Final answer:
The term 'wild type' refers to the most common phenotype or genotype found in the natural environment and represents the norm against which other alleles, known as variants, are measured in genetics.
Step-by-step explanation:
The term describing the most common expression of a particular allele combination in a population is wild type.
In biology, especially genetics, the most common phenotype or genotype found in the natural environment is referred to as the wild type. This standard form is represented by the symbol "+" and occurs with the highest frequency in a given population. When multiple alleles exist for the same gene, the prevalence of these alleles can vary. The wild type is the benchmark against which other allele combinations, known as variants, are compared. These variants can be either recessive or dominant compared to the wild type allele.
Moreover, in population genetics, the concept of allele frequency plays a crucial role. This frequency represents the rate at which a particular allele appears within a population, and a change in this frequency over time is considered evolution. The alleles with the highest advantage in terms of survival and reproduction will generally increase in frequency, illustrating natural selection at work.