Chapter 3 Natural Selection Assigned reading chapter 3.

Chapter 3 Natural SelectionChapter 3 Natural Selection

Assigned reading chapter 3.Assigned reading chapter 3.

Evolution by Natural SelectionEvolution by Natural Selection

Darwin envisaged process similar to Darwin envisaged process similar to artificial selection that had produced artificial selection that had produced organisms we see today. He called it organisms we see today. He called it Natural SelectionNatural Selection..

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.



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.

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


Phenotype Phenotype

An organism’s physical characteristics An organism’s physical characteristics such as height, coloration, shape as well such as height, coloration, shape as well as its behavior are its as its behavior are its phenotypephenotype..

Phenotype is the result of both the Phenotype is the result of both the organism’s genotype and the influence of organism’s genotype and the influence of the environment acting on the the environment acting on the genotypegenotype..

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 considerable phenotypic variation There is considerable phenotypic variation in coat color in oldfield mice.in coat color in oldfield mice.

For natural selection to occur the variation For natural selection to occur the variation must be heritable. Hoekstra et al. have must be heritable. Hoekstra et al. have shown that several genes affect coat color shown that several genes affect coat color in these mice.in these mice.

Genetics of coat colorGenetics of coat color

The first gene is the melanocortin-1 The first gene is the melanocortin-1 receptor gene (Mc1R). This gene receptor gene (Mc1R). This gene switches between producing a dark switches between producing a dark pigment (Eumelanin) and a light pigment pigment (Eumelanin) and a light pigment (Phaeomelanin) depending on how it is (Phaeomelanin) depending on how it is affected by signals from other genes. affected by signals from other genes.


If a protein called alpha-MSH binds to the If a protein called alpha-MSH binds to the McR1 gene then the dark pigment McR1 gene then the dark pigment eumelanin is produced.eumelanin is produced.

If alpha-MSH cannot bind to the Mc1R gene If alpha-MSH cannot bind to the Mc1R gene either because of the presence of a different either because of the presence of a different protein called ASP (agouti-signaling protein) protein called ASP (agouti-signaling protein) or because a mutation in the Mc1R gene so or because a mutation in the Mc1R gene so it cannot bind alpha-MSH then the light-it cannot bind alpha-MSH then the light-colored pigment phaeomelanin is produced.colored pigment phaeomelanin is produced.


Hoekstra et al. have shown in populations Hoekstra et al. have shown in populations with large numbers of light-colored mice with large numbers of light-colored mice that two mutations are common: one that that two mutations are common: one that prevents alpha-MSH binding to Mc1R and prevents alpha-MSH binding to Mc1R and the second a mutant agouti allele that the second a mutant agouti allele that produces much more ASP than usual. produces much more ASP than usual.

Both mutant alleles result in light-colored Both mutant alleles result in light-colored mice. Thus there is a clear genetic basis mice. Thus there is a clear genetic basis for the observed variation in coat color.for the 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) carried out an experiment Kaufman (1974) carried out an experiment in which pairs of mice (one dark-coated, in which pairs of mice (one dark-coated, one light coated) along with an owl were one light coated) along with an owl were placed in large cages located in habitats placed in large cages located in habitats with different backgrounds (light or dark with different backgrounds (light or dark and with different vegetation densities).and with 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. carried out a follow-up Kaufman et al. carried out a follow-up experiment in which they made silicone mouse experiment in which they made silicone mouse models painted light or dark to mimic either the models painted light or dark to mimic either the dark or light background.dark or light background.

They placed the models in different habitats They placed the models in different habitats and measured from beak and claw marks how and measured from beak and claw marks how often the models were attacked. They found often the models were attacked. They found clear differences in attack rates. Models that clear differences in attack rates. Models that matched their background were attacked matched their background were attacked much less.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.

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.

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 selectionConstraints on natural selection

There are limits to what natural selection There are limits to what natural selection can produce.can produce.

Natural selection cannot for example Natural selection cannot for example produce organisms that violate the laws of produce organisms that violate the laws of physics for example. physics for example.


For example, the size of unicellular organisms For example, the size of unicellular organisms is limited because they depend on diffusion is limited because they depend on diffusion across the cell membrane to obtain their across the cell membrane to obtain their essential requirements.essential requirements.

Because the volume of a cell increases as a Because the volume of a cell increases as a cubic function of linear dimension, but surface cubic function of linear dimension, but surface area only increases as a square function, area only increases as a square function, above a certain size, a cell becomes too big above a certain size, a cell becomes too big for diffusion to supply its needs.for diffusion to supply its needs.


A lack of genetic variation also constrains A lack of genetic variation also constrains natural selection.natural selection.

Natural selection depends on mutation Natural selection depends on mutation and sexual reproduction to produce and sexual reproduction to produce variation, but if a trait is not produced by variation, but if a trait is not produced by these processes it cannot be selected for.these processes it cannot be selected for.

The nature of Natural SelectionThe nature of Natural Selection

Many misconceptions about how selection Many misconceptions about how selection operates and evolution occurs.operates and evolution occurs.

Points to remember about natural Points to remember about natural selectionselection

Natural selection acts on individuals, Natural selection acts on individuals, but its effects accumulate in but its effects accumulate in

populationspopulations

Individual finches live or die during a Individual finches live or die during a drought (the selection event). drought (the selection event).

But change occurs in the characteristics of But change occurs in the characteristics of the the populationpopulation, not in individuals., not in individuals.

Natural selection acts on Natural selection acts on individuals, but its effects individuals, but its effects accumulate in populationsaccumulate in populations

During drought individual finch’s beaks During drought individual finch’s beaks did not change, but average beak did not change, but average beak dimensions changed because more dimensions changed because more small-beaked birds died than large-small-beaked birds died than large-beaked birds.beaked birds.

Evolution causes changes in Evolution causes changes in allele frequenciesallele frequencies

Evolution only occurs when traits have a Evolution only occurs when traits have a geneticgenetic basis. basis.

If beak dimensions were environmentally If beak dimensions were environmentally induced, no evolution could take place. induced, no evolution could take place. After drought, frequencies of phenotypes in After drought, frequencies of phenotypes in next generation might have been the same next generation might have been the same as before.as before.

Natural selection does not plan Natural selection does not plan ahead.ahead.

Each generation is result of selection by Each generation is result of selection by environmental conditions of the environmental conditions of the previousprevious generation.generation.

Evolution always one generation behind Evolution always one generation behind environmental changes.environmental changes.

New traits evolve even though New traits evolve even though selection acts on existing selection acts on existing

traits. traits. This occurs because: This occurs because: 1. mutation produces new alleles.1. mutation produces new alleles.

2. In sexually reproducing organisms 2. In sexually reproducing organisms meiosis and fertilization recombine meiosis and fertilization recombine existing alleles to produce new genotypes.existing alleles to produce new genotypes.

New traits evolve even though New traits evolve even though selection acts on existing traits.selection acts on existing traits.

Artificial selection for oil content in corn.Artificial selection for oil content in corn.

After 60 generations oil levels were well After 60 generations oil levels were well above starting values.above starting values.

Fig 3.12

New traits evolve even though New traits evolve even though selection acts on existing traits.selection acts on existing traits.

Natural selection can also modify existing Natural selection can also modify existing features over time for a new purpose e.g. features over time for a new purpose e.g. Panda’s thumb.Panda’s thumb.

Trait used in novel way and eventually Trait used in novel way and eventually developed into a new structure referred to developed into a new structure referred to as a as a preadaptationpreadaptation. This does NOT . This does NOT mean there is pre-planning by natural mean there is pre-planning by natural selection.selection.

Natural selection does not Natural selection does not produce ‘perfect’ solutionsproduce ‘perfect’ solutions

Panda’s thumb not a perfect solution.Panda’s thumb not a perfect solution.


On Daphne Major during drought finches with On Daphne Major during drought finches with narrow beaks survived better than those with narrow beaks survived better than those with wider bills.wider bills.

At end of drought, however, selection for deeper At end of drought, however, selection for deeper bills and bigger body size resulted in wider beaks bills and bigger body size resulted in wider beaks even through deeper narrower beaks would have even through deeper narrower beaks would have been a better solution.been a better solution.

Presumably same genes control all three traits. Presumably same genes control all three traits.

And solution is not perfect.And solution is not perfect.


Similarly, many characteristics of Similarly, many characteristics of organisms are the result of compromises organisms are the result of compromises between different selection pressures.between different selection pressures.

The wings of various species of auk The wings of various species of auk (seabirds that dive and swim) are a (seabirds that dive and swim) are a compromise between the need to “fly” (i.e. compromise between the need to “fly” (i.e. swim) underwater and in the air, two very swim) underwater and in the air, two very different media.different media.

Natural selection is nonrandom, Natural selection is nonrandom, but not progressivebut not progressive

There is no “goal” of natural selection.There is no “goal” of natural selection.

Evolution makes organisms better adapted Evolution makes organisms better adapted to their environments, but there is no trend to their environments, but there is no trend towards being more advanced.towards being more advanced.

E.g. Tapeworms have no digestive tract. E.g. Tapeworms have no digestive tract. They are simpler than their ancestors.They are simpler than their ancestors.

Selection does not act for the Selection does not act for the “good of the species”“good of the species”

Apparently altruistic acts (e.g. giving an Apparently altruistic acts (e.g. giving an alarm call) are favored because they alarm call) are favored because they enhance relatives’ survival.enhance relatives’ survival.

Infanticide in lions benefits individual male Infanticide in lions benefits individual male lions not the species as a whole.lions not the species as a whole.

Chapter 3 Natural Selection Assigned reading chapter 3.

Documents

Transcript of Chapter 3 Natural Selection Assigned reading chapter 3.