chapter 8 natural selection: empirical studies in the wild assigned reading chapter 8

Chapter 8 Natural Selection: Chapter 8 Natural Selection: Empirical studies in the wildEmpirical studies in the wild

Assigned reading chapter 8.Assigned reading chapter 8.

Recall Darwin proposed evolution was the Recall Darwin proposed evolution was the inevitable outcome of 4 postulates:inevitable outcome of 4 postulates:

1. There is 1. There is variation in populationsvariation in populations. .

Individuals within populations differ.Individuals within populations differ.

2. 2. Variation is heritable.Variation is heritable.

Evolution by Natural SelectionEvolution by Natural Selection


3. In every generation some organisms 3. In every generation some organisms are more successful at surviving and are more successful at surviving and reproducing than other. reproducing than other. Differential Differential reproductive success.reproductive success.

4. 4. Survival and reproduction are not Survival and reproduction are not randomrandom, but are related to variation among , but are related to variation among individuals. Organisms with best individuals. Organisms with best characteristics are ‘naturally selected.’characteristics are ‘naturally selected.’


If 4 postulates are true then the population If 4 postulates are true then the population will change from one generation to the will change from one generation to the next. next.

Evolution will occur.Evolution will occur.

Recall -- Darwinian fitness: ability of an Recall -- Darwinian fitness: ability of an organism to survive and reproduce in its organism to survive and reproduce in its environment.environment.

Fitness measured Fitness measured relativerelative to others of its to others of its speciesspecies


Adaptation is a characteristic or trait of an Adaptation is a characteristic or trait of an organism that increases its fitness relative organism that increases its fitness relative to individuals that do not possess it. to individuals that do not possess it.


Natural Selection and coat color in the Natural Selection and coat color in the oldfield mouse: is there variation?oldfield mouse: is there variation?

The oldfield mouse is widely distributed in The oldfield mouse is widely distributed in the southeastern U.S. It is preyed upon by the southeastern U.S. It is preyed upon by a variety of visually hunting predators such a variety of visually hunting predators such as hawks and owls.as hawks and owls.

The mouse displays considerable The mouse displays considerable variationvariation in coat color both within and in coat color both within and between populations across its range.between populations across its range.

Natural Selection and coat color Natural Selection and coat color in the oldfield mousein the oldfield mouse

Most populations of the mouse are dark Most populations of the mouse are dark colored, but populations on beaches and colored, but populations on beaches and barrier islands have lighter colored coats.barrier islands have lighter colored coats.

Hoekstra et al. carried out a series of Hoekstra et al. carried out a series of experiments to evaluate the hypothesis experiments to evaluate the hypothesis that natural selection favors a match that natural selection favors a match between coat color and background color. between coat color and background color.

Is variation in coat color Is variation in coat color heritable?heritable?

There is lots of phenotypic variation in coat There is lots of phenotypic variation in coat color in oldfield mice.color in oldfield mice.

For natural selection to occur the variation For natural selection to occur the variation must be heritable. Several genes affect must be heritable. Several genes affect coat color in these mice.coat color in these mice.

Genetics of coat colorGenetics of coat color

Melanocortin-1 receptor gene (Mc1R). Melanocortin-1 receptor gene (Mc1R). This gene produces either a dark pigment This gene produces either a dark pigment (Eumelanin) or a light pigment (Eumelanin) or a light pigment (Phaeomelanin) depending on signals it (Phaeomelanin) depending on signals it receives from other genes. receives from other genes.


If a protein called alpha-MSH binds to the If a protein called alpha-MSH binds to the Mc1R gene then the dark pigment Mc1R gene then the dark pigment eumelanin is produced.eumelanin is produced.

If alpha-MSH cannot bind then a light-If alpha-MSH cannot bind then a light-colored pigment phaeomelanin is colored pigment phaeomelanin is produced.produced.


In populations with many light-colored mice two In populations with many light-colored mice two mutations are common: mutations are common:

(1) mutant that prevents alpha-MSH binding to (1) mutant that prevents alpha-MSH binding to Mc1R Mc1R

(2) mutant allele that produces an excess of a (2) mutant allele that produces an excess of a protein called ASP that competes with alpha-protein called ASP that competes with alpha-MSH to bind to the MC1R. MSH to bind to the MC1R.

Both mutant alleles result in light-colored mice. Both mutant alleles result in light-colored mice. Thus there is a clear genetic basis for the Thus there is a clear genetic basis for the observed variation in coat color.observed variation in coat color.

Does variation affect fitness?Does variation affect fitness?

Does coat color affect the survival and Does coat color affect the survival and ultimately reproduction (i.e. fitness) of ultimately reproduction (i.e. fitness) of oldfield mice?oldfield mice?

Two experiments suggest it does.Two experiments suggest it does.


Kaufman (1974) experiment Kaufman (1974) experiment Pairs of mice (one dark-coated, one light Pairs of mice (one dark-coated, one light

coated) and an owl were placed in large coated) and an owl were placed in large cages located in habitats with different cages located in habitats with different backgrounds (light or dark and with backgrounds (light or dark and with different vegetation densities).different vegetation densities).


In all cases mice that better matched the In all cases mice that better matched the background survived better than mice that background survived better than mice that matched less well.matched less well.


Kaufman et al. made silicone mouse Kaufman et al. made silicone mouse models painted light or dark.models painted light or dark.

Placed the models in different habitats and Placed the models in different habitats and measured how often the models were measured how often the models were attacked. attacked.

Clear differences in attack rates. Models Clear differences in attack rates. Models that matched their background were that matched their background were attacked much less.attacked much less.

Natural Selection and coat color Natural Selection and coat color in the oldfield mousein the oldfield mouse

Thus for oldfield mice all 4 postulates are Thus for oldfield mice all 4 postulates are satisfied. There is (i) variation in coat color satisfied. There is (i) variation in coat color and it is (ii) heritable.and it is (ii) heritable.

There is (iii) differential reproductive success There is (iii) differential reproductive success (or in this case differential survival which is a (or in this case differential survival which is a necessary precursor to reproduction).necessary precursor to reproduction).

That differential reproductive success is (iv) That differential reproductive success is (iv) related to the variation (different coat colors related to the variation (different coat colors survive better in different habitats).survive better in different habitats).

Another example of natural Another example of natural selection: Darwin’s finchesselection: Darwin’s finches

Evolution of beak shape in Darwin’s Evolution of beak shape in Darwin’s Finches.Finches.

Peter and Rosemary Grant’s (and Peter and Rosemary Grant’s (and colleagues) work on Medium Ground colleagues) work on Medium Ground Finches Finches Geospiza fortisGeospiza fortis

On Daphne Major since 1973.On Daphne Major since 1973.


Postulate 1. Is the population variable?Postulate 1. Is the population variable?

Finches vary in beak length, beak depth, Finches vary in beak length, beak depth, beak width, wing length and tail length. beak width, wing length and tail length.


Postulate 2: Is variation among individuals Postulate 2: Is variation among individuals heritable?heritable?

Variation can be a result of environmental Variation can be a result of environmental effects.effects.

Heritability: proportion of the variation in a Heritability: proportion of the variation in a trait in a population that is due to variation trait in a population that is due to variation in genes.in genes.


Peter Boag compared average beak depth Peter Boag compared average beak depth of parents with that of their adult offspring. of parents with that of their adult offspring.

Strong relationship between offspring and Strong relationship between offspring and parent beak depths.parent beak depths.

FIG 3.7


Postulate 3: Do individuals differ in their Postulate 3: Do individuals differ in their success at survival and reproduction?success at survival and reproduction?

1977 drought 84% of 1977 drought 84% of G. fortisG. fortis individuals individuals died, most from starvation. In two other died, most from starvation. In two other droughts 19% and 25% of the population droughts 19% and 25% of the population died.died.


Seed densities declined rapidly during Seed densities declined rapidly during drought and the small soft seeds were drought and the small soft seeds were consumed first. consumed first.

Average size and hardness of remaining Average size and hardness of remaining seeds increased over the course of the seeds increased over the course of the drought.drought.

FIG 3.8b

FIG 3.8A

Fig 3.8c


Postulate 4: Are survival and reproduction Postulate 4: Are survival and reproduction nonrandom?nonrandom?

Do those who survive and reproduce have Do those who survive and reproduce have different characteristics than those that different characteristics than those that don’t?don’t?


As drought progressed small soft seeds As drought progressed small soft seeds disappeared and large, hard disappeared and large, hard TribulusTribulus seeds became a key food item.seeds became a key food item.

Only birds with deep, narrow beaks could Only birds with deep, narrow beaks could open them.open them.


At end of the 1977 drought the average At end of the 1977 drought the average survivor had a deeper beak than the survivor had a deeper beak than the average non-survivor and also a larger average non-survivor and also a larger body size.body size.

FIG 3.9

Did the population evolve?Did the population evolve?

Chicks hatched in 1978 had deeper beaks Chicks hatched in 1978 had deeper beaks on average than those hatched in 1976.on average than those hatched in 1976.

Population evolved.Population evolved.

Strong association between parent andoffspring beak sizes. Hence narrow-senseheritability is high.

There is a difference in beak dimensions(selection differential) between breedersand original population.

Response to selection in that beak dimensions increased in the offspring.

Fig 3.10


Variation in weather from year to year on Variation in weather from year to year on Daphne Major over 30 years has led to Daphne Major over 30 years has led to variation in the traits that are favored by variation in the traits that are favored by selection.selection.

Population has evolved over time.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) thendeclined to pre-drought dimensions.

Agents of selection operating in opposite Agents of selection operating in opposite directions– gall flies. directions– gall flies.

Gall flies induce plants to produce galls in which Gall flies induce plants to produce galls in which the larva develops in a protected environment.the larva develops in a protected environment.

Gall diameter is variable. Some individuals Gall diameter is variable. Some individuals produce large galls and others small ones. produce large galls and others small ones.

Relatives produce similar size galls and Relatives produce similar size galls and there is heritable variation in gall size.there is heritable variation in gall size.

Stabilizing selection on gall sizeStabilizing selection on gall size

There are two major predators of larvae in There are two major predators of larvae in galls – birds and parasitic wasps.galls – birds and parasitic wasps.

Parasitic wasps cannot reach larvae Parasitic wasps cannot reach larvae enclosed in very large galls, but birds spot enclosed in very large galls, but birds spot large galls more easily and consume the large galls more easily and consume the larvae. There is thus stabilizing selection larvae. There is thus stabilizing selection on gall size with intermediate sized galls on gall size with intermediate sized galls favored.favored.

Milk drinking: evidence for Milk drinking: evidence for natural selectionnatural selection

Milk contains the sugar lactose and young Milk contains the sugar lactose and young mammals produce an enzyme, lactase, to mammals produce an enzyme, lactase, to break it down. Most humans (about 70%) break it down. Most humans (about 70%) stop producing lactase after weaning, but stop producing lactase after weaning, but many western Europeans retain the ability many western Europeans retain the ability to digest lactose into adulthood.to digest lactose into adulthood.


Humans began to domesticate cattle in NW Humans began to domesticate cattle in NW Europe about 10,000 years ago and this new Europe about 10,000 years ago and this new food source favored individuals able to digest food source favored individuals able to digest milk into adulthood.milk into adulthood.

The frequency of alleles for lactose tolerance The frequency of alleles for lactose tolerance are highest in NW European populations and are highest in NW European populations and lowest in SE Europe the in populations furthest lowest in SE Europe the in populations furthest from the origin of cattle domestication.from the origin of cattle domestication.


A similar pattern is found when comparing A similar pattern is found when comparing animal herding societies with nearby non-animal herding societies with nearby non-herding populations.herding populations.

The herders have much higher tolerance The herders have much higher tolerance for lactose than their non-herding for lactose than their non-herding neighbors. neighbors.

Humans as agents of selectionHumans as agents of selection

Humans act as strong agents of selection.Humans act as strong agents of selection.

This has occurred through deliberate This has occurred through deliberate choice (artificial selection for desired traits choice (artificial selection for desired traits in crops and domesticated animals) and in crops and domesticated animals) and inadvertently through environmental inadvertently through environmental change.change.

Artificial SelectionArtificial Selection Artificial Selection. Humans have Artificial Selection. Humans have

selectively bred for desirable traits in selectively bred for desirable traits in domestic animals and plants for millenia.domestic animals and plants for millenia.

Process has produced our crop plants, Process has produced our crop plants, garden plants, pets, and domestic garden plants, pets, and domestic animals.animals.

Recall: Darwin closely studied pigeon Recall: Darwin closely studied pigeon breeding as a process analogous to breeding as a process analogous to natural selection.natural selection.

Artificial SelectionArtificial Selection

Cauliflower, broccoli, kale, brussels Cauliflower, broccoli, kale, brussels sprouts all descended from wild cabbage. sprouts all descended from wild cabbage.

All these crops can be crossed and All these crops can be crossed and produce fertile offspring.produce fertile offspring.

Cauliflower: edible bit is the inflorescence Cauliflower: edible bit is the inflorescence or flower stalk.or flower stalk.


Cauliflower has large dense infloresence. Cauliflower has large dense infloresence. This results from mutant ‘loss of function’ This results from mutant ‘loss of function’ alleles of two genes that affect flower alleles of two genes that affect flower structure and infloresence density.structure and infloresence density.


Early farmers choosing among their crops Early farmers choosing among their crops selected those with largest infloresences. selected those with largest infloresences. Process has resulted in cauliflowers that Process has resulted in cauliflowers that are homozygous for both loss of function are homozygous for both loss of function alleles.alleles.

Pesticides and herbicides act as Pesticides and herbicides act as agents of selectionagents of selection

Resistance to pesticidesResistance to pesticides

Insects and plants treated with chemicals Insects and plants treated with chemicals designed to kill them have rapidly designed to kill them have rapidly developed resistance.developed resistance.

Heavy spraying creates an environment in Heavy spraying creates an environment in which any mutations that offer resistance which any mutations that offer resistance are strongly selected for and spread are strongly selected for and spread rapidly.rapidly.

Resistance to pesticides in housefliesResistance 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 Rapid evolution of herbicide resistanceresistance


Farmers are now using evolutionary Farmers are now using evolutionary biology to reduce rate of evolution of biology to reduce rate of evolution of resistance.resistance.

Resistance frequently comes with a cost Resistance frequently comes with a cost and in pesticide-free environments non-and in pesticide-free environments non-resistant pests may have an advantage resistant pests may have an advantage and outcompete resistant forms.and outcompete resistant forms.


To maintain non-resistant genes in pest To maintain non-resistant genes in pest populations farmers are now setting aside populations farmers are now setting aside pesticide free refuges that are not pesticide free refuges that are not sprayed.sprayed.

For example farmers using BT-corn (corn For example farmers using BT-corn (corn containing a gene that produces a natural containing a gene that produces a natural pesticide) must set aside 20% of their pesticide) must set aside 20% of their plantings as non-BT cornplantings as non-BT corn


States in which large areas of refuges States in which large areas of refuges were used have shown much slower rates were used have shown much slower rates of BT-resistance in pests than states of BT-resistance in pests than states where smaller areas of refuges were set where smaller areas of refuges were set aside.aside.

Hunting and fishing as agents of Hunting and fishing as agents of selection selection

Humans have intensively fished all the world’s Humans have intensively fished all the world’s oceans and that fishing pressure has resulted oceans and that fishing pressure has resulted in fish populations evolving in response.in fish populations evolving in response.

For example, because under fishing pressure For example, because under fishing pressure few individuals survive to breed late in life, few individuals survive to breed late in life, fish such as cod today mature much younger fish such as cod today mature much younger and at smaller sizes than they did 20 years and at smaller sizes than they did 20 years ago.ago.

Hunting and fishing as agents of Hunting and fishing as agents of selection selection

In a similar fashion selective shooting by In a similar fashion selective shooting by trophy hunters of males with larger horns trophy hunters of males with larger horns has led to the evolution of smaller horns in has led to the evolution of smaller horns in hunted populations. hunted populations.

Evolution of shorter male horns due Evolution of shorter male horns due to huntingto hunting

chapter 8 natural selection: empirical studies in the wild assigned reading chapter 8

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