Evolution Unit: Evolution of Populations Biology Chapter 16 Evolution Unit: Evolution of Populations

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

B. Evolutionary biologists connected. Darwin’s work and Mendel’s work B. Evolutionary biologists connected Darwin’s work and Mendel’s work during the 1930’s. 1. Changes in ________ produce heritable variation on which _______________ can operate. 2. genes natural selection   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 ______________ ________________ 2. Population: ____________________ _______________________________ studied in populations. a group of individuals of 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 Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

change in the relative frequency of alleles Evolution is any change in the relative frequency of alleles MICROEVOLUTION: ______________ __________________________________________ in a population. Microevolution refers to ___________ change in allele frequency over time. small scale

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 100 80 60 40 20 Frequency of Phenotype (%) Attached Earlobes (ff) Free Earlobes (FF, Ff) Phenotype

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: Dark-colored peppered moths in regions of England with industrial pollution.

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

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 High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

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

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

C. Two types of genetic drift: 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 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 there would be genetic equilibrium.

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).

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

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.

1. Behavioral Isolation: Members of two populations are capable of interbreeding but have differences in mating displays or courtship rituals. a. b. c. specific scents (pheromones of insects). color patterns/strutting. 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.

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.

Reproductive Isolation Section 16-3 Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation Temporal isolation Geographic isolation produced by produced by produced by Behavioral differences Different mating times Physical separation 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.