What is a species?

Guybrush · 03-02-2010, 05:24 AM

The species concept has been a little awkward and troublesome since the start. In his earlier editions of Systema Naturae, Carl von Linnè considered swedes to be a species with different kinds of swedes as sub-taxa. The simple truth is that differences between organisms are often not clear cut and biologists today are still having trouble defining what a species is - what line you have to cross before you go from one species to another.

People in general are not as weighed down by academic considerations and may have a more clear cut idea of how to define species. As the world is not really managed by biologists, the general perception of species is immensly important to stuff like conservation and less academically hindered definitions may give valuable insight.

So, that was the intro

Now, here's a question for you to answer :

How would you define a species?

Please, don't look up the answer before you write because you won't find one (at least not by consensus, only different ideas) and I'd like to see some unadulterated opinions. No definition is too silly and if yours contains one good idea and one bad one, we can possibly ditch the bad and build on the good!

noise · 03-02-2010, 06:15 AM

species is a blurry category indeed.

first and foremost, the label "species" can only be applied retroactively. this is because it is impossible to draw a line across an evolutionary path, marking the moment when species A became species B. a parent can never be a different species than its offspring.

as such, it must be a very general category, and must be analogical rather than digital.

generally speaking, two organisms are of a different species if they will not interbreed. i imagine this is as much social as it is biological.

of course, this definition has problems. some animals are physically capable of interbreeding, but will not for social reasons. and vice versa...

FETCHER. · 03-02-2010, 06:39 AM

I consider a species a group of animals that can interbreed and produce fertile offspring (My own words btw) as that's what I was taught, for instance, a horse and a donkey are not the same species in my opinion as they produce a mule which in turn is sterile.

Maybe you can help me answer this tore as I always wondered it.

Why is the mule sterile? Why is a "Liger" sterile? I was never taught that and I always wondered why two animals from the same family couldn't interbreed (well they can technically) but they cannot produce fertile offspring? It's always confused me.

Guybrush · 03-02-2010, 07:57 AM

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.

Quote:

Originally Posted by kayleigh.

Why is the mule sterile? Why is a "Liger" sterile? I was never taught that and I always wondered why two animals from the same family couldn't interbreed (well they can technically) but they cannot produce fertile offspring? It's always confused me.

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.

FETCHER. · 03-02-2010, 09:47 AM

Quote:

Originally Posted by tore

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.

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.

P A N · 03-02-2010, 10:31 AM

what an interesting question.

i recently watched a documentary called "earthlings." despite the fact that there were video clips depicting what man does to species lower on the totem pole and it brought me to tears on multiple occasions (ps, i love eating meat... a lot), it brought a new point to my perspective going by the name "Speciesism," wherein humans believe that because we are more ingenuous, we are able to without guilt do things to animals that if happened to us via the hand of our leaders, would no doubt result in violent uprising.

it made me think a little about the fact that we're the only species capable of looking at all the different species and making a decision to try and understand them, whereas they just go about their lives being a part of the system.

there is also some science floating around concerning the universe and the possibility that it might be conscious, like a species of it's own. if you examine the way the universe works, you'll find that yes, indeed, it WORKS, making it possible to call it a SYSTEM. an autonomous one at that, and thusly alive...? it doesn't reproduce the entire thing (obviously another convo, involving multidimensional thinking, so to speak), but planets die and are born within it... galaxies... quasars... all just parts of a seemingly ceaseless show.

is anything classifiably ALIVE a member of some species? and if so, to what species does the universe belong, for it too exhibits the traits of life?

that might sound like a really dumb question, and it's on the fly, so jumbly it is. but, if we can look at the universe as the mother of all things, the complex within which our planet and many other planets reside, on which we and our dogs reside, inside which the minutiae of reality resides, the word "species" becomes obsolete or irrelevant, particularly when considering evolution as it is there that we see nothing is quite as static as we'd like it to be.

i guess i'm saying that classifying things as species is not a tangible concept... still not sure if that's what i believe though, it's just what came out.

maybe there is just life.

333 · 03-02-2010, 11:20 AM

Very interesting thread, Tore. I love your thread ideas. I'm going to have to sit on this question for a bit, though. Interesting discussion so far, though. I had no idea that some hybrids were sterile.

Guybrush · 03-02-2010, 03:24 PM

Quote:

Originally Posted by kayleigh.

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.

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.

FETCHER. · 03-02-2010, 05:52 PM

Thank you, I think you answered my question as much as it can really be answered and defined.

Guybrush · 03-02-2010, 05:58 PM

Quote:

Originally Posted by kayleigh.

Thank you, I think you answered my question as much as it can really be answered and defined.

And you can only do so much in a forum posts .. There are several text books that cover these problems in more detail.

Back to the species definition, sexual isolation has been mentioned - that members of a species can have reproductive offspring or - turned on it's head, that they are reproductively isolated from other species.

But how about similarities? Shouldn't members of a species also look similar and have similar ecology?

03-02-2010, 05:24 AM	#1 (permalink)
Guybrush Juicious Maximus III Join Date: Nov 2008 Location: Scabb Island Posts: 6,525	What is a species? The species concept has been a little awkward and troublesome since the start. In his earlier editions of Systema Naturae, Carl von Linnè considered swedes to be a species with different kinds of swedes as sub-taxa. The simple truth is that differences between organisms are often not clear cut and biologists today are still having trouble defining what a species is - what line you have to cross before you go from one species to another. People in general are not as weighed down by academic considerations and may have a more clear cut idea of how to define species. As the world is not really managed by biologists, the general perception of species is immensly important to stuff like conservation and less academically hindered definitions may give valuable insight. So, that was the intro Now, here's a question for you to answer : How would you define a species? Please, don't look up the answer before you write because you won't find one (at least not by consensus, only different ideas) and I'd like to see some unadulterated opinions. No definition is too silly and if yours contains one good idea and one bad one, we can possibly ditch the bad and build on the good! __________________ Something Completely Different

03-02-2010, 06:15 AM	#2 (permalink)
noise thirsty ears Join Date: Sep 2009 Location: Boulder Posts: 742	species is a blurry category indeed. first and foremost, the label "species" can only be applied retroactively. this is because it is impossible to draw a line across an evolutionary path, marking the moment when species A became species B. a parent can never be a different species than its offspring. as such, it must be a very general category, and must be analogical rather than digital. generally speaking, two organisms are of a different species if they will not interbreed. i imagine this is as much social as it is biological. of course, this definition has problems. some animals are physically capable of interbreeding, but will not for social reasons. and vice versa... __________________ my flac collection

03-02-2010, 10:31 AM	#6 (permalink)
P A N DO LIKE YOU. Join Date: Feb 2010 Posts: 629	what an interesting question. i recently watched a documentary called "earthlings." despite the fact that there were video clips depicting what man does to species lower on the totem pole and it brought me to tears on multiple occasions (ps, i love eating meat... a lot), it brought a new point to my perspective going by the name "Speciesism," wherein humans believe that because we are more ingenuous, we are able to without guilt do things to animals that if happened to us via the hand of our leaders, would no doubt result in violent uprising. it made me think a little about the fact that we're the only species capable of looking at all the different species and making a decision to try and understand them, whereas they just go about their lives being a part of the system. there is also some science floating around concerning the universe and the possibility that it might be conscious, like a species of it's own. if you examine the way the universe works, you'll find that yes, indeed, it WORKS, making it possible to call it a SYSTEM. an autonomous one at that, and thusly alive...? it doesn't reproduce the entire thing (obviously another convo, involving multidimensional thinking, so to speak), but planets die and are born within it... galaxies... quasars... all just parts of a seemingly ceaseless show. is anything classifiably ALIVE a member of some species? and if so, to what species does the universe belong, for it too exhibits the traits of life? that might sound like a really dumb question, and it's on the fly, so jumbly it is. but, if we can look at the universe as the mother of all things, the complex within which our planet and many other planets reside, on which we and our dogs reside, inside which the minutiae of reality resides, the word "species" becomes obsolete or irrelevant, particularly when considering evolution as it is there that we see nothing is quite as static as we'd like it to be. i guess i'm saying that classifying things as species is not a tangible concept... still not sure if that's what i believe though, it's just what came out. maybe there is just life.

03-02-2010, 11:20 AM	#7 (permalink)
333 Music Addict Join Date: Mar 2009 Posts: 965	Very interesting thread, Tore. I love your thread ideas. I'm going to have to sit on this question for a bit, though. Interesting discussion so far, though. I had no idea that some hybrids were sterile.