Why Does Species Stratification Occur in Biology?

[DivergentSpectrum]

i really don't know how to ask this as I am not very familiar with biology, but why are the species stratified?
by stratified i mean they are separated into different, discrete classes
for example, there are frogs and fish, but nothing inbetween
theres apes and people but no inbetwen. why would the intermediary steps presumably die out?
this is something i never understood and would like to know the scientific explanation

 

[Danger]

To start with, you can't make assumptions. We still haven't discovered probably 95% of the species that exist on Earth. You say that there is nothing between fish and frogs, but what about lungfish? Birds are technically dinosaurs, so right there is an overlap with lizards.

 

[jim mcnamara]

I think I can still identify a lot of North American grass species. If you can find a copy of Hitchcock (Agnes Chase)
1950, revision, Manual of grasses of the United States. Washington, D.C.: U.S. Dept. of Agriculture. OCLC 847512944

Look at the wonderful illustrations of, oh say, all the grass species in the genus Panicum. After you go cross-eyed at the level of infinitesimally small details required to begin to sort these guys apart, you will change your mind. And be glad that Agnes did that sorting for the rest of us. PS: she was 81 when she published the book.

PS: humans have (presumably) killed off many species of larger animals, or we domesticated and or killed off lots of other competing species so we no longer have some previous members of genera. Look up gaur for a living example of hamburger on the hoof circa 8000BC.

 

[Guco]

Fish are not a discrete class in an evolutionary sense, it's just the word we use to describe a specific type of animal that lives in water. If you look at the phylogeny of fish, it's not a monophyletic group. In other words, the common ancestor of all fish is also the common ancestor of many other types of animals besides fish.

It would be best not to talk about intermediate species (when comparing currently existing species) because that word implies that evolution has a direction, when it does not. Species might have specific traits that reflect an intermediate between ancestral trait and more recently evolved trait. No species that is currently alive is more primitive than another, even if some species might superficially resemble ancient species in terms of what they look like. All species continuously evolve and if their physical appearance remains stable over time, that just means it is a highly successful bodyplan and that there is no evolutionary pressure to change it.

Basically the premise of your question (currently existing intermediate species) is somewhat flawed. Humans did not evolve from any currently existing apes. Rather, all currently existing apes and all humans evolved from a single ancestor species and they all evolved and changed over time. Some species might be closer related to others (this means that the split between those species occurred more recently) and some species might deviate more from related species in terms of physical appearance.

 

[Guco]

Wanted to add some wiki pages you might find interesting to read:
http://en.wikipedia.org/wiki/Niche_differentiation
http://en.wikipedia.org/wiki/Speciation
http://en.wikipedia.org/wiki/Cryptic_species_complex

 

[DaveC426913]

+1 for Guco's post #4. He gets at the real misunderstandings in the OP's post.

 

[zyxwv99]

I think the original question is entirely valid. There is a parallel situation in historical linguistics, where a language evolves into dialects, which in turn evolve into languages that are mutually unintelligible. The process often starts with a language spreading out over a wide geographical area, then evolving into a dialect continuum. What matters is not so much the number of miles as the closeness of the contact. For example, a mountain range can partially separate two parts of a continuum. So can political boundaries, as when an empire splits up into separate countries. Langauge mutations, known as innovations, can spread and "contaminate" a particular area, but sometimes only up to a point. Then, beyond some point, the innovation is rejected as "too foreign."

Charles Darwin pointed out how something similar happened in the Galapagos Islands with the origin of species.

Of course one can always find counter-examples, as these are just general tendencies, not absolute laws. For example, German and Dutch are still part of a language continuum. However, the "missing link" is now limited to small villages and rural areas. In the big city it is more advantageous to speak the standard form of the national language, either proper Dutch or standard High German, depending on which side of the border people live on.

 

[Danger]

I think the original question is entirely valid.

You think incorrectly.

the innovation is rejected as "too foreign."

And again, so what?

Charles Darwin pointed out how something similar happened in the Galapagos Islands with the origin of species.

Not even close, if you're trying to equate it with your preceding statement.

it is more advantageous to speak the standard form of the national language, either proper Dutch or standard High German, depending on which side of the border people live on.

And again... in exactly what way do you think that the ability to speak the same language is as evolutionarily advantageous as being more adapted to obtain food and shelter and water. Trade was accomplished long before common languages, simply because people can communicate one way or another if they want to. That's a matter of brain structure and function, along with environmental influences. All of that is sociological, not biological. When has evolution ever worked on such a short scale that language could have anything to do with it?

edit: Sorry, I tripped over my own lips for a second there. Brain structure and function are most assuredly biological. That other crap isn't.

 

[zyxwv99]

You think incorrectly.

And again, so what?

Not even close, if you're trying to equate it with your preceding statement.

And again... in exactly what way do you think that the ability to speak the same language is as evolutionarily advantageous as being more adapted to obtain food and shelter and water. Trade was accomplished long before common languages, simply because people can communicate one way or another if they want to. That's a matter of brain structure and function, along with environmental influences. All of that is sociological, not biological. When has evolution ever worked on such a short scale that language could have anything to do with it?

edit: Sorry, I tripped over my own lips for a second there. Brain structure and function are most assuredly biological. That other crap isn't.

I'm no expert on evolutionary biology, but have read literally dozens of books on the subject. Mainly the sort advertized and reviewed in Scientific American, e.g., nearly every book Stephen Jay Gould ever wrote, plus Richard Dawkins, Daniel C. Dennett, etc. I was just trying to explain my understanding of the subject based on these sources. Social evolution was obviously an analogy.

For example, during the Cambrian explosion, the basic forms of nearly every conceivable kind of multicellular organism evolved. Over time some basic models of organism-plans fell by the wayside, while other models flourished. Once a winning model comes along, it tends to get replicated with variations. The result is a tree structure, where the space in between the branches represents impossible or failed models.

Wherever you see evolutionary tree structures (in terms of lines of descent) the same principles are usually at work. You can see the same in business, where one company like Hewlett-Packard spins off new businesses as its employees quit to start their own companies. Today a very large percentage of the high-tech companies in Silicon Valley can trace their lineage to Hewlett-Packard, and still count "the HP Way" as a core part of their approach to business. The HP Way is what Dawkins would call a "meme," i.e., a piece of mental software that people feel motivated to replicate.

 

[Danger]

I'm no expert on evolutionary biology

Bingo!
The whole subject matter of this thread is evolutionary biology. I know that you mean well, but your input is sociologic rather than biologic.

 

[DivergentSpectrum]

I came up with a couple of theories:
For one, like attracts like. in other words,a species will only be sexually attracted to each other if they appear similar in some way.
For example, if you were a neanderthal, and came across another neanderthal that happened to have a slightly bigger head to contain its highly evolved brain, you would probably dismiss it as a potential mate, because let's be honest, that super intellegent humanoid looks kinda goofy.
Also there is the phenomena of mothers abandoning one of their children if she detects something different.

But this brings up a problem, for one if there was some kind of advantageous mutation, the holder of the mutation would be kicked out of the pack, and less likely to survive on its own. Furthermore, it would probably need to find one of its own with the same mutation inorder to reproduce. So even if an advantageous mutation occurred, it would be unlikely to survive.

If a new species was born with some random mutation that still allowed them to reproduce, it would probably only be able to reproduce with others with the exact same mutation to produce viable offspring.

for example, horses and donkeys are closely related, but if you breed them you get a mule, and mules are infertile.

So I am guessing that what i stated earlier is true; if a new species was to evolve, it would need to find a mate with the same mutation in order to reproduce.
This brings up the "adam and eve" (i apologize full-heartedly for using that terminology :p) problem. If the available gene pool of a newly evolved species was only two trailblazing lifeforms, where would the offspring find mates? this means that when a new species evolves and is successful, there needs to be not only two mutations occurring at the same time, but 3 or more all within a similar geographical area.
What are the chances of that? I am aware that the timespan of evolution is huge, but regardless doesn't this mean evolution isn't as gradual and continuous as we say it is, but instead is a sort of "quantum leap" from one state to another?

 

[Choppy]

DivergentSpectrum,

There are a lot of common fallacies in your line of thinking. A new species is not "born" from an existing one. Individuals give birth to other individuals of the same species that have some small diversities. Over long periods of time those diversities can change the characteristics of the organisms within that population if there is any reason why some or those characteristics convey some kind of survival advantage.

If a subset of that species is isolated somehow, and subject to different selection pressures, that subset will develop different characteristics from the population it was isolated from. Over a long time, the isolated population can become so different, that if reintegrated with the original population, they cannot effectively breed. That's when you have something that we would classify as a separate species.

 

[DivergentSpectrum]

ahhh ok i see what your saying. what you described should provide a mechanism for many genetic abberations of the same kind to occur in a certain geographical area.
But, I am still wondering. As i understand, number of chromosomes is an integer, right? for example humans have 46 chromosomes. Suppose that number changed (and didnt result in something horrible like down syndrome) What would have to occur for the new species to actually be able to breed?

 

[DaveC426913]

ahhh ok i see what your saying. what you described should provide a mechanism for many genetic abberations of the same kind to occur in a certain geographical area.

Not sure where you're going with this but the mutations are random.

 

[DivergentSpectrum]

ok then, let me rephrase it
what choppy described should provide a mechanism for many genetic abberations of the same kind to occur in a certain geographical area and have a higher chance of surviving.

But number of chromosomes is still an integer, right? what happens when the number of chromosomes changes?I know that people with down syndrome have one extra chromosome, but they are also sterile. Is this always the case? If suppose some animal gave birth to an animal with a new number of chromosomes, would it be sterile? or could it only breed with others that also have the different number?

 

[Pythagorean]

The number of chromosones is probably not as significant as the contents of them.

 

[DivergentSpectrum]

The number of chromosones is probably not as significant as the contents of them.

please explain

 

[mfb]

But this brings up a problem, for one if there was some kind of advantageous mutation, the holder of the mutation would be kicked out of the pack, and less likely to survive on its own. Furthermore, it would probably need to find one of its own with the same mutation inorder to reproduce. So even if an advantageous mutation occurred, it would be unlikely to survive.

No. We all have random mutations. Two random humans of the same sex differ in something like a few million DNA base pairs (!) and we still have children with other humans. This selection effect might have an effect on populations that are on the edge of forming two different species, but it has no effect on the success of individual mutations (unless they completely change the way an individual looks like - and even then it does not completely rule out reproduction, it just means a third arm is not as advantageous as it might look like).

for example, horses and donkeys are closely related, but if you breed them you get a mule, and mules are infertile.

And they are counted as two independent species.

Suppose that number changed (and didnt result in something horrible like down syndrome) What would have to occur for the new species to actually be able to breed?

Nothing special - it just should not result in something like down syndrome.

 

[Choppy]

It might help if you think of a chromosome as the way the DNA is packaged. A "mutation" is a change in the sequence of DNA base pairs and not a change in the number or arrangement of chromosomes. Your DNA is changing all the time. I seem to recall that background radiation alone can lead to a significant DNA damaging event once per day per cell in the human body. In the vast majority of cases, the damage is either repaired or the cell is destroyed.

So you have a mutation and that codes for a different amino acid. This can lead to a difference in the protein that sequence builds. Maybe. And that may ultimately lead to a difference in the phenotype (the physical/biochemical characteristics of the organism). But it's important to recognize that there is always diversity in the phenotype of a given species. These slight changes, in most cases, make little difference to the overall survival probability. But is some cases, individuals come along with a phenotype that offers a distinct advantage. Those individuals survive and pass on those genes more often than those without it. Over time, more individuals in the population have that advantage.

Those with the advantage can still breed with those that don't have it. But over time, these little advantages accumulate. And they change too because the environment isn't always the same. Predators also evolve for example. Or they can die out. The environment is always changing. And the only rule is that those that survive get to pass their genes on.

 

[Guco]

Human males with XYY sex chromosomes (normally XY) and females with XXX (normally XX) exist and are usually fertile. Because the Y chromosome contains few genes (most of them are involved in sperm production) and extra X chromosomes are mostly inactivated, these people are relatively healthy. However, as far as I know they do not pass their duplicate chromosomes on to their children.

The concept of species is not perfect. It is a useful classification system but it has its limitations, especially when dealing about speciation (the process by which species arise) or species that existed in the past. It works best for currently existing species that procreate strictly by sexual reproduction. But even then there many examples where it's not so clear if we are dealing with a single species or multiple species.

 

[zyxwv99]

Stephen Jay Gould, in his book Ontogeny and Phylogeny, explains one mechanism of evolution of that often gets overlooked. When an egg cell gets fertilized, three things happen: cell division, growth, and differentiation (a stem cell splitting up into two different daughter cells). At first only cell division happens. Since the egg is the largest cell in the body, it can do that for a certain number of times before the cells reach normal size. During that time, no growth or differentiation takes place. Then the other two things start happening, but in an orderly sequence. In many cases, changing the number of cell divisions by one or two before differentiation occurs can result in an animal with a radically different body.

 

[Danger]

For one, like attracts like. in other words,a species will only be sexually attracted to each other if they appear similar in some way.

I take it that you've never seen a Brazilian porn movie.

super intellegent humanoid looks kinda goofy.

That's a matter of opinion. I rather like them.
By the bye, we contain something like 13% Neanderthal DNA. That pretty much proves that they didn't go extinct; they were absorbed into the Cro-Magnon gene pool to become us.

But this brings up a problem, for one if there was some kind of advantageous mutation, the holder of the mutation would be kicked out of the pack

Aside from it being on fire or having its reproductive parts in the wrong place, just how do you think that the "pack" would even be aware of a mutation? I'm a mutant, in that I have unusual intelligence and an immune system so freakishly powerful that it's attacking me. No one can detect either by looking at me, and I'm relieved to say that it hasn't adversely affected my mating habits.

If a new species was born with some random mutation that still allowed them to reproduce, it would probably only be able to reproduce with others with the exact same mutation to produce viable offspring.

Ridiculous.

I think that a huge part of the problem here is that you are making the same fundamental mistake that anti-evolution freaks do. "Survival of the fittest" refers to a species, not an individual. The compulsion to run into a burning building to save your daughter is not conducive to your survival, but it is to that of your descendants and other humans in general.

https://www.physicsforums.com/threads/stratification-of-species.785763/goto/post?id=4936342#post-4936342

Jay Gould, in his book Ontogeny and Phylogeny, explains one mechanism of evolution of that often gets overlooked. When an egg cell gets fertilized, three things happen: cell division, growth, and differentiation (a stem cell splitting up into two different daughter cells). At first only cell division happens. Since the egg is the largest cell in the body, it can do that for a certain number of times before the cells reach normal size. During that time, no growth or differentiation takes place. Then the other two things start happening, but in an orderly sequence. In many cases, changing the number of cell divisions by one or two before differentiation occurs can result in an animal with a radically different body.

Doesn't matter. It still ends up on my plate with some chives and a fried mushroom.