Biology Chapter 16 Evolution Unit Evolution of Populations

Biology Chapter 16 Evolution Unit: Evolution of Populations

16 -1 Genes and Variation A. As Darwin developed his theory of evolution, he was not aware of how heritable traits _______ passed from one generation to the next and how variation appeared in organisms.

B. Evolutionary biologists connected Darwin’s work and Mendel’s work during the 1930’s. genes 1. Changes in ____ produce heritable variation on which natural selection can operate. ________ 2. Discovery of DNA demonstrated the molecular nature of mutation and genetic variation.

II. How Common is Genetic Variation? A. Individual fishes, reptiles, and mammals are typically heterozygous for between 4 -8% of their genes. B. Variation and Gene Pools 1. Genetic variation is studied _______ in populations. ________ a group of individuals of 2. Population: __________ the same species that interbreed. ________________

3. Gene pool: all genes, including all the different alleles, that are present in a population.

4. Relative frequency is the number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur. Example: Fur color in a population of mice 40% B (black fur) 60% b (brown fur)

Relative Frequencies of Alleles Figure 16– 2 Section 16 -1 Sample Population 48% heterozygous black 16% homozygous black 36% homozygous brown Frequency of Alleles allele for brown fur allele for black fur

Evolution is any 5. MICROEVOLUTION: _______ change in the relative frequency of _________________ alleles _____ in a population. Microevolution refers to small scale ______ change in allele frequency over time.

III. Sources of Genetic Variation A. Two sources of genetic variation 1. Mutation a. Ultimate source of variation. b. Any change in a sequence of DNA c. Most mutations are bad. Example: UV, radiation, toxins

d. Mutations that produce changes in an organism’s phenotype and increase an organism’s fitness, or its ability to reproduce in its environment, will be passed on.

2. Genetic shuffling that results from sexual reproduction. a. Independent assortment during meiosis produces 8. 4 million possible combinations. b. Crossing-over.

IV. Single-Gene and Polygenic Traits A. The number of phenotypes produced for a given trait depends on how many genes control the trait. 1. Single-gene trait: Single gene that has two alleles. Example: Free earlobes (FF, Ff) or attached earlobes (ff). Free Attached

Phenotypes for Single-Gene Trait Frequency of Phenotype (%) 100 80 60 40 20 0 Attached Earlobes Free Earlobes (ff) (FF, Ff) Phenotype

2. Polygenic traits: Traits that are controlled by two or more genes. One polygenic trait can have many possible genotypes or phenotypes. Example: Height, eye color, skin color.

16 -2 Evolution as Genetic Change I. Natural Selection on Single-Gene Traits A. Reminder: Evolution is any change over time in the relative frequencies of alleles in a population. Populations, not individual organisms, evolve over time. B. Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.

Effect of Color Mutations on Lizard Survival (Figure 16 -5): 1. Organisms of one color may produce fewer offspring than organisms of other colors. Example: Red lizards are more visible to predators and therefore, may be more likely to be eaten and not pass on that red gene.

II. Natural Selection on Polygenic Traits Natural selection can affect the distribution of phenotypes in any of three ways: (1) directional selection (2) stabilizing selection (3) disruptive selection.

A. Directional Selection 1. One of the two possible extremes is favored. Example: Darkcolored peppered moths in regions of England with industrial pollution.

Section 16 -2 Directional Selection Figure 16– 6 Key Directional Selection Food becomes scarce. Low mortality, high fitness High mortality, low fitness

B. Stabilizing Selection 1. Intermediate characteristics are favored. Examples: Human babies with very high or very low birth weights have lower survival than babies with intermediate weights.

Stabilizing Selection Figure 16– 7 Section 16 -2 Stabilizing Selection Key Low mortality, high fitness Percentage of Population High mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place.

C. Disruptive Selection 1. Natural selection moves characteristics toward both extremes, and intermediate phenotypes become rarest. Example: Populations of West African birds with either large or small, but not intermediate size beaks.

Disruptive Selection Figure 16– 8 Section 16 -2 Disruptive Selection Low mortality, high fitness High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Number of Birds in Population Largest and smallest seeds become more common. Beak Size

III. Genetic Drift A. In small populations, an allele can become more or less common simply by chance. B. Genetic drift is a random change in allele frequency.

Genetic Drift

C. Two types of genetic drift: 1. Genetic bottleneck: If a population crashes, then there will be a loss of alleles from the population. Example: Northern Elephant Seals, Cheetahs.

Genetic Bottleneck

2. Founder effect: A population can become limited in genetic variability if it’s founded by a small number of individuals. Example: Polydactyly in Amish.

Figure 16 -9: Founder Effect Sample of Original Population Descendants Founding Population A Founding Population B

IV. Hardy-Weinberg and Genetic Equilibrium A. What would be necessary for no change to take place? 1. Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. 2. If allele frequencies remained constant then it there would be genetic equilibrium.

3. If allele frequencies do not change, THEN the population will not evolve. 4. Hardy-Weinberg Equation: p 2 + 2 pq + q 2 = 1 a. The population is made of: homozygous dominant genotypes (p 2) + heterozygous genotypes (2 pq) + homozygous recessive genotypes (q 2). b. The sum of the frequencies must always equal the entire population (100%).

c. Example: If 10% of the population exhibits attached earlobes (homozygous 90% recessive phenotype: ff), then ___ of the population is FF or Ff and exhibits the dominant free ____ phenotype (____ earlobes). Solution Dominant Recessive p + q =1 (free) + (attached earlobes) = 1 p + 10% = 100% p = 100% - 10% p = 90%

5. Five conditions necessary for Hardy. Weinberg Equilibrium NOTE: Hardy-Weinberg equilibrium rarely exists in natural populations but understanding the assumptions behind it gives us a basis for understanding how populations evolve.

Conditions necessary for Hardy-Weinberg Equilibrium a. The population is very large. b. The population is isolated (no migration of individuals, or alleles, into or out of the population). c. Mutations do not alter the gene pool. d. Mating is random. e. All individuals are equal in reproductive success (no natural selection).

IV. Agents of Change Agent 1. Mutation Example Alteration in an organism’s DNA. Ultimate source of variation.

Sickle Cell Mutation

IV. Agents of Change Agent 2. Gene Flow Example The movement of alleles from one population to another. Occurs when individuals move between populations.

IV. Agents of Change Agent 3. Genetic Drift Example The CHANCE alteration of gene frequencies in a small population. Can occur when populations are reduced in size (genetic bottleneck) or when a few individuals start a new population (founder effect).

IV. Agents of Change Agent 4. Nonrandom Mating Example Occurs when one member of a population is not equally likely to mate with any other member.

Queen Victoria & Hemophilia

IV. Agents of Change Evidence 5. Natural Selection Example Some individuals will be more successful than others in surviving and reproducing. Certain traits give them a better “fit” with the environment.

Large Ground Finch Small Tree Finch Woodpecker Finch

16 -3 The Process of Speciation I. How do we get new species? A. What is a Species? 1. Species: a group of interbreeding organisms that breed with one another and produce fertile offspring. This means that the individuals of the same species share a common gene pool.

Diversity in Humans

2. If a beneficial genetic change occurs in one individual, then that gene can be spread through the population as that individual and its offspring reproduce.

B. Isolating Mechanisms (Leads to a new species!) Reproductive Isolation – members of two populations cannot interbreed and produce fertile offspring.

Reproductive Barriers

PRE-Mating Reproductive Isolation – involves mechanisms which do not allow mating to occur in the first place. 1. Behavioral Isolation: Members of two populations are capable of interbreeding but have differences in mating displays or courtship rituals. a. specific scents (pheromones of insects). b. color patterns/strutting. c. specific sounds or calls.

Courtship Dance

Different Mating Songs

2. Geographic/Ecological Isolation: Two populations are separated by geographic barriers such as rivers, mountains, or bodies of water.

When has speciation occurred?

3. Temporal Isolation: Two or more species live in the same habitat but have different mating/reproductive seasons. a. Brown trout and Rainbow trout are found in the same streams but Rainbow trout spawn in the Spring and Brown trout spawn in the Fall. b. Three similar species of orchid living in the same tropical habitat each release pollen on different days; therefore, they cannot pollinate one another.

Section 16 -3 Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation Geographic isolation Temporal isolation produced by Behavioral differences Physical separation Different mating times which result in Independently evolving populations which result in Formation of new species

NOTE: Several isolating mechanisms can compound one another to insure mating doesn’t occur. This permits two species to occupy the same valuable habitat and prevents wastage of valuable gametes.

POST-Mating Reproductive Isolation – (fertilization occurred and zygote formed) 1. Hybrid inviability: hybrid zygotes fail to develop or fail to reach sexual maturity. 2. Hybrid sterility: Hybrids fail to produce functional gametes. Example: horse x donkey => mule (sterile).

Hybrid Sterility + Donkey Horse = Mule (sterile)

II. Testing Natural Selection in Nature A. Peter and Rosemary Grant from Princeton University worked to band measure finches on the Galapagos Islands for over twenty years. By documenting natural selection in the wild, the Grants provided evidence of the process of evolution.

B. Grants realized Darwin’s hypothesis relied on two TESTABLE assumptions: 1. Variation must occur in beak size and shape 2. Natural Selection takes place. a. Different finches compete and eat different food. b. During the rainy season, there is plenty of food.

c. During dry-season drought, some foods become scarce forcing birds to become feeding specialists. d. Each species selects the type of food its beak handles best. Example: Birds with big, heavy beaks can crack open big, thick seeds that no other birds can open. e. Grants observed that average beak size in that population increased dramatically over time. This is an example of directional selection.

III. Speciation in Darwin’s Finches Speciation in the Galapagos finches occurred by the following events: A. Founders Arrive: A few finches from the South American mainland (species A) flew to the Galapagos Islands. B. Geographic Isolation: Some birds from species A crossed to another island in the Galapagos group.

C. Changes in the Gene Pool: Over time, populations on each island became adapted to their local environments causing a separate species to form. On the second island, the larger seeds would favor individuals with larger, heavier beaks thus forming species B. D. Reproductive Isolation: If birds from the second island cross back to the first island mating does not occur between the two populations, then reproductive isolation has occurred. The two populations have become separate species.

E. Ecological Competition: As these two species compete on the first island, the more specialized birds have less competition. During a dry season, individuals that are most different from each other have the highest fitness. Over time, species evolve in a way that increases the differences between them. The species-B birds on the first island may evolve into a new species, C. F. Continued Evolution: Isolation on different islands, genetic change, and reproductive isolation led to 13 different finch species found there today.

Finch Species

IV. Studying Evolution Since Darwin A. Data from genetics, physics, biochemistry, geology and biology supports theory that living species descended with modification from common ancestors that lived in the ancient past.

B. Unanswered Questions 1. No scientist suggests that all evolutionary processes are fully understood. 2. Why is understanding evolution important? a. Drug resistance in bacteria and viruses. b. Pesticide resistance in insects. c. Evolutionary theory helps us respond to these changes to improve human life.