Evolution is a fundamental biological process driving the diversity of life on Earth. It involves changes in the genetic makeup of populations over generations. Natural selection is a primary force shaping these changes, influencing which inherited traits become more or less common within a population.
Understanding Alleles and Natural Selection
Genes carry instructions for various traits. An allele represents a specific version of a gene, such as those determining eye color or blood type. An individual inherits two alleles for each gene, one from each parent. The proportion of a specific allele within a population’s gene pool is known as its allele frequency.
Natural selection is a process where organisms better suited to their environment tend to survive and produce more offspring. This process acts on an organism’s observable traits, which are often influenced by its genetic makeup. Over time, individuals with advantageous traits are more likely to pass on the alleles associated with those traits to the next generation.
The Mechanism of Allele Frequency Change
Natural selection drives changes in allele frequencies through differential survival and reproduction. Individuals within any population exhibit variation in their traits due to different alleles. Some inherited traits provide an advantage, allowing certain individuals to better survive, acquire resources, or avoid predators. For instance, a rabbit with camouflage suited to its environment may avoid a hawk.
Individuals with advantageous traits are more likely to reproduce and pass on their beneficial alleles. Over successive generations, as more individuals inherit these alleles, their frequency within the population’s gene pool increases. Conversely, alleles linked to less beneficial traits become less common because individuals carrying them are less likely to survive and reproduce effectively. This process results in a population becoming better adapted to its environment.
Different Modes of Selection
Natural selection manifests in several distinct patterns, or modes, each affecting allele frequencies differently. Directional selection favors individuals at one extreme of a phenotypic range, causing the average trait value in the population to shift over time. For example, during the Industrial Revolution, darker peppered moths became more prevalent as they blended better with soot-darkened trees, leading to an increase in alleles for dark coloration.
Stabilizing selection favors intermediate phenotypes and acts against extreme variations, leading to a reduction in genetic diversity. Human birth weight is a classic example, where infants with intermediate weights tend to have higher survival rates than those at very low or very high extremes. This narrows the range of variation within the population.
Disruptive selection, in contrast, favors individuals at both extremes of the phenotypic spectrum over intermediate forms. This can lead to increased genetic variation and the emergence of two distinct phenotypes. For example, in a population of mice living in a habitat with both light sand and dark grass, very light mice (blending with sand) and very dark mice (hiding in grass) might be favored, while medium-colored mice are more easily detected by predators.
Interacting Evolutionary Forces
Natural selection is a powerful evolutionary force, but it operates alongside other factors that influence allele frequencies within a population. Mutation, for instance, introduces new alleles into the gene pool, serving as the ultimate source of all genetic variation. Though mutation rates are generally low, these new alleles provide the raw material upon which natural selection and other forces can act.
Gene flow involves the movement of alleles between different populations, typically through migration of individuals. This exchange can introduce new alleles into a population or alter the frequencies of existing ones. Gene flow can either enhance or counteract the effects of natural selection, depending on whether the introduced alleles are advantageous or disadvantageous in the new environment.
Genetic drift refers to random fluctuations in allele frequencies, particularly pronounced in small populations. These chance events, such as random survival or reproduction, can cause alleles to become more or less common, irrespective of their adaptive value. Genetic drift can lead to the loss of some alleles or the fixation of others, reducing genetic variation over time. The interplay among natural selection, mutation, gene flow, and genetic drift collectively shapes the complex dynamics of allele frequencies in populations, leading to continuous evolutionary change.