Perhaps slightly surprisingly, this is a question still without a strict answer. Generally it's thought to be if two groups of organisms can't interbreed, then you have separate species, but this doesn't always work (bacteria, for example, screw up this definition by reproducing asexually).
So aside from a definition debate, how do we work out whether or not something should be named as a new species, and also where it fits in with other currently known species?
There are 2 ways to go about this, and depending on the field you're in, you can use one or the other or (preferably) both.
The first method used is the traditional morphological method. This involves picking apart what the organism looks like and using those traits to group it with things that are similar. It's the method that has been used since the first ancient attempts were made to classify things, and realistically humans have been doing it for centuries. How did we determine that a mouse and a worm are different species? Because they look like they are.
Thankfully methods in morphological study are more advanced than that now. Traits studied might be form and function, as well as internal structures of that plant/animal. However different traits are also given different weighting, according to their importance, as not all characteristics are necessarily useful for classifying animals/plants/fungi. For example red hair in Orangutans and humans would certainly be a trait that would be largely ignored, as there are so many others that are far more important for determining whether or not they're different species.
However it is exactly this sort of uninformative characteristic that makes a purely morphological study potentially unreliable (although good systemics know this and choose better characters/leave out really dubious ones). It is important to recognise this, and make sure your characteristics are reliable, for example colour is usually a bad character for animal species (but is often ok for plants), as it can be really variable within a species.
Let's make a case-study of two similar species, say... the plant I'm working on, Cassytha pubescens (Lauraceae), and a dodder species, Cuscuta australis (Convolvulaceae). The reason I'll use these two is that they're frequently confused, and are a good example of the pros and cons of using morphology alone to describe species.
(Cuscuta image from http://www.natureloveyou.sg/Cuscuta%20australis/Main.html; Cassytha photo is my own)
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| Cuscuta australis |
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| Cassytha pubescens |
So you can see from the above photos that the two plants are superficially similar. They're both rootless, twining parasitic vines that can spread quite rapidly. Single plants can cover rather large areas, with dense mats forming over the host vegetation. Their leaves are so reduced they appear like scales, and they both attach to host plants using haustoria. These traits, without looking at any others, would immediately create an idea that they should be grouped together.
However an immediate difference we can see is the colour - the Cassytha is green, whereas the Cuscuta is yellow. This is due to their different modes of parasitism - Cassytha is a hemiparasite (takes water and the associated dissolved nutrients from its host) and Cuscuta is a holoparasite (takes photosynthates from it's host, which is why it's orange - it doesn't typically photosynthesise much). In addition to this, Cassytha is a perennial plant and Cuscuta is an annual. So on outward appearance we might group them together, but we can already see that their physiology is very different.
Other differences can be easily found in the flowers and fruits of the plants. Typically when classifying plants by their morphology any characters you can find that are not reliant on flowers and fruits are good. This is because vegetative characteristics (eg veins in leaves, or leaf arrangement etc) are there the whole year around (ok, excluding deciduous trees!) but flowers and fruits are only on the plant at certain times of the year. Having said that, flowers and fruits are really important for classifying plants. It can just makes it a pain to identify them in the field if they're not in flower/fruit!
The above examples both have leaves that are so tiny they're called scales - they're only a few mm long and sit flush with the stem at the nodes where the vine branches. So while they can be used for taxonomic purposes, flower and fruit characters are also really useful here.
In the photo of Cassytha, you can see the flowers are yellow, whereas in Cuscuta they're white. Cassytha flowers are tiny, and open about half-way in comparison to Cuscuta flowers. They're anatomically different (which can get complicated, so I won't explain that in detail) and you can also see the distribution of the flowers is different - the position of flowers along the stem bearing them and also the structure of the inflorescence is different - in Cassytha they're borne on spikes and have about 3-5 flowers in a cluster, in Cuscuta they're closer to the stem and have many in tight clusters. The fruits are really different too (although you can't see them in the photos), with Cassytha having fleshy fruits that are vertebrate dispersed, and Cuscuta having light, dry papery fruits.
So all the physiology and flower/fruit characters are pointing to them being very different species, from different families.
There are many other differences between these two plants, I've just picked out a few obvious ones. So you can see how once you start to look closely, sufficient characters can be found to place them into different families.
This is why morphological studies work. For some areas of taxonomy, this is essential - for example palaentology. Morphological studies are the only method they have to determine which ancient organisms belong together. But for those lucky enough to be working on living things, we can also use the newer, shinier methods created by our new(ish) knowledge of genetics.
So, to explain how we can do this, we turn to genetics 101, dot-point style:
- All living things have DNA (we won't go into the 'is a virus a living thing' debate here)
- This DNA is made up of 4 amino acid bases, Adenine, Guanine, Thymine, Cytosine, known as A, G, T and C.
- These pair up with each other (A with T, G with C)
- These pairs of bases run in sequence and create a double-helix of DNA.
- A specific region of DNA sequence makes up a 'gene' and is responsible for a certain function; ie it might be for blue eyes.
- Because all members of a species must have compatible genes, you tend to get subtle changes between species that allow us to tell them apart if we look at their DNA (if we can figure out which genes are best to look for this in).
- If the DNA can be extracted and sequenced, you end up with a string of A, T, G and Cs. By comparing these between your samples, you can determine if there are large enough differences to call them different species.
- Usually more than one gene is used - the more the better - as there is some variability within species as well as between them.
So from genetics combined with morphology we could, theoretically, end up with two different answers to whether or not our Cassytha and Cuscuta are in fact different species. However usually the two methods complement each other, and neither are without their own problems. The morphology results can be influenced by which characters you choose, or even how you score them, and the genetics could be influenced by how many genes you used, whether you took them from chloroplasts (which are like our mitochondria and only inherited from the 'female' plant) or the nucleus of the cells, or even which genes you picked. However with good experimentation and care these problems can be easily minimised.
Taxonomy isn't really that complicated. Both of these methods rely on the same principle - detecting differences between species based on the characters you chose to use (whether they be genetic or mophological ones). There also ways to represent the relationships between species visually, and these are typically known as phylogenetic trees. The process for making these is simple, but because you need so many characters to justify splits bewteen species, it can get mindbogglingly complicated with all the recombining and maths that becomes involved. This is why we let computers do the legwork for us now.
Having said that, I might make a post on how to construct a phylogenetic tree to finish up the taxonomy thread, as it's a concept that I think is way better understood if you actually make one from scratch, by hand. It's so easy to just plug in numbers and have a computer spit out an answer without actually understanding any of the processes involved.
So that's what I'll talk about next time :)
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