What determines a species?

Question

I come from a non biological background, how do you tell the difference between different species. A horse and a zebra look the same, but have different coats anddifferent sizes. What is the difference between these and, say a 6'6 European person and a 5'2 Asian person. What makes a speciesaspecies and not just variations of the same

Answer 1

Ultimately species are populations of animals (or plants, fungi microbes etc.) that reproduce with each other but are completely reproductively isolated from other populations of animals. For example a cheetah and a leopard are both cat species that live on the plains of Africa. Their habitats overlap, superficially they look quite similar but they do not even attempt to mate with each other. They are separate species. If you looked at their chromosomes or DNA sequences then they would be quite different. Ultimately, the lineages that led to Cheetah and leopard have been separated from one another for such a long time that they have become completely genetically differentiated from one another and all aspects of their biology have become so different that you can easily recognise them as what we would call species.

In the case you mentioned horse vs zebra, much of the above holds true. They are adapted to different ecological niches and their chromosomes and DNA sequences are also different. In the wild the two would never meet and reproduce - horses are of Asian and zebras of African origin. This case is slightly complicated by the fact that they can reproduce in captivity, although the offspring are sterile. All of this boils down to the fact that zebras and horses are both descendants of a single ancestor and that the time since separation is not so great that all reproductive behaviour or genetic differentiation is so complete that the possibility of producing offspring has vanished. However, you should also note that ancient wild horse populations overlapped with yet another equine called the onager and they never crossed in the wild. The stress of unnatural situations can cause behaviour that would never happen in the wild.

If we then look at human populations, the differences you point out are relatively trivial. No more or less than the differences you would see between breeds of domestic animal such as sheep or dogs. The real point is that human evolutionary history is very young compared to that of say zebras or horses and there simply hasn't been enough time for humans to differentiate into different species. We all share the same number of chromsomes and the genes are all found in the same order. That is not to say that there is no genetic structure in humans, there is, and we know that there are some barriers to reproduction between different human populations but these are cultural rather than biological.

Finally, it would be worth putting your example into some sort of context. First, height in Europeans has increased with better nutrition and is not a fixed genetic difference between them and Asians for example. If you visit a house from the 16th Century and look at the size of beds and heights of doorways this will become obvious. Second, you distinguish between Europeans and Asians. In fact, populations from say India and Europe are much more closely related to each other than either are to the Japanese for example. The fact that we all speak Indo-European languages which developed only around 5000 years ago, shows that there is very recent common ancestry.

Answer 2

What Determines A Species

Answer 3

There are indeed several different ways of determining "species". The main one is what's known as the Biological Species Concept. This is what other posters have already referred to - if two organisms can mate and produce sterile offspring, they are considered to be the same species.

Of course, this is only useful for sexually-reproducing organisms. Plus, it's useless for categorising fossils of extinct animals - we can't breed them to see what happens! So there are a whole range of other concepts that can be used, depending on the circumstance, usually based on the physical similarities and differences.

The other thing to note is that "species" is an artificial distinction. Nature isn't divided up into neat categories the way that people like. It's a continuum, and this is best illustrated by the example of ring species such as the herring gull. The gull is found in a ring around the northern hemisphere. If you start in Britain and go east, populations of gulls differ slightly - but each population can breed with its neighbour. By the time you go all the way around the world and get back to Britain, the changes will have accumulated so that you now have a completely different species that can't interbreed with the original one. This just shows how difficult the concept of species is.

It's worth pointing out, however, that under every single way of categorising species, a European person and an Asian person would be the same.

Answer 4

I'm not a biologist, so this won't be the best answer, but this is how I understand it:

There's no real, cut and dry definition of species. The concept of species is somewhat artificial, and it should be seen as a continuum rather than a series of discrete steps.
The best simple definition I know of is the ability to reproduce. Genetically, a Great Dane could have offspring with a Corgi, even though they are quite different in some ways, because they are the same species. However, a horse and a donkey can reproduce (offspring = mule), but they are not considered the same species.

The process by which distinct races develop is this: geographical separation to the point where 2 groups of the same species can no longer interbreed. Presumably, all humans came from the same group (in eastern Africa). For whatever reason, subgroups left for different locales. Once separated, each group underwent natural selection, and eventually developed new traits, distinct from other groups due to their unique climate and genomes of the members. To date, no 2 races of people have shown the inability to reproduce with one another.

But seriously, don't take my word for it.

Answer 5

If they can mate and produce fertile offspring they are the same species.
A Horse and a Zebra can mate and produce Zorse that are infertile, therefore the horse and zebra, although they look alike, are different species.
There are probably more scientific answers to do with numbers of chromosomes but this is the simplest way I know to explain it.

Answer 6

In biology, a species is one of the basic units of biological classification

A usable definition of the word "species" and reliable methods of identifying particular species are essential for stating and testing biological theories and for measuring biodiversity.

A species consists of individual organisms which are very similar in appearance, anatomy, physiology and genetics due to having relatively recent common ancestors.

Traditionally, multiple examples of a proposed species must be studied for unifying characters before it can be regarded as a species.

The commonly used names for types of animal in English and many other languages often do not correspond to species.

For example: dogs are a sub-species of wolf; but the word "deer" refers to a family of 34 species, such as the Red Deer and Mule Deer.


The Hierarchy of LIFE:-

LIFE

Domain

Kingdom

Phylum

Class

Order

Family

Genus

Species

Answer 7

there are several different species 'theories'
one is that the difference lies in the number of differences from a set dna sequence accepted as 'normal' for that species [the animal/plant is allowed to have so many variations before being considered a different species] but the one that most biologists go with is that so long as the creatures can mate and produce viable offspring [meaning that the offspring produced can then go on to produce more offspring]
the problem with this is that it leaves out bacteria and other one-celled or even multicelled organisms that do not reproduce sexually but it is the easiest definition to use for sexually reproducing organisms.

as far as the people go, humans can reproduce with any other human of the opposite sex, the differences in height, eye shape/color, hair color/texture and skin colors are just normal variations that contribute to the wonderful diversity we have as humans

Answer 8

The short answer is, 2 organisms are the same species if they can produce a fertile offspring. If they can't, they're different species.

Answer 9

Usually, that is too long for me. I think it depends on the guy as well as his face though. Certain guys can easily pull off long hair and other people can't.

Answer 10

I have pretty lengthy hair, I like them but I actually think I would have them cut short in about some two yrs