Guybrush

Some interesting definitions so far You both suggest to define a species by reproductive isolation as it were, all organisms that can have fertile offspring together form a species. This makes sense because you would think that these will evolve independently from all other organisms and indeed that's pretty much what's traditionally now thought of as the biological species concept.

There are some deep underlying problems with this species concept, but I think I'll wait until a few more definitions come in before I comment any more on them.

That's a deceptively difficult question to answer. As you've probably guessed, it has to do with genetic stuff. You probably know that animal genes are organized into long strings of DNA and associated proteins called chromosomes. Diploid animals have one set of chromosomes from dad and one from mum. These chromosomes are copied every time a cell splits into two. It's quite simple, you get a duplicate of each chromosome and they align themselves at different sides of the cell core (nucleus) and then the whole thing splits down the middle and you get two new cells with the same two sets of chromosomes each. That's called mitosis.

However, when sperm and eggs are created, there's a slightly different process called meiosis. Instead of one cell dividing to two, one cell shall become four sperm or four eggs, each containing only half the amount of genetic material as the parent cell. They will be haploid, containing only one set of chromosomes with some from mom and some from dad. To ensure that each new egg/sperm cell gets a nice mixing of genes and chromosomes from both mom and dad in that 1 set, there's a step in meiosis where homologous chromosomes from mom and dad pair up and exchange genetic material.

When two organisms that are very dissimilar create a hybrid (f.ex horse and donkey), the diploid mule offspring's cells can have two very different sets of chromosomes. When meiosis is to take place in the hybrid, chromosomes can have a hard time pairing up with their homologues from the other dad/mum set or the mixing between them creates problems .. possibly because their homologues in the other set are so different or even because they're simply not present! Horses and Donkeys have different amount of chromosomes which would result in some chromosomes being unable to pair up with homologues. This can cause a multitude of problems, many which end in reduced ability to reproduce and even sterility.

I'm not sure if that made it understandable. Possibly, this meiosis illustration might help a little bit.

edit :

Gurgh, had to brush up a bit on that explanation. I think it's understandable now.

edit 2 :

By the way, some "species" do hybridize quite a lot in nature, for example many birds. Hybrids are not always sterile, but may simply have lower reproductive ability which translates to lower fitness.

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FETCHER.

I didn't even think about the bolded. I feel stupid for not realizing something as important as that. I'm assuming either horse or donkey has more information than the other, leaving spare chromatids over? I never learned any of this during school, I'm just guessing btw. So if two animals in a similar situation to horse and donkey reproduced but had an equal number of chromosomes the product would be fertile? Or would the only time this happened be within the same species? Meaning that in every circumstance different species never have an equal number of chromosomes? I think I may be complicating things.

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Guybrush

You may be complicating things, yes, but don't feel stupid I find most people have little real knowledge about this and that often includes myself! Considering horses and donkeys are quite similar, if they had the same amount of chromosomes, I'm thinking their chances at producing fertile offspring should at least be much better, so I'm guessing that yes, they could create fertile offspring! That's just a guess though. There are still more ways for genetic material to be different and even incompatible other than the count of chromosomes. For example, on what chromosomes certain genes are located can differ and so I'm sure there could be a lot of possible negative effects from unhealthy genetic interactions between chromosomes from differing species. Simply in order to illustrate, imagine if species A has a vital gene on chromosome nr. 1 and it hybridizes with another species B that has the same gene, but on chromosome nr. 2. If a hybrid's sperm cell contains chromosome number 1 from species B and chromosome nr. 2 from species A, the sperm would not contain the gene at all. Maybe that's an overcomeable problem, but imagine that variations of the same problem also happens with other genes. I'm not saying that's what happens, genes in different locations, but it illustrates how differing genetics could cause trouble. Plus, I'm sure that before you get to the point where you actually have sperm/egg, there could be problems with pairing and recombination as well with some unhealthy results, but let's not focus too much on that otherwise this post will get horribly lengthy.

Also, there are a couple more here with knowledge in biology, Vegangelica being one of them, and they might know more about this than I do.

As I wrote above, many hybrids are fertile. Very many in fact - and not all of them are animals either. Hybrids are very common with plants for example and the variation in the amount of chromosomes going on in the plant kingdom quickly gets high and quite chaotic. If we stick to animals, birds are good examples of animals that hybridize a lot. Not all birds of course, but many species. Negative effects from what I'm taking an educated guess at what must be negative genetic interactions and so on usually cause these hybrids to have a lower fitness than most average members of their parent species. For this reason, it should be generally adaptive to try and avoid producing hybrids, but of course birds can get confused as well when different species look so alike!

Anyways, there's a lot of stuff going on so it's hard to say "this explains all of that" if you know what I mean, but at least I can add something to help solve pieces of the mystery.


To zevokes, about you wondering what species the planet is, back in the days of Linnè when the binomial taxonomy as we know it was brand new, people actually tried to organize minerals the same way, with families and genera and species and so on. They stopped at some point because there's no real evolutionary relationship between minerals like there is between organisms (even if they had no concept of evolution at the time). Your point was a little more profound, but I thought it might be an interesting nugget of trivia.

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