Evolution of PopulationsChapter 16

Dr. Donna HowellBiology I

Blacksburg High School

Genes and Variation

• In the 1930’s, experts finally connected the work of Gregor Mendel and Charles Darwin.• They realized for the

first time that changes in genes produced variation in offspring.• They then said that

natural selection works on this variation.

Variation and Gene Pools

• One way scientists study genetic variation is through the study of populations – both animal and human.• Because members of a

population interbreed, they share a common group of genes called a “gene pool.”• The relative frequency

of an allele is the number of times that the allele occurs in a gene pool.

Variation and Gene Pools• Gene pools

are important in the field of evolution because evolution is the change in relative frequency of alleles in a population.

Sources of Genetic Variation

• One major source of genetic variation is mutations in DNA.• These can occur

due to mistakes in DNA replication or due to environmental factors such as chemicals, radiation, etc.• Can affect an

organisms “fitness.”

Sources of Genetic Variation

• Another source of genetic variation is the shuffling of genes.• This occurs during

the production of eggs and sperm.• The 23 pairs of

chromosomes can produce 8.4 million different genetic combinations.

Single-Gene and Polygenic Traits

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

controlled by a single gene. • Others are

controlled by two or more genes.

Single-Gene and Polygenic Traits

• An example of a single-gene trait is the gene that codes for the widow’s peak in your hair.• An example of a

polygenic trait is skin color in humans.• Because multiple

genes code for this, there is a very wide range of possible skin colors.

Natural Selection on Single-Gene Traits

• Natural selection doesn’t act on genes – it acts on whole organisms because either the organism lives or dies with all of its genes.• Natural selection

on single-gene traits results in changes in allele frequencies and thus evolution.

Natural Selection on Single-Gene Traits• An example is the

population of lizards to the right. • Mutations have

occurred that produced red and black lizards.• The red ones are

more visible to predators, so disappear.• The black ones can

get warmer in the sun, eat more, and thus survive.

Natural Selection on Polygenic Traits• When there is

more than one gene that controls a trait, natural selection is more complex.• Can affect in three

ways:

• Directional selection• Stabilizing selection• Disruptive selection

Natural Selection on Polygenic Traits• Directional

selection is when individuals at one end of the curve have higher fitness than individuals at the middle or other end of the curve.• Example: a food

shortage causes the supply of small seeds to run low, and beak sizes get bigger as a result because only big seeds left.

Natural Selection on Polygenic Traits• Stabilizing

selection is when individuals near the center of a curve have higher fitness that those at either end.• Example: human

babies born at average mass are more likely to survive than babies born either much smaller or much larger than average.

Natural Selection on Polygenic Traits• Disruptive

selection is when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle• Example: average-

sized seeds become less common, and larger and smaller seeds become more common.

Genetic Drift• Genetic drift is a

random change in allele frequency due to a smaller population.• Can occur when a

small group of individuals colonizes a new habitat.• The founder effect

occurs when a small subgroup of a population migrates away from the rest.

Genetic Equilibrium• Genetic

equilibrium is when allele frequencies in a population remain constant.• If allele

frequencies remain constant, evolution does not occur.• The Hardy-

Weinberg Principle says that allele frequencies will remain constant unless something causes it to change.

Genetic Equilibrium• Five conditions are

required for genetic equilibrium to take place:

1. Random mating2. Population must be

large3. No movement into

or out of population4. No mutations5. No natural selection

Genetic Equilibrium• Five conditions are

required for genetic equilibrium to take place:

1. Random mating2. Population must be

large3. No movement into

or out of population4. No mutations5. No natural selection

Speciation• Natural selection

can lead to new allele frequencies in a population.• But how does this

lead to the formation of whole new species?• The formation of

new species is called speciation.

Speciation – Reproductive Isolation• As new

species evolve, populations exhibit reproductive isolation.

Speciation – Reproductive Isolation• One way

reproductive isolation can occur is through behavioral isolation.• Ex:

different courtship rituals or mating songs

Speciation – Reproductive Isolation• Another

way reproductive isolation can occur is geographic isolation.• Ex: get

stuck on two different sides of a river

Speciation – Reproductive Isolation• Another

way reproductive isolation can occur is by temporal (time) isolation.• Ex:

reproduce at different times of year

Speciation of Darwin’s Finches• Darwin

studied a group of finches in the Galapagos, and documented the process of speciation.

Speciation of Darwin’s Finches• Darwin

found that speciation occurred by:• Founding of

new population• Geographic

isolation• Changes in

gene pool• Reproductive

isolation• Ecological

competition

1. Founders Arrive• To begin, a

few species of finches either flew or were blown to another island. • They survived

and reproduced.

2. Geographic Isolation• Later, some

birds crossed to another island and stayed there. • They survived

and reproduced and no longer shared the same gene pool with birds on the original island.

3. Gene Pool Changes• Over time,

populations on the different islands became adapted to their local environments.• Natural

selection caused beaks to change according to what birds ate.

4. Reproductive Isolation• Once the gene

pool changes, different species can no longer mate with one another and produce offspring.• Example: mate at

different times of the year, different courtship rituals, etc.

5. Ecological Competition• Now that

different species are living together, they must compete for available food (seeds). • Their beaks will

evolve to eat whatever kinds of food they can find.

The End