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Speciation: Anagenesis vs. Cladogenesis

Speciation: Anagenesis vs. Cladogenesis
handsworthrevolution
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Posted Apr 7, 2011 - 10:58 AM:
Subject: Speciation: Anagenesis vs. Cladogenesis
Darwin got most everything right on his first go-round, but failed to realize one thing; he was far too attached to phyletic gradualism. By stating unequivocally that evolution was a gradual process he opened the door to objections. Even today, philosophers like Dennett have espoused Darwinism under these strict terms.
I follow Stephen Jay Gould’s pleas for pluralism. When speaking of evolution, why must one restrict their thought to a hard line speciation by anagenesis or cladogenesis? Is there not enough room for both theories of speciation?
I am inclined to believe that cladogenesis is a more plausible theory of speciation, but that does not mean I throw anagenesis out the window. There may be instances of speciation where a mutation spreads gradually throughout a population, becoming more and more prevalent, until the new population differs enough from the ancestral population to warrant speciation; but I believe this type of speciation is rare, due to environmental pressures and the dilution effect of the large group. A more likely account of speciation lies in cladogenesis.
The laboratories of speciation are more often found on the peripheries of the group, in an isolated environment. This peripatric speciation emphasizes the importance of genetic drift. Natural selection still plays it role, but one must realize that a majority of mutations are neither beneficial nor deleterious, but neutral. Natural selection plays its role when needed; it just takes a back seat in speciation. My reading of Gould led me to research the neutral theory of molecular evolution, put forth by the Motoo Kimura. I find that this theory is compatible with peripatric speciation, for it shows that genetic drift plays a much more important role in the mutation of a small group, which in turn could lead to speciation. This type of speciation is consistent with Darwin’s other claims; it is just inconsistent with his phyletic gradualism.

One of my main interests is the philosophy of biology... whether or not this is a science is debated, but I figured this would be the correct board for this topic.
thinker100
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Posted Apr 7, 2011 - 12:24 PM:

Alright, my favorite topic. First off, I am assuming when you say Darwin was too attached to phyletic gradualism you are referring to speciation of one species? If so, I agree to an extent that species to species evolution is faster than others. What I think you are trying to explain is Goulds theory of punctuated equilibrium. This is theory explains that most species stay extant and unchanged for awhile. When they do change, they change quickly and violently into two separate species by process of cladogenesis. But punctuated equilibrium can also explain why speciation happens slowly, and in some areas but not others.

To answer your question, I would say one does not need to restrict there thoughts to only these two at all. Speciation happens many ways (yes mostly like cladogenesis) both slow fast branching or rarely by the replacing method of anagenesis. You only talked about one natural mode of speciation. There are 3 more, and the one you chose is actually a sub catagory of allopatric. Then there is parapatric and sympatric. Peripatric speciation is actually when a small group of a main population can no longer mate with the main population due to isolation mechanisms. Possibly geographic. When the two return, the changes in the subgroup have changed so much it is a new species. You describes sympatric more. It can happen through hybrids as well. Also, you described gene transposition. The rate of mutations is too high in each organism to keep them in the population gene pool if natural selection is not working. Natural selection selects the most fit organisms, the gene pool stays consistent this way. If you wanted I have a rough draft term paper on creationism vs evolution in the classroom and its filled with this topic. Lineages everything...if you want I can send it to u?
Geospiza
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Posted Apr 8, 2011 - 5:49 AM:

handsworthrevolution wrote:
Darwin got most everything right on his first go-round, but failed to realize one thing; he was far too attached to phyletic gradualism. By stating unequivocally that evolution was a gradual process he opened the door to objections. Even today, philosophers like Dennett have espoused Darwinism under these strict terms.
I follow Stephen Jay Gould’s pleas for pluralism. When speaking of evolution, why must one restrict their thought to a hard line speciation by anagenesis or cladogenesis? Is there not enough room for both theories of speciation?
I am inclined to believe that cladogenesis is a more plausible theory of speciation, but that does not mean I throw anagenesis out the window. There may be instances of speciation where a mutation spreads gradually throughout a population, becoming more and more prevalent, until the new population differs enough from the ancestral population to warrant speciation; but I believe this type of speciation is rare, due to environmental pressures and the dilution effect of the large group. A more likely account of speciation lies in cladogenesis.
The laboratories of speciation are more often found on the peripheries of the group, in an isolated environment. This peripatric speciation emphasizes the importance of genetic drift. Natural selection still plays it role, but one must realize that a majority of mutations are neither beneficial nor deleterious, but neutral. Natural selection plays its role when needed; it just takes a back seat in speciation. My reading of Gould led me to research the neutral theory of molecular evolution, put forth by the Motoo Kimura. I find that this theory is compatible with peripatric speciation, for it shows that genetic drift plays a much more important role in the mutation of a small group, which in turn could lead to speciation. This type of speciation is consistent with Darwin’s other claims; it is just inconsistent with his phyletic gradualism.


I am pleased to encounter someone with an interest in the philosophy of biology however you have badly conflated several different issues. Gradualism pertains to the rate of speciation. Anagenesis/cladogenesis pertains to the degree of reproductive isolation in an evolving population. Natural selection and genetic drift pertain to the mechanisms of evolution. Finally, the number of individuals in an evolving population pertains to the sensitivity of that population’s evolutionary trajectory to stochastic processes.

Given the subject heading of your thread, I will assume that you are most interested in pursuing the anagenesis/cladogenesis issue. An important thing to note about these concepts is that they are derived theoretically, and not experimentally. It is easier to imagine speciation by cladogenesis because it is less complicated and more pronounced: a population separates into two by some isolating mechanism or event (be it geographical, ecological or otherwise) and this isolation leads to evolutionary independence. The isolating mechanism is an important key to the process of speciation; in fact, depending on the species concept you employ it can be determinative of the issue of whether speciation has taken place. Anagenesis is conceptually more problematic, because without an isolating mechanism, the moment of speciation seems completely arbitrary. Some theorists think it is meaningless to speak of speciation in the absence of isolation. These theorists prefer to say merely that at some time in the past the lineage was composed with certain gene frequencies, and at present the population has a different composition, and that both populations form a continuum.

As I have already hinted at, much of this discussion hinges on which concept of species we utilize. This opens up an entirely different debate, sometimes referred to as “the species problem”.
Geospiza
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Posted Apr 8, 2011 - 5:51 AM:

thinker100 wrote:
I have a rough draft term paper on creationism vs evolution in the classroom and its filled with this topic.


The topic of speciation is complex enough already without bringing religion into things. Do civilization a favour and just forget about creationism entirely. Science is impartial to religion. If you care about your scientific credibility, you won’t bring it up in the current context.
handsworthrevolution
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Posted Apr 8, 2011 - 9:12 AM:

Geospiza wrote:


I am pleased to encounter someone with an interest in the philosophy of biology however you have badly conflated several different issues. Gradualism pertains to the rate of speciation. Anagenesis/cladogenesis pertains to the degree of reproductive isolation in an evolving population. Natural selection and genetic drift pertain to the mechanisms of evolution. Finally, the number of individuals in an evolving population pertains to the sensitivity of that population’s evolutionary trajectory to stochastic processes.

Given the subject heading of your thread, I will assume that you are most interested in pursuing the anagenesis/cladogenesis issue. An important thing to note about these concepts is that they are derived theoretically, and not experimentally. It is easier to imagine speciation by cladogenesis because it is less complicated and more pronounced: a population separates into two by some isolating mechanism or event (be it geographical, ecological or otherwise) and this isolation leads to evolutionary independence. The isolating mechanism is an important key to the process of speciation; in fact, depending on the species concept you employ it can be determinative of the issue of whether speciation has taken place. Anagenesis is conceptually more problematic, because without an isolating mechanism, the moment of speciation seems completely arbitrary. Some theorists think it is meaningless to speak of speciation in the absence of isolation. These theorists prefer to say merely that at some time in the past the lineage was composed with certain gene frequencies, and at present the population has a different composition, and that both populations form a continuum.

As I have already hinted at, much of this discussion hinges on which concept of species we utilize. This opens up an entirely different debate, sometimes referred to as “the species problem”.


Thanks for pointing out the conflation... I am relatively new to this sort of philosophy; there's a good bit of studying I need to do. The concept of species I am working with is inter-breeding groups of organisms. This then raises the question of whether that is the mark of a species. IS a species a group of organisms that is ever morphing? IF so, then how can we ever be sure where the boundaries of this group are?

But I digress.

Isolation seems to be the key to speciation; therefore I find that the theories of sympatric and parapatric speciation are inadequate. I will not go as far as saying that these types of speciation do not happen, I do find allopatric and peripatric speciation to be more viable theories.
You may be able to help me as I try to understand more. Why is isolation of a portion of a species group so potent a speciator? My limited understanding of genetics stops me from further pondering this question. I am somewhat familiar with genetic drift... Am I correct in saying that genetic drift plays a much larger role in the speciation process when speaking in terms of small groups?
thinker100
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Posted Apr 8, 2011 - 9:13 AM:

Well I have to do an argumentative term paper. 40% of the us thinks god put us here. There is still definitely a debate. Also I only brought it up for the scientific part of it.
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Posted Apr 8, 2011 - 9:59 AM:

handsworthrevolution wrote:
The concept of species I am working with is inter-breeding groups of organisms.

The “Biological Species Concept” is a good operational hypothesis when dealing with vertebrate animals. However it becomes problematic for discussions of plants, and is utterly useless in discussion of asexual organisms.

handsworthrevolution wrote:
Isolation seems to be the key to speciation; therefore I find that the theories of sympatric and parapatric speciation are inadequate. I will not go as far as saying that these types of speciation do not happen, I do find allopatric and peripatric speciation to be more viable theories. You may be able to help me as I try to understand more. Why is isolation of a portion of a species group so potent a speciator?

Isolation is what speciation is all about; the individuation (on some criterion) of one population of living organisms from another. Perhaps you just need to broaden your understanding of isolation. It doesn’t have to be geographical; it can be ecological, behavioural, or temporal as well. Consider a subpopulation of birds that breeds slightly later in the year than another. Both populations could overlap significantly in terms of geography, but may take advantage of foods that become available at different times of the year. We would say that each subpopulation develops a different “phenology”. It should not be too difficult for you to imagine this leading to cladogenetic speciation that is sympatric.

handsworthrevolution wrote:
Am I correct in saying that genetic drift plays a much larger role in the speciation process when speaking in terms of small groups?

Genetic drift is the accumulation of gene frequencies as a result of stochastic processes. The origin of this genetic variation is somewhat of an unanswered question, but the idea is that there is no discriminating of the resultant gene frequencies because, like you mention, the majority of these novel gene frequencies are neutral in terms of the phenotype. It is a statistical truth that small sample size (i.e. a small population) is less genetically stable, and more vulnerable to stochastic events than larger sample sizes. Therefore, to the extent that genetic drift plays any role in evolution, it makes sense that its effect would be more pronounced in smaller populations.
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Posted Apr 8, 2011 - 10:01 AM:

thinker100 wrote:
Well I have to do an argumentative term paper. 40% of the us thinks god put us here. There is still definitely a debate. Also I only brought it up for the scientific part of it.


If your paper is for a sociology course or a political science course or even a religious or philosophical course then okay, but it is not a suitable ecology & evolution topic.
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Posted Apr 8, 2011 - 12:53 PM:

its political science and mainly just an english argumentative paper with much information on ecology and evolution, that is why I said that.

@Handsworthrevolution.

You must understand nature is not restricted to the taxonomic ranks we have created. Species are not constrained to them and do not follow them. Secondly, a species, yes, is ever morphing (evolving) yet not always with a result of speciation. The chimpanzee is still around today. It evolved but a large population remained the same. Some 200 million plus years ago, a subspecies evolved due to evolution mechanisms into eventually the pan robustus, and eventually into the austrilopithicus, The first homo (i think). From then more evolution happened to eventually the neandrethal and the homo sapien and now us homo sapien sapien. Yet the other half portion of the population ramined until present. \

lastly sympatric and parapatric are viable. Sympatric has happened for example with a fish in the ice age. As time progressed and natural selection favored the larger fish, and the fish favored mates with the same size, they eventually split into seperate species. The spined fish and marine fish. Both different sizes and have different foraging areas yet continue to share the same habitat. Parapatric is when a population splits yet continues to be conected. Think of it spreading horizontally across a habitat. The two ends encounter different environments causing natural selection, gene flow, mutations (all mechanisms) to change the two ends. When they meet, speciation could be complete. If not and interbreeding of the two subgroups occur, hybrids form. If the hybrid is less fit, it causes reproductive isolation and speciation essentially has taken place (horse and donkey). If the hybrid is more fit and can then interbreed with its parent subgroup, hybridization is controlled and speciation will not happen. Unless the hybrid, through natural selection, favors one parent population over the other, the non chosen subgroup will diverge from possibly both. It can become complicated, but they are all viable.
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