Origin and Preservation of Genetic Variability

Genotype and Phenotype

Animal evolution involves a change across generations in the traits that characterize a population. These traits can be morphological (e.g., the shape of a finch’s beak) or functional (e.g., a bat’s use of echolocation for catching moving prey). The traits that form an organism are its phenotype. Traits develop through interactions between genetic and nongenetic information. This genetic information is the organism’s genotype.

The development of some traits is dependent on one or a few genes. The genetics of these traits is called Mendelian because this 1-to-l relationship held for the traits of garden peas studied by Gregor Mendel (1822-1884), the founder of modern genetics. But the phenotype-genotype relationship is complex. Most traits are influenced by many genes, a phenomenon known as polygeny. Polygeny is the rule; any complex morphological or functional trait, such as organs and behavior, is the result of the interplay of many genes with nongenetic factors. Moreover, a particular gene can affect the development of many traits, a phenomenon called pleiotropy. A change in the composition of a protein can affect numerous traits, whether because of a cascade of effects or because of independent effects of a change in the protein (Paaby & Rockman, 2013). For example, a human genetic disorder called Marfan syndrome affects height, impacts the shape of fingers and toes, and leads to heart failure, all seemingly unrelated effects that derive from a mutation in a single gene.

Furthermore, the same set of genes can lead to significant phenotypic variance when development occurs in different environments. For example, the age at which humans reach sexual maturity depends on physiological processes involving proteins known as hormones. However, the process is modulated by nongenetic factors. In women, nutrition is responsible

Biological Evolution 15 for a range of variation in age of sexual maturity from 10 to 20 years of age. This is referred to as the norm of reaction for sexual maturity, in this example. Similarly, the behavioral and neural development of an adult mammal depends on early experience. Norms of reaction highlight the principle that the phenotype is the result of complex genetic and nongenetic interactions.

New species evolve when phenotypic changes in members of two populations lead to their reproductive isolation. One general assumption of this definition is that phenotypic change involves also genotypic change. However, phenotypes can change because of the expression of genetic information that was previously silent. Therefore, phenotypic change does not necessarily imply genotypic change.