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I.
Evolution
II.
Darwin and Selection
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
Selection can create phenotypes beyond the initial
range of expression.. There are no adult wolves as
small as chihuahuas.
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
2. Reading Malthus
“In October 1838, that is, fifteen months after I had begun my systematic enquiry,
I happened to read for amusement Malthus on Population…” - The
Autobiography of Charles Darwin 1809-1882 (Barlow 1958).
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
2. Reading Malthus
Thomas Malthus (1766-1834)
Essay On the Principle of Population (1798)
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
2. Reading Malthus
Thomas Malthus (1766-1834)
Essay On the Principle of Population (1798)
P1: All populations have the capacity to ‘overreproduce’
P2: Resources are finite
C: There will be a “struggle for existence”… most
offspring born will die before reaching reproductive
age.
A. Transitional Observations
1. ‘Artificial Selection’ and Domesticated Animals and Plants
2. Reading Malthus
“In October 1838, that is, fifteen months after I had begun
my systematic enquiry, I happened to read for amusement
Malthus on Population and being well prepared to
appreciate the struggle for existence which everywhere goes
on from long-continued observation of the habits of animals
and plants, it at once struck me that under these
circumstances favourable variations would tend to be
preserved, and unfavourable ones to be destroyed. The
result of this would be the formation of new species. Here,
then, I had at last got a theory by which to work; but I was so
anxious to avoid prejudice, that I determined not for some
time to write even the briefest sketch of it. In June 1842 I
first allowed myself the satisfaction of writing a very brief
abstract of my theory in pencil in 35 pages; and this was
enlarged during the summer of 1844 into one of 230 pages,
which I had fairly copied out and still possess.” - The
Autobiography of Charles Darwin 1809-1882 (Barlow 1958).
B. Natural Selection
P1: All populations have the capacity to ‘over-reproduce’
P2: Resources are finite
C: There will be a “struggle for existence”… most offspring born will die
before reaching reproductive age.
P3: Organisms in a population vary, and some of this variation is heritable
C2: As a result of this variation, some organisms will be more likely to
survive and reproduce than others – there will be differential reproductive
success.
C3: The population change through time, as adaptive traits accumulate in
the population.
Corollary: Two populations, isolated in different environments, will diverge
from one another as they adapt to their own environments. Eventually,
these populations may become so different from one another that they are
different species.
B. Natural Selection
"It is interesting to contemplate an entangled bank, clothed with many plants of many
kinds, with birds singing on the bushes, with various insects flitting about, and
with worms crawling through the damp earth, and to reflect that these elaborately
constructed forms, so different from each other, and dependent on each other in
so complex a manner, have all been produced by laws acting around us. These
laws, taken in the largest sense, being Growth with Reproduction; Inheritance
which is almost implied by reproduction; Variability from the indirect and direct
action of the external conditions of life, and from use and disuse; a Ratio of
Increase so high as to lead to a Struggle for Life, and as a consequence to Natural
Selection, entailing Divergence of Character and the Extinction of less-improved
forms. Thus, from the war of nature, from famine and death, the most exalted
object which we are capable of conceiving, namely, the production of the higher
animals, directly follows. There is grandeur in this view of life, with its several
powers, having been originally breathed into a few forms or into one; and that,
whilst this planet has gone cycling on according to the fixed law of gravity, from so
simple a beginning endless forms most beautiful and most wonderful have been,
and are being, evolved". - The Origin of Species (Darwin 1859).
III. Natural Selection
A. The Nature of Selection
- Types of Selection
III. Natural Selection
A. The Nature of Selection
- Types of Selection
Stabilizing selection for
size at birth in humans
III. Natural Selection
A. The Nature of Selection
- Types of Selection
Directional selection for
beak size
III. Natural Selection
A. The Nature of Selection
- Types of Selection
Disruptive selection for beak
width in African finches feeding
on two species of sedges with
soft and hard seeds.
Can result in sympatric speciation – speciation within a
reproducing population, in which different subpopulations
become adapted to different niches in the same environment
and produce poorly adapted hybrids when they breed with
other subpopulations.
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on organism’s sex
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on sex
Kin Selection: fitness depends on relatedness within group
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on sex
Kin Selection: fitness depends on relatedness within group
Frequency-dependent Selection: fitness depends on genotype frequency
Papilio memnon females have
many possible phenotypes that
mimic toxic species. Abundant
morphs are selected against;
because bird predators learn
they are tasty and prey on
them. Negative frequency
dependence.
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on sex
Kin Selection: fitness depends on relatedness within group
Frequency-dependent Selection: fitness depends on genotype frequency
Heliconius erato is a toxic
species in Central America.
Whichever morph becomes
more abundant (even by
chance) becomes EVEN MORE
abundant because birds learn
to avoid it more readily.
Positive frequency
dependence.
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on sex
Kin Selection: fitness depends on relatedness within group
Frequency-dependent Selection: fitness depends on genotype frequency
Female guppies prefer to mate with the rarest male phenotype;
selects for polymorphism over time.
III. Natural Selection
A. The Nature of Selection
- Modes of Selection
Natural Selection
Sexual Selection: fitness of a genotype/trait depends on sex
Kin Selection: fitness depends on relatedness within group
Frequency-dependent Selection: fitness depends on genotype frequency
Artificial Selection
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1. Phenotypic Plasticity
Why do populations differ?
1) Genetic adaptation
2) Evolutionary diverge due to
other agents of change
3) Phenotypic plasticity – the
direct effect of environment
on phenotype
Daphnia grow spines and
head-spines when
developing in the presence
of fish (predators)
Gastrimargus grasshoppers developing in
different seasons (with predictable
sustrate colors) develop into different
colored nymphs and adults
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
So, suppose we observe variation
in plant size among genetically
different plants growing in a field:
This variation in phenotype might
be due to a combination of genetic
and environmental differences
between them.
V(phen) = V(env) + V(gen)
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
IF these plants were all grown
under the same environmental
conditions (‘common garden’
experiment), then there is no
variation in the environment and
the variation we observe can be
attributed to genetic differences.
V(phen) = 0 + V(gen)
(whether these genetic differences are
ADAPTIVE is another question…)
IIi. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
IF these plants were all grown
under the same environmental
conditions (‘common garden’
experiment), then there is no
variation in the environment and
the variation we observe can be
attributed to genetic differences.
V(phen) = 0 + V(gen)
(But this is only true for this environment!)
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
In a different environment, phenotypic
and genetic variation may be expressed
differently.
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
So, in a large population experiencing a
range of environments:
V(phen) = V(env) + V(gen) + V(g*e)
V(g*e) is a genotype by environment
interaction; reflecting the fact that
genotypes may respond in different ways
to changes in the environment.
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Suppose we had populations of each genotype, and
these were the mean heights of these populations.
Height
Genotype
C
F
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Suppose we had populations of each genotype, and
these were the mean heights of these populations.
Height
Genotype
C
F
XS
XM
Stanford
Mather
Environment
V(env)
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Suppose we had populations of each genotype, and
these were the mean heights of these populations.
Height
Genotype
C
F
XC
V(gen)
XF
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Suppose we had populations of each genotype, and
these were the mean heights of these populations.
Height
Genotype
C
F
XCS
XCM = XFS
XFM
The effect of
environment IS
THE SAME for the
two genotypes:
(g*e) = 0.
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
So, in this comparison:
V(phen) = V(env) + V(gen) + V(g*e)
Sig.
Sig.
ns
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Suppose we compare B and E.
Height
Genotype
B
E
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Environmental effects are significant
Height
Genotype
B
E
XS
V(env)
XM
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Genetic effects are insignificant; means don’t differ.
Height
Genotype
B
E
XC
XE
V(gen) = 0
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
There is a significant ‘G x E’ interaction.
Height
Genotype
B
E
XCS
XCM >> XFS
XFM
The effect of
environment IS
NOT THE SAME
for the two
genotypes!!
Stanford
Mather
Environment
III. Natural Selection
B. Testing for Selection/Adaptation
1.
2.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
In a different environment, phenotypic
and genetic variation may be expressed
differently.
So, in this comparison:
V(phen) = V(env) + V(gen) + V(g*e)
Sig.
ns
Sig.
We should expect adaptations to specific environmental conditions to be
reflected by such “genotype by environment” interactions in characters that
affect reproductive success.
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
3.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Selection for Acclimation Ability
III. Natural Selection
A. The Nature of Selection
B. Testing for Selection/Adaptation
1.
2.
3.
Phenotypic Plasticity
Genetics, environment, or genetic adaptation to environment?
Selection for Acclimation Ability
Raised at 20oC
Raised at 45oC