SELECTING FOR SUPERBUGS
NATURAL SELECTION Differential survival and reproduction of individuals in response to environmental pressure that leads to change in allele frequencies in a population over time.
ADAPTATION The process by which populations become better suited to their environment as a result of natural selection.
Ultimately, the interplay between an organism’s traits, or its phenotype (which is largely determined by its genes, or genotype), and its environment is what determines what traits will predominate in a population. When the environment favors the survival and reproduction of individuals with certain traits, those traits become more common in the population. This process of differential survival and reproduction of individuals within a population in response to environmental pressure is known as natural selection. Much in the way plant and animal breeders have for centuries practiced artificial selection to produce individuals with desired traits, the environment also, in a sense, selects individuals with certain traits.
When natural selection acts on a population over time, advantageous traits become more common, and the population becomes better suited to its environment. In other words, evolution by natural selection leads to adaptation. This is what we see with antibiotic-resistant bacteria: the population has become better suited, or adapted, to an environment in which antibiotics are abundant because individual bacteria carrying resistance genes are more fit in this environment. Charles Darwin was one of the first people to figure out how natural selection works and to study its results—for example, in birds on the Galápagos Islands, where various finch species had evolved different sizes of beak as adaptations to different food sources (see Milestones in Biology: Adventures in Evolution).
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Note that evolution by natural selection occurs in populations, not individuals. Individual organisms do not experience a change in allele frequencies over time. Therefore, individual organisms do not evolve (INFOGRAPHIC 14.6).
In any genetically diverse population, individual fitness varies. When an organism’s environment favors specific genetic variants to survive and reproduce over others, natural selection occurs. Those with high fitness tend to reproduce more successfully. Over generations, the frequency of alleles that confer higher fitness increase while those that confer lower fitness decrease. This non-random change in allele frequencies over generations is called evolution by natural selection.
DIRECTIONAL SELECTION A type of natural selection in which organisms with phenotypes at one end of a spectrum are favored by the environment.
STABILIZING SELECTION A type of natural selection in which organisms near the middle of the phenotypic range of variation are favored by the environment.
DIVERSIFYING SELECTION A type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment.
Natural selection doesn’t always affect populations in the same way. By studying how natural selection has shaped populations, scientists have defined three major patterns of natural selection. When the predominant phenotype in the population has shifted in one particular direction, we say that directional selection has occurred. For example, when bacterial populations evolve from populations sensitive to drugs into ones that resist drugs—that is, toward antibiotic resistance—they are exhibiting directional selection.
When the phenotype of the population settles around the middle of the phenotypic spectrum, we call this stabilizing selection. Or, a population can “spread out,” so that the population shows extremes of the phenotypic spectrum; this pattern is known as diversifying selection (INFOGRAPHIC 14.7).
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The particular pattern of natural selection a population follows depends on the interaction of phenotypes with the environment. So, for example, in the absence of antibiotics, populations of staph bacteria might have followed stabilizing or diversifying selection. Instead, directional selection led to the MRSA that killed Carlos Don, Rebecca Lohsen, and Ricky Lannetti.