Section 18.2Evolution as Genetic Change

Standards

• LS 4.2 Using a model that demonstrates the change in allele frequencies resulting in evolution of a population over many generations, identify causative agents of change.

Key Questions

1. How does natural selection affect single-gene and polygenic traits?

2. What is genetic drift?

3. What conditions are required to maintain genetic equilibrium?

Vocabulary

• Directional selection

• Stabilizing selection

• Disruptive selection

• Genetic drift

• Bottleneck effect

• Founder effect

• Genetic equilibrium

• Hardy-Weinberg principle

• Sexual selection

• Gene flow

How Natural Selection Works

• Pesticide resistance example

Natural Selection on Single-Gene Traits

• Natural selection on single-gene traits can produce changes in allele frequencies that may be reflected by simple changes in phenotype frequencies.

• Lizard example:• Population of brown lizards undergoes mutations that cause red color and

black color

• Red- more visible to predators (less likely to survive and reproduce)

• Black- body temp. warms up faster (more likely to survive and reproduce)

• Allele and phenotype frequencies in this population will change over time

Natural Selection on Polygenic Traits

• Both the trait and the effects of natural selection on the trait are more complex with polygenic traits.

• Natural selection on polygenic traits can affect the relative fitness of phenotypes in three types of selection:

1. Directional selection

2. Stabilizing selection

3. Disruptive selection

1. Directional Selection

• Directional selection- when individuals at one end of the curve have higher fitness than individuals elsewhere on the curve

• Example:• After the drought on Daphne Major,

birds with larger beaks were more likely to survive and reproduce

2. Stabilizing Selection

• Stabilizing selection- when individuals near the center of the curve have higher fitness than individuals at either end

• Example:• On an island where seed size is highly

changeable, birds with medium beaks can eat both large and small seeds (more likely to survive and reproduce than birds who can only eat one seed type)

• Human birth weight

3. Disruptive Selection

• Disruptive selection- when phenotypes at both the upper and lower ends of the curve have higher fitness than individuals near the middle

• Example:• On an island where medium-sized seeds

are not common, birds with unusually small or large beaks would both have higher fitness than medium beaks

Genetic Drift

• In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.

• Genetic drift- random change in allele frequency

Genetic Drift

• Example• The number of horns in rhino

species

• It is not a matter of one or two horns being better

• Natural selection simply favored horns over no horns

Genetic Bottlenecks

• Bottleneck effect- a change in allele frequency following a dramatic reduction in the size of a population • Usually after a natural disaster

• The gene pool of the surviving population may differ from the original population by chance

• Severe bottleneck can sharply reduce a population’s genetic diversity

The Founder Effect

• Founder effect- allele frequencies change as a result of the migration of a small subgroup of a population

Evolution Versus Genetic Equilibrium

• How, why, and when do populations evolve?

• One way to answer this question is to imagine a hypothetical population that does not evolve.

• Genetic equilibrium- a population that is not evolving, so the allele frequencies in its gene pool are not changing

Can a species ever reach genetic equilibrium?

• Horseshoe Crab• changed little from the time they appeared in

the fossil record

• been around for 450 million years (before dinosaurs)

• in a state of equilibrium

The Hardy-Weinberg Principle

• Hardy-Weinberg Principle- allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change

• The Hardy-Weinberg principle makes predictions for populations.

The Hardy-Weinberg Principle

• Equations:

• p + q = 1

• p2 + 2pq + q2 = 1

• What the symbols means:

• p = allele A (dominant)

• q = allele a (recessive)

• p2 = frequency of AA

• 2pq = frequency of Aa

• q2 = frequency of aa

The Hardy-Weinberg Principle

• Given:• If the frequency of allele A is 40% and the frequency of allele a is 60%...

• Answers: • Frequency of allele A = p = 0.4 = 40%

• Frequency of allele a = q = 0.6 = 60%

• Frequency of AA = p2 = (0.4)2 = 0.16 = 16%

• Frequency of Aa = 2pq = (2)(0.4)(0.6) = 0.48 = 48%

• Frequency of aa = q2 = (0.6)2 = 0.36 = 36%

The Hardy-Weinberg Principle

• Predicts that five conditions can disturb genetic equilibrium and cause evolution to occur:

1. Nonrandom mating

2. Small population size

3. Genetic flow from immigration or emigration

4. Mutations

5. Natural selection

1. Nonrandom Mating

• In genetic equilibrium, individuals must mate at random.

• But…

• Sexual selection- females of many animal species select mates based on size, strength, or coloration• Genes for traits selected for or against

are not in equilibrium

2. Small Population Size

• Genetic drift does not usually have major effects on large populations, but can effect small populations.

• Evolution due to genetic drift happens more easily in small populations.

3. Gene Flow from Immigration or Emigration

• Gene flow- the movement of genes into or out of a population

• If allele frequency in the population changes, gene flow has caused evolution to occur.

• Immigration- individuals who join a population (could bring in new alleles)

• Emigration- individuals who leave a population (may remove alleles)

4. Mutations

• Mutations can introduce new alleles into a gene pool, changing allele frequencies and causing evolution to occur.

5. Natural Selection

• If different phenotypes have different fitness, natural selection will disrupt genetic equilibrium, and evolution will occur.

The Hardy-Weinberg Principle

• Almost any population in nature is likely to experience one or more of these five conditions.

• So, most of the time, in most populations, in typical habitats, evolution happens.

1. Nonrandom mating

2. Small population size

3. Genetic flow from immigration or emigration

4. Mutations

5. Natural selection

Section 18.2 Exit Ticket

1. What are the five conditions that need to be met for a population of organisms to remain in genetic equilibrium?

2. What five conditions can cause a population to evolve over time?

3. How can one analyze changes in allele frequencies over time to infer whether or not a population is evolving over many generations?

The End