Chapter 11: The Evolution of Populations

11.1: Genetic Variation Within Populations

• Objectives: Describe the significance of genetic variation within a population.

• Identify sources of genetic variation.• Warm Up: What are the chances that you

can put together a good team if just ten players try out? What about 30 players?

• Words to Know: Gene Pool, Allele Frequency, Phenotype, Gene, Allele, Meiosis, Gamete

Genetic Variation • Natural selection acts on different phenotypes (physical characteristics) in a

population.• In order to have different phenotypes, a population must have genetic variation.• A population with a lot of variation likely has a wide range of phenotypes.• Genetic Variation is stored in a population’s Gene Pool – the combined alleles

(genes) of all of the individuals in a population.• An Allele Frequency is a measure of how common a certain allele is in the

population.• What is the relationship between allele frequencies and a gene pool?

Genetic Variation Comes from Several Sources

• Mutation• A mutation is a

random change in the DNA of a gene.

• This change can form a new allele.

• If these changes are in reproductive cells they can be passed on to new generations.

Genetic Variation Comes from Several Sources

• Recombination• New allele combinations

form in offspring through a process called recombination.

• Most recombination occurs during meiosis (producing 4 sex cells) in crossing over.

• This shuffling results in many new combinations of gametes (sex cells- sperm and egg) .

• Why aren’t mutations in nonreproductive cells sources of genetic variation?

11.2: Natural Selection in Populations

• Objectives: Describe how natural selection acts on the distribution of traits in a population.

• Explain three ways natural selection can change the distribution of a trait in a population.

• Warm Up: When a drought caused more large0geak ground finches to survive than small-beaked ground finches both of the same species, what was natural selection acting upon?

• Words to Know: Normal Distribution, Microevolution, List 3 Ways Natural Selection can change the Distribution of a trait (with definition) 1.Directional Selection, 2.Stabilizing Selection, 3.Disruptive Selection

Natural Selection Acts on Distributions of Traits

• If we were to line up a group of people on a football field by height, relatively few people would fall at the extreme tall and short ends. A majority of people would fall in the middle range.

• When frequency is highest near the mean value (middle) and decreases towards each extreme, it is called a Normal Distribution.

• The graphed result is a bell shaped curve.

• What other types of data might follow a normal distribution?

Natural Selection Can Change the Distribution of a Trait

• Macroevolution is the observable change in the allele frequencies of a population over time.

• Microevolution occurs on a small scale, within a single population.

• This can result in one of three paths.

1. Directional Selection• Directional Selection favors phenotypes at one extreme of a trait’s

range.• Ex: Drug Resistant Bacteria.

– This phenotype was selected Against when there were no antibiotics.– Now that there are antibiotics, this phenotype is now more successful and

selected for.• Directional selections moves the curve on the graph left or right.

2. Stabilizing Selection• In Stabilizing Selection the intermediate phenotype (median) is

favored.• Ex: Gall flies and their predators.

– Woodpeckers feed on larger gall fly larva.– Parasitic wasps lay eggs on the smaller gall fly larva.– Only the gall fly larva that are medium size have a chance to survive.

• Stabilizing Selection on a graph looks like Normal Selection.

3. Disruptive Selection• Disruptive Selection occurs when

both extreme phenotypes are favored, while individuals with intermediate phenotypes are selected against by something in nature.

• Ex: Feather color in male lazuli buntings (a bird).– Feathers range from dull brown

to bright blue.– Dull Brown and Bright blue

feather buntings have a better chance of getting a mate than regular blue birds do.

• The brown males are successful because the aggressive bright blue males do not see them as a threat.

• The regular blue birds are frequently attacked because they are seen as a threat.

• The graph in disruptive selection has two curves, one at each extreme.

• If bluish brown coloring became advantageous for young males, what type of selection would likely occur in a lazuli bunting population?

LE 23-12

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11.3: Other Mechanisms of Evolution

• Objectives: Explain how gene flow, genetic drift, and sexual selection can lead to the evolution of populations.

• Warm Up: What effect has immigration had on the genetic variation of the U.S. population?

• Words to Know: Gene Flow, Speciation, Genetic Drift, Bottleneck Effect, Founder Effect, Sexual Selection

Gene Flow • When an organism joins a new

population and reproduces, its alleles become part of that population’s gene pool.

• At the same time its genes are removed from its previous population.

• The movement of alleles from one population to another is called Gene Flow.

• Gene flow increases genetic variation.

• A lack of gene flow can cause speciation.

• How does gene flow affect neighboring populations?

Genetic Drift

• Genetic Drift is the change in allele frequencies due to chance.

• Two processes commonly cause populations to become small enough for genetic drift to occur.

Bottleneck Effect

• The Bottleneck Effect is genetic drift that occurs after an event greatly reduces the size of a population.

• Disasters such as earthquakes, floods, or fires may reduce the size of the pop. Drastically. The result is that a small surviving pop. Is unlikely to be representative of the original pop.

• Ex: Overhunting of northern elephant seals during the 1800’s.– The population got reduced to

about 20 seals that did not represent the genetic diversity of the original population.

Founder Effect• The Founder Effect is genetic drift that occurs after a

small number of individuals colonize a new area.• The gene pools of these new populations are often very

different from those of the larger population.• Ex: The Amish of Lancaster Pennsylvania have a high

rate of a rare form of Dwarfism.– Because they are such a small community, this trait is common

in the population.

Effects of Genetic Drift

• The population loses genetic variation.

• Less likely to have individuals to adapt.

• Alleles that are lethal can become more common.

• What is genetic drift more likely to occur in smaller populations?

Sexual Selection • Mating has an important effect

on the evolution of a population.• Sexual Selection occurs when

certain traits increase mating success.

• There are two types of sexual selection:– 1. Intrasexual Selection

involves competition among males. Whoever wins get the girl.

– 2. Intersexual Selection occurs when males display certain traits that attract the females, such as peacock feathers.

• Male Irish elks, which are now extinct, had 12-foot-wide antlers. Describe how sexual selection could have caused such an exaggerated trait to evolve.

11.4: Hardy-Weinberg Equilibrium

• Objectives: Identify the conditions that define Hardy-Weinberg Equilibrium

• Explain the predictive value of the Hardy-Weinberg equation.

• Warm Up: What does it mean to be in a state of equilibrium?

• Words to Know: Hardy-Weinberg Equilibrium.

Hardy-Weinberg Equilibrium • Hardy and Weinberg showed

that genotype frequencies in a population will stay the same over time as long as 5 conditions are met:– 1. Must be a very Large

Population (NO genetic drift).– 2. No emigration or immigration

(NO gene flow).– 3. No mutations (NO new

alleles).– 4. Must have Random Mating

(NO sexual selection allowed).– 5. No Natural Selection (all traits

must equally aid in survival).• Real populations rarely meet all

five requirements• How are models used by

population biologists?

The Hardy-Weinberg Equation

• For traits in a simple dominant-recessive systems, biologists can predict genotype frequencies using the Hardy-Weinberg equation.

• P2 + 2pq + q2 = 1 p = frequency of the dominant allele q = frequency of the recessive allele.

• In Hardy-Weinberg Equilibrium, the equation ALWAYS equals 1.

• If the equation DOES NOT equal 1, evolution is occurring.

Hardy Weinberg Equation

There are Five Factors that can Lead to Evolution

• 1. Genetic Drift – allele frequency can change due to chance alone.

• 2. Gene Flow – the movement of alleles from one place to another change the allele frequencies of the population.

• 3. Mutation – new alleles can form through mutations, and these create the genetic variation needed for evolution.

• 4. Sexual Selection – certain traits may improve mating success which cause an increase in that allele frequency.

• 5. Natural Selection – Certain traits may be an advantage for survival so alleles for these traits increase in frequency.

• Why do real populations rarely reach Hardy-Weinberg equilibrium?

Isolation of Populations • If gene flow between two populations stops for any reason, the

population are said to be isolated.• As they adapt to their new environments, their gene pools may change.1. Reproductive Isolation occurs when member of different populations

can no longer mate successfully with one another.• Speciation is the rise of two or more species from on existing species.• Why is reproductive isolation considered to be the final stage in

speciation?

11.5: Speciation Through Isolation

• Objectives: Explain how isolation of populations can lead to speciation.

• Describe how populations can become isolated.• Warm Up: How do we know that a three-foot-tall

Irish wolfhound and a six-inch-high Chihuahua are the same species? At what point would the two breeds become separate species?

• Words to Know: List the 4 ways Speciation can Occur (with definition) 1. Reproductive Isolation 2. Behavioral Isolation 3. Geographic Isolation 4. Temporal Isolation.

Behavioral Barriers

2. Behavioral Isolation is isolation caused by differences in courtship or mating behaviors.

• Ex: Fireflies. Male and female fireflies produce patterns of light flashes that attract only their own species.

Geographic Barriers

3. Geographic Isolation involves physical barriers that divide a population into two or more groups.

• These can be rivers, mountains, and dried lakebeds.

• Ex: The isthmus of Panama separated many aquatic species that then evolved separately.

Temporal Barriers

4. Temporal Isolation exists when Timing prevents reproduction between populations.

• Some members of a population may show signs of courtship at different times.

• Ex: flowers bloom at different times of year.

• What are the differences and similarities between behavioral isolation and temporal isolation?

11.6: Patterns in Evolution

• Objectives: Describe different types and rates of evolution.

• Compare different types and rates of extinction.

• Warm Up: What are some different adaptations in plants and animals that seem to relate directly to features of their environment?

• Words to Know: Convergent Evolution, Divergent Evolution, Coevolution, Extinction, Punctuated Equilibrium, Adaptive Radiation

Evolution through Natural Selection is NOT Random.

• Natural Selection is NOT random.• Individuals with traits that are better adapted for their environment

have a better chance of surviving and reproducing than do individuals without these traits.

• Natural selection always pushes traits in an advantageous direction.

• The environment controls the direction taken by natural selection.

Convergent Evolution

• Different species often must adapt to similar environments.

• Evolution toward similar characteristics in unrelated species is called Convergent Evolution.

• Ex: Shark and Dolphins are not related yet they have evolved similar tail fins.

Divergent Evolution

• When closely related species evolve in different directions, they become increasing different in Divergent Evolution.

• Ex: Red Fox and Kit fox: though closely related they have developed different characteristics based on the environment they live in.

• Are the shells of turtles and snails examples of convergent or divergent evolution? Explain.

Coevolution

• Coevolution is the process in which two or more species evolve in response to changes in each other.

• Ex: Bees and flowers.

Evolutionary Arms Race• Coevolution can also occur in competitive relationships.• Ex: Many plants produce defense chemicals to discourage herbivores

from eating them.– Natural selection then favors the herbivores who can overcome the effect of

the toxins.• Ex: Thick shells and spines of murex snails are an adaptive response to

predation by crabs.– In turn, crabs have evolved strong claws.

• What do you think will happen in future generations of crabs and snails?

Species Can become Extinct• The elimination of a species from Earth is Extinction.• There are two types of extinctions involved in evolution:

– 1. Background extinctions – occur continuously at a slow rate. (usually effect one or few species in a small space).

– 2. Mass Extinctions – many species are destroyed suddenly. (ice age, meteors).

• What are the differences and similarities between background extinctions and mass extinctions?

Speciation Often Occurs in Patterns

• There are repeating patterns in the history of life.

• The Theory of Punctuated Equilibrium states that episodes of speciation occur suddenly in geologic time and are followed by long periods of little evolutionary change.

• This opposed Darwin’s theory of gradualism (slow, steady evolution).

• Adaptive Radiation the diversification of one ancestral species into many descendant species.

• Ex: Following the mass extinction at the end of the Cretaceous Period 65 million years ago, Adaptive radiation of mammals occurred.

• The adaptive radiation of mammals followed the extinction of the dinosaurs. How do these events support the theory of punctuated equilibrium?