The most common definition of 'species' is a group of organisms capable of interbreeding and producing fertile offspring. Notwithstanding, the way biologists (specifically zoologists and botanists) divide species often has little to do with the capability of interbreeding, but mostly with physical features, such as colour or size.
In his book The Ancestor's Tale, Richard Dawkins states (p. 346) that "depending on whether your ichthyologist is a lumper or a splitter, the number of species of cichlid [fish] in Lake Victoria is between 200 and 500". He explains that species are actually divided either based upon their ability to interbreed OR their willingness to interbreed. Many species of fish and birds have evolved into many "races" sporting different colours, so that they typically choose to mate with individuals similar to them. That doesn't mean that they cannot produce fertile offspring with the different-looking races. They simply choose not too. Well, in most cases, as hybridisation does occur from time to time.
The same thing actually happens a lot with humans. Geographical isolation has led us to evolved into different-looking races, and up to this day the majority of people choose to mate with people of their own racial group, even in cosmopolitan societies like the USA. That doesn' mean that we are different species. So why would there be hundreds of species of cichlid fish if they can all interbreed together ? Closer to us, why do we categorise dogs, wolves, jackals, dingos and foxes as different species if they can interbreed with each others but lack the opportunity (geographic isolation) or just choose not to (cultural difference) ?
I have quoted a passage from Richard Dawkins' The Ancestor's Tale (p. 349-350) to illustrate my point :
"According to the geographical isolation theory, speciation begins with the accidental geographical division of a single ancestral species into separate populations. No longer able to interbreed, the two populations drift apart, or are pushed by natural selection in different evolutionary directions. Then, if they subsequently meet after this divergence, they either can't interbreed or don't want to. They often recognise their own species by some particular feature, and studiously avoid similar species who lack it. Natural selection penalises mating with the wrong species, especially where the species are close enough for it to be a temptation, and close enough for hybrid offspring to survive, to consume costly parental resources, and then turn out to be sterile, like mules. Many zoologists have interpreted courtship displays as aimed mainly against miscegenation. This may be an exaggeration, and there are other important selection pressures bearing upon courtship. But it is still probably correct to interpret some courtship displays, and some bright colours and other conspicuous advertisements, as 'reproductive isolation mechanisms' evolved through selection against hybridisation.
As it happens, a particularly neat experiment was done on cichlid fish by Ole Seehausen, now at the University of Hull, and his colleague Jacques van Alphen at the University of Leiden. They took two related species of Lake Victoria cichlids, Pundamilia pundamilia and P. nyererei (named after one of Africa's great leaders, Julius Nyerere of Tanzania). The two species are very similar, except that P. nyererei has a reddish colour, whereas P. pundamilia is bluish. Under normal conditions, females in choice tests prefer to mate with males of their own species. But now, Seehausen and van Alphen did their critical test. They gave females the same choice, but in artificial monochromatic light. This does dramatic things to perceived colour, as I remember vividly from schooldays in Salisbury, a city whose streets happened to be lit by sodium lights. Our bright red caps, and the bright red buses, all looked dirty brown. This is what happened to both the red and the blue Pundamilia males in Seehausen and van Alphen's experiment. Red or blue in white light, they all went dirty brown. And the result? The females no longer distinguished between them, and mated indiscriminately. Offspring of these matings were fully fertile, indicating that female choice is the only thing that stands between these species and hybridisation. The Grasshopper's Tale gives a similar example. If the two species were a bit more different, their offspring would probably be infertile, like mules. Later still in the process of divergence, isolated populations reach the point where they couldn't hybridise even if they wanted to. Whatever the basis of the separation, failure to hybridise defines a pair of populations as belonging to different species. Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that 'intervention' does not necessarily mean it is geography itself that made the active change — as when a valley floods or a volcano erupts. The same effect is achieved if geographical barriers existed all along, wide enough to impede gene flow, but not so formidable that they are never crossed by occasional founder populations. In the Dodo's Tale we met the idea of sporadic individuals having the luck to cross to a remote island, where they then breed in isolation from their parent population."
In his book The Ancestor's Tale, Richard Dawkins states (p. 346) that "depending on whether your ichthyologist is a lumper or a splitter, the number of species of cichlid [fish] in Lake Victoria is between 200 and 500". He explains that species are actually divided either based upon their ability to interbreed OR their willingness to interbreed. Many species of fish and birds have evolved into many "races" sporting different colours, so that they typically choose to mate with individuals similar to them. That doesn't mean that they cannot produce fertile offspring with the different-looking races. They simply choose not too. Well, in most cases, as hybridisation does occur from time to time.
The same thing actually happens a lot with humans. Geographical isolation has led us to evolved into different-looking races, and up to this day the majority of people choose to mate with people of their own racial group, even in cosmopolitan societies like the USA. That doesn' mean that we are different species. So why would there be hundreds of species of cichlid fish if they can all interbreed together ? Closer to us, why do we categorise dogs, wolves, jackals, dingos and foxes as different species if they can interbreed with each others but lack the opportunity (geographic isolation) or just choose not to (cultural difference) ?
I have quoted a passage from Richard Dawkins' The Ancestor's Tale (p. 349-350) to illustrate my point :
"According to the geographical isolation theory, speciation begins with the accidental geographical division of a single ancestral species into separate populations. No longer able to interbreed, the two populations drift apart, or are pushed by natural selection in different evolutionary directions. Then, if they subsequently meet after this divergence, they either can't interbreed or don't want to. They often recognise their own species by some particular feature, and studiously avoid similar species who lack it. Natural selection penalises mating with the wrong species, especially where the species are close enough for it to be a temptation, and close enough for hybrid offspring to survive, to consume costly parental resources, and then turn out to be sterile, like mules. Many zoologists have interpreted courtship displays as aimed mainly against miscegenation. This may be an exaggeration, and there are other important selection pressures bearing upon courtship. But it is still probably correct to interpret some courtship displays, and some bright colours and other conspicuous advertisements, as 'reproductive isolation mechanisms' evolved through selection against hybridisation.
As it happens, a particularly neat experiment was done on cichlid fish by Ole Seehausen, now at the University of Hull, and his colleague Jacques van Alphen at the University of Leiden. They took two related species of Lake Victoria cichlids, Pundamilia pundamilia and P. nyererei (named after one of Africa's great leaders, Julius Nyerere of Tanzania). The two species are very similar, except that P. nyererei has a reddish colour, whereas P. pundamilia is bluish. Under normal conditions, females in choice tests prefer to mate with males of their own species. But now, Seehausen and van Alphen did their critical test. They gave females the same choice, but in artificial monochromatic light. This does dramatic things to perceived colour, as I remember vividly from schooldays in Salisbury, a city whose streets happened to be lit by sodium lights. Our bright red caps, and the bright red buses, all looked dirty brown. This is what happened to both the red and the blue Pundamilia males in Seehausen and van Alphen's experiment. Red or blue in white light, they all went dirty brown. And the result? The females no longer distinguished between them, and mated indiscriminately. Offspring of these matings were fully fertile, indicating that female choice is the only thing that stands between these species and hybridisation. The Grasshopper's Tale gives a similar example. If the two species were a bit more different, their offspring would probably be infertile, like mules. Later still in the process of divergence, isolated populations reach the point where they couldn't hybridise even if they wanted to. Whatever the basis of the separation, failure to hybridise defines a pair of populations as belonging to different species. Each of the two species is now free to evolve separately, free from contamination by the genes of the other, even though the original geographical barrier to such contamination is no more. Without the initial intervention of geographical barriers (or some equivalent), species could never become specialised to particular diets, habitats or behaviour patterns. Notice that 'intervention' does not necessarily mean it is geography itself that made the active change — as when a valley floods or a volcano erupts. The same effect is achieved if geographical barriers existed all along, wide enough to impede gene flow, but not so formidable that they are never crossed by occasional founder populations. In the Dodo's Tale we met the idea of sporadic individuals having the luck to cross to a remote island, where they then breed in isolation from their parent population."