Natural selection is the fundamental process driving evolution, but the precise target of its action often causes confusion. Understanding this process requires differentiating between an organism’s genotype and its phenotype. A genotype is the specific genetic makeup, representing the combination of alleles it carries for a trait. The phenotype is the observable expression of that genetic code, encompassing all physical, physiological, and behavioral characteristics.
Selection Acts on Observable Traits
Natural selection acts directly on the phenotype, as this is the part of the organism that interacts with the environment. The environment, whether a predator, climate condition, or food source, assesses an individual based solely on its observable traits. An organism’s survival and reproductive success, or fitness, are determined by how well its characteristics allow it to navigate its surroundings.
For example, a rabbit’s fur color is a phenotype that directly influences its ability to avoid detection by predators. A rabbit with white fur in a snowy habitat is camouflaged, giving it a high chance of survival. Conversely, a rabbit with brown fur in the same environment is exposed and easily caught. The predator does not evaluate the rabbit’s DNA sequence; it simply catches the poorly camouflaged individual.
Similarly, the size and shape of a finch’s beak determine its efficiency in cracking seeds. If only large, hard seeds remain during a drought, only finches with larger, stronger beaks will be able to feed, survive, and reproduce.
Selection is a process of differential filtering, where individuals with advantageous phenotypes are more likely to survive and pass on their heritable traits. The selective pressure of the environment sorts individuals based on their performance, a direct consequence of their expressed characteristics. Any trait, from metabolic efficiency to mating behavior, must be expressed in some functional way to be subjected to selection. Therefore, the phenotype is the immediate subject of natural selection.
The Mechanism of Genetic Change
While the phenotype is the target of selection, the ultimate consequence is a change in the population’s genetic structure. When a phenotype confers a survival or reproductive advantage, the underlying genotype is disproportionately passed to the next generation. Differential reproductive success translates phenotypic sorting into genotypic change.
Over successive generations, the frequency of beneficial alleles associated with the successful phenotype increases within the gene pool. Conversely, alleles linked to less successful phenotypes become less common because the individuals carrying them are less likely to reproduce. This shift in allele frequencies is the definition of evolution. The genotype is the heritable unit that changes its distribution across the population as a result of selection.
Consider a gene with two alleles controlling a trait: one favorable and one unfavorable. If all organisms with the favorable phenotype survive and all with the unfavorable one perish, the frequency of the beneficial allele will rapidly increase in the next generation. The genotype is not acted upon directly in a single individual. Instead, selective pressure on the phenotype causes a statistical change in the collection of genotypes across the population. Therefore, the genotype represents the inherited material that is refined and altered over time as a consequence of phenotypic selection.
Hidden Variation and Recessive Alleles
The relationship between selection and genetic change is complicated by hidden genetic variation not fully expressed in the phenotype. This is evident with recessive alleles, which can be masked by a dominant allele in a heterozygous individual. If a recessive allele causes a harmful trait when two copies are present, selection acts strongly against the homozygous recessive phenotype.
The selection mechanism cannot efficiently remove the recessive allele when it is carried by a heterozygote. The heterozygote expresses the dominant, often beneficial, phenotype and experiences the same high fitness as a homozygous dominant individual. The recessive allele is protected from selection in these carrier individuals, allowing it to persist in the gene pool at low frequencies. This demonstrates how the genotype is preserved despite selection acting against one of its potential phenotypic expressions.
Another form of hidden variation occurs with polygenic traits, which are influenced by multiple genes. Many different combinations of alleles can result in similar or identical phenotypes, creating a genetic buffer. This buffering means that selection against one specific phenotype does not eliminate all the underlying genetic variation, as multiple genotypes can produce the same successful trait. The persistence of these hidden alleles highlights the complexity of the selection process, confirming that while selection acts on the observable trait, the genetic structure of a population is resilient.