Chapter 3 Natural Selection
 Assigned
reading chapter 3.
Evolution by Natural Selection
 Darwin
envisaged process similar to
artificial selection that had produced
organisms we see today. He called it
Natural Selection.
Evolution by Natural Selection
 Recall
Darwin proposed evolution was the
inevitable outcome of 4 postulates:

 1.
There is variation in populations.
Individuals within populations differ.
 2.
Variation is heritable.
Evolution by Natural Selection
 3.
In every generation some organisms
are more successful at surviving and
reproducing than other. Differential
reproductive success.
 4.
Survival and reproduction are not
random, but are related to variation among
individuals. Organisms with best
characteristics are ‘naturally selected.’
Evolution by Natural Selection
 If
4 postulates are true then the population
will change from one generation to the
next.
 Evolution will occur.
Evolution by Natural Selection
 Darwinian
fitness: ability of an organism to
survive and reproduce in its environment.
 Fitness
species
measured relative to others of its
Evolution by Natural Selection
 Adaptation
is a characteristic or trait of an
organism that increases its fitness relative
to individuals that do not possess it.
Phenotype
 An
organism’s physical characteristics
such as height, coloration, shape as well
as its behavior are its phenotype.
 Phenotype
is the result of both the
organism’s genotype and the influence of
the environment acting on the genotype.
Natural Selection and coat color in the
oldfield mouse: is there variation?
 The
oldfield mouse 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.
Natural Selection and coat color
in the oldfield mouse
 Most
populations of the mouse are dark
colored, but populations on beaches and
barrier islands have lighter colored coats.
 Hoekstra et al. carried out a series of
experiments to evaluate the hypothesis
that natural selection favors a match
between coat color and background color.
Is variation in coat color
heritable?
 There
is considerable phenotypic variation
in coat color in oldfield mice.
 For
natural selection to occur the variation
must be heritable. Hoekstra et al. have
shown that several genes affect coat color
in these mice.
Genetics of coat color
 The
first gene is the melanocortin-1
receptor gene (Mc1R). This gene
switches between producing a dark
pigment (Eumelanin) and a light pigment
(Phaeomelanin) depending on how it is
affected by signals from other genes.
Genetics of coat color
 If
a protein called alpha-MSH binds to the
McR1 gene then the dark pigment
eumelanin is produced.
 If alpha-MSH cannot bind to the Mc1R
gene either because of the presence of a
different protein called ASP (agoutisignaling protein) or because a mutation
in the Mc1R gene so it cannot bind alphaMSH then the light-colored pigment
phaeomelanin is produced.
Genetics of coat color
 Hoekstra
et al. have shown in populations
with large numbers of light-colored mice
that two mutations are common: one that
prevents alpha-MSH binding to Mc1R and
the second a mutant agouti allele that
produces much more ASP than usual.
 Both mutant alleles result in light-colored
mice. Thus there is a clear genetic basis
for the observed variation in coat color.
Does variation affect fitness?
 Does
coat color affect the survival and
ultimately reproduction (i.e. fitness) of
oldfield mice?
 Two experiments suggest it does.
Does variation affect fitness?
 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).
Does variation affect fitness?
 In
all cases mice that better matched the
background survived better than mice that
matched less well.
Does variation affect fitness?
 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.
Natural Selection and coat color
in the oldfield mouse
 Thus
for oldfield mice all 4 postulates are
satisfied. There is (i) variation in coat
color and it is (ii) heritable.
 There is (iii) differential reproductive
success (or in this case differential survival
which is a necessary precursor to
reproduction).
 That differential reproductive success is
(iv) related to the variation (different coat
colors survive better in different habitats).
Another example of natural
selection: Darwin’s finches
 Evolution
of beak shape in Darwin’s
Finches.

Peter and Rosemary Grant’s (and
colleagues) work on Medium Ground
Finches Geospiza fortis
 On
Daphne Major since 1973.
Evolution of beak shape in
Darwin’s Finches.
 Postulate
 Finches
1. Is the population variable?
vary in beak length, beak depth,
beak width, wing length and tail length.
Evolution of beak shape in
Darwin’s Finches.
 Postulate
2: Is variation among individuals
heritable?
 Variation
can be a result of environmental
effects.
 Heritability: proportion of the variation in a
trait in a population that is due to variation
in genes.
Evolution of beak shape in
Darwin’s Finches.
 Peter
Boag compared average beak depth
of parents with that of their adult offspring.
 Strong
relationship between offspring and
parent beak depths.
FIG 3.7
Evolution of beak shape in
Darwin’s Finches.
 Postulate
3: Do individuals differ in their
success at survival and reproduction?
 1977
drought 84% of G. fortis individuals
died, most from starvation. In two other
droughts 19% and 25% of the population
died.
Evolution of beak shape in
Darwin’s Finches.
 Seed
densities declined rapidly during
drought and the small soft seeds were
consumed first.
 Average
size and hardness of remaining
seeds increased over the course of the
drought.
FIG 3.8b
FIG 3.8A
Fig 3.8c
Evolution of beak shape in
Darwin’s Finches.
 Postulate
4: Are survival and reproduction
nonrandom?
 Do those who survive and reproduce have
different characteristics than those that
don’t?
Evolution of beak shape in
Darwin’s Finches.
 As
drought progressed small soft seeds
disappeared and large, hard Tribulus
seeds became a key food item.
 Only
birds with deep, narrow beaks could
open them.
Evolution of beak shape in
Darwin’s Finches.
 At
end of the 1977 drought the average
survivor had a deeper beak than the
average non-survivor and also a larger
body size.
FIG 3.9
Did the population evolve?
 Chicks
hatched in 1978 had deeper beaks
on average than those hatched in 1976.
 Population
evolved.
Fig 3.10
Evolution of beak shape in
Darwin’s Finches.
 Variation
in weather from year to year on
Daphne Major over 30 years has led to
variation in the traits that are favored by
selection.
 Population
has evolved over time.
Fig 3.11 A
Over the course of 30 years (1970 to 2000) beak size
evolved. Rose sharply during drought (red line) then
declined to pre-drought dimensions.
Fig 3.11 B
Over same 30-year period birds evolved more pointed
beaks and (next slide) significantly smaller body size.
Fig 3.11 C
Constraints on natural selection
 There
are limits to what natural selection
can produce.
 Natural
selection cannot for example
produce organisms that violate the laws of
physics for example.
Constraints on natural selection
 For
example, the size of unicellular
organisms is limited because they depend
on diffusion across the cell membrane to
obtain their essential requirements.
 Because the volume of a cell increases as
a cubic function of linear dimension, but
surface area only increases as a square
function, above a certain size, a cell
becomes too big for diffusion to supply its
needs.
Constraints on natural selection
A
lack of genetic variation also constrains
natural selection.
 Natural
selection depends on mutation
and sexual reproduction to produce
variation, but if a trait is not produced by
these processes it cannot be selected for.
The nature of Natural Selection
 Many
misconceptions about how selection
operates and evolution occurs.
 Points
to remember about natural
selection
Natural selection acts on individuals,
but its effects accumulate in
populations
 Individual
finches live or die during a
drought (the selection event).
 But
change occurs in the characteristics of
the population, not in individuals.
Natural selection acts on
individuals, but its effects
accumulate in populations
 During
drought individual finch’s beaks
did not change, but average beak
dimensions changed because more
small-beaked birds died than largebeaked birds.
Evolution causes changes in
allele frequencies
 Evolution
only occurs when traits have a
genetic basis.
 If
beak dimensions were environmentally
induced, no evolution could take place.
After drought, frequencies of phenotypes in
next generation might have been the same
as before.
Natural selection does not plan
ahead.
 Each
generation is result of selection by
environmental conditions of the previous
generation.
 Evolution always one generation behind
environmental changes.
New traits evolve even though
selection acts on existing
traits.
 This
occurs because:
 1. mutation produces new alleles.
 2.
In sexually reproducing organisms
meiosis and fertilization recombine
existing alleles to produce new genotypes.
New traits evolve even though
selection acts on existing traits.
 Artificial
 After
selection for oil content in corn.
60 generations oil levels were well
above starting values.
Fig 3.12
New traits evolve even though
selection acts on existing traits.
 Natural
selection can also modify existing
features over time for a new purpose e.g.
Panda’s thumb.
 Trait
used in novel way and eventually
developed into a new structure referred to
as a preadaptation. This does NOT
mean there is pre-planning by natural
selection.
Natural selection does not
produce ‘perfect’ solutions
 Panda’s
thumb not a perfect solution.
Natural selection does not
produce ‘perfect’ solutions

On Daphne Major during drought finches with
narrow beaks survived better than those with
wider bills.

At end of drought, however, selection for
deeper bills and bigger body size resulted in
wider beaks even through deeper narrower
beaks would have been a better solution.


Presumably same genes control all three
traits. And solution is not perfect.
Natural selection does not
produce ‘perfect’ solutions
 Similarly,
many characteristics of
organisms are the result of compromises
between different selection pressures.
 The
wings of various species of auk
(seabirds that dive and swim) are a
compromise between the need to “fly” (i.e.
swim) underwater and in the air, two very
different media.
Natural selection is nonrandom,
but not progressive
 There
is no “goal” of natural selection.
 Evolution
makes organisms better adapted
to their environments, but there is no trend
towards being more advanced.
 E.g.
Tapeworms have no digestive tract.
They are simpler than their ancestors.
Selection does not act for the
“good of the species”
 Apparently
altruistic acts (e.g. giving an
alarm call) are favored because they
enhance relatives’ survival.
 Infanticide
in lions benefits individual male
lions not the species as a whole.