BIO 171 1st Edition Lecture 8 Outline of Last Lecture I. Darwin and WallaceII. Phylogenetics Outline of Current Lecture I. Complex traitsII. Evolution by Natural SelectionCurrent LectureComplex Traits -Discrete vs. Polygenic traits oDiscrete traits= phenotypes fall into a few distinct classes (also known as Mendelian traits) -Ex) purple or white pea flowers, yellow or green pea seeds, ABO blood type oPolygenic (meaning many genes) traits= phenotypes display a bell-shaped or normal distribution (2 or more genes effect a phenotype)-Ex) human height, skin color, eye color, hair color, blood pressure -These traits are more prevalent because they are found everywhere Many common diseases are polygenic -Are also referred to as complex traits, continuous traits, or quantitative traits-Hypothesis to explain polygenic inheritance in color of wheat kernels:oRecessive alleles contribute 0 units of pigment, and dominant alleles contribute 1 unit ofpigmentoThe F1 generation (between homozygous recessive and homozygous dominant) shows incomplete dominance- a color that is in the middle of both extremesoThe F2 generation would then express polygenic inheritance (each offspring containing different combinations of recessive and dominant alleles)-As the number of dominant alleles increases, so does the pigment contribution, expressing a normal distribution of pigment color 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.-Genotype-by-environment interactions= When the phenotype is a result of an interplay between genes and the environment oAn example of this is emphysema because inheritance of the gene that codes for the disease (genotype), as well as smoking cigarettes (environment) affects a person's chances ofgetting it. -Heritability is defined as the proportion of phenotypic variation in a population that is attributed to genetic variation oIt can change depending on the population and conditions being studied -Allele frequency= the number of a certain allele present in the population, divided by the total number of alleles.oWhen a population exhibits only one allele at a particular gene, the population is fixed for that allele -Single Nucleotide Polymorphisms (SNPs)= sites in the genome where there are base pair differences among chromosomes (must be common enough to be present in a ransom sample of 50 diploid individuals, or 1% of the population)oThey can happen anywhere in DNA (including non-coding DNA)oGeneticists use SNPs for genetic mapping, or finding the gene responsible for a variable trait-Ex) the trait for blue eyes vs. brown eyes Evolution by Natural Selection -Populations evolve based on the ways the selective pressures (i.e. predators) act on individual organisms within the populationoEx) if it is harder to see darker squirrels in a dark forest, then the selective pressure will act on the lighter squirrels (darker squirrels have selective advantage) causing the populationof squirrels over time to evolve to be darker because of their ability to survive-Fitness= the number of surviving offspring that an individual produces oAlleles increase or decrease in frequency based upon their influence on an individual's fitness -Positive selection vs. negative selection oPositive selection= natural selection that increases the frequency of an allele that is advantageous (or beneficial)oNegative selection= natural selection that decreases the frequency of an allele that is deleterious (or harmful)-Balancing selection acts to maintain two or more alleles in a population oWhen the heterozygote's fitness is higher than that of either of the homozygotes, it results in selection that ensures both alleles remain in the population at intermediate frequencies. (This is called the heterozygote advantage and it leads to balancing selection)-Types of natural selection:oStabilizing selection= maintains the status quo and acts against extremes (reduces the amount of variation in a trait)oDirectional selection= one tail of phenotypic distinction is favored, or 1 allele. (leads to change in a trait over time)a. Artificial selection= a type of directional selection based on human priorities (i.e. humans, not nature, select what they want)oDisruptive selection= Favors extremes and is against intermediate