Chapter 17 – Evolution of Populations 17.2 - Evolution as Genetic Change in Populations

How Natural Selection Works – Review In genetic terms, what does fitness mean? -Passing on genes onto offspring Adaptation: Any genetically controlled trait that increases an individual’s ability to pass along its alleles.

Natural Selection on Single-Gene Traits Natural selection on a single-gene trait can lead to changes in allele frequencies and thus to differences in phenotype frequency.   Why are red lizards less likely to survive? Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.

Natural Selection on Polygenic Traits Polygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. Natural selection can affect the relative fitness of a phenotype and result in one of three types of selection. Direction Selection Stabilizing Selection Disruptive Selection

Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts.

Stabilizing Selection Individuals near the center of the curve have higher fitness than individuals at either end. Center of the curve at its current position, narrows the overall graph.

Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Acts against individuals of an intermediate type Can create 2 distinct phenotypes

Changes in Gene Pools Due to Chance Genetic Drift: a change in the gene pool (allele frequency) of a population due to chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population. Evolution can occur with out natural selection. Populations can evolve through genetic drift as well/

Examples of Genetic Drift Bottleneck Effect - Chance decreases the size and genetic variation of the population, decreases adaptability Ex. - when a population is reduced in size (due to a disaster) and ultimately the gene pool of the population is also reduced in size

Examples of Genetic Drift Founder Effect - when few individuals colonize an isolated environment, reducing the gene pool of the population Chance decreases genetic variation Change in the gene pool is determined by the founders of the colony

Other Mechanisms of Gene Pool Changes Gene Flow - the exchange of genes with another population Occurs when fertile individuals or their gametes migrate between populations (ex: wind and pollen) Tends to reduce genetic differences between populations

Natural Selection and Fitness Only natural selection leads to adaptation Genetic Drift, Gene Flow, and Mutations do not necessarily lead to adaptation These do not tend to increase fitness within a population

Warm-up: Use the choices on the right to describe the scenarios on the left Scenario 1: 4 moose were taken from the Canadian mainland to Newfoundland, in 1904. These 2 males and 2 females rapidly formed a large population of moose that now flourishes in Newfoundland. Scenario 2: The fossil remains of pygmy (or dwarf) mammoths (1.5 m to 2 m tall) have been found on Santa Rosa and San Miguel Islands off the coast of California. This population of pygmy mammoths is descended from a population of mammoths of normal size (4 m tall). Dwarfing is common in island populations and is not the result of chance events. Scenario 3: Bearded Vultures in the Alps was nearly hunted to extinction. The population is currently small and lacks genetic variation. Which mechanism is most likely to have contributed to the 3 scenarios to the left? A. Bottleneck Effect B. Founder effect C. Natural Selection The correct answer is the founder effect. Student Misconceptions and Concerns 1. The concept of a genetic bottleneck contributing to a loss of diversity can be difficult for some students to understand. Without an appreciation for population genetics, students might think that a recovery in terms of sheer numbers is all that is needed. Yet the loss of genetic diversity might ultimately doom a population that is later faced with widespread disease. Teaching Tips 1. To demonstrate the concept of genetic drift, consider a demonstration with several colored beans (different kinds of dried beans are cheap and easy to find in grocery stores). Fill a bottle with ten beans of five different colors. Then remove ten beans as a population sample. It is very likely that the sample will not adequately represent the diversity of the original bean collection. In fact, it is common for not all colors to be represented. As an alternative, draw eight cards out of a shuffled deck of 52 cards. Are all four suits represented? Are they represented equally? 2. Sampling error, a cause of genetic drift, can be difficult for some students to understand, but the following exercise may help. Have students work in pairs to flip a coin ten times. For each pair, list the number of heads on the board as just a long list of numbers (no need to keep track of heads and tails). This can be done quickly, as long as you have been sure to bring enough pennies. Now analyze the results. There should be considerable variation from the expected five. Now determine the average of all the groups and the collective sample size. By pooling the results, and increasing the sample size, the means should be closer to five. 3. The loss of genetic diversity in a population due to bottlenecks is a significant problem in conservation. When a species is reduced to relatively few individuals, and then is brought back to abundance by extraordinary efforts, the species is not fully recovered. The lost genetic diversity may be a prerequisite for the long-term survival of the species. Consider challenging your students to identify modern endangered species not mentioned in the textbook that might suffer from a genetic bottleneck. 4. The following thought exercise may help students understand founder-effect “bias.” Ask them to imagine that all the students present in today’s class are the only survivors of some global catastrophe. Would your class adequately represent the biological diversity of the current human population?

Evolution VS Genetic Equilibrium If a population is not evolving, allele frequencies in its gene pool do NOT change this is genetic equilibrium Sexual reproduction and allele frequency: A population of sexually reproducing organisms could remain in genetic equilibrium (random mating)

Hardy-Weinberg Principle Hardy-Weinberg principle - states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. 5 conditions that disturb genetic equilibrium and cause evolution to occur:

5 Conditions that Disturb Genetic Equilibrium Nonrandom mating a. Sexual selection: When individuals of a species select mates based on heritable traits (size, strength, color, etc) Small population size (Genetic drift happens easily) Immigration/Emigration (Introduction of new alleles to a gene pool, or removing alleles from a gene pool) Natural Selection (If different genotypes have different fitness, evolution will occur) Mutation Most of the time evolution is happening.