Transcript 15.2 Notes
15.2 Notes I. Population Genetics and Evolution A. Organisms do not adapt new traits over their lifetimes. 1. Natural selection acts on ALL organisms in a population. 2. As a population’s genes change, the characteristics of that population also change. 3. All of a population’s genes is collectively known as a gene pool. a. If a populations genes don’t change over many generations, the population is in genetic equilibrium. b. Populations in genetic equilibrium are not changing. B. Mutations can sometimes be responsible for changing the genes in a population. 1. Mutations are usually fatal. C. Genetic drift can also cause a population’s genes to change. 1. Small populations can experience genetic drift. D. Mutations and Genetic drift influence mostly smaller populations. 1. Natural selection affects mostly large, less isolated populations. 1. Stabilizing selection – favors average organisms. Selection for average size spiders Normal variation 2. Directional selection - favors one of the extreme variations of a trait. Normal variation Selection for longer beaks 3. Disruptive selection – individuals with either extreme are selected for. Selection for light limpets Normal variation Selection for dark limpets II. The Evolution of Species A. The evolution of a new species – Speciation. B. Species is defined as: 1. A group of living things. 2. Can breed with others in that group. 3. Breeding results in FERTILE offspring. C. Fertile-able to reproduce by forming egg and sperm cells. D. There are three major events that lead to species formation: 1. A barrier forms that separates members of a species (Geographic Isolation) . Speciation Rates Loxodonta africana Elephas maximus 0 1 2 Elephas 3 4 Mammuthus primigenius Loxodonta Mammuthus 5 Primelephas 6 Ancestral species about 55 million years ago Question 1 The fur of an Arctic fox turns white in the winter. Is this an example of natural selection? Why or why not? The answer is no. An individual cannot evolve a new phenotype (in this case, changing the color of its fur) within its lifetime in response to its environment. Question 2 Which type of natural selection does NOT favor the evolution of new species? A. divergent B. disruptive C. stabilizing D. directional The answer is C. Stabilizing selection reduces variation in a population. Question 3 Which of the following rarely affects a population’s genetic equilibrium? A. genetic drift B. lethal mutations C. gene flow D. disruptive selection The answer is B. Organisms with lethal mutations do not survive. Therefore, organisms with lethal mutations cannot produce enough offspring to affect a population’s genetic equilibrium. Question 4 Why are the Galapagos Islands rich in unique species of organisms? A. The islands are an area exhibiting an abnormal number of mutations. B. The islands are geographically isolated. C. The island species have been subjected to increased gene flow. D. The island species have been subjected to stabilizing selection. The answer is B. Geographic isolation has helped to keep the islands’ species unique.