Section1: Genetic Variation Preview Bellringer Key Ideas Population Genetics Phenotypic Variation Measuring Variation and Change Sources of Genetic Variation Summary

Bellringer Imagine that you are in charge of a goat ranch.  The cost of fencing is high, so you must implement a breeding program that will produce shorter-legged goats within the next 20 years.  Write down how you would meet this goal.

Key Ideas How is microevolution studied? How is phenotypic variation measured? How are genetic variation and change measured? How does genetic variation originate?

Population Genetics Charles Darwin knew that heredity influences characteristics, but he did know about genes. We can now study and predict genetic variation and change that underlie evolution. Microevolution is evolution at the level of genetic change in populations.

Population Genetics, continued Microevolution can be studied by observing changes in the numbers and types of alleles in populations, called population genetics. The study of genetics and evolution are advancing together. The link from microevolution to macroevolution—speciation—can be studied in detail.

Visual Concept: Population and Gene Movement

Phenotypic Variation The variety of phenotypes that exists for a given characteristic depend on how many genes affect it. Polygenic characters are influenced by several genes. Examples include human eye color and height. Biologists study polygenic phenotypes by measuring each individual in the population and then analyzing the distribution of the measurements.

Visual Concept: Single Allele, Multiple Allele and Polygenic Traits

Phenotypic Variation, continued A distribution is an overview of the relative frequency and range of a set of values. Often, some values in a range are more common than others. A normal distribution, or bell curve, is one that tends to cluster around an average value in the center of the range.

Normal Distribution

Measuring Variation and Change The particular combination of alleles in a population at any one point in time makes up a gene pool. Genetic variation and change are measured in terms of the frequency of alleles in the gene pool of a population. A frequency is the proportion or ratio of a group that is of one type. To study genetic change, the frequency of each allele in a population can be tracked over time.

Genotype Frequencies Vs. Allele Frequencies

Sources of Genetic Variation Evolution cannot proceed if there is no variation. The major source of new alleles in natural populations is mutation in germ cells. Mutation generates new alleles at a slow rate. Only mutations in germ cells (egg and sperm) are passed on to offspring.

Summary Microevolution can be studied by observing changes in the numbers and types of alleles in populations. Biologists study polygenic phenotypes by measuring each individual in the population and then analyzing the distribution of the measurements. Genetic variation and change are measured in terms of the frequency of alleles in the gene pool of a population. The major source of new alleles in natural populations is mutation in germ cells.