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Member Posts: 20323 From: the other end of the sidewalk Joined: Member Rating: 3.5 |
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Author | Topic: Definition of Species | ||||||||||||||||||||||||||||||
RAZD Member Posts: 20323 From: the other end of the sidewalk Joined: Member Rating: 3.5 |
I think everyone is fairly familiar with the biological species definition, and some may be familiar with the morphological definition. Here is a site for reference on these definitions:
U of Michigan Lectures - The Process of Speciation quote: Where we can study living populations of sexual species we can use the first definition, but when we deal with the fossil record or with asexual species we would have to use the second definition. There is also another definition in the forum glossary: http:///WebPages/Glossary.html#S quote: Buried in Message 118 is this comment:
The news article listed by Murkywater above has a link to the actual Journal article: http://www.allenpress.com/pdf/mamm-87-04-24_643..662.pdf quote: Now my first impression is that this is really just the biological definition of species using genetics to determine reproductive isolation, and one that would be useful for finding "cryptic" species, one that could be applied to asexual species, and even extended to some fossils (where DNA is recoverable). quote: Reading further it seems that they establish a somewhat arbitrary delineation for "type" species concept based on <0.5% difference in the mitochondrial cytochrome-b gene: quote: I would think you really want to compare the total genome to ensure you are picking up where the genetic change is occurring within the population(s). I also think you would want to do a statistical mapping of all the variations by frequency and see if you have one or more peaks in the data, and let those peaks define your species (or incipient species depending on the degree of overlap and geographical separation). Furthermore, where you draw the line (>0.5%) could determine arbitrary species designations, with several monophylic species with wide viable hybrid zones, as opposed to one polyphylic species, being a matter of somewhat subjective interpretation. quote: Now it is not surprising to me that there would be grades of separation to be found in the data, as various populations would be at different points in developing isolating mechanisms to achieve speciation, as this is a gradual process after all. Nor am I surprised to think that there could be subpopulations that don't have the same proportions of alleles as other subpopulations, due to geographic factors that would make direct mating difficult, and where gene flow would lag temporally. The question though is whether any member of subpopulation (A) could mate with a member of subpopulation (B) and produce viable (hybrid) offspring ... and this is not really answered when you have the subpopulations defined by average genetic similarity around mean values (there will always be a distribution in any population) and only observe mating between some of the two populations (that may or may not be statistically high in either subpopulation). You don't know who is in the "hybrid zone" when mating occurs. Why is the definition of species important and what is the use for the definition of species? Speciation is is the dividing line between what are considered microevolutionary and macroevolutionary processes and mechanisms, between the generation of homogeneous change within a population (evolution), and the generation of heterogeneous change (diversification) between diverging (especially for new) species. Thus I would define any population with a single peak frequency distribution as a species, any population with two peaks and a high "saddle" between them as incipient species, and any population with two peaks and a low "saddle" as different species. Analysis of this type of pattern for species like horses, zebras and donkeys would give you an idea of the saddle height necessary for speciation. Comments? Enjoy. Edited by RAZD, : Edited by RAZD, : fixed symbols, signature by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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AdminNosy Administrator Posts: 4754 From: Vancouver, BC, Canada Joined: |
Thread moved here from the Proposed New Topics forum.
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Doddy Member (Idle past 4245 days) Posts: 563 From: Brisbane, Australia Joined: |
I don't think it is important. It will always drawing an arbitrary line on a gradual process, and I don't think it even matters.
Micro and macro evolution, being defined as speciation and morphological changes, are dependant on species as a unit (at least in biology, rather than creation science). But species aren't units, and so dividing micro from macro is just as problematic and useless as diving species from each other. It just leads to the question of 'why is macroevolution an important term?'. In conclusion, a one-size fits all method of determining species will always cause problems. It's subjective, and thus useless outside of the obviously distinct species. That's my opinion anyway. What do you mean "You can't prove a negative"? Have you searched the whole universe for proofs of a negative statement? No? How do you know that they don't exist then?!
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RAZD Member Posts: 20323 From: the other end of the sidewalk Joined: Member Rating: 3.5 |
It's two aspects of the same issue. But speciation - the division of one species into two or more species - is what accounts for the diversity in types of organisms, and having a usable definition of species lets you determine when speciation has occurred, and then you can study how long it takes and what the specific mechanisms are.
Yet we know that many populations of organisms are genetically isolated by lack of mechanism for sharing genes (sex or horizontal transfer), and that as a result they have different resources to use when reacting to changing ecological situations or opportunities. The concept of species allows better understanding of the mechanisms that lead to (a) continual change in a genetic line of a population of otherwise similar organisms or (b) diversification of life into different ecological opportunities. Enjoy. compare Fiocruz Genome and fight Muscular Dystrophy with Team EvC! (click) we are limited in our ability to understand by our ability to understand RebelAAmericanOZen[Deist ... to learn ... to think ... to live ... to laugh ... to share.
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Ben! Member (Idle past 1957 days) Posts: 1154 From: San Diego, CA Joined: |
I wouldn't phrase it that way, but reading purely what's in your post, it seems the same to me.
The number seems arbitrary as they haven't defined what "often" means. Also seems that it's lacking any theoretical underpinning, and is pulled straight from the data with no attempt at providing additional theoretical "motivation" for choosing the number.
Razd, sorry, I didn't understand what frequency distribution you were talking about here, or in the other part of your post. Could you elaborate? Sorry I couldn't come up with any better comments... but thanks for the information.
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RAZD Member Posts: 20323 From: the other end of the sidewalk Joined: Member Rating: 3.5 |
Ben, long time no cross paths.
Well, every individual would be genetically unique, so there is no genetic "type" in a strict sense, but a distribution. You would need some way to map this distribution that wouldn't bias the data, such as take a sample from a sister species that you know is isolated reproductively and genetically, and then count the differences between each individual in the population and that sister species sample. It should show a frequency distribution like a bell curve if the population is relatively homogeneous for genotype (one species), but it should have multiple peaks if it is heterogeneous for genotype (two or more species or incipient species). Because of gene mixing (reproductive imperfection) you probably could not make a cladogram for the population using the whole genomes, as there would be cross branching (some genes from one parent line others from the other at every level).
That's kind of my impression: more work needed to refine the application. Enjoy. compare Fiocruz Genome and fight Muscular Dystrophy with Team EvC! (click) we are limited in our ability to understand by our ability to understand RebelAAmericanOZen[Deist ... to learn ... to think ... to live ... to laugh ... to share.
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Doddy Member (Idle past 4245 days) Posts: 563 From: Brisbane, Australia Joined: |
Branching is what accounts for the structure of the tree, but it is completely useless to try to distinguish the exact line between a bough, a branch and a twig. You don't really need to know what the precise definition of a twig is to study how they form.
So, we need to have a definition for species to study how species form. Well, that just brings me to the questions of: why do we want to know exactly when speciation has occured? We don't really need to know that to understand the processes involved. What do you mean "You can't prove a negative"? Have you searched the whole universe for proofs of a negative statement? No? How do you know that they don't exist then?!
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RAZD Member Posts: 20323 From: the other end of the sidewalk Joined: Member Rating: 3.5 |
Well even if you don't I do. Understanding this process can help understand why some speciations are (relatively) abrupt and morphologically distinct while othes are cryptic. Understanding the cryptic species in mosquitos was important to distinguish those that were carriers of malaria and those that were not, when they have all been considered to be one species before. Enjoy. compare Fiocruz Genome and fight Muscular Dystrophy with Team EvC! (click) we are limited in our ability to understand by our ability to understand RebelAAmericanOZen[Deist ... to learn ... to think ... to live ... to laugh ... to share.
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Medis Member (Idle past 2226 days) Posts: 34 Joined: |
Not sure if this is the right place to post this but...
How do you define the complexity of an organism? One evidence of the occurrence of macroevolution is the fossil record observation that invertebrates were on the earth much longer than vertebrates. Thus showing that "complex" organisms evolved from "simple" organisms. But how do you define a complex organism? Having more cells? Having a skeletal structure? Is it possible to say that an organism with a skeletal structure is "more complex" than an organism without it and why? Edited by Malangyar, : No reason given. Edited by Malangyar, : No reason given.
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Dr Adequate Member Posts: 16107 Joined: Member Rating: 8.3 |
We don't.
No, it isn't. Well, to be precise, which I am even in my sleep, it is possible to say it, but I challenge anyone to mean anything by it. Edited by Dr Adequate, : No reason given.
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Medis Member (Idle past 2226 days) Posts: 34 Joined: |
Are you sure this is true, I mean I'm looking at a wikipedia article right now saying "More complex organisms such as...". I mean can't you say that a multicellular organism is more complex than a single-celled organism?
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Chiroptera Member Posts: 6829 From: Oklahoma Joined: Member Rating: 6.6 |
Sure, you can say it. But what does it mean? Edited by Chiroptera, : No reason given. In many respects, the Bible was the world's first Wikipedia article. -- Doug Brown (quoted by Carlin Romano in The Chronicle Review)
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Medis Member (Idle past 2226 days) Posts: 34 Joined: |
That it has more parts and thus has a more interconnected whole than an organism with fewer cells.
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Dr Adequate Member Posts: 16107 Joined: Member Rating: 8.3 |
Yes. I was taking you up on a particular example, and I challenge you, anyone else, the whole world, to demonstrate that vertebrates, in general, are more "complex" than invertebrates in any meaningful sense whatsoever.
Not without knowing more about them. But the whole idea of "complexity" is that complex organism, the "red herring". It's not an interesting concept within evolutionary thought. Edited by Dr Adequate, : No reason given. Edited by Dr Adequate, : No reason given. Edited by Dr Adequate, : No reason given.
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Medis Member (Idle past 2226 days) Posts: 34 Joined: |
So what should you use instead? That organisms aren't more complex than others, just different from others?
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