Tens of millions of species inhabit the Earth. From the massive Californian redwood, to the tiny crescent-cup liverwort; the rare blue whale and its prey, the common krill; and from the primitive E. coli to its complex human host, the variation of the natural world is astounding. Surely, on our pale blue dot, there’s only limited room for species to live, so how is it that organisms have evolved to be so diverse?
Speciation is the process by which new species arise. There are several recognised modes, although in reality it works under more of a sliding scale. At one end is allopatric speciation, when members of the same species are geographically separated (through continental drift, etc.). At the other is sympatric speciation, where the organisms are isolated not geographically, but genetically, through processes such as genetic polymorphism. Polymorphism involves relatively small variations in DNA that have such a profound effect on the phenotype that two organisms with slight differences in their genotype are prevented from reproducing, and a single species is effectively split into two daughter species, which evolve separately.
Important to understanding the origin of biodiversity is the biological ‘niche’. Niches are present in every ecosystem, and can be thought of as equivalent to the ‘job’ or ‘role’ played by the organism that occupies that niche. As an example, a garbageman makes a living by processing the waste of others, much in the way that a mushroom does. Of course, this is an awkward over-simplification, as niches are not so black and white; they can be finely graded by something as simple as how much sunlight a patch of rock receives.
All organisms can be broadly split into one of two types: they are either a specialist, or a generalist. A specialist expertly exploits one niche, and is able to gain the maximum amount of resources from that one source. A generalist is an organism that inexpertly utilises a great number of niches and so is adaptable, but relatively wasteful. Specialists do better during stable periods when food sources are guaranteed, but when times are changeable, the generalists reign supreme.
Darwin’s finches first arrived at the Galápagos Islands as a single, common ancestor. That common ancestor evolved on the Southern American mainland, where it had adapted to fit with the niche environment its species occupied. However, once the finches arrived at the Galápagos Islands, they found a great many unoccupied niches, and selective pressures acted on those individuals that were better suited to different tasks (for example, birds with slightly larger beaks developed beaks larger still, so that they are more able to crack nuts open), until one generalist species had become thirteen specialists. Darwin noted as he observed these finches: “…one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.” This insightful observation was just one of many that led him to publish his theory of evolution.
In conclusion, diversity is caused by selection pressures to fill all of the exploitable niches available. The precise environment of every niche is different, and thus, the selection pressures placed upon a species are dependent on what niche that organism exploits. In short; organisms evolve to fit their environment. The diversity of life reflects the diversity of environments on earth.