[Stevie Bain (OP)]

Geoff would like to know:

What is the definition of a species and are there different species of human?

Can you help answer this question?

[chiralSPO]

I'm not a biologist, but I can provide a rule of thumb for determining if animals are the same species:

If a male of one and female of the other can mate (and vice versa), and their offspring can also reproduce they are probably not different species. There can be a significant amount of variation within a species (all dogs, wolves, and coyotes can (and do) interbreed). The variation between humans is fairly narrow compared to the variation of dogs (largely because of purposeful diversification through breeding campaigns).

[Kryptid]

All living humans are of the same species, but there have been other species of human in the past, such as Homo erectus, Homo naledi and Homo habilis.

[alancalverd]

There is no consistent definition of species.

Production of fertile offspring is an indicator that male and female are of the same species, but most of the living things on this planet reproduce asexually and a fair number of any species turn out to be sterile - e.g. worker bees - so it's a proof but not a test!

It is quite likely that early homo sapiens interbred with homo neanderthalensis   which suggests that the differentiation was more a question of race than species, and this raises the possibility that all our ancestors  swere genetically sufficiently  similar that there was never a sharp distinction. Indeed this would be  a more probable interpretation of evolution: that the differences between successive generations in any location were negligibly small, but geographically separate groups evolved in different directions to the point at which they were not mutually fertile or attractive.

The problem with prehistory is that we have very few samples, widely separated in place and time, of what was once a thriving population. Whilst logic and chemistry suggest that evolution is quantised in such small steps as to be effectively continuous, the only evidence we have is of huge jumps. But absence of evidence is not evidence of absence!   

I think the  answer to the OP is that we define species simply for the convenience of labelling things that appear in some way to be different but connected. You can't interchange a flat blade and a crosshead screwdriver, but they are clearly very different from a hammer, so within the family of tools we have the genus screwdriver and two distinct species.

[jeffreyH]

And in the distance we see the crosshead screwdriver darting across the plane towards toolbox mountain. Majestic and enthralling in its migration from the hand of man.

[evan_au]

One thing that will differentiate species is the number of chromosomes.
If the male and female have different numbers of chromosomes, the offspring are less likely to be fertile, as they will get two copies of some genes (=usual and good), and three copies or 1 copy of others (not so good).

See: https://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count

Humans all have 46; I expect Neanderthals & Denisovans did too, while Chimpanzees have 48.

Someone mentioned in another thread that foxes have a different number of genes from dogs, but dogs have the same number as wolves. Dogs, most Wolves & Dingos have 48; different species of fox have many different numbers in the range 34-66 chromosomes.

[glot]

Thanks. All very informative.

[puppypower]

Lions and tigers are considered separate species, yet they can mate and produce viable offspring. My guess is much of the convention was established before we knew about genetics, and more based on the assumptions of natural selection.

[glot]

So, when a new plant or animal is discovered, someone must make a decision is it a new species or a variation on an existing species.

[chris]

So, when a new plant or animal is discovered, someone must make a decision is it a new species or a variation on an existing species.

Yes, good point. How do they do that?

[glot]

I always thought Ligers were infertile. I found this an interesting read on the subject.
http://www.huffingtonpost.com/quora/when-a-lion-and-tiger-mat_b_2001932.html [nofollow]

I have come to the conclusion that when someone names a new species, they need to justify it is a new species rather than prove it. Would that be correct?

[chris]

I have come to the conclusion that when someone names a new species, they need to justify it is a new species rather than prove it. Would that be correct?

I think anatomy is a pretty powerful tool for identification; at the moment we don't require genetic evidence but perhaps, given how cheap sequencing has become, that will be the gold standard?

[cheryl j]

I often see this question pop up on political forums, where someone claims the different "races" are different, or almost different species. The fact that they can interbreed would seem to discount this. Another thing to consider, as well, is that while you see these maps showing the migration of humans across the globe, those maps often do not show back tracking, reconnecting,  or intermingling with nearby groups. And unlike groups of migrating humans, genes them selves can essentially travel independently, so to speak, in different directions, depending on whether they are more adventitious in one environment or another. People often categorize anyone with dark skin, whose ancestors lived near the equator,  as belonging to a particular "race", even if they are no more genetically related to one other,  than to  other groups. 

[Zer0]

Good Question Geoff. 👍
And Wonderful Answers folks! 👌
If only each person on this planet could somehow read this page/OP, they would realize the fact that We/All of Us are a humongous Family. 👪 🌎🌍🌏
Hopefully then a quarter of the socioeconomic problems that plague Humanity would disappear/vanish overnight. ✌