Chapter 8: Evolution Lesson 8.3: Microevolution and the Genetics of Populations ​ ​ Microevolution refers to varieties within a given type. Change happens within a group, but the descendant is clearly of the same type as the ancestor. This might better be called variation, or adaptation, but the changes are "horizontal" in effect, not "vertical." Such changes might be accomplished by "natural selection," in which a trait ​ ​ ​ ​ within the present variety is selected as the best for a given set of conditions, or accomplished by "artificial selection," such as when dog breeders produce a new breed of dog. Lesson Objectives ● Distinguish what is microevolution and how it affects changes in populations. ● Define gene pool, and explain how to calculate allele frequencies. ● State the Hardy-Weinberg theorem ● Identify the five forces of evolution. Vocabulary ● adaptive radiation ● gene pool ● migration ● allele frequency ● genetic drift ● mutation ● artificial selection ● Hardy-Weinberg theorem ● natural selection ● directional selection ● macroevolution ● population genetics ● disruptive selection ● microevolution ● stabilizing selection ● gene flow Introduction Darwin knew that heritable variations are needed for evolution to occur. However, he knew nothing about Mendel’s laws of genetics. Mendel’s laws were rediscovered in the early 1900s. Only then could scientists fully understand the process of evolution. Microevolution is how individual traits within a population change over time. In order for a population to change, some things must be assumed to be true. In other words, there must be some sort of process happening that causes microevolution. The five ways alleles within a population change over time are natural selection, migration (gene flow), mating, mutations, or genetic drift. The Scale of Evolution We now know that variations of traits are heritable. These variations are determined by different alleles. We also know that evolution is due to a change in alleles over time. How long a time? That depends on the scale of evolution. ● Microevolution occurs over a relatively short period of time within a population or species. The ​ Grants observed this level of evolution in Darwin’s finches which will be discussed later in this lesson. ● Macroevolution occurs over geologic time above the level of the species. The fossil record reflects ​ this level of evolution. It results from microevolution taking place over many generations and will be discussed in the next lesson of this chapter. 231 Genes in Populations One common misconception about evolution is the idea that individuals can evolve. Individuals do not evolve. Their genes do not change over time. Individuals can only accumulate adaptations that help them survive in the environment. Evolution takes a long time, spanning several generations, to happen. While it is possible for individuals to mutate and have changes made to their DNA, this does not mean the individual has evolved. So if individuals cannot evolve, then how does evolution happen? Populations can evolve. The unit of evolution is the population. A population consists of organisms of the same species that live in the same area and can interbreed. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. Populations of individuals in the same species have a collective gene pool in which all future offspring will draw their genes from. This allows natural selection to work on the population and determine which individuals are more “fit” for their environments. The aim is to increase those favorable traits in the gene pool while weeding out the ones that not favorable. Natural selection cannot work on a single individual because there are not competing traits in the individual to choose between. Gene Pool The genetic makeup of an individual is the individual’s genotype. A population consists of many genotypes. Altogether, they make up the population’s gene pool. The gene pool consists of all the ​ available genes of all the members of the population that are able to be passed down from parents to offspring. For each gene, the gene pool includes all the different alleles for the gene that exist in the population. The more diversity there is in a population of a species, the larger the gene pool. The gene pool can change in an area due to migration of individuals into or out of a population. If individuals that have certain traits are the only ones in the population and they emigrate out of or immigrate into a new population their genes will travel with them. The size of the gene pool directly affects the evolutionary trajectory of that population. As natural selection works on a population, the gene pool changes. The favorable adaptations become more plentiful and the less desirable traits become fewer or even disappear from the gene pool completely. Populations with larger gene pools are more likely to survive as the environment changes than those with smaller gene pools. For example, in bacteria populations, individuals that are antibiotic resistant are more likely to survive any sort of medical intervention and will live long enough to reproduce. Therefore, the gene pool has now changed to include only bacteria that are antibiotic resistant. Allele Frequencies Allele frequency or genetic variation is how often an allele occurs in a gene pool relative to the ​ other alleles for that gene. In genetic variation, the genes of organisms within a population change. ​ ​ Genetic variation occurs mainly through DNA mutation, gene flow (movement of genes from one population to another) and sexual reproduction. Due to the fact that environments are unstable, populations that are genetically variable will be able to adapt to changing situations better than those that do not contain genetic variation. Look at the example in Table 8.3. The population in the table has 100 members. In a sexually ​ ​ reproducing species, each member of the population has two copies of each gene. Therefore, the total number of copies of each gene in the gene pool is 200. The gene in the example exists in the gene pool in two forms, alleles A and a. Knowing the genotypes of each population member; we can count the ​ ​ ​ ​ number of alleles of each type in the gene pool. The table shows how this is done. 232 Table 8.3: Number of Alleles in a Gene Pool (for one gene with two Alleles, A and a) ​ ​ -------------------------------------------------------------------------------------------------------------------------------------------------------- Genotype Number of Individuals Number of Allele A Number of Allele a ​ ​ in the Population Contributed to the Contributed to the with that Genotype Gene Pool by that Gene Pool by that Genotype Genotype -------------------------------------------------------------------------------------------------------------------------------------------------------- AA 50 50 × 2 = 100 50 × 0 = 0 Aa 40 40 × 1 = 40 40 × 1 = 40 aa 10 10 × 0 = 0 10 × 2 = 20 Totals 100 140 60 -------------------------------------------------------------------------------------------------------------------------------------------------------- Let the letter p stand for the frequency of allele A. Let the letter q stand for the frequency of allele a. We ​ ​ ​ ​ ​ ​ ​ ​ can calculate p and q as follows: ​ ​ ​ ​ ● p = number of A alleles/total number of alleles = 140/200 = 0.7 ​ ​ ​ ● q = number of a alleles/total number of alleles = 60/200 = 0.3 ​ ​ ​ ● Notice that p + q = 1. ​ ​ ​ ​ Evolution occurs in a population when allele frequencies change over time. What causes allele frequencies to change? That question was answered by Godfrey Hardy and Wilhelm Weinberg in 1908. Hardy and Weinberg and Microevolution Hardy was an English mathematician. Weinberg was a German doctor. Each worked alone to come up with the founding principle of population genetics. Today, that principle is called the Hardy-Weinberg theorem. It shows that allele frequencies do not change in a population if certain conditions are met. Such a population is said to be in Hardy-Weinberg equilibrium. The conditions for equilibrium are: In order for this equation to work, it is assumed that all of the following conditions are not met at the same time: 1. Mutation at a DNA level is not occurring. Therefore, no new alleles are being created. 2. Natural Selection is not occurring. Thus, all members of the population have an equal chance of reproducing and passing their genes to the next generation. 3. The population is infinitely large. 4. All members of the population are able to breed and do breed. 5. All mating is totally random. This means that individuals do not choose mates based on genotype. 6. All individuals produce the same number of offspring. 7. There is no emigration or immigration occurring. In other words, no one is moving into or out of the population. The list above describes causes of evolution. If all of these conditions are met at the same time, then there is no evolution occurring in a population. Since the Hardy Weinberg Equilibrium Equation is used to predict evolution, a mechanism for evolution must be happening. However, when all these conditions are met, allele frequencies stay the same. Genotype frequencies also remain constant. In addition, genotype frequencies can be expressed in terms of allele frequencies, as Table 8.4 shows. ​ ​ 233 Table 8.4: Genotype Frequencies in a Hardy-Weinberg Equilibrium Population ​ ​ (for one gene with two alleles, A and a) --------------------------------------------------------------------------------------------------------------------------------------------------------