How does microevolution affect a gene pool

School Days Except for their plastic lunch coolers, you might think that this picture of children on their way to school came from the s. Genes in Populations Individuals do not evolve because their genes do not change over time. Forces of Evolution The factors that cause allele frequencies to change are called the forces of evolution.

Genetic Drift Genetic drift is a random change in allele frequencies that occurs in a small population. The bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire or disease epidemic.

By chance, allele frequencies of the survivors may be different from those of the original population. The founder effect occurs when a few individuals start or found a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left.

The Amish population in the U. The population has grown to almost , individuals who rarely interact with people outside the Amish community. One of the founders carried a recessive allele for a rare condition called Ellis-van Creveld syndrome - a type of dwarfism that results in extra fingers and short limbs as seen in this image.

Today the Amish population has far more cases of this syndrome than any other population in the world. Mutation Mutation creates new genetic variation in a gene pool. Gene Flow Gene flow occurs when individuals move into or out of a population. Natural Selection Natural selection occurs when there are differences in fitness among members of a population. Disruptive selection occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution.

An example is a sexual dimorphism. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different average heights and body shapes. Stabilizing selection occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive, and this keeps birth weight within a relatively narrow range.

Directional selection occurs when one of two extreme phenotypes is selected for. This shifts the distribution toward that extreme. Larger beaks were selected for during drought, so beak size increased over time. Natural selection may affect the distribution of a polygenic trait. The top panel shows the disruptive selection in the oyster shell shades. The lightest and darkest shades are more prevalent.

The middle panel shows the stabilizing selection. Most lizards have median-sized tails. The bottom bottom panel shows the directional selection of the giraffe's neck size. Feature: Human Biology in the News Recently reported research may help solve one of the most important and long-lasting mysteries of human biology.

Review Why are populations, rather than individuals, the units of evolution? What is a gene pool? List and define the four forces of evolution. Why is mutation needed for evolution to occur, even though it usually has little effect on allele frequencies? What is the founder effect? Give an example. Identify three types of natural selection for polygenic traits. Explain why genetic drift is most likely to occur in a very small population. In some species, females prefer to mate with males that have certain genetically determined characteristics, such as bright coloration or a large, showy tail.

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Do you look a bit like your brothers and sisters? Do you look a bit like your parents? The similarities are because, unless you were adopted, you and the other members of your family have genetic material Fish Adapting to a Changing World Scientists have been studying stickleback fish to learn more about how animals cope when their environment changes. Since the Earth is changing quickly because of global warming and other global changes, Can there be Life in the Environment of Jupiter?

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There also may be an Autotrophs Autotrophs are organisms that can "make their own food" from an inorganic source of carbon carbon dioxide given a source of energy. Most autotrophs use sunlight in the process of photosynthesis to make Mathematically, we can determine whether microevolution is occuring by assessing whether a population is in Hardy-Weinberg Equilibrium.

The five "forces" that can cause shifts in gene frequency microevolution are:. Microevolution Microevolution is defined as changes in the frequency of a gene in a population.

Gene Flow Migration : when there is mixing of genes from previously isolated populations that have diverged, this can rapidly change gene frequencies in the newly merged population. Mutation : when an advantageous mutation spontaneously arises in an organism, this mutated gene can increase in frequency over generations if it conveys an advantage over those who do not have it.

If a neutral mutation one that is neither beneficial nor harmful arises in a population, it can increase in a population by genetic drift.