CHAPTER 16 EVOLUTION AND POPULATIONS

GENES AND VARIATION

Fitness, adaptation, species, and evolutionary change are now defined in genetic terms.

Gene pool: all the genes (and its alleles) present in a population.

Relative Frequency: the number of times an allele appears in a gene pool compared with the number of times other alleles for that same gene occur.

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

SOURCES OF GENETIC VARIATION Mutations

- are random- occur in the DNA- most are harmless but some can be lethal

Gene Shuffling- occurs during sexual reproduction- is responsible for most differences- each chromosome has a pair of homologous

chromosomes that may sort independently during meiosis; 23 8.4 million different gene combos

- crossing over Sexual reproduction produces different phenotypes,

but it does not change the relative frequency. The probability remains the same.

SINGLE GENE TRAITS The number of phenotypes produced for a

given trait depends on how many genes control the trait.

A single-gene trait is a trait that is controlled by a single gene with two alleles.

ex: widow’s peak vs. straight hairline Just because a trait is dominant does

not mean that more people have it. A recessive allele may contribute to an

organism’s fitness, therefore leading to a population with more organisms having the recessive allele.

POLYGENIC TRAITS Polygenic traits are controlled by two or more

genes. Each gene often has two or more alleles. As a result, one polygenic trait may have

many possible genotypes and phenotypes.ex: height in humans

Refer to pg. 396 in your textbook showing a bell-shaped curve (or normal distribution).

EVOLUTION AS GENETIC CHANGE Natural selection never directly acts on genes. It acts on the entire organism- not a single gene. It is the organism that survives and reproduces or

dies without reproducing. Natural selection can only affect which individuals

survive and reproduce and which do not. If an organism dies without reproducing, then it

does not contribute its genetics to the populations gene pool; if it reproduces before it dies, then it contributes its genetics to the population’s gene pool.

If an individual produces many offspring, then its alleles stay in the gene pool and contribute to the relative frequency of the allele.

NATURAL SELECTION ON SINGLE-GENE TRAITS

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

An example is the story of the peppered moth…

Break for activity….

HHMI DVD on mice…

NATURAL SELECTION ON POLYGENIC TRAITS

When traits are controlled by more than one gene, the effects of natural selection are more complex.

Remember, the distribution of a polygenic trait can be seen on a typical bell curve.

There won’t be much difference in fitness between individuals who are next to each other on the curve, but the individuals at either end of the curve will be very different.

Natural selection can affect the distribution of phenotypes in any of three ways: 1) directional selection, 2) stabilizing selection, or 3) disruptive selection.

DIRECTIONAL SELECTIONIndividuals at one end of the curve have more fitness than individuals in the middle or at the other end of the curve. The range of phenotypes shifts as some individuals fail to survive and reproduce while others succeed.

See page 398 for the example.

Population after selection

Original population

Directional Selection

STABILIZING SELECTION

Selection Against Both Extremes

Population after selection

Original population

Individuals near the center of the curve have more fitness than the individuals at either end of the cure.

DISRUPTIVE SELECTION

Selection against the mean

Population after selection

Original population

Individuals at the upper and lower ends of the curve have higher fitness than the individuals near the middle.

GENETIC DRIFT Natural selection is not the only source of evolutionary

change. In small populations, an allele can become more or less

common simply by chance. Probability laws are applied to genetics to make genetic

predictions. But the smaller the population, the more unlikely it

becomes that the laws of probability can be applied. Genetic Drift: In small populations, individuals

can carry a particular allele that may leave more descendants than other individuals, just by chance. Over time, a series of occurrences of this type can cause an allele to become common in a population.

GENETIC DRIFT

Sample of original population

Founding Population A

Founding Population B

Descendants

FOUNDER AFFECT It is a situation in which allele frequencies

change as a result of the migration of a small subgroup of a population.

Therefore, two small groups from a large, diverse population could produce new populations that differ from the original group.

island

Founder Effect: a few individuals from a population start a new population with a different allele than the original population.

Founder Effect also referred to as the Bottle Neck Effect.

EVOLUTION VERSUS GENETIC EQUILIBRIUM Hardy-Weinberg Principle: allele frequencies in

a population will remain constant unless one or more factors cause those frequencies to change.

Genetic equilibrium is when allele frequencies remain constant and do not change; populations do not evolve.

5 conditions required to maintain genetic equilibrium from generation to generation are:

1. random mating2. population must be large3. there can be no movement in or out of a

population4. no mutations5. no natural selection

THE PROCESS OF SPECIATION Natural selection and chance

occurrences can change allele frequencies, but how do entirely new species form?

Species: a group of organisms that breed with one another and produce fertile offspring.

In order for a species to evolve into a new species, the gene pools of two populations must become separated.

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

When the members of two populations cannot interbreed and produce fertile offspring, reproductive isolation has occurred.

ISOLATING MECHANISMS (TYPES OF REPRODUCTIVE ISOLATION)

Behavioral isolation Geographic isolation Temporal isolation

BEHAVIORAL ISOLATION Occurs when two populations

are capable of interbreeding, but have differences in courtship rituals or other reproductive strategies that involve behavior.

Ex: Eastern Meadowlark vs. Western Meadowlark

They have overlapping habitats.

Won’t mate with each other because they use different songs to attract mates.

Notice these birds are singing different mating songs even though they are the same species. This prevents them from hooking up.

GEOGRAPHIC ISOLATION Two populations are separated

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

Ex: the Albert squirrel in the Southwest.

Its population got separated when the Colorado River split the area.

The Albert and Kaibab squirrels have many similarities, but fur color differs.

1. Original beetle population

2. River arises, effectively

splitting the population.

3. After many generations, each population evolves genetic

differences.

4. After the river dries up, genetic differences prevent interbreeding.

TEMPORAL ISOLATION Two or more species

reproduce at different times.

Ex: three species of orchid are found in the same rain forest.

They release pollen only on a single day.

Since they each release pollen on different days, they cannot pollinate each other.

SPECIATION IN DARWIN’S FINCHES Read pgs. 408-409