Chapter 16 POPULATION GENETICS

In order to understand the genetics behind populations we must revisit Darwin

Charles Darwin

Darwin’s Theory: All organisms compete for limited space

Organisms produce more offspring than can survive

Natural selection states that organisms best suited to the environment survive while those not suited may eventually die

Charles Darwin

Darwin’s Evolution: Variation exists within a species Some variations are favorable Survival of the fittest

The strongest will survive and reproduce The weak will die out Organisms better adapted to the environment will

survive Adaptations will happen gradually

Gradualism

Charles Darwin

Gene Pools

Biologists today study a particular population Gene pool- combined genetic information of all the

members of that population Relative frequency- the number of times an allele

appears in a population as compared with the other alleles

Sources of Genetic Variation Mutations Genetic reshuffling during sexual reproduction

Single vs Polygenetic Variation

Inheritable variation can be expressed in a number of ways Single trait- controlled by a single gene

Example widows peak Since single gene controls the trait usually there are

only two phenotypes Polygenic trait- controlled by a multiple genes

Example height in humans Due to the multiple number of genes controlling this

allele there are multiple phenotypes that result End up with a bell shaped curve (most people fall

around the average, you have some that are well above and some well below average

Evolution as Genetic Change

Natural Selection on Single Gene TraitsLizard example, peppered moths

Natural Selection on Polygenic Traits

Can effect the distribution of phenotypes in any number of three ways: Disruptive Selection

Selection can act against the middle of a normal distribution after an environmental change, this is selection against the most common variation (ex. African Swallowtale Butterfly

Directional Selection After several generations, the normal distribution shifts in

the direction of change (ex. DDT and insects) Stabilizing Selection

Environments may go through long periods of stability, when conditions remain about the same. Organisms that are best adapted to the existing environment will be favored, and there is selection against the extremes

Disruptive Selection

Disruptive Selection   When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place. In this example, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds.

Disruptive Selection

Directional Selection

Directional Selection   Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. In this example, a population of seed-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low. The dotted line shows the original distribution of beak sizes. The solid line shows how the distribution of beak sizes would change as a result of selection.

Directional Selection

Stabilizing Selection

Stabilizing Selection   Stabilizing selection takes place when individuals near the center of a curve have higher fitness than individuals at either end. This example shows that human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average

Stabilizing Selection

Natural Selection on polygenic Traits

Natural Selection

Other Sources of Genetic Variation

Genetic Drift- In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. May occur when small group colonizes new habitat Not caused by natural selection but by chance situation in which allele frequencies change as a

result of the migration of a small subgroup of a population is known as the founder effect.

Founders effect

One example of the founder effect is the evolution of several hundred species of fruit flies found on different Hawaiian Islands. All of those species descended from the same original mainland population. Those species in different habitats on different islands now have allele frequencies that are different from those of the original species.

Understand?

Genetic drift is A. colonization of a new habitat by small

groups of individuals. B. random change in allele frequencies. C. migration of a small subgroup of a

population.

Understand?

Genetic drift is A. colonization of a new habitat by small

groups of individuals. B. random change in allele frequencies. C. migration of a small subgroup of a

population.

Population Genetics

Population Group of organisms that live in the same are &

interbreed

Evolution can only occur when there is a change in the kinds or % of genes in the gene pool of a population (allele frequencies)

Hardy-Weinberg Principle

States that allele frequency will stay constant unless one or more factors cause those frequencies to change Describes the conditions that must be met in order

for the allele frequencies to remain constant It describes genetic equilibrium

Five conditions

Hardy-Weinberg Principle

1. No Mutations

2. Random Mating

3. No Genetic Drift

4. No Natural Selection

5. No Gene Flow

THESE CONDITIONS CAN BE MET FOR LONG PERIODS OF TIME. IF HOWEVER THESE CONDITIONS ARE NOT MET THEN THE GENETIC EQUILIBRIUM WILL BE DISRUPTED AND THE POPULATION WILL EVOLVE

Key Concepts

Can you answer the following: Describe three patterns of natural selection on

polygenic traits. Which one leads to two distinct phenotypes?

How does genetic drift lead to a change in a population's gene pool?

What is the Hardy-Weinberg principle? Describe how natural selection can affect traits

controlled by single genes.

The Process of Speciation

Isolating Mechanisms- As new species evolve, populations become reproductively isolated from each other. Reproductive isolation through:

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.

Behavioral Isolation

The eastern meadowlark (left) and western meadowlark (right) have overlapping ranges. They do not interbreed, however, because they have different mating songs.

Geographic Isolation

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

Temporal Isolation

Two or more species reproduce at different times. three similar species of orchid all live in the same rain

forest. Each species releases pollen only on a single day. Because the three species release pollen on different days, they cannot pollinate one another.

Testing Natural Selection

Peter and Rosemary Grant continued Darwin’s observations on the finches of Galapagos Islands When food for the finches was scarce, individuals

with the largest beaks were more likely to survive, as shown in the graph below. Beak size also plays a role in mating behavior, because big-beaked birds tend to mate with other big-beaked birds. The Grants observed that average beak size in that finch population increased dramatically over time.

Understand?

This graph shows that A. the larger a bird's

beak, the smaller are its chances of survival.

B. the smaller a bird's beak, the greater are its chances of survival.

C. the larger a bird's beak the greater are its chances of survival.

Understand?

This graph shows that A. the larger a bird's

beak, the smaller are its chances of survival.

B. the smaller a bird's beak, the greater are its chances of survival.

C. the larger a bird's beak the greater are its chances of survival.

Understand?

What type of natural selection did the Grants observe in the Galápagos? A. disruptive selection B. directional selection C. stabilizing selection

Understand?

What type of natural selection did the Grants observe in the Galápagos? A. disruptive selection B. directional selection C. stabilizing selection

Speciation in Darwin’s Finches

Speciation- When one or more new organisms evolve from a single ancestral species Founders Effect- few finches arrive from mainland Separation of populations- some birds cross to other

islands Changes in gene pool- over time populations become

adapted to their environment Reproductive isolation- no longer will mat e with one

another Ecological competition- compete for available

resources…best suited to environment wins Continued Evolution- repeats process time and time

again. Over many generations it produced 13 different species of finches (see page 410 in your book)

Understand?

When two species do not reproduce because of differences in mating rituals, the situation is referred to as A. temporal isolation. B. geographic isolation. C. behavioral isolation.

Understand?

When two species do not reproduce because of differences in mating rituals, the situation is referred to as A. temporal isolation. B. geographic isolation. C. behavioral isolation.

Understand?

One finding of the Grants' research on generations of Galápagos finches was that A. natural selection did not occur in the finches. B. natural selection can take place often and very

rapidly. C. beak size had no effect on survival rate of the

finches.

Understand?

One finding of the Grants' research on generations of Galápagos finches was that A. natural selection did not occur in the finches. B. natural selection can take place often and very

rapidly. C. beak size had no effect on survival rate of the

finches.

Understand?

All of the following played a role in speciation of Galápagos finches EXCEPT A. no changes in the gene pool. B. separation of populations. C. reproductive isolation.

Understand?

All of the following played a role in speciation of Galápagos finches EXCEPT A. no changes in the gene pool. B. separation of populations. C. reproductive isolation.