BIOl 151 1st Edition Lecture 5 Outline of Last Lecture I Hardy Weinberg Equation II Conditions of Evolution III Small Population size Outline of Current Lecture I Types of Selection II Preservation of Genetic Variation III Barriers isolating gene pools Current Lecture I Types of Selection Positive selection increases the frequency of certain alleles resulting in adaptation and increased fitness this selection can promote fixation of certain alleles On the other hand negative selection decreases frequencies of harmful alleles that reduce fitness Balancing selection is the maintenance of two or more alleles in a population This selection provides a heterozygote advantage or better resistance to an ailment Natural selection can alter the frequencies of heritable traits through three main types of selection stabilizing selection disrupting selection and directional selection Stabilizing selection acts against extreme phenotypes and favors intermediate forms of the phenotype Disrupting selection favors the two extremes of a phenotype when the environment is disrupted Directional Selection is similar to disruptive but favors only one extreme of a particular phenotype Artificial Selection occurs when humans select individuals with desired traits and uses them as breeding stock Some examples of artificial selection are domesticated dogs and domesticated plants Sexual selection occurs to enhance the individual s ability to reproduce successfully II Preservation of Genetic Variation These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute There are several factors that ensure that there is genetic variation within a population such factors are diploidy and balancing selection Diploidy occurs when genetic variation is hidden as a recessive gene This means that an individual with a heterozygous gene for a particular gene could interbreed with an individual with a homozygous dominant and produce an individual with a homozygous recessive gene This is important because it ensures genetic variation among the population Balancing selection maintains that a population will have genetic variation III Barriers isolating gene pools There are two types of barriers that isolate gene pools these barriers are pre zygotic and post zygotic barriers Pre zygotic barriers prevent mating between closely related species and post zygotic barriers prevent complete formation of viable and fertile offspring There are several types of pre zygotic barriers Habitat or ecological isolation prevents closely related species from contacting and mating by means of physical barriers such as mountains or rivers Temporal Isolation separates species by having different times of breeding Behavioral Isolation means that each species has specific courtship rituals or signals for attracting mates Mechanical Isolation means that each species has specific physical shapes and therefore are incompatible with other species Gamete Isolation occurs because gametes are incapable of fusion because recognition of sperm and egg is species specific