Chapter 17 – Evolution of Populations 17.1 Genes and Variation

THINK ABOUT IT Darwin developed his theory of evolution without knowing how heritable traits passed from one generation to the next. What role does genetics play in understanding evolution?  

Genetics Joins Evolutionary Theory How is evolution defined in genetic terms? Microevolution is any change in the relative frequency of alleles in the gene pool of a population over time.

Genetics Joins Evolutionary Theory Heritable traits are controlled by genes. Changes in genes and chromosomes create variation. Example = Children receive their genes from the same parents all look different. (Exception Twins)

Genotype and Phenotype in Evolution Genotype – an individuals particular combination of alleles. Genotype with environmental conditions produces = phenotype. Phenotype - all physical, functional, and behavioral characteristics of an organism.

Genotype and Phenotype in Evolution Natural selection acts directly on phenotype. Some phenotypes are better suited to their environment than others. **Better suited individuals produce more offspring and pass on their genes to the next generation.**

Populations & Their Gene Pools Gene Pool: consists of all the alleles in all the individuals that make up a population Frequency of alleles: how often certain alleles occur in the gene pool (expressed as a decimal or a %) Figure 14-24 Each plant in this hypothetical population of wildflowers has 2 alleles for flower color. In all, there are 14 red-flower alleles (R) and 6 white-flower alleles (r). The frequency of each allele is calculated as a ratio based on the total of 20.

Allele Frequency - Example Example - this diagram shows the gene pool for fur color in a population of mice.  

Populations and Gene Pools Natural selection operates on individuals, but resulting changes in allele frequencies show up in populations. Populations, rather than individuals, evolve!!!!  

Sources of Genetic Variation Three sources of genetic variation: Mutation Genetic recombination during sexual reproduction Lateral gene transfer

Mutations Change in the genetic material of a cell! Mutations that impact phenotype may or may not affect fitness. Can be: Lethal Beneficial  Detrimental Mutations matter in evolution only if they can be passed from generation to generation.

Genetic Recombination in Sexual Reproduction Combination of genes from different parents Half of the DNA comes from each parent. Each offspring receives a unique set of genetic information. (Exception = Twins) This is the result of crossing over and independent assortment during meiosis!  

Lateral Gene Transfer When organisms pass genes from one individual to another that is not its offspring. Increases genetic variation in the individual who acquires the new genes. Example = Formation of antibiotic resistant bacteria.

Single-Gene and Polygenic Traits KEY: What determines the number of phenotypes for a given trait? ANSWER: The number of phenotypes produced for a trait depends on how many genes control the trait.

Single-Gene and Polygenic Traits  The number of phenotypes produced for a trait depends on how many genes control the trait. Single-Gene Traits Polygenic Traits

Single-Gene Traits Single-gene trait = is a trait controlled by only one gene. May have just two or three distinct phenotypes.   The most common form of the allele can be dominant or recessive.

Single-Gene Trait - Example Dominant allele isn’t always the most common in a population! In a snail population some have dark bands on their shells while others don’t. Allele for no bands = dominant Allele for bands = recessive

Polygenic Traits Polygenic traits = traits controlled by two or more genes. Each gene of a polygenic trait often has two or more alleles. Results in many different genotypes and phenotypes!   Examples: Height and skin color

Polygenic Trait - Example Human height, which varies from very short to very tall, is an example of a polygenic trait. The bell-shaped curve in the graph is typical of polygenic traits.