Message 175 of 450 (572442)
08-05-2010 9:09 PM
Reply to: Message 157 by Big_Al35
08-05-2010 8:34 AM
Species Definition vs what matters (and why)
Thanks for staying on topic, even if you are making a number of assertions from an under-informed base (not ignorant, but not fully cognizant either):
|...I am not a paleontologist or biologist.|
And a great way to learn is to ask questions. Hope this isn't too long ...
To the issue of "fish" and "shark" and any other commonly used layman's term for organisms, this is necessarily fraught with misconceptions and mistakes. Taxonomists use many multitudes of measured traits to determining homologous (by descent) versus analogous (derived independently) traits in the formation of trees of descent.
To your basic issue of species definition, there is no single definition that works. Even with DNA sequencing there will be problems, and as you point out, our knowledge of the DNA of fossils is ... slim to none.
|But we can't just make up ad hoc species and lump these fossils under that category. It couldn't be defined as a species under any of our definitions anyway. We don't have access to the DNA, we don't know their behaviour and we can never know if they could interbreed. It has become an exercise in futility.|
But this assumes that DNA knowledge is critical to determining different species: it isn't. It kind of seems that you want to include DNA in the definition of species, so that then you can claim that species cannot be determined without knowing the DNA. This is employing a logical fallacy in your approach.
The real question is: what matters?
Species labels are useful to human scientists to differentiate bits and pieces of information from the fossil record, from the genetic record, and from the world around us.
Let me introduce the concept of "lumpers" vs "splitters"
Lumpers put lots of populations of organisms into one lumped group and label that a single species, with a number of varieties (slightly different traits, theoretically able to interbreed.
Splitters divide up populations into many small groups and label each one a different species, while the next larger group, genus, incorporates other closely related species that may be able to interbreed but typically don't.
Greenish warblers (Phylloscopus trochiloides) inhabit forests across much of northern and central Asia. In central Siberia, two distinct forms of greenish warbler coexist without interbreeding, and therefore these forms can be considered distinct species. The two forms are connected by a long chain of populations encircling the Tibetan Plateau to the south, and traits change gradually through this ring of populations. There is no place where there is an obvious species boundary along the southern side of the ring. Hence the two distinct 'species' in Siberia are apparently connected by gene flow.
Here we have between two and five species (if you're a splitter) and one species with five varieties (if you're a lumper), based on how much you count gene flow through the (small) hybrid zones.
Organisms are generally lumpers compared to humans, relatively unconcerned about what is or isn't a species (it's basically a them kind and us kind approach, although them kind can be broken up into eat this, avoid that, etc). You could call this the "organism definition" of species, and lump a lot of different organisms into big categories. This becomes highly unweildy when populations change from the "eat this" species to the "avoid that" species ... and back.
All the definitions we have of species have similar problems, and this leads to long discussions between lumpers and splitters.
But the point to keep in focus is that they matter to us as a means of describing groups of organisms, and how they fit into the broad scheme of things.
As far as evolution is concerned species is important for two reasons: (1) to define a discrete population (a breeding population) to monitor the process of evolution within the population, and (2) to define when speciation (the division of a breeding population into two or more daughter populations that become reproductively isolated from the others) occurs.
Let me introduce you to the concepts of "arbitrary speciation" and "non-arbitrary speciation" so you can see how this works.
In essence, "arbitrary speciation" occurs when a descendant population is no longer capable of breeding with an ancestral population, however this can end up with an awful lot of species definitions (especially if you are a splitter eh?) -- virtually every other generation. This would get pretty unwieldy very rapidly.
So an arbitrary speciation designation is normally made based on the descendant population at some point becoming as distinguishable from the ancestral population to the same arbitrary approximate difference as exists between two closely related species.
The changes accumulated in the descendant population are similar in degree to the differences between closely related species.
"Non-arbitrary speciation" occurs when two (or more) daughter populations become reproductively isolated from the parent population (as above) and each other, leading to different evolution in different ecologies, and reaching the point where interbreeding does not occur.
The numbers down the left hand side indicate the depth (in feet) at which each group of fossils was found. As is usual in geology, the diagram gives the data for the deepest (oldest) fossils at the bottom, and the upper (youngest) fossils at the top. The diagram covers about five million years.
The numbers across the bottom are a measure of body size. Each horizontal line shows the range of sizes that were found at that depth. The dark part of each line shows the average value, and the standard deviation around the average.
The dashed lines show the overall trend. The species at the bottom is Pelycodus ralstoni, but at the top we find two species, Notharctus nunienus and Notharctus venticolus. The two species later became even more distinct, and the descendants of nunienus are now labeled as genus Smilodectes instead of genus Notharctus.
As you look from bottom to top, you will see that each group has some overlap with what came before. There are no major breaks or sudden jumps. And the form of the creatures was changing steadily.
Here we have a number of "arbitrary speciation" events, marked by the different species designations along the general trend from lower left to upper right. We also have a couple of instances when the breeding population divides into two groups with a horizontal gap between them, yet overlapping ancestral populations.
These divisions are non-arbitrary speciation events: the two daughter populations after a split are not interbreeding, and this results in the horizontal gap between them.
I have accessed the original article and made the following colored lines on the same basic data to emphasize what is going on:
We see the "drunken walk" of evolution within an overall trend, and if we draw a line down from the left end of Pelycodus trigonodus we see that it is outside the range of traits for Pelycodus ralstoni while a line from the right end up to the top is outside the range for Pelycodus jarrovi.
The divisions of populations like this is evidence of non-arbitrary speciation events in the fossil record even though we do not have DNA samples from any of these fossils.
The staggering along trend that accumulates differences over time, while maintaining overlaps with ancestral and descendant populations shows that arbitrary speciation depends on where you subjectively draw the line, but approximates the same degree of change as occurs between daughter species in non-arbitrary events.
The non-arbitrary events only occur in sexual species, as this depends on the (sexual) biological definition of species.
Arbitrary speciation events would still occur in asexual species, but where you make the differentiation depends on a subjective analysis of when the accumulated changes are sufficient to justify a new arbitrary label for a bunch of organisms in order to describe things.
And whether you are a lumper or a splitter.
we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
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| ||Message 157 by Big_Al35, posted 08-05-2010 8:34 AM|| ||Big_Al35 has responded|