Species hypothesized to have the same ancestors are placed in one genus, based on similarities. The similarity of species is judged based on comparison of physical attributes, and where available, their DNA sequences. The binomial is written in italics when printed and underlined when handwritten.
Whether a variation is adaptive or non-adaptive depends on the environment: different environments favor different traits. Since the environment effectively selects which organisms live to reproduce, it is the environment (the “fight for existence”) that selects the traits to be passed on. This is the theory of evolution by natural selection.
Although the current scientific understanding of species suggests that there is no rigorous and comprehensive way to distinguish between different species in all cases, biologists continue to seek concrete ways to operationalize the idea. One of the most popular biological definitions of species is in terms of reproductive isolation.
Definitions of species
Practically, biologists define species as populations of organisms that have a high level of genetic similarity. This may reflect an adaptation to the same niche, and the transfer of genetic material from one individual to others, through a variety of possible means.
Consequently, any single, universal definition of “species” is necessarily arbitrary. Instead, biologists have proposed a range of definitions; which definition a biologists uses is a pragmatic choice, depending on the particularities of that biologist’s research.
The process through which species are formed by evolution is called speciation. A species that gives rise to another species is a paraphyletic species, or paraspecies. A single evolutionary lineage of organisms within which genes can be shared, and that maintains its integrity with respect to other lineages through both time and space.
At some point in the evolution of such a group, some members may diverge from the main population and evolve into a subspecies, a process that may eventually lead to the formation of a new species if isolation (geographical or ecological) is maintained.