Genes and Variation March 9th/10th, 2009 Remember… • In order for natural selection to occur, there MUST be variation. • Variation in the genes then creates variations in the phenotype. • Natural selection then “chooses” those phenotypes that are most favorable. Genetic variation is studied in Populations • A population is a group of individuals of the same species that interbreed. • Because members of the population interbreed, they share a common group of genes called a gene pool. • A gene pool consists of all the genes, including all the different alleles, that are present in a population. • The relative frequency of an allele is the number of times that the allele occurs in a gene pool, compared to the number of times other alleles for the same gene occur. What are the main sources of genetic variation in a population? • There are two main sources of genetic variation in a population; 1. mutations 2. the shuffling of genes that results from sexual reproduction. Mutations • A mutation is any change in a sequence of DNA (insertion, deletion, point shift, etc.) – Mutations can be caused by replication errors, radiation, and/or chemicals in the environment. Gene Shuffling • Gene shuffling is the random mixing-up of the genetic information. – Occurs during gamete formation (meiosis) when chromosomes cross over, as well as when they are randomly pulled apart during anaphase I and II. • Each of these babies has inherited a collection of traits. Some, such as hair color, are visible, while others, such as the ability to resist certain diseases, are not. Single-Gene Traits and Polygenic Traits. • A single-gene trait is a trait that is controlled by a single gene that has two alleles. • For example, widow’s peak in humans. – The allele for widow’s peak (W) is dominant over the allele for no widow’s peak (w). • Many traits in humans are controlled by two or more genes, and are therefore called polygenic (many-genes). • A polygenic trait can have many possible genotypes and phenotypes. – For example, height. People are not either tall or short. They can be tall, short, and everything in between. How Does Natural Selection Work on Polygenic Traits? • Class activity: Student Height 1. Directions: Using meter sticks, record the height of each student at your table to the nearest inch. Record the measurements in your data table. 2. When you are done, have a representative from your group come up to the front and record your numbers in the data table on the board. 3. Make sure to record each groups’ measurements in your data table, as they are written on the board. Types of Selection • Natural selection can affect the distributions of phenotypes in any of three ways: directional selection, stabilizing selection, or disruptive selection. Directional Selection • Directional Selection – when individuals at one end of the curve have a higher fitness than individuals in the middle or other end. For Example: Really tall people are more fit than average or really short people. Those that are very tall are then selected for, all others are selected against. Stabilizing Selection • Stabilizing Selection – when individuals near the center of the curve have a higher fitness than at either end of the curve. For Example: Average height people are more fit than really short or really tall people. Those that are average are then selected for, all others are selected against. Disruptive Selection • Disruptive Selection – when individuals at the upper and lower ends of the curve have a higher fitness than individuals near the middle. For Example: Very tall and very short people are more fit than average height people. Those that are very tall or very short are then selected for, all others are selected against.