Chapter 16 POPULATION GENETICS

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 Traits • Lizard 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.

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? • 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? • 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.

Chapter 16 POPULATION GENETICS