Final answer:
The definition of species has changed over time due to various factors and advancements in scientific knowledge, including molecular genetics and the study of evolution. Traditional criteria based on physical characteristics have been supplemented with genetic differentiation and reproductive isolation for defining distinct species.
Step-by-step explanation:
The definition of species has changed over time due to various factors and advancements in scientific knowledge. Initially, species were defined solely based on physical characteristics, called morphological traits. However, with the advent of molecular genetics and DNA analysis, scientists realized that organisms may appear similar but have different genetic makeup, leading to the concept of molecular or genetic species. Additionally, the study of evolution and the understanding of speciation processes have contributed to the changes in the definition of species.
For example, the traditional definition of a species as a group of organisms capable of interbreeding and producing fertile offspring may be problematic in cases of closely related species that are capable of hybridization and produce viable but sterile offspring. In such cases, genetic differentiation and reproductive isolation become important criteria to define distinct species.
Overall, the changes in the definition of species have resulted from a combination of new scientific discoveries, technological advancements, and an improved understanding of the complexity of biodiversity.