Genetic variation is a fundamental aspect of life, describing the differences in DNA among individuals within a species. These variations are passed from one generation to the next, influencing traits and characteristics. While genes are inherited, not all versions of these genes, known as alleles, remain equally present across generations. Over extended periods, some alleles can become the sole variant for a particular gene within a group, a process that significantly shapes the genetic makeup of populations. Understanding this phenomenon, called allele fixation, offers insights into how populations evolve.
What is Allele Fixation?
An allele refers to a specific variant form of a gene, which is a segment of DNA that carries instructions for a particular trait. For instance, a gene for eye color might have alleles for blue, brown, or green eyes. In genetics, a population is defined as a group of individuals of the same species that live in the same geographic area and can interbreed, sharing a common gene pool. This interbreeding allows for the exchange of genetic material among its members.
Allele fixation occurs when only one allele exists for a particular gene in a population. This means the frequency of that allele reaches 100%, or 1.0, within the population. Once an allele becomes fixed, there is no longer any genetic variation at that specific gene locus within that population. All individuals in that population will carry the identical version of that gene.
The Path to Fixation
Alleles typically exist at various frequencies within a population, meaning different versions of a gene are present in differing proportions among individuals. For example, in a population of flowers, a gene for petal color might have a red allele at 70% frequency and a white allele at 30%. Over successive generations, the prevalence of these alleles can fluctuate.
Through various influences, the frequency of one allele can gradually increase while others decrease. This shift in allele frequencies is a continuous process in evolving populations. Eventually, one allele can become so prevalent that all other variants for that specific gene are lost from the population, resulting in its fixation.
Why Alleles Become Fixed
Alleles can become fixed due to several evolutionary mechanisms that alter their frequencies within a population. Two primary forces driving this are natural selection and genetic drift. These processes, while distinct, both contribute to how certain alleles become universally present.
Natural selection favors alleles that provide a survival or reproductive advantage to individuals carrying them. If an allele confers a benefit, such as improved camouflage or increased resistance to disease, individuals possessing this allele are more likely to survive, reproduce, and pass it on to their offspring. Over many generations, this differential success can lead to a steady increase in the advantageous allele’s frequency until it becomes fixed in the population. This process is adaptive, meaning it generally leads to populations becoming better suited to their environment.
Genetic drift, conversely, describes random fluctuations in allele frequencies that occur purely by chance. This mechanism is particularly influential in small populations, where random events can have a more significant impact on the genetic makeup of the next generation. An allele can increase in frequency and even become fixed simply due to chance, regardless of whether it confers any advantage or disadvantage. Unlike natural selection, genetic drift is a non-adaptive process, meaning it does not necessarily lead to improved fitness for the population.
What Fixation Means for a Population
When an allele becomes fixed in a population, it has significant consequences for genetic diversity. The complete loss of all other variants for that gene means that genetic variation at that specific locus is entirely eliminated. This reduction in diversity can limit a population’s resilience and capacity to adapt to future environmental shifts. If conditions change, and the fixed allele is no longer advantageous, the population may lack the alternative genetic variants needed to cope with the new challenges.
The absence of genetic diversity at a fixed locus means there are no alternative alleles for natural selection to act upon if conditions change. For example, if an allele providing disease resistance becomes fixed, but a new pathogen emerges that the fixed allele cannot defend against, the entire population could be vulnerable.