The difference between species and speciation, explained

Did you ever wonder how two birds that look very similar can be members of different species… …but a Chihuahua and a St. Bernard are the same species? Historically, biologists have defined species as groups of organisms that are reproductively isolated. This means they can breed with each other but can’t successfully breed with members of another species. Successful is the key word. A horse and a donkey can mate to produce a hybrid animal — a mule or a hinny — but those animals are almost always sterile. A pair of mules cannot go on to produce further offspring, they are infertile. Dog breeds may look very different, but dogs don’t have reproductively isolated populations. When different breeds mate, they successfully produce healthy, fertile hybrids. The process of one species splitting into two or more species is called speciation. Speciation happens when a barrier to reproduction occurs and different populations within the species become reproductively isolated. The most well-documented way for speciation to occur is when two populations become geographically separated. Remember those similar-looking birds? Charles Darwin studied many species of closely related finches in the Galapagos Islands. These finches descended from a single species, but as they moved across the islands, their populations became separated and developed in reproductive isolation. Speciation that occurs when populations of a species become geographically isolated is called allopatric speciation. For Darwin’s finches, the ancestral species that came to the Galapagos diverged into many populations isolated on different islands. Isolated from each other by the ocean, they evolved different evolutionary strategies; some living in trees and others on the ground, some eating insects and others eating plants. Sometimes, other factors cause reproductive isolation. When speciation occurs among populations of a species that share the same geographic location, this is called sympatric speciation.
Biologists are still learning about sympatric speciation, but they think there are many ways it can occur. One example is different plant species sometimes produce hybrids that are polyploid, meaning they have more sets of chromosomes than their progenitors. This difference makes the hybrids genetically isolated from either of their two progenitor species.