Species are groups of living organisms that share a common ancestry and are capable of interbreeding and producing fertile offspring. A species is often defined as a group of individuals that can mate with one another and produce fertile offspring that are also able to mate with each other.
Humans, for example, are members of the species Homo sapiens. Other members of the Homo genus include Neanderthals (Homo neanderthalensis) and Denisovans (Homo denisova), but these two groups are now extinct. Humans and Neanderthals share a common ancestor who lived around 600,000 years ago, while the Denisovans shared a common ancestor with Neanderthals around 400,000 years ago.
All members of a species have some degree of genetic variation, which is the basis for evolution. When environmental conditions change, certain variants may become more advantageous than others and this can lead to changes in the frequencies of these variants over time. If the changes are large enough, they can result in the formation of new species.
One well-known example is Darwin’s finches (Geospizinae), which live on the Galapagos Islands off the coast of Ecuador. These birds show a great deal of variation in beak shape, which is an adaptation to different food sources available on different islands. Over time, this has led to the formation of several different finch species from a single ancestral species.