DAY 3 1

Population Genetics and Evolution  Darwin developed his theory of natural selection without knowing about genes.  The principles of today’s modern theory are rooted in population genetics and other related fields of study and are expressed in genetic terms. 2

Populations, not individuals, evolve  Can an organism respond to natural selection by acquiring or losing characteristics? Recall that genes determine most features Within its lifetime, an individual cannot evolve to a new phenotype by natural selection in response to its environment 3

…  Natural selection acts on the range of pheontypes in a population. Each member of a population has the genes that characterize the traits of the species (as pairs of alleles). All of the genes of the population’s individuals make up the population’s genes Evolution occurs as a population’s genes and their frequencies change over time. 4

…  How?  Gene pool – picture all of the alleles of the population’s genes as being together in a large pool.  The percentage of any specific allele in the gene pool is called the allelic frequency. Scientists calculate the allelic frequency of an allele in the same way that a baseball player calculates a batting average.  Genetic Equilibrium - a population in which the frequency of alleles remains the same over generations. 5

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Changes in Genetic Equilibrium  A population that is in genetic equilibrium is not evolving.  If allelic frequencies remain the same, so do phenotypes.  Changes in genetic equilibrium result in evolution.  Mutations  Genetic drift – the alteration of allelic frequencies by chance events. Can greatly affect small populations Life in the Galapagos Islands  Gene flow – the transport of genes by migrating individuals. When an individual leaves a population, its genes are lost from the gene pool. 7

Natural Selection acts on Variation  Recall that some variations increase or decrease an organism’s chance of survival in an environment. These variations can be inherited and are controlled by alleles. There are three different types of natural selection that act on variation: stabilizing, directional, and disruptive. 8

Stabilizing Selection  favours individuals with an “average” value for a trait, and selects against those with extreme values. Human birth weight is an example. Until recent medical advance, infants that were too small tended not to survive and infants that were too large died during birth. 9

Stabilizing selection 10

Directional Selection  favours individuals possessing values for a trait at one extreme of the distribution, and selects against the average and other extreme.  The development of antibiotic resistant bacteria is an example of directional selection. Only those bacteria that can tolerate the presence of an antibiotic survive. 11

Directional Selection 12

Disruptive Selection  favours individuals at both ends of the distribution and selects against the average.  It is also known as diversifying selection. 13

Disruptive Selection 14

Natural vs. Artificial Selection  Both are mechanisms of change in the gene pool of a population  In artificial selection - humans ensure individuals with the more desirable traits are allowed to reproduce  In natural selection, those individuals who are best suited to their environment survive and reproduce. 15

Natural Selection in a Candy Dish  Yummy! 16