Species
A species is a group of similar organisms that can breed with one another and produce fertile offspring; it is the basic unit of classification.
Definition
A species is a group of living things that are alike and can have babies together whose babies can also have babies, so a poodle and a German shepherd are the same species because they can produce puppies together, while a horse and a donkey are different species because their mule offspring cannot reproduce; looking alike is not the test, since two birds can look nearly identical yet be different species. More formally, the biological species concept defines a species as a group of organisms that can interbreed in nature and produce fertile offspring while being reproductively isolated from other such groups, named with a two-part scientific name such as Homo sapiens, and biologists estimate around $8.7$ million eukaryotic species exist, of which only $1.5$ to $2$ million have been described; new species form when populations become separated and change independently until they can no longer interbreed. Because the biological species concept cannot handle asexual organisms, fossils, or geographically separated populations, biologists also use morphological, ecological, and phylogenetic concepts, and speciation proceeds when gene flow is interrupted, geographically (allopatric), along a range (parapatric), or within one area (sympatric, often via polyploidy in plants), followed by divergence through selection and drift until reproductive isolating mechanisms arise, though species boundaries are often porous, with hybridization and introgression common.
Example
Lions and tigers share a genus and can produce hybrids in captivity, but the hybrids are usually sterile and the species never interbreed in the wild. Cichlid fishes in Lake Victoria diversified into several hundred species in roughly $15{,}000$ years, largely through sexual selection on male coloration under different light conditions, and ring species such as California's Ensatina salamanders form a chain of interbreeding populations whose two ends behave as distinct species where they meet.
Key Insight
The difficulty of defining species is itself evidence for evolution: if life were static, its boundaries would be sharp, but because lineages split gradually, biologists inevitably find populations in every stage of separation.