibits extensive coat color variation across localities in Florida. Red areas indicate the distribution of
beach populations; gray areas denote the distribution of inland populations. Characteristic phenotypes for each population are
indicated by the coat coloration sketches, but coat color varies within populations as well. Adapted from Hoekstra et al. (2006).
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Ecology and Evolution
 Most populations of the mouse are dark colored, but
populations on beaches and barrier islands have
lighter colored coats.
 Does coat color affect the survival and ultimately
reproduction (i.e. fitness) of oldfield mice?
 Two experiments suggest it does.
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Ecology and Evolution
 Kaufman (1974) carried out an experiment in which
pairs of mice (one dark-coated, one light coated)
along with an owl were placed in large cages
located in habitats with different backgrounds (light
or dark and with different vegetation densities).
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Ecology and Evolution
 In all cases mice that better matched the background survived
better than mice that matched less well.
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Ecology and Evolution
 Kaufman et al. carried out a follow-up experiment in which they
made silicone mouse models painted light or dark to mimic
either the dark or light background.
 They placed the models in different habitats and measured
from beak and claw marks how often the models were
attacked. They found clear differences in attack rates. Models
that matched their background were attacked much less.
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Ecology and Evolution
Figure 3.10 Predation, coat color, and fitness in the oldfield mouse, using plastic models in the field.
Hoekstra and colleagues placed light and dark silcone mouse models in light and dark environments to test predation rates. (A) The
experimental sites: a light beach environment and a dark inland environment. (B) Proportion of attacks against light and dark mice in
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Pearson
Education, Inc.
the
light
environment.
(C) Proportion of attacks against light and dark mice in the dark environment. Adapted from Vignieri et al.
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Ecology and Evolution
Observations made by Charles Darwin that were the building blocks of his
theory on Evolution by Natural Selection
1) Individuals of a population are different from one another.
Variation among individuals arises through:
►Mutation
►Sexual Reproduction (genetic recombination)
2) Many of these behavior, morphological or physiological differences,
which we term traits, are inherited from the parents.
3) All plants and animals produce more offspring than are needed to simple
replace the parents.
4) Among the different individuals in a population, some are better suited for
survival than others.
5) Different ancestors leave different numbers of descendants.
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Ecology and Evolution
Charles Darwin's Theory of Evolution by
Natural Selection
Natural Selection is the differential success in reproduction, and
its product is adaptation of organisms to their environment.
What this means:
Organisms that possess the characteristics (traits) that are
most helpful in a given environment are more likely to survive
and increase their numbers.
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Why check for Hardy-Weinberg?
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Why check for Hardy-Weinberg?
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The Hardy-Weinberg equation can be used to test
whether a population is evolving
The Hardy-Weinberg Equilibrium Principle has two
fundamental conclusions:
1. The allele frequencies in a population will not
change, generation after generation.
2. If the allele frequencies in a population are given by
p and q, then the genotype frequencies will be given
by p2, 2pq q2.
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Activity - The Hardy-Weinberg Equilibrium Principle
Consider a population of 500 flower plants.
320
CRCR
160
CWCW
20
CRCW
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Conditions for Hardy-Weinberg Equilibrium
 The Hardy-Weinberg theorem describes a
hypothetical population that is not evolving.
 It is a model which has assumptions.
1. No mutations
2. Random mating
3. No natural selection
4. Extremely large population size
5. No gene flow
 If the population is not in H-W equilibrium, then one of
the assumptions has been violated.
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Non-random mating alters genotype frequency, not
allele frequency
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Non-random mating alters genotype frequency, not
allele frequency
Second assumption of H-W is violated:
2. If the allele frequencies in a population are given by p and
q, then the genotype frequencies will be given by p2, 2pq
q2.
Allele frequencies of
A1 = 0.5 ,A2 = 0.5 at
the start and at
generation 3.
But! Genotype
frequencies have
changed.
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Natural selection, genetic drift, and gene flow can
alter allele frequencies in a population
Zimmer
and Emlen
2013
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Education,
Inc.
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
 Natural selection
 Genetic drift
 Gene flow
 Why not mutation?
Although mutation can change allele frequencies, even a
beneficial mutation will not increase in frequency without a
selection pressure.
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Violation of Hardy-Weinberg - Mutation
 Mutation is not a strong force for
evolution.
 ∆32 is a 32 base pair deletion
in the CCR5 gene. The gene
encodes a cell surface protein
called CCR-5.
 The 32 base pair deletion
confers resistance to it carriers
from HIV.
 But most carriers do not live in
areas with high incidence of
HIV.
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Violation of Hardy-Weinberg - Mutation
 But if an allele mutates from one form (A1) to another
form (A2) at a constant rate and is not eliminated, it can
change the allele frequency of a population – over a very
long period of time.
Bergstrom
and
Dugatkin
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Education,
Inc. 2012
Violation of Hardy-Weinberg - Genetic Drift
 The smaller a sample, the more likely it is that chance
alone will cause deviation from a predicted result.
 Genetic drift describes how allele frequencies
fluctuate unpredictably from one generation to the
next.
 Genetic drift tends to reduce genetic variation through
losses of alleles, especially in small populations.
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Violation of Hardy-Weinberg - Genetic Drift
 Ten simulations of random genetic
drift using 3 different population
sizes.
 The random loss of one allele in
small populations leads to fixation of
the other allele.
 Large populations do not exhibit
dramatic effects of drift because it is
unlikely that all of one allele would be
lost due to chance.
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The Founder Effect
 The founder effect occurs when a few individuals
become isolated from a larger population.
 Allele frequencies in the small founder population
can be different from those in the larger parent
population due to chance.
Zimmer & Emlen 2013
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Genetic Drift- The Bottleneck Effect
 The bottleneck effect can result from a drastic reduction in
population size due to a sudden environmental change or other
indiscriminate reduction in population size.
 By chance, the resulting gene pool may no longer be reflective of
the original population’s gene pool.
 If the population remains small, it may be further affected by
genetic drift.
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Case Study: Impact of Genetic Drift on the
Greater Prairie Chicken
 Loss of prairie habitat caused a severe reduction in
the population of greater prairie chickens in Illinois.
 The surviving birds had low levels of genetic
variation, and only 50% of their eggs hatched.
 Prairie chicken video.
http://www.youtube.com/watch?v=s2_wdMmEupQ
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Figure 21.11
Pre-bottleneck Post-bottleneck
(Illinois, 1820) (Illinois, 1993)
Greater prairie chicken
Range
of greater
prairie
chicken
Population
size
Number
of alleles
per locus
Percentage
of eggs
hatched
1,000–25,000
50
5.2
3.7
93
50
Kansas, 1998
(no bottleneck)
750,000
5.8
99
Nebraska, 1998
(no bottleneck)
75,000–
200,000
5.8
96
Location
Illinois
1930–1960s
1993
http://www.youtube.com/watch?v=s2_wdMmEupQ#t=0
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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 populations.
4. Genetic drift can cause harmful alleles to become
fixed.
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Violation of Hardy-Weinberg - Gene Flow
 Gene flow consists of the
movement of alleles among
populations.
 Alleles can be transferred through
the movement of fertile individuals
or gametes (for example, pollen).
 Gene flow tends to reduce genetic
variation among populations over
time
Freeman & Herron 2007
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Violation of Hardy-Weinberg - Gene Flow


Gene flow can decrease the fitness of a population
Consider, for example, the great tit (Parus major) on the Dutch
13% first time breeders
43% first time breeders
island of Vlieland
 Immigration of birds from the mainland
introduces alleles that decrease fitness in
island populations.
 Natural selection reduces the frequency of
these alleles in the eastern population where
immigration from the mainland is low.
 In the central population, high immigration
from the mainland overwhelms the effects of
selection.
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Directional, Disruptive, and Stabilizing Selection
 There are three modes of natural selection
 Directional selection favors individuals at one end
of the phenotypic range
 Disruptive selection favors individuals at both
extremes of the phenotypic range
 Stabilizing selection favors intermediate variants
and acts against extreme phenotypes
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Frequency of
individuals
Figure 21.13
Original
population
Original
Evolved
population population
Phenotypes (fur color)
(a) Directional selection
(b) Disruptive selection
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(c) Stabilizing selection
Directional Selection
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Disruptive Selection
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Stabilizing Selection
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Overview: The Smallest Unit of Evolution
• Question: Do organisms evolve during their
lifetimes?
Answer: No
• Question: What is the smallest unit of evolution?
Answer: The population
Natural selection acts on individuals, but
only populations evolve.
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Natural selection is the only mechanism that
consistently causes adaptive evolution
• Evolution by natural selection involves both
chance and “sorting”
– New genetic variations arise by chance
– Beneficial alleles are “sorted” and favored by natural
selection
• Only natural selection consistently results in
adaptive evolution, an increase in the frequency
of alleles that improve fitness
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Relative Fitness
 The phrases “struggle for existence” and
“survival of the fittest” are misleading as they
imply direct competition among individuals.
 Reproductive success is generally more subtle
and depends on many factors.
 Relative fitness is the contribution an individual
makes to the gene pool of the next generation,
relative to the contributions of other individuals.
 Selection indirectly favors certain genotypes by
acting directly on phenotypes.
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Natural Selection does not predict perfection
 There is a temptation to regard the match between organism
and environment as an example of evolved perfection.
 But, there is nothing in the process of evolution by natural
selection that implies perfection.
 Natural selection can only favor those that are fittest from
among the range of genetic variants available. But, no
population contains the full range of genetic variation that
might exist, so the range of genetic variants might be
limited.
 Organisms come to match their environments by being ‘the
fittest available’ or ‘the fittest yet’; they are not ‘the best
imaginable’.
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Proximate and Ultimate explanations
 A proximate explanation is the immediate cause of
a phenomenon.
It explains how a trait operates or is regulated.
 An ultimate explanation provides more general
reasons, involving evolutionary arguments and
adaptations. Ultimate explanations are remote in
time and space.
It explains why a trait exists.
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Why Natural Selection Cannot Fashion Perfect
Organisms
1. Selection can act only on existing
variations.
2. Evolution is limited by historical
constraints.
3. Adaptations are often compromises.
4. Chance, natural selection, and the
environment interact.
Natural selection operates on a “better than” basis.
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Why Natural Selection Cannot Fashion Perfect
Organisms – Historical Factors
 Organisms live where they do for reasons that are often, at least
in part, accidents of history.
 Moreover, they have lived in environments that were quite
different from the present: some of the features that they
acquired in other environments now hang like millstones around
their necks.
 These characters that they have acquired limit and constrain
where they can now live and what they do.
 These characters can also constrain how these organisms
respond to additional changes in their environment.
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Why Natural Selection Cannot Fashion Perfect
Organisms – Historical Factors
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Activity - Hardy-Weinberg
Peppered moth, Biston betularia
(B. Kettlewell, 1959)
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