Chapter

16

Evolution of Populations

16-1 Genes and Variation

• As Darwin developed his theory of evolution, he worked under a serious handicap

• He didn’t know how heredity worked

• This lack of knowledge left two big gaps in Darwin’s thinking

1. He had no idea how heritable traits pass from one generation to the next

2. He had no idea how variation appeared, even though variation in heritable traits was central to Darwin’s theory

• During the 1930’s Evolutionary biologists connected Mendel’s work to Darwin’s

• By then biologists understood that genes control heritable traits

How Common Is Genetic Variation?

• Many genes have at least 2 forms or alleles

• Animals such as horses, dogs, mice, and humans often have several alleles for traits such as body size or coat color

Variation and Gene Pools• Genetic variation is studied in

populations

Population• A group of individuals of the same

species that interbreed

• Because members of a population interbreed, they share a common group of genes called a gene pool

Gene pool• All the genes including all the

different alleles, that are present in a population

Relative Frequency• The number of times that the allele

occurs in a gene pool, compared with the number of times other alleles for the same gene occur

• In genetic terms, evolution is any change in the relative frequency of alleles in a population

Sources of Genetic Variation

• The 2 main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction

Mutations• Any change in a sequence of DNA• Mutations can occur because of• Mistakes in DNA replication• Radiation or chemicals in the

environment

• Some mutations don’t affect the phenotype but some do

Gene shuffling during sexual reproduction

• Mutations are not the only source of variation

• Most heritable differences are due to gene shuffling that occurs during the production of gametes

• The 23 pairs of chromosomes can produce 8.4 milliondifferent combinations of genes

• Crossing over further increases the number of different genotypes that can also appear in offspring

Single – Gene and Polygenic Traits

• The number of phenotypes produced for a given trait depends on how many genes control the trait

Single – gene trait• Controlled by a single gene that

has two alleles

• Variation in these genes leads to only 2 distinct phenotypes

Polygenic traits• Traits controlled by two or more genes• Each gene of a polygenic trait has two

or more alleles• As a result one polygenic trait can have

many possible genotypes and phenotypes

Ex.) height

Polygenic traits

16-2 Evolution as Genetic Change

• A genetic view of evolution offers a new way to look at key evolutionary concepts

• If each time an organism reproduces, it passes copies of its genes to its offspring…

• We can therefore view evolutionary fitness as an organism’s success in passing genes to the next generation

• We can also view an evolutionary adaptation as any genetically controlled physiological, anatomical, or behavioral trait that increases an individuals ability to pass along its genes

Evolution as Genetic Change

• Remember that evolution is any change over time in the relative frequency of alleles in a population.

• This reminds us that it is populations, not individual organisms that can evolve overtime

Natural Selection on Single – Gene Traits

• Natural selection on single gene traits can lead to changes in allele frequencies and thus to evolution

Natural Selection on Polygenic Traits

• Natural selection can affect the distributions of phenotypes in any of three ways

1. Directional Selection• When individuals at one end of

the curve have higher fitness than individuals in the middle or at the other end

2. Stabilizing Selection• When individuals near the center

of the curve have higher fitness than individuals at either end of the curve

3. Disruptive Selection• When individuals at the upper and

lower ends of the curve have higher fitness than individuals near the middle

• Can create 2 distinct phenotypes

Genetic Drift• Natural Selection is not the only

source of evolutionary change

• In small populations, an allele can become more or less common by chance

Genetic Drift• A random change in allele frequency

Genetic Drift• These individuals may carry alleles in

different relative frequencies than did the larger population from which they came

• If so, the population that they found will be genetically different from the parent population

• This cause is not natural selection, but chance

Founder effect• A situation in which allele

frequency changes as a result of the migration of a small subgroup of a population

Evolution vs. Genetic Equilibrium

• To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place

Hardy – Weinberg principle

• States that allele frequency in a population will remain constant unless one or more factors cause these frequencies to change

Genetic equilibrium

• The situation in which allele frequencies remain constant

5 conditions are required to maintain genetic equilibrium

1. There must be random mating

2. The population must be very large

3. There can be no movement into or out of the population

4. No mutations

5. No natural selections

16-3 The Process of Speciation

• Factors such as natural selection and chance events can change the relative frequencies of alleles in a population

• But how do these changes lead to speciation?

Speciation• The formation of new species

Isolating Mechanisms• Since members of the same species

share a common gene pool, in order for a species to evolve into 2 new species, the gene pools must be separated into 2

• As new species evolve, populations become reproductively isolated from each other

Reproductive isolation• When the members of 2

populations cannot interbreed and produce fertile offspring

Behavioral Isolation• When two populations are

capable of interbreeding but have differences in courtship rituals or other reproductive strategies

Eastern & Western Meadowlark

Geographical Isolation• When two populations are

separated by geographic barriers such as rivers, mountains, or bodies of water

Albert & Kaibab Squirrels

Temporal Isolation• When 2 or more species reproduce

at different times

Rana aurora - breeds January - March

Rana boylii - breeds late March - May

Testing Natural Selection in Nature

Q: Can evolution be observed in nature?

A: YES

The Grants

Testing Natural Selection in Nature

• Darwin hypothesized that finches had descended from a common ancestor and overtime, natural selection shaped the beaks of different bird populations as they adapted to eat different foods

• The Grants, realized that Darwin’s hypothesis relied on two testable assumptions

1. There must be enough heritable variation in these traits to provide raw materials for natural selection

2. Differences in beak size and shape must produce differences in fitness that cause natural selection to occur

Variation• The Grants identified and

measured every variable characteristic of the birds on the island

• Their data indicated that there is a great variation of heritable traits among the Galapagos finches

Natural Selection• During the…

• Rainy season – enough food for everyone, no competition

• Dry season – some foods become scarce

• At that time, differences in beak sizes can mean the difference between life and death

• Birds become feeding specialists

Natural Selection• The Grants discovered that individual

birds with different size beaks had different chances of survival during a drought

Speciation in Darwin’s Finches

• Speciation in the Galapagos finches occurred by founding of a new population, geographical isolation, changes in the new population’s gene pool, reproductive isolation and ecological competition

Founders Arrive• Many years ago, a

few finches from South American mainland

• Species A, flew or were blown to one of the Galapagos Islands

Geographic Isolation• Later on, some birds from

species A crossed to another island in the Galapagos group

• The finches then became unable to fly from island to island and become isolated from each other and no longer share a common gene pool

Changes in the Gene Pool

• Overtime, populations on each island became adapted to their local environments

Reproductive Isolation• Now imagine that a

few birds from the second island cross back to the first island

• Q: Will the population A birds, breed with the population B birds?

• A: Probably not

Ecological Competition• As these two new

species live together in the same environment, they compete with each other for available seeds

• The more different birds are, the higher fitness they have, due to less compitition

Continued Evolution• This process of isolation

on different islands, genetic change, and reproductive isolation probably repeated itself time and time again across the entire Galapagos island chain

• Over many generations, it produced the 13 different finch species found there today

Studying Evolution Since Darwin

• It is useful to review and critique the strength and weakness of evolutionary theory

• Darwin made bold assumptions about heritable variation, the age of the Earth, and the relationships among organisms

• New data from genetics, physics, and biochemistry could have proved him wrong on many counts, and they did not

• Scientific evidence supports the theory that living species descended with modification from common ancestors that lived in the past

Limitations of Research• The Grants data shows how

competition and climate change affects natural selection

• However, they did not observe the formation of a new species

Unanswered Questions• Many new discoveries have led to

new hypotheses that refine and expand Darwin’s original ideas

• No scientist suggests that all evolutionary processes are fully understood. Many unanswered questions remain

Why Understanding Evolution is Important?

• Evolution continues today

Ex.)

• Drug resistance in bacteria and viruses

• Pesticide resistance in insects

• Evolutionary theory helps us understand and respond to these changes in ways that improve human life