Transcript Chapter 8 Natural Selection Empirical studies

Chapter 8 Natural Selection:
Empirical studies in the wild
 Assigned
reading chapter 8.
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
 Recall
-- 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.
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 lots of phenotypic variation in coat
color in oldfield mice.
 For
natural selection to occur the variation
must be heritable. Several genes affect
coat color in these mice.
Genetics of coat color
 Melanocortin-1
receptor gene (Mc1R).
This gene produces either a dark pigment
(Eumelanin) or a light pigment
(Phaeomelanin) depending on signals it
receives from other genes.
Genetics of coat color
 If
a protein called alpha-MSH binds to the
Mc1R gene then the dark pigment
eumelanin is produced.
 If alpha-MSH cannot bind then a lightcolored pigment phaeomelanin is
produced.
Genetics of coat color
 In
populations with many light-colored
mice two mutations are common:
 (1) mutant that prevents alpha-MSH
binding to Mc1R
 (2) mutant allele that produces an excess
of a protein called ASP that competes with
alpha-MSH to bind to the MC1R.
 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) experiment
 Pairs of mice (one dark-coated, one light
coated) and 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. made silicone mouse
models painted light or dark.
 Placed the models in different habitats and
measured how often the models were
attacked.
 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.
Strong association between parent and
offspring beak sizes. Hence narrow-sense
heritability is high.
There is a difference in beak dimensions
(selection differential) between breeders
and original population.
Response to selection in that beak
dimensions increased in the offspring.
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.
Agents of selection operating in opposite
directions– gall flies.
Gall flies induce plants to produce galls in which
the larva develops in a protected environment.
Gall diameter is variable. Some individuals
produce large galls and others small ones.
Relatives produce similar size galls and
there is heritable variation in gall size.
Stabilizing selection on gall size
 There
are two major predators of larvae in
galls – birds and parasitic wasps.
 Parasitic
wasps cannot reach larvae
enclosed in very large galls, but birds spot
large galls more easily and consume the
larvae. There is thus stabilizing selection
on gall size with intermediate sized galls
favored.
Milk drinking: evidence for
natural selection
 Milk
contains the sugar lactose and young
mammals produce an enzyme, lactase, to
break it down. Most humans (about 70%)
stop producing lactase after weaning, but
many western Europeans retain the ability
to digest lactose into adulthood.
Milk drinking: evidence for
natural selection
 Humans
began to domesticate cattle in
NW Europe about 10,000 years ago and
this new food source favored individuals
able to digest milk into adulthood.
 The
frequency of alleles for lactose
tolerance are highest in NW European
populations and lowest in SE Europe the
in populations furthest from the origin of
cattle domestication.
Milk drinking: evidence for
natural selection
A
similar pattern is found when comparing
animal herding societies with nearby nonherding populations.
 The
herders have much higher tolerance
for lactose than their non-herding
neighbors.
Humans as agents of selection
 Humans
 This
act as strong agents of selection.
has occurred through deliberate
choice (artificial selection for desired traits
in crops and domesticated animals) and
inadvertently through environmental
change.
Artificial Selection
 Artificial
Selection. Humans have
selectively bred for desirable traits in
domestic animals and plants for millenia.
 Process has produced our crop plants,
garden plants, pets, and domestic
animals.
 Recall: Darwin closely studied pigeon
breeding as a process analogous to
natural selection.
Artificial Selection
 Cauliflower,
broccoli, kale, brussels
sprouts all descended from wild cabbage.
 All
these crops can be crossed and
produce fertile offspring.
 Cauliflower:
edible bit is the inflorescence
or flower stalk.
Artificial Selection
 Cauliflower
has large dense infloresence.
This results from mutant ‘loss of function’
alleles of two genes that affect flower
structure and infloresence density.
Artificial Selection
 Early
farmers choosing among their crops
selected those with largest infloresences.
Process has resulted in cauliflowers that
are homozygous for both loss of function
alleles.
Pesticides and herbicides act as
agents of selection
Resistance to pesticides
 Insects
and plants treated with chemicals
designed to kill them have rapidly
developed resistance.
 Heavy
spraying creates an environment in
which any mutations that offer resistance
are strongly selected for and spread
rapidly.
Resistance to pesticides in houseflies
Inverted triangle indicates first occurrence of resistance and R indicates when most
Populations were resistant. Bar width indicates extent of the pesticides use.
Rapid evolution of herbicide
resistance
Resistance to pesticides
 Farmers
are now using evolutionary
biology to reduce rate of evolution of
resistance.
 Resistance
frequently comes with a cost
and in pesticide-free environments nonresistant pests may have an advantage
and outcompete resistant forms.
Resistance to pesticides
 To
maintain non-resistant genes in pest
populations farmers are now setting aside
pesticide free refuges that are not
sprayed.
 For
example farmers using BT-corn (corn
containing a gene that produces a natural
pesticide) must set aside 20% of their
plantings as non-BT corn
Resistance to pesticides
 States
in which large areas of refuges
were used have shown much slower rates
of BT-resistance in pests than states
where smaller areas of refuges were set
aside.
Hunting and fishing as agents of
selection
 Humans
have intensively fished all the
world’s oceans and that fishing pressure
has resulted in fish populations evolving in
response.
 For
example, because under fishing
pressure few individuals survive to breed
late in life, fish such as cod today mature
much younger and at smaller sizes than
they did 20 years ago.
Hunting and fishing as agents of
selection
 In
a similar fashion selective shooting by
trophy hunters of males with larger horns
has led to the evolution of smaller horns in
hunted populations.
Evolution of shorter male horns due
to hunting