Evolutionary Mechanisms: Microevolution - Lecture Notes | BI 102, Lab Reports of Biology

Download Evolutionary Mechanisms: Microevolution - Lecture Notes | BI 102 and more Lab Reports Biology in PDF only on Docsity! Evolutionary Mechanisms: Microevolution Objectives  Identify the mechanisms that produce change in allele frequencies.  Test hypotheses about allele frequencies using simulations  Draw conclusions about how effective different evolutionary mechanisms can be. You, as an individual, have a whole set of genes, your genome, which are segments of the DNA molecules we call chromosomes. The genes code for building and maintaining you as a living creature. Can you (or any individual) evolve in the biological sense? In other words, will your alleles, the different versions of a particular gene, change over your lifetime? You share genes with other humans and it is possible to determine allele frequencies for a population, members of a species that share a particular area at a specified time. The sum total of these genes in a population is called the gene pool. Population membership changes over time. Think of some ways in which this could happen. If population membership changes, would you expect that, under certain conditions, allele frequencies in this population could change over time as well? This is the topic of today’s lab—how allele frequencies can change over time or the mechanisms of evolution. If allele frequencies are significantly changing over time, the population has evolved. To observe evolution, we would need to look at several generations (perhaps 100’s or 1000’s) of a population. This would be difficult to achieve in a three-hour laboratory. Even bacteria need approximately a half-hour to reproduce! Instead, you will use simulations (simplified models of the real thing) to determine if and how evolution of population occurs. The primary goal of this lab is to give you insight about how evolution can and does work. Review of Mendelian Genetics It is important to distinguish an individual’s appearance, or phenotype, from the alleles present in that individual’s cells. Every individual carries two alleles of a given gene; the description of these alleles is a genotype. Recessive alleles are those that are masked or hidden by another allele. For example, the allele for blue eye color (recessive) is masked by an allele for a pigment such as brown. Pigment is thus dominant over blue. By convention, recessive alleles are denoted be lowercase letters and dominant alleles by uppercase letters. If “P” stands for pigment and “p” stands for nonpigmented eye color, then blue-eyed individuals have the genotype “pp” since they have two recessive alleles, while individuals with pigmented eyes are either “Pp” or “PP”. We cannot identify the correct genotype for the pigmented phenotype unless we gain more information about parents and/or offspring, since it takes only one dominant allele to mask a recessive allele. Note that some genotypes involve identical pairs (“pp” and “PP”, called homozygous (alike) and some involve a mixed pair (“Pp”, called heterozygous). Most human traits, such as facial features, height, dexterity, etc. are controlled by numerous alleles, and many of these have complex interactions among themselves and with the environment. These traits can be very difficult to study. Some human physical and biochemical traits are controlled through the inheritance of single genes with two or more alternate alleles, for example traits such as blood type. We will examine eight such traits in this class. Bi 102 Laboratory Microevolution 1 1. For each of the following traits, determine your phenotype and note it in Table 1. Record your genotype for each of the traits. Remember that if you have a recessive characteristic you must have both recessive alleles, but if you have a dominant phenotype you have no way of knowing if you carry a recessive allele (unless you have children). In this case, use a dash (–) to represent the unknown second allele. 2. When you have determined your phenotypes for those listed in Table 1, record your results on the board. Fill in the table with the class results, and then determine the percentage of individuals who have each trait. To calculate percentages you will need the total number of students in your section. Letter Characteristic Your Phenotype Your Genotype Lab Totals Number Dom Rec Percentage Dom Rec D Dimpled chin or non-dimpled chin E Free earlobes or attached earlobes W Widow’s peak or straight hairline T Taster of PTC or nontaster F Left thumb on top Right thumb on top B Bent little finger or finger not bent N Normal thumb or Hitchhiker’s thumb R Tongue roller or nonroller Population Simulations with Beans To determine if a population is evolving it is necessary to keep track of allele frequencies in the population over time. Alleles, for sexually reproducing organisms, of course, are inherited as a pair, each associated with a homologous chromosome. One allele is inherited from the father (via sperm) and the other from the mother (via the ovum or egg). Therefore, each individual in the population will contribute two alleles. To simplify things, you will only consider two versions of a gene. Let’s say we have a dominant allele, A, and a recessive allele, a. We need to know the frequency of each in the initial population so that it is possible to track any changes in frequency over time. Let’s say the frequency of A is represented by the letter p. And q represents the frequency of a. Because, for this example, there are only A’s and a’s in the population and the sum of all frequencies should equal 100% (or 1.00), then what is the relationship of p and q to the total frequency of alleles in the population? Write the equation showing the relationship between the total frequency (i.e. 1.00) and the frequencies (p and q) of A and a in the population (see Question 1 on the lab assignment sheet to turn in). Bi 102 Laboratory Microevolution 2