Darwin called his book ‘On the Origin of Species’, but what exactly does the origin of species mean?
A species is a group of animals or plants that are able to interbreed and produce fertile offspring.
A new species is formed when two populations of animals or plants accumulate so many different genetic changes that successful reproduction is no longer possible.
Typically this happens when two populations become geographically separated – when continents move, or a river changes course or a mountain range appears. Sometimes plants and animals become isolated on islands or in lakes and will begin to diverge.
But species can occasionally diverge even when they share the same space. This might be because of behaviour changes that limit mating, or groups of organisms may develop breeding seasons that do not coincide. Or animals may simply prefer to mate with members of the opposite sex with a particular characteristic.
Speciation usually results from the splitting of a single lineage, but sometimes two species may fuse to create a third. Heliconius heurippa, for example, appears to be a hybrid of H. melpomene and H. cydno. Interestingly, mate preferences maintain the three separate species: males only go for wing patterns characteristic of their own species.
Inter-species crosses usually don’t generate viable offspring, because of chromosomal or genetic incompatibilities. Even if offspring are produced, they are usually infertile – like the mule.
But how did those incompatibilities appear in the ﬁrst place? In fact, even simple genetic changes can create reproductive barriers. Recently, experiments on closely related species have revealed speciﬁc genes that cause reproductive isolation.
Typically, such genes affect reproductive biology (usually in males) and are under selective pressures. Male sterility in two subspecies of fruit fly and mice has been pinned down to simple genetic changes. A major challenge now is to identify how these changes lead to biological incompatibility and how they relate to the environmental factors exerting selective pressure.Lead image:
NASA’s Marshall Space Flight Centre/Flickr CC BY NC