Except for their plastic lunch coolers, you might think that this picture of children on their way to school came from the 1800s. In fact, the picture is a photograph that was taken in 2006. The children are part of a religious community called the Amish, whose founders first came to the U.S. in the 1700s.
Amish people shun modern conveniences such as electricity and automobiles. Their lives are more similar to the lives of their founders than to those of most other people in the U.S. today. As you will learn when you read this concept, the Amish are an example of one way in which populations may evolve. Genes in PopulationsIndividuals do not evolve because their genes do not change over time. Instead, evolution occurs at the level of the population. A population consists of organisms of the same species that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group. This means that most mating takes place within the population. Evolutionary change that occurs over relatively short periods of time within populations is called microevolution. The science that focuses on evolution within populations is population genetics. It is a combination of evolutionary theory and Mendelian genetics. The Gene PoolThe genetic makeup of an individual is the individual’s genotype. A population consists of many individuals and therefore many genotypes. All the genotypes together make up the population’s gene pool. The gene pool consists of all the genes of all the members of the population. For each gene, the gene pool includes all the different alleles of the gene that exist in the population. An allele is referred to as a version of a gene. For a given gene, the population is characterized by the frequency of the different alleles in the gene pool. Allele frequency is how often an allele occurs in a gene pool relative to the other alleles for the same gene. Forces of EvolutionThe factors that cause allele frequencies to change are called the forces of evolution. There are four such forces: mutation, gene flow, genetic drift, and natural selection. Genetic DriftGenetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, just by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads and 50 percent tails. Due to such chance variations in small populations, allele frequencies drift over time. There are two special conditions under which genetic drift occurs. They are called the bottleneck effect and founder effect.
MutationMutation creates new genetic variation in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes. Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. However, mutations provide the genetic variation needed for other forces of evolution to act. Gene FlowGene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. Allele frequencies may change in the population the migrants leave as well as in the population the migrants enter. An example of gene flow occurred during the Vietnam War in the 1960s and 1970s. Many young American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war. However, they left copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Do you think the gene pool of the U.S. was also affected? Why or why not? Natural SelectionNatural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation than do other members of the population. This causes allele frequencies to change over time. The example of sickle cell anemia, which is shown in the following table and described below, shows how natural selection can keep even a harmful allele in a gene pool.
The allele (S) for sickle cell anemia is a harmful, autosomal recessive allele. It is caused by a mutation in the normal allele (A) for hemoglobin (the oxygen-carrying protein on red blood cells). Malaria is a deadly tropical disease that is common in many African populations. Heterozygotes (AS) with the sickle cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool. The sickle cell example shows that fitness depends on phenotypes and also on the environment. What do you think might happen if malaria were to be eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele? The sickle cell trait is controlled by a single gene. Natural selection for polygenic traits, which are controlled by multiple genes, is more complex, although it is less complicated if you consider just phenotypes for polygenic traits rather than genotypes. There are three major ways that natural selection can affect the distribution of phenotypes for a polygenic trait. The three ways are shown in the graphs in Figure \(\PageIndex{3}\).
Feature: Human Biology in the NewsRecently reported research may help solve one of the most important and long-lasting mysteries of human biology. The mystery is why people with the AS genotype for sickle cell hemoglobin are protected from malaria. As you read above, their sickle cell hemoglobin gives them higher fitness in malaria areas than normal homozygotes (AA) who have only normal hemoglobin. The malaria parasite and its mosquito vector were discovered in the late 1800s. The genetic basis of sickle cell hemoglobin anemia and the resistance to malaria it confers were discovered around 1950. Since then, scientists have assumed, and some evidence has suggested, that the few sickle-shaped red blood cells of heterozygotes make them less hospitable hosts for the malaria parasite than the completely normal red blood cells of AA homozygotes. This seems like a reasonable hypothesis, but is it the correct one? The new research suggests a different hypothesis. Working with genetically engineered mice as model organisms, researchers in Portugal discovered that an enzyme that produces the gas carbon monoxide is expressed at much higher levels in the presence of sickle cell hemoglobin than normal hemoglobin. Furthermore, the gas seems to protect the infected host from developing the lesions and symptoms of malaria, even though it does not seem to interfere with the life cycle of the malaria parasite in red blood cells. These findings may lead to new therapies for treating malaria, which is still one of the most serious public health problems in the world. The findings may also shed light on other abnormal hemoglobin variants that are known to protect against malaria. Review
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What causes genetic drift quizlet?Genetic drift that occurs when a few individuals become isolated from a larger population, with the result that the new population's gene pool is not reflective of the original population. Graded variation in some traits of individuals that parallels a gradient in the environment.
What focuses on the evolutionary relationships between related species?A phylogenetic tree, also known as a phylogeny, is a diagram that depicts the lines of evolutionary descent of different species, organisms, or genes from a common ancestor.
Who proposed that animal populations remained stable over time because of the high proportion of animal offspring not surviving maturity?Darwin himself had ten children; three died before maturity. Darwin reasoned that populations of all species have the capacity to grow. Simply put, species produce more offspring than can survive. However, his observations showed that most populations remained stable due to environmental limits.
Which of the evolutionary forces is most likely to decrease variation between populations?Which of the evolutionary forces is most likely to decrease variation between two populations? Genetic drift is the force of evolution that is most powerful when acting on very large populations.
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