Final answer:
Sexual Selection is the theory that explains the development of secondary sexual characteristics within species, which involves the preferential choice by one sex for certain traits in individuals of the opposite sex.
Step-by-step explanation:
The theory that explains the development of secondary sexual characteristics in species is Sexual Selection. This concept is distinct from natural selection, although both are mechanisms of evolution. Sexual selection pertains to the preference by one sex for certain characteristics in individuals of the opposite sex. For example, in many species, secondary sexual characteristics like the colorful plumage of male peacocks or the elaborate dances of male birds of paradise evolved because they are attractive to females, leading to a higher chance of mating and passing on genes responsible for these traits.
While Darwin's original theory of natural selection accounted for many aspects of species change over time, it was not until he proposed the idea of sexual selection in "The Descent of Man, and Selection in Relation to Sex" that the explanation for such characteristics within species was more thoroughly articulated. It is important to distinguish this from Mendelian Genetics, which explains the mechanisms of inheritance and the assortment of genes from one generation to the next.