In each generation, a random set of individuals reproduces to produce the next generation. Genetic drift in a population can lead to the elimination of an allele from a population by chance. In these cases, the signal of genetic drift is easily swamped out by the stronger effects of selection or gene flow, so we often ignore drift except in small or endangered populations, where a random draw of alleles can dramatically change the population’s chance of survival in the next generation. The random drifting of allele frequencies always happens, but the effect is subtle in larger populations. Populations are constantly under the influence of genetic drift. By random chance, not every allele will make it through, and some will be overrepresented while other decline in frequency regardless of how well those alleles encode for phenotypic suitability to the environment, so sometimes drift reduces the average fitness of a population for its environment. Evolution by genetic drift causes changes in populations by chance aloneĮvolution by genetic drift occurs when the alleles that make it into the next generation in a population are a random sample of the alleles in a population in the current generation. But mutation combined with one of the other mechanisms of evolution ( genetic drift, natural selection, non-random mating, and gene flow) can result in meaningful changes in allele frequencies in a population. Mutation rates are actually pretty low for most genes, ranging from 10^-6 for the average human gene to 10^-10 for the average bacterial gene (from ).īecause mutation rates are low relative to population growth in most species, mutation alone doesn’t have much of an effect on evolution. Mutation is the raw stuff of evolution because it creates new heritable phenotypes, irrespective of fitness or adaptation. The variation that is created in a population through the random process of mutation is called standing genetic variation, and it must be present for evolution to occur. Mutations occur at random in the genome, but mutations of large effect are often so bad for the organism that the organism dies as it develops, so mutations of smaller effect or even neutral mutations are theoretically more common in a population. For sexual organisms, mutations are passed to the next generation if they occur in the egg or sperm cells used to create offspring. In the single-celled asexual organisms, such as bacterial, the whole cell and its DNA is passed on to the next generation because these organisms reproduce via binary fission. Mutation generates variationĮvolution by mutation occurs whenever a mistake in the DNA occurs in the heritable cells of an organism. Each type of evolution can be characterized by how it affects fitness, adaptation, the average phenotype of a trait in a population, and the genetic diversity of the population. These are evolution by: mutation, genetic drift, natural selection, and gene flow. There are four key mechanisms that allow a population, a group of interacting organisms of a single species, to exhibit a change in allele frequency from one generation to the next.
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