Heredity describes how traits are passed from parents to offspring, forming the basis of biological continuity. Over generations, these inherited characteristics can shift within populations, a process known as evolution. Natural selection is a key mechanism driving this change, favoring certain traits that enhance an organism’s survival and reproduction. This process can significantly influence traits determined by a single gene.
Single Gene Traits
A single gene trait is a characteristic primarily determined by the alleles, or different versions, of just one gene. Each individual inherits two copies of every gene, one from each parent, and these allele combinations dictate the observable trait, known as the phenotype. For example, the ability to taste phenylthiocarbamide (PTC) is a single gene trait, where specific alleles determine whether an individual perceives a bitter taste. Similarly, features like attached or unattached earlobes are considered single gene traits. These straightforward genetic foundations make single gene traits useful for understanding basic inheritance patterns.
Natural Selection
Natural selection drives evolutionary change in populations. It begins with variation among individuals, where organisms exhibit differences in their traits. Some of these traits are heritable, meaning they can be passed from parents to offspring.
A key aspect is differential survival and reproduction. In any given environment, individuals with certain heritable traits may be better suited to survive and produce more offspring. Over time, this leads to an accumulation of these beneficial traits. The environment effectively “selects” for individuals with traits that improve their fitness, their ability to survive and reproduce in that specific setting.
The Direct Impact on Single Gene Traits
Natural selection directly influences single gene traits by altering the frequency of their underlying alleles within a population. When a specific allele for a single gene trait provides an advantage in survival or reproduction, individuals carrying that allele are more likely to pass it on to their offspring. This increased reproductive success leads to a higher proportion of that advantageous allele in the gene pool across generations. Conversely, alleles that confer a disadvantage will decrease in frequency because individuals possessing them are less likely to survive and reproduce.
Consider a hypothetical insect population where body color is determined by a single gene with two alleles: one for dark coloration and one for light coloration. If the environment changes, perhaps due to industrial pollution, dark-colored insects might become better camouflaged against soot-covered surfaces, making them less visible to predators. Light-colored insects, being more exposed, would be more frequently preyed upon, reducing their chances of survival and reproduction. As a result, the allele for dark coloration would increase in frequency over time, while the allele for light coloration would decline.
Patterns of Trait Evolution
Natural selection can drive single gene traits to evolve in distinct patterns, influencing how these characteristics are distributed within a population over time.
Directional Change
Directional change favors one extreme of a trait, causing the population’s average trait value to shift. For instance, if an allele for a single gene trait provides enhanced resistance to a new disease, individuals with that allele will increasingly dominate the population.
Stabilizing Change
Stabilizing change favors individuals with intermediate trait values, selecting against extremes. This pattern maintains the average trait value within a population, reducing variation. For example, if a particular body size, determined by a single gene, is most advantageous in a stable environment, natural selection will keep the population centered around that size.
Disruptive Change
Disruptive change favors individuals at both extremes of the trait distribution over intermediate forms. This can lead to the emergence of two distinct trait variations. For example, if a food source is available in two distinct sizes, selection might favor individuals with small and large beaks (determined by a single gene) to efficiently access these resources.
The Broader Evolutionary Landscape
While natural selection plays a significant role in shaping single gene traits, other evolutionary mechanisms also contribute to their dynamics within populations. Genetic drift, for example, involves random fluctuations in allele frequencies, particularly noticeable in smaller populations. This chance-driven process can cause alleles for single gene traits to become more or less common, irrespective of their adaptive value.
Gene flow, the movement of individuals and their genes between populations, also influences single gene traits. It can introduce new alleles into a population or alter existing allele frequencies, potentially counteracting or reinforcing the effects of natural selection. Furthermore, mutation, the ultimate source of new genetic variation, continuously introduces new alleles for single gene traits into the gene pool. These new alleles provide the raw material upon which natural selection and other evolutionary forces can act. These interacting forces collectively determine the evolutionary trajectory of single gene traits.