Chapter 8: Evolution and Natural Selection Darwin’s dangerous idea: evolution by natural selection



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Chapter 8: Evolution and Natural Selection

Darwin’s dangerous idea: evolution by natural selection

Objectives

Be able to explain evolution in action.
Be able to explain Darwin’s journey to an idea.
Be able to describe and explain the four mechanisms that can give rise to evolution.

Objectives

Be able to explain how populations of organisms can adapt to their environment through natural selection.
Be able to explain how the evidence for the occurrence of evolution is overwhelming.
Evolution in Action

8.1 We can see evolution occur right before us.
How long could a fruit fly survive without food.
Only about 20 hours.
They can’t live long without food because their tiny bodies don’t hold very large caloric reserves.
Could you breed fruit flies who could live longer than 20 hours on average?
1. Start with a large population.

Population = group of organisms of the same species in a particular geographic habitat.
2. Wait for 80% to starve to death.

3. Then feed the survivors.

4. When they breed and produce eggs, collect them and grow them.

When these eggs hatch, do you think the flies in this new generation will live longer than 20 hours without food?


What happened?

Evolution

a genetic change in the population
Natural selection

the consequence of certain individual organisms in a population being born with characteristics that enable them to survive better and reproduce more than the offspring of other individuals in the population

Does evolution occur?

The answer is an unambiguous: YES.
We can watch it happen in the lab whenever we want.

Researchers carried out the starvation-resistance experiment described above five separate times.

The results were the same every time.
Reference: Methuselah Flies: A Case Study in the Evolution of Aging

By Michael R. Rose

Experiments in Evolution

Dogs? If we selected only the smallest puppies in the litter, would size of dogs decrease over time?

Yes! Because these experiments have already been done.

What if only the fastest rabbits escaped the predators, would running speed increase over time?

Topics of Evolution in this Chapter

How does evolution occur?
What types of changes can evolution cause in a population?
Five primary lines of evidence
Evolution by natural selection
Darwin’s Journey to an Idea

8.2 Before Darwin, most people believed that all species had been created separately and were unchanging.

Darwin’s Impact

Darwin through into question the commonly held beliefs about the natural world, and changed our perspectives on the origins of humans and our relationship to all other species.

But he did not do it by himself.


LaMarkian Inheritance: Theory of evolution by acquired traits.

Charles Lyell

Geologist
1830 book Principles of Geology

Geological forces had shaped the earth and were continuing to do so.
The idea that the physical features of the earth were constantly changing would most closely parallel Darwin’s idea that the living species of the earth, too, were gradually—but constantly—changing.
Take-home message 8.2

People used to think that the earth was 6,000 years old and that species were unchanging.
In the 18th and 19th centuries, scientists began to change their beliefs, and it began to be accepted that the earth was at least millions of years old.
These changes helped shape Darwin’s thinking.

8.3 A job on a ‘round-the-world survey ship allowed Darwin to indulge and advance his love of nature.

Age 16, University of Edinburgh, medical studies
Studied theology at Cambridge University
His real love: study of nature

He was intellectually challenged by what he saw.

How, for example, could he explain marine fossils high in the Andes mountains hundreds of miles from the sea?


An then he came to the Galapagos Islands several years into the voyage

The Galapagos Islands

The living laboratory of evolution

This group of volcanic islands was home to many unusual species, from giant tortoises to extremely docile lizards that made so little effort to run away that Darwin had to avoid stepping on them.

Two important and unexpected patterns:1. Traits exhibited by species
Darwin originally thought that all the Galapagos finches were of the same species, but with different physical characteristics or traits.

The Zoology Society, however, could see from their physical differences that there were 13 unique species—a different species from every one of the Galapagos Islands that Darwin had visited.

They all resembled very closely the single species of finches living on the closest mainland, in Ecuador.

2. Similarity between the fossils of extinct species and the living species in that same area

Glyptodonts and armadillos


If glyptodonts had lived in South America in the past, and armadillos were currently living there, why was only one of the species still alive?

And why were the glyptodont fossils found only in the same places that modern armadillos live?

Darwin deduced that glyptodonts resembled armadillos because they were their ancestors.

The wheels were turning in Darwin’s head but he didn’t have a great “Eureka!” moment until several years after his voyage when he was back in England. Darwin was reading “for amusement” a book called Essay on the Principle of Population by the economist Thomas Malthus.

In Essay on the Principle of Population. Malthus prophesied doom and gloom for populations—including humans—based on his belief that populations had the potential to grow much faster than food supplies could.

Darwin speculated that, rather than certain catastrophe for all, maybe the best individuals would “win” in the ensuing struggle for existence and the worst would “lose.” The favorable variations are preserved.

The Book that Would “Rock the World”

1842 first draft of a 35 page paper.
14 years in a drawer


Darwin published only after Alfred Russel Wallace independently came up with the same idea.


It came in the form of a letter from Wallace in Malaysia.

In the letter, Alfred Russel Wallace, who had also read Malthus’s book on population growth, laid out a clear description of the process of evolution by natural selection. He asked Darwin to “publish it if you think it is worthy.”

Crushed at having been scooped, Darwin wrote that “all my originality will be smashed.”

He had numerous friends among the most prominent scientists of the day, however, and they arranged for a joint presentation to the Linnaean Society of London of Wallace’s and Darwin’s work.

As a result, both Darwin and Wallace are credited for the first description of evolution by natural selection.


In 1859, he published The Origin of Species (although it actually carried the more unwieldy title: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life).

The book was an instant hit, selling out on its first day, provoking public discussion and debate, and ultimately causing a wholesale change in the scientific understanding of evolution.


Four mechanisms can give rise to evolution.

8.6 Evolution occurs when the allele frequencies in a population change.
Suppose you were put in charge of a large population of tigers in a zoo.

Occasionally, unusual all-white tigers are born.

This white phenotype is the result of possessing a rare pair of alleles that suppresses the production of most fur pigment

Because visitors flock to zoos to see the white tigers, you want to increase the proportion of your tiger population that is white.

How would you go about doing that?
Witnessing Evolution

Alter the population

Increase the white phenotype through breeding white tigers to each other.
As the generations go by…

Higher proportion of white tigers
Evolution = change on allele frequencies of the population
Individuals do NOT evolve.

Populations evolve
Allele frequencies
It is helpful to think of each allele as having some “market share” of all of the alleles. Over time, the white alleles make up a bigger market share in the population.

Natural Selection

An efficient mechanism of evolution…
and a powerful force in adapting populations to their environment.
Evolution and natural selection, however, are not the same thing. Natural selection is one of 4 mechanisms or agents in evolution.

Agents of Evolutionary Change

1. Mutation

2. Genetic drift

3. Migration

4. Natural selection
Evolution, by definition, is genetic change in a population.
8.7 Mutation—a direct change in the DNA of an individual—is the ultimate source of all genetic variation.

Mutation

An alteration of the base-pair sequence of an individual’s DNA
When this alteration occurs in a gene, the change in the DNA sequence may change the allele.


What causes mutations?

The process of cells dividing can go awry.
Environmental phenomena

Mutagens such as exposure to high energy radiation or mutagenic chemicals
Mutations are random

Beneficial?

Detrimental?

Tanning beds bombard the body with ultraviolet radiation.Can they cause mutations?

Mobile phones release radiation.

Can they cause brain tumors?Some studies suggest long-term is linked to increased incidence of brain tumors.More research needed here to confirm or refute this.

Mutation is the ultimate source of genetic variation in a population.

Nearly all mutations reduce an organism’s fitness.

Suppose that you have written a ten-page paper.
Randomly select one letter in the paper and change it to another letter
Is the change more likely to make your paper better or worse?
Only the rare mutation improves an organism’s fitness.

8.8 Another evolutionary agent: Genetic drift.Genetic drift is a random change in allele frequenciesin a population.
Imagine a couple with a particular trait, such as smooth chin, but who carried a recessive for cleft chin.

Genotypes are Cc and Cc.
Suppose by random chance they have two children, and both inherit the recessive alleles. Their genotype is cc and they have a cleft chin!

The proportion (or percentage) of recessive alleles in the population has increased, simply due to chance!

Natural selection is not involved here, because the trait of cleft chin has nothing to do with fitness.

This is the important factor that distinguishes genetic drift from natural selection:

The change in allele frequencies is not related to the alleles’ influence on reproductive success.
The impact of genetic drift is much greater in small populations than in large populations.
Fixation

Genetic drift can lead to fixation for one allele for a gene in a population. One allele is eliminated and the other is 100%
If this happens, there is no more variability in the population for this gene.
Genetic drift reduces the genetic variation in a population.
Two special cases of genetic drift, the founder effect and population bottlenecks, are important in the evolution of populations.

Founder Effect

A small number of individuals may leave a population and become the founding members of a new, isolated population.
The founders may have different allele frequencies than the original “source” population, particularly if they are a small sample.
Why are Amish people more likely to have extra fingers and toes?
Amish population in the United States established by a small number of founders, who happened to carry the allele for “polydactyly”…having extra fingers and toes.

Today this trait, while still rare, occurs much more frequently among the Amish than among the rest of the U.S. population
Occasionally, a famine, disease or rapid environmental change may cause the deaths of a large proportion (sometimes as much as 90% or more) of the individuals in a population.

Because the population is reduced to a small fraction of its original size, this reduction is called a bottleneck.
Population Bottlenecks: another type of genetic drift
Just such a population bottleneck occurred in the cheetah near the end of the last ice age, about 10,000 years ago.

Although the cause is unknown—possibly environmental cataclysm or human hunting pressures—it appears that nearly all cheetahs died.

And although the population has rebounded, all cheetahs living today can trace their ancestry back to a dozen or so lucky individuals that survived the bottleneck.

There is almost no genetic variation left in the current population of cheetahs. (A cheetah will accept a skin graft from any other cheetah much as identical twins will.)
Migration: the third element of evolutionary change.

Migration of individuals from one population to another population can also introduce or remove alleles from a population.

Immigration brings new alleles in.

Emigration takes alleles away.

Migration is also called ‘gene flow.’


Gene Flow

Migration rate (gene flow) is influenced by the mobility of the organisms, as well as the barriers that separate populations (rivers, mountains, deserts, etc).

Human activities also influence rates of gene flow.

Take-home message 8.9

Migration, or gene flow, leads to a change in allele frequencies in a population as individuals move into or out of the population.

8.10 When three simple conditions are satisfied, evolution by natural selection occurs.

There must be variation for the particular trait within a population.
That variation must be inheritable.
Individuals with one version of the trait must produce more offspring than those with a different version of the trait.
Condition 1: Variation for a Trait

Variation is all around us.

Variation is the raw material on which evolution feeds.

Condition 2: Heritability

We call the transmission of traits from parents to their children through genetic information inheritance or heritability.
Condition 3: Differential Reproductive Success

There are more organisms born than can survive.
Condition 3: Differential Reproductive Success

Organisms are continually struggling for existence.
Condition 3: Differential Reproductive Success

Some organisms are more likely to win this struggle and survive and reproduce.

Differential Reproductive Success

From all the variation existing in a population, individuals with traits most suited to reproduction in their environment generally leave more offspring than individuals with other traits.
That’s it. Natural selection—certainly one of the most influential and far-reaching ideas in the history of science—occurs when these three basic conditions are met

Most agricultural pests evolve resistance to pesticides. 

How does this happen?
Through natural selection, populations of organisms can become adaptedto their environment.

8.11 Traits causing individuals to have more offspring than others become more prevalent.
The word “fitness” in discussing evolution, has little to do with an organism’s ability to survive or to its physical strength or health.

Rather, fitness has everything to do with an organism’s reproductive success. Physical strength and health impart “fitness” only to the degree that they lead to reproduction!
Survival of the Fittest”

Reproductive success
Fitness

a measure of the relative amount of reproduction of an individual with a particular phenotype, as compared with the reproductive output of individuals with alternative phenotypes
Fruit Fly Example

One fly carries the genes for a version of a trait that allows it to survive a long time without food.
The other fly has the genes for a different version of the trait that allows it to survive only a short while without food.
Which fly has the greater fitness?
The alleles carried by an individual with high fitness will increase their market share in a population over time.

So alleles for long life increases fitness only if the individual has more offspring.
There are three important elements to an organism’s fitness:

An individual’s fitness is measured relative to other genotypes or phenotypes in the population.

There are three important elements to an organism’s fitness:

Fitness depends on the specific environment in which the organism lives.

There are three important elements to an organism’s fitness:

Fitness depends on an organism’s reproductive success compared to other organisms in the population.
Fitness is always relative. Its value is not fixed in stone but varies with circumstance.

"Survival of the fittest" is a misnomer.

Why?

It implies that health and strength alone are measures of evolutionary fitness.

8.12 Organisms in a population can become better matched to their environment through natural selection.

Take-home message 8.12

Adaptation—the process by which organisms become better matched to their environment and the specific features that make an organism more fit—occurs as a result of natural selection.
Example: Porcupine quills make porcupines almost impervious to predation

Natural Selection does not lead to perfect organisms.

In this place it takes all the running you can do, to keep in the same place.” (Red Queen to Alice, in Through the Looking Glass.)
Consider one of the many clearly documented cases of evolution occurring: that of the beak size of Galapagos finches.

Over the course of a multi-decade study, the biologists Rosemary and Peter Grant closely monitored the average size of the finches’ beaks.

They found that the average beak size fluctuated according to the food supply.

During dry years—when the finches had to eat large, hard seeds—bigger, stronger beaks became the norm.

During wet years, smaller beaked birds were more successful since there was a surplus of small softer seeds.

The ever-changing “average” finch beak illustrates that adaptation does not simply march toward some optimal endpoint What’s fit in one time and place may not be fit in another.

Evolution in general, and natural selection specifically, do not guide organisms toward “better-ness” or perfection.
If the environment changes, the alleles causing the traits favored by natural selection may change, too.

Why doesn’t natural selection lead to the productionof perfect organisms?

Factors that Prevent Populations from Progressing Inevitably toward Perfection

Environments change quickly.

Variation is needed as the raw material of selection. One needs the right mutation at the right time! One cannot order up the perfect mutation.

There may be multiple different alleles for a trait, each causing an individual to have the same fitness.

Take-home message 8.13

Natural selection does not lead to organisms

perfectly adapted to their environment because:
Environments can change more quickly than natural selection can adapt organisms to them.
All possible alleles are not produced by mutation.
There is not always a single optimum adaptation for an environment, but multiple optimums.

8.14 Artificial selection is just a special case of natural selection.
Animal breeders and farmers utilize natural selection when they modify their animals and crops because the three conditions for natural selection are satisfied.

Because the differential reproductive success is determined by humans and not nature, this type of natural selection is also called artificial selection.
Natural selection can change traits in several ways.

Directional Selection

Individuals with one extreme from the range of variation in the population have higher fitness.

Turkeys on poultry farms have such large breast muscles that they can’t get close enough to each other to mate.

How can such a trait evolve?

Stabilizing Selection

Individuals with intermediate phenotypes are most fit.
How is medical technology undoing the work of natural selection in optimizing the number of babies with normal birth weights?

Disruptive Selection

Individuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the lowest. i.e. Coho salmon.

Take-home message 8.15

Natural selection can change populations in several ways:

directional selection, in which the average value for the trait increases or decreases

stabilizing selection, in which the average value of a trait remains the same while extreme versions of the trait are selected against

disruptive selection, in which individuals with extreme phenotypes have the highest fitness

8.16 Natural selection can cause the evolution of complex traits and behaviors.

For instance, can natural selection improve maze running ability in rats?

William Thompson trained a group of rats to run through a maze for a food reward.

There was a huge amount of variation in the rats’ abilities: some rats learned much more quickly than others how to run the maze.

Thompson then selectively bred the fast learners (called “maze bright” rats) with each other and the slow learners (the “maze dull” rats) with each other. Over several generations, he developed two separate populations: rats descended from a line of fast maze learners and rats descended from a line of slow maze learners.

After only six generations, the maze dull rats made twice as many errors as the maze bright rats before mastering the maze.

Fifty years later, it still is unclear which actual genes are responsible for maze running behavior, yet the selection experiment still demonstrates a strong genetic component to the behavior.
How can a wing evolve if 1% of a wing doesn’t help an organism fly or glide at all?
One vexing case involved the question of how natural selection could ever produce a complex organ such as a fly wing, when 1% or 2% of a wing—that is, an incomplete structure—doesn’t help an insect to fly.
The key to answering this question is that 1% of a wing doesn’t actually need to function as a wing at all to increase an individual’s fitness.

Often, structures appear because they serve some other purpose.

In the case of little nubs or “almost-wings” on an insect, experiments using models of insects demonstrated that they allow much more efficient temperature control, allowing an insect to gain heat from the environment when the insect is cold and to dissipate heat when the insect is hot.

But only up to a point.

Often, structures appear because they serve some other purpose. 


Functional Shifts
It is right around this point that the proto-wing starts to confer some aerodynamic benefits

Natural selection may continue to increase the length of this “almost wing,” but now the fitness increase is due to a wholly different effect.


Such functional shifts from one function to another may explain the evolution of numerous complex structures that we see today and may be common in the evolutionary process.

It is indeed remarkable that this theory [evolution] has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge.

The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory.

Pope John Paul II, 1996

Five primary lines of evidence:

The fossil record
Biogeography
Comparative anatomy and embryology
Molecular biology
Laboratory and field experiments
8.17 The fossil record documents the process of natural selection.

The evolutionary history of horses is among the most well-preserved in the fossil record.

First appearing in North America about 55 million years ago, horses then radiated around the world.


It is tempting to imagine a simple linear progression from ancient horses through 55 million years to modern horses.

But that’s just not how evolution works. It is more like a branching tree.
8-18. Geographic patterns of species’ distributions reflect their evolutionary histories.

History Matters in Evolution

Who arrived first?
In Australia marsupial mammals diversified into forms that elsewhere were filled by placental mammals. Australia was isolated by an ocean that placentals were not able to cross until man introduced them.


8.19 Comparative anatomy and embryology reveal common evolutionary origins.
If you observe any vertebrate embryo, it will pass through a stage in which it has little gill pouches on the sides of the neck.

It will also pass through a stage in which it possesses a long bony tail.

This is true whether it is a human embryo or that of a turtle or a chicken or a shark.

Such common embryological stages indicate that the organisms share a common ancestor

Homologous Structures

Vestigial Structures

The human appendix serves no function.

Why are we all born with one?

It is vestigial in us.

Convergent Evolution

8.20 Molecular biology reveals that common genetic sequences link all life forms.

The genetic code provides our fourth line of evidence that evolution occurs.

DNA Similarities and Differences

DNA Similarity between Two Species

Take-home message 8.20

8.21 Laboratory and field experiments enable us to watch evolution in progress.
The End

Review Questions

Define the following terms:

Adaptation

Bottleneck

Evolution

Fitness

Founder effect

Gene flow

Genetic drift

Natural selection

Population

In the multiple choice do

all except 13 and 18. Also do short answer 1 and2.


Both bats and insects have wings for flight. This is most likely an example of _____________ evolution because they arrived at the adaptation independently.

Wolves tend to take the slow and weak and unwary deer from the herd, producing a faster stronger and warier deer over many generations. This is an example of evolution by means of __________________ (artificial selection, natural selection )

________ is the ultimate source of genetic variation in a population. (natural selection, genetic drift, mutation, migration)
A type of natural selection that favors the mean values of a phenotype is called __________________ (directional, stabilizing, or disruptive).

A type of natural selection that favors the extreme values of a phenotype is called ________________ (directional, stabilizing, or disruptive).

A type of natural selection that favors one extreme of a phenotype is called ________________ (directional, stabilizing, or disruptive).

Which of the following scientists is credited with discovering the theory of evolution by natural selection?

A) Darwin B) Cuvier C) LaMark D) Wallace E) both a and d.
Genetic drift occurs in a) large populations b) intermediate sized populations c) small populations. Genetic drift produces change in a population and is due to a) natural selection which selects the most fit traits b) chance events which determine the survival of individuals.

What kind of agents can mutate DNA?

___________ and __________.
Match the case studies with the term which best describes them below.

A) founder effect b) bottleneck c) gene flow d) stabilizing selection e) directional selection ab) disruptive selection ac) natural selection

1. The loss of genetic variability in cheetahs.

2. The spread of genetic resistance to penicillin in bacteria.

3. The higher frequency of polydactyly in the Pennsylvania Amish community then in the general population.

4. The evolution male size in Coho salmon.

5. The evolution of birth weight in human babies.

Do the following factors usually a) increase or b) decrease genetic variation in a population?

Mutation _____________ (increase or decrease genetic variation)

Migration into a population ______________ (increase or decrease variation variation)

Natural Selection _________________(increase or decrease genetic variation)

Genetic drift ____________________(increase or decrease genetic variation)
Short Answer

1. Distinguish between evolution and natural selection.

2. List the four mechanisms that can give rise to evolution.

3. What is genetic drift? Why is it a more potent agent of change in small populations?

4. What functional shift explains the evolution of a complex structure such as wings in insects?

5. What are the three reasons there are no perfectly adapted organisms?
List the 5 types of evidence used to support the theory of evolution, and give an example of each.


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