Only the resistant bacteria will continue to proliferate in the presence of the antibiotic and increase in number over time. A bactericidal antibiotic is added, which kills most of the susceptible bacteria in the population, while the resistant bacteria survive.
The starting point in this example is a large bacterial population where most bacteria are susceptible to antibiotics, but a couple of bacteria are antibiotic-resistant by chance. Natural selection of antibiotic resistant bacteria. These bacteria can now increase in numbers and the end result is a population of mainly resistant bacteria.įigure 1. If we were to treat the bacterial population with that specific antibiotic, only the resistant bacteria will be able to multiply the antibiotic selects for them. Some spontaneous mutations (or genes that have been acquired from other bacteria through horizontal gene transfer) may make the bacterium resistant to an antibiotic (See: Resistance mechanisms for information about how bacteria resist antibiotic action). Mutations are one way for bacteria to become resistant to antibiotics. Mutations can provide resistance to antibiotics If a mutation gives the bacterium an advantage in a particular environment, this bacterium will grow better than its neighbors and can increase in numbers – it is selected for. Here, Darwin’s theory of natural selection comes in. While some mutations are harmful to the bacteria, others can provide an advantage given the right circumstances. Mutations can also form due to external factors like radiation or harmful chemicals.
These mutations are random and can be located anywhere in the DNA. Every time the bacterium goes through this process there is a chance (or risk, depending on the end result) that errors occur so-called mutations. Before the bacterium can divide, it needs to make two identical copies of the DNA in its chromosome one for each cell. When bacteria multiply, one cell divides into two cells. Mutationsīacteria grow and multiply fast and can reach large numbers. Resistant bacteria survive antibiotic treatment and can increase in numbers by natural selection.
Natural selection and genetic drift act on the variation that exists in a population.Evolution results from natural selection acting upon genetic variation within a population.The Hardy-Weinberg equation describes expectations about the gene pool of a population that isn't evolving, that is large, mates randomly, doesn't experience mutation, natural selection, or gene flow.Evolution is often defined as a change in allele frequencies within a population.Environmental changes may provide opportunities that can influence natural selection.Depending on environmental conditions, inherited characteristics may be advantageous, neutral, or detrimental.Over time, the proportion of individuals with advantageous characteristics may increase due to their likelihood of surviving and reproducing.Natural selection is dependent on environmental conditions.Natural selection acts on phenotype as an expression of genotype.Natural selection acts on the variation that exists in a population.Evolution results from selection acting upon genetic variation within a population.There is a fit between organisms and their environments, though not always a perfect fit.Information on controversies in the public arena relating to evolution.Alignment with the Next Generation Science Standards.The big issues – Pacing, diversity, complexity, and trends.Macroevolution – Evolution above the species level.Microevolution – Evolution within a population.
Mechanisms: the processes of evolution – Selection, mutation, migration, and more.The history of life: looking at the patterns – Change over time and shared ancestors.An introduction to evolution: what is evolution and how does it work?.
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