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ANTHRO 1: Introduction to Biological Anthropology R. Mitchell, Instructor GENES IN POPULATIONS INTRODUCTION Evolution, that is, change in living things over time, occurs through the process of many small genetic changes that take place from generation to generation. That’s why the modern definition of evolution is a genetic one: a change in the frequency of alleles from one generation to the next. For this definition to be useful, you need to have a working definition of the terms population, allele, and allele frequency. A population is a group of freely interbreeding individuals. Alleles are different forms or versions of the same gene. Allele frequency refers to the percentage of times a particular allele appears in a population. For any given trait (such as blood type or eye color), if you count the number of people with each form of a trait (how many people with Type A blood or green eyes), your total count is a frequency of individuals with each form of a trait. Frequencies are expressed in percentages. So what actually causes evolutionary change? Or, rather, what causes allele frequencies to change? Evolution relies on four main “forces” that cause a change in gene frequency in a population over time. These are: Natural Selection, Gene flow, Genetic drift, and Mutation. The first three processes cause change in the frequency of various genes in the population by “redistributing” the existing alleles, while the fourth, mutation, is the only one to introduce new variation into the gene pool. Natural Selection – Natural selection refers to the process by which those individual organisms that possess a favorable “variation” or “trait” within a particular environmental context are better adapted and thus more likely to survive and reproduce than those individuals that are less well-adapted. Therefore, the genes associated with the well-adapted traits will be passed onto the next generation with greater frequency than the genes for the less well-adapted traits. The effect of this differential reproductive success of individuals results in a change in gene frequency within the population. Over time, populations may become reproductively isolated from each other (geographically, behaviorally) such that splintered populations may no longer interbreed with members of the populations from which they were originally derived, that is, they may constitute a new species. Thus, natural selection acts on individuals, while evolution occurs at the level of the population. Gene Flow – Gene flow is the exchange of genes between populations. As individuals (and the genes they take with them) move to and from different populations, their movement alters the gene frequency in the populations they leave behind, as well as the population they join. This flow or exchange of genes between populations can result in evolutionary change as new genotypes and thus, phenotypes become established in the gene pool. Genetic Drift – Genetic drift involves the random “fission” (or splitting/separating) of populations into entirely new or distinct populations with different gene frequencies. Two frequent causes of the fluctuation in gene frequencies are the founder effect and genetic bottleneck. If, for example, a small group of individuals separates itself from the rest of the population to “found” (i.e. establish) a new population, it will not have the same genetic diversity as the original population, called the parent population. The small group that has broken away therefore contributes exclusively to the gene pool of the next generation. This is called the “founder effect” and could lead to phenomenon known as a “genetic bottleneck” since the “founding” group and its descendants carry only a small proportion of all the alleles (and of the variation) that were present in the original population. Genetic bottlenecks can also occur when there is a sudden, sharp decline in a populations size due to environmental factors (such as natural disasters, predation, habitat destruction, disease epidemics, etc.). These are random events that will remove some alleles from a population, even if they are beneficial and contribute to evolutionary or reproductive success since the individuals that carry those genes may perish due to these natural events. This results in a drastic reduction of the total genetic diversity of the original gene pool. The founder effect and genetic bottlenecks can have a major impact on small populations including: A higher proportion of recessive genes, which can be “lost” more easily in a large population A greater chance of two recessive alleles coming together in zygote formation More recessively expressed traits, sometimes including genetic diseases Loss of genetic diversity overall, because fewer individuals contribute their genes to the gene pool The effect of genetic drift on allele frequencies is larger in small populations and smaller in large populations. Mutation - A gene is a sequence of DNA bases that specifies the order of amino acids in a protein. A gene may take one of several alternative forms, called alleles. If an allele is altered during various genetic processes---mitosis, meiosis, DNA replication, or protein synthesis---a mutation has occurred. For such changes to have evolutionary significance, they must occur in the sex cells, which are passed between generations (Remember: evolution is defined as a change in allele frequencies between generations). Mutations provide entirely new sources of variation on which natural selection can act since alterations in an organism’s genotype can translate into a radically different phenotype (physical characteristics/traits). Thus, mutations can bring about major changes to an organism’s biology making evolution appear to occur relatively quickly rather than gradually. The punctuated equilibrium model of evolution proposes just this; that species tend to remain stable over long periods of time and evolutionary change occurs in sudden bursts (it is important to note, however, that in an evolutionary framework, “sudden” can mean thousands or tens of thousands of years). Taken together, these processes along with genetic recombination through sexual reproduction and the effects of the epigenome on gene expression and activity all impact the number and types of alleles present in a population. Consequently, genotypes and the phenotypes they produce are constantly being reshuffled and redistributed. These processes not only provide raw material for natural selection and evolution to work, but can lead to the production of new species in both a short as well as a long period of time.