Evolutionary history can be represented by a tree of life on which extinction stops some branches from growing while speciation – the process that produces new species – maintains or increases nature's diversity. And although Charles Darwin's revolutionary book is titled On the Origin of Species, it doesn't really explain how or why species might originate.
What is a species anyway?
It's controversial. A species is a population of organisms whose members share common characteristics, but biologists don't agree on one definition for what a species is: plants and fungi are typically defined by features (morphology), for instance, whilst animal species have been traditionally based on whether members interbreed to create fertile offspring – and species are now often distinguished by DNA.
Taxonomists describing new species must publish peer-reviewed scientific papers detailing why they believe a species is new to science, listing out it’s morphological, behavioural and genetic differences from similar species.
Why does speciation occur?
Changes to an environment – triggered by factors such as climate, migration or interactions with other species – can reveal niches whose resources (especially food) are ready to be exploited. Such ecological pressures or opportunities can drive a population to evolve into two potentially-distinct groups or 'incipient species' if they each adapt to a niche through evolution by natural selection.
Once survival of the fittest has pushed incipient species to be specialised for their respective niches, any hybrids formed by interbreeding would be less well-adapted and less likely to survive, reinforcing the divergence between species. This was suggested by Alfred Russel Wallace (who co-proposed the theory of natural selection) in his book Darwinism, and is now known as the ‘Wallace effect’.
So what prevents interbreeding?
If members from two incipient species keep combining genetic material and make offspring, they share a gene pool and may not diverge into distinct populations. That outcome is prevented when a pair of sister species accumulate differences in physiology or behaviour that create reproductive barriers that block any gene flow between the two species before or after a zygote (fertilised egg) develops – in organisms with sexual reproduction, they're pre-zygotic or post-zygotic.
Post-zygotic barriers don't prevent mating but a hybrid might be infertile, for instance, whereas pre-zygotic barriers include signals such as a courtship song or dance that's essential for recognising compatible mates. Even organisms that can't move or choose partners directly have barriers that block interbreeding: in flowering plants, for example, orchid species might look identical but have subtle distinctions in colour or odour that only attract certain pollinators.
How do new species originate?
There are two main routes. Allopatric speciation occurs when sister species inhabit non-overlapping geographic ranges, after a (sometimes only temporary) structure -- like a river or frozen water -- physically separates members of a population or lets individuals migrate to new locations.
The most famous example is Darwin's finches, 13 species scattered across the Galápagos archipelago that show striking variation in size and shape, from the small tree finch that uses its curved beak to extract insects to the large ground finch that uses a short, strong beak to crack-open nuts and seeds. Genetic analysis suggests the finches are descended from songbirds called grassquits that hopped onto the Galápagos islands from mainland South America using a glacier during the last ice age.
The other route to speciation occurs when sister species live together (and presumably emerged) in overlapping geographic ranges. This process, sympatric speciation, seems less common across the tree of life but is also the dominant route in certain groups – such as fish in oceans and lakes: among thousands of cichlids in the African Great Lakes, for example, over 96 per cent originated as species whose ancestors colonised the micro-habitats found at various depths.
Which organisms contribute most to biodiversity?
Symbiotic bacteria will be geographically separated if they live in hosts that become new species. Because an estimated 79 per cent of speciation events occur in those endosymbiotic bacteria – which make up most living things – and most animals are insects, the majority of Earth's biodiversity is probably generated by microbes and their insect hosts.
