Biology 1 Notes- Chapter 16 (pages 393-410) Evolution of Populations 16–1 Genes and Variation A. How Common is Genetic Variation? B. Variation and Gene Pools C. Sources of Genetic Variation 1. Mutations 2. Gene Shuffling Single-Gene and Polygenic Traits 16–2 Evolution as Genetic Change A. Natural Selection on Single-Gene Traits B. Natural Selection on Polygenic Traits 1. Directional Selection 2. Stabilizing Selection 3. Disruptive Selection C. Genetic Drift D. Evolution Versus Genetic Equilibrium 1. Random Mating 2. Large Population 3. No Movement Into or Out of the Population 4. No Mutations 5. No Natural Selection 16–3 The Process of Speciation A. Isolating Mechanisms 1. Behavioral Isolation 2. Geographic Isolation 3. Temporal Isolation B. Testing Natural Selection in Nature 1. Variation 2. Natural Selection 3. Rapid Evolution C. Speciation in Darwin’s Finches 1. Founders Arrive 2. Separation of Populations 3. Changes in the Gene Pool 4. Reproductive Isolation 5. Ecological Competition 6. Continued Evolution D. Studying Evolution Since Darwin

How Common is Genetic Variation? Genetic variation is studied within populations. Because members within populations interbreed, they share a common group of genes called a gene pool. A gene pool is made up of all of the genes (including all of the alleles) that are present in a population. The relative frequency of an allele is the number of times that allele shows up, compared to the number of times other alleles for that same gene occur. In genetic terms, evolution is any change in the relative frequency of alleles in a population. (change from 10-20% allele frequency= evolution)

Sources of Genetic Variations Gene pool for a fur color in mice 50 alleles total 20 are B black 30 b brown 48% heterozygous black 36% homozygous brown 16% homozygous black allele for brown fur allele for black fur Frequency of Alleles Two main sources of genetic variation Mutations Genetic shuffling from sexual reproduction

Phenotypes for Single-Gene Trait Frequency of Phenotype (%) 100 80 60 40 20 Widow’s peak No widow’s peak Phenotype A single gene trait is one that is controlled by one gene with 2 alleles for that gene. Widows peak is an example of a single gene trait and the frequency of phenotype is shown in the graph.

Generic Bell Curve for Polygenic Trait Polygenic traits are traits that are controlled by 2 or more genes. The graph shows there are many possibilities for phenotypes. Also, most people have the average or medium version of the phenotype, with a small number of individuals at either extreme. Frequency of Phenotype Phenotype (height)

Evolution as Genetic Change Natural Selection on Single Gene Traits Natural selection can lead to changes in allele frequency, and thus evolution. Natural Selection on Polygenic Traits Some variations increase or decrease an organism’s chance of survival in an environment. Fitness is the ability of an organism to survive and reproduce in its environment. The range of phenotypes of a polygenic trait normally fit a bell curve. Natural Selection can affect the distribution of phenotypes. There are three different types of natural selection that act on variations: stabilizing, directional, and disruptive.

Directional Selection Normal variation Selection for longer beaks Directional Selection takes place when individuals at one end of the curve have a higher fitness than those in the middle or at the other end. Natural selection favors one of the extreme variations of a trait. Directional Selection Food becomes scarce. Key Low mortality, high fitness High mortality, low fitness High mortality, low fitness

Stabilizing Selection Selection for average size spiders Normal variation Stabilizing Selection Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection Stabilizing selection is a natural selection that favors average individuals in a population. This keeps the curve in the same place, but makes it more narrow.

Disruptive Selection In disruptive selection, individuals with either extreme of a trait’s variation are selected for (individuals at the ends of the curve have higher fitness than individuals near the middle) Over time, it is less likely that species with extreme variations will mate, therefore giving rise to new species. Selection for light limpets Normal variation dark limpets Disruptive Selection Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Key Low mortality, high fitness High mortality, low fitness

Genetic Drift Genetic drift is the change of allelic frequencies by chance events. In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become common in a population. A 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.

Evolution Versus Genetic Equilibrium Genetic Equilibrium is when allele frequencies remain constant (there is no evolution or change in the population) The Hardy –Weinberg Principle states that allele frequencies in a population will remain constant as long as the following five conditions are met: If any of the above are not met, then the population will evolve Random mating- no preference in mate selection Large population size- so that small changes will not be significant No migration- no gene flow: no new alleles brought into the population No mutations- no new alleles added to the population No Natural selection- all organisms are reproductively successful therefore no genes are favored

The Process of Speciation Recall that a species is defined as a group of organisms that look alike and can interbreed to produce fertile offspring in nature. The evolution of new species is called speciation. Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. There are three types of reproductive isolation: behavioral, temporal, and geographic.

Types of Reproductive Isolation Behavioral Isolation Two organisms are capable of interbreeding, but they have different courtship rituals. Ex: different mating songs of birds eastern and western meadowlarks Temporal Isolation Two or more species reproduce at different times. Ex: Pollen released at different times from orchids. Geographic Isolation Geographic isolation occurs whenever a physical barrier divides a population . Ex: tree frogs

Geographic Isolation When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations. Tree frogs are a single population. The formation of a river may divide the frogs into two populations. Over time, the divided populations may become two species that may no longer interbreed, even if reunited.

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

Testing Natural Selection in Nature Peter and Rosemary Grant have worked for the past twenty years to show that Darwin’s hypothesis is correct. They realized that Darwin relied on two assumptions: In order for beak size and shape to evolve in finches, there must have been many varieties in those traits for natural selection to work on. Differences in beak size and shape must produce differences in fitness that cause natural selection to occur.

Variation The Grants found that the finches of the Galapagos Islands had a great variety of heritable traits. Many of the characteristics appeared in a bell-shaped distributions typical of polygenic traits.

Natural Selection Individual birds with different sized beaks had different chances of survival during a drought. (food, mating) The average beak size in that finch population increased dramatically over time. Natural Selection This change in beak size is an example of directional selection operating on an anatomical trait

Rapid Evolution The Grants found that natural selection takes place frequently—and sometimes very rapidly. Changes in the food supply caused measurable fluctuations in the finch populations over a period of only decades. This is very different from the slow, gradual evolution that Darwin envisioned.

Speciation in Darwin's Finches The Grants' work demonstrates that finch beak size can be changed by natural selection. Speciation in the Galapagos finches occurred by founding of a new population, geographic isolation, changes in the new population's gene pool, reproductive isolation, and ecological competition. Small groups of finches moved from one island to another, became reproductively isolated, and evolved into new species.

Founders Arrive A few finches travel from South America to one of the islands, there they survive and reproduce. Separation of populations Some birds from species A cross to a second island. The two populations no longer share a gene pool. Changes in the gene pool Seed sizes on the second island favor birds with larger beaks. The population on the second island evolves into a population, B, with larger beaks. Eventually, population A and B will evolve into a separate species.

Reproductive Isolation Even if a few birds from population B move back to the first island, they will not breed with the birds from population A. The differences in beak size and mating behavior will lead to reproductive selection. Ecological Competition The two species will compete with each other for seeds when they live together.

Continued Evolution The process of isolation on different islands, genetic change, and reproductive isolation will continue to repeat itself. Thirteen different species of finches live on the Galapagos islands today.

Studying Evolution since Darwin Why is understanding evolution so important? Evolution continues today. For example, bacteria and viruses are evolving resistance to drugs. Insects are evolving resistance to pesticides. Evolutionary theory can help us understand and respond to these changes in ways that can improve human life. There are still many unanswered questions that need to be addressed.