Speciation in Evolution

[The Shrike]

I'm not sure how this CAN be taken too far. What I mean is, in Decapoda, there are certain traits that differentiate between the various suborders and families.

Emphasis mine. I'm totally with you, but for the fact that Hedges' analysis addressed species.

Jumbling things all together rather indicates that one has not properly acknowledged the limitations of each concept.

Hedges is claiming "the" rate at which new species arise. Were I doing this research, I would have described first what one species concept I was applying and by what criteria I would identify a new species. I would make sure I applied the same criteria in each case.

I don't know enough about the species in question to know if the example is valid, but I suspect that there are more differences than merely the beak length. I may be wrong, but in most of the cases I've seen that's been the case.

Beak length, depth, width, gonydeal angle - all are important in morphological distinctions among birds, but there's also a great deal of overlap in these characters among closely related species.

 

[Darwin123]

Okay, so how do perceptible mutations NOT accumulate?

You are right. I stated a hypothesis that was far to strong to be valid. Perceptible mutations can accumulate, so that they can contribute to evolution.

am even willing to grant, ad arguendum, that it is the most common mode for evolution. But we cannot say it is the only, or the necessary mode of evolution--because experiments show otherwise, and the mechanism demands otherwise.

I agree with this weaker hypothesis that you present. The most common mode of evolution is the accumulation of small mutations.

I was addressing those replies which suggested that the 'punctuated equilibrium' was caused by saltations. This is not what the article being discussed had said.

One poster even suggested, based on the article, that the giant tortoise on each island could have been caused by a single de nova mutation. It may have happened that way on one islands. However, this is really improbable for all the islands. Suppose a tortoise was born on the island, ten times as large as the other turtles on the island, through a de nova mutation causing an allele that was dominant for gigantism. I propose that he would have a difficult time finding compatible mates.

It depends on how one defines 'barely perceptible'. However, the effect on phenotype of most de nova mutations de nova than the variability of phenotype already there in the population. In fact, most mutations produce recessive phenotypic effects. So the immediate phenotypic effect of most de nova mutation is really imperceptible.


There is no evidence that the variation in the de Vries primroses were de nova mutations. The fact that many of these mutations were found close together suggests that they may have been related through a common ancestor with a de nova mutation.

There may have been more than one de nova mutation in the garden from which these new primroses 'escaped'. We know that the radical expression for these genes occurred suddenly. However, de Vries don't know how many mutations accumulated to make an obvious difference.

The mutations in de Vries study will probably not form a new 'species' of primrose. Many scientists now believe that those mutations in de Vries study where caused by polyploidy. If so, the descendants will have problems for a few generations finding compatible gametes. There may be a reproduction barrier with 'monoploid' primroses. After they propagate a few generations, finding compatible gametes may not be such a problem. At that point, they may have a problem finding compatible pollinators. Pollinators that have adapted to the original type of primrose may not be attracted to these new primroses. Their success at that point may depend on what mutations occur in the pollinators.

 

[Dinwar]

A fair point on the de Vrise experiments. I know that some of the mutations, at least, necessarily were perpetuated--after all, that's what caught his eye--but given natural populations and the way gene transfer works, it'd be pretty easy to swamp those out.

http://sysbio.oxfordjournals.org/content/57/1/15.full.pdf+html

Here's another example where specific genes are affected, resulting in some substantial morphological changes. I'd argue that many of these do not fall under the heading "small mutations", as "small" means, by your definition, barely perceptable. The transition was stepwise--not saltation, but certainly not the gradual accumulation of imperceptible alterations, either.

I'm including this to illustrate what I mean by discrete mutations having real evolutionary affects. I'm not talking hopeful monsters here. I think you and I are saying very similar things, just in different ways, and want to be as clear as possible so that we can determine where, if anywhere, we disagree.

I agree with this weaker hypothesis that you present. The most common mode of evolution is the accumulation of small mutations.

There are two issues with this. First, mutations either happen or they don't--a nucleotide can either be A, T, C, or G. So really, all mutations are discrete, not gradational.

The second issue (and to be clear, I only included the first to illustrate what I meant by "the mechanism demands otherwise") is that any mutation, in order to be selected for or against, must have some measurable affect on the organism. If it doesn't, there's no way for it to affect fitness. It can be within the normal range of the population--and in many cases will necessarily be (many traits are combinations of genetics and environment; take human height for example)--but it has to have some affect, and that affect must be mesurable. Otherwise, we're looking at things like third-nucleotide variability.

Emphasis mine. I'm totally with you, but for the fact that Hedges' analysis addressed species.

From a phylogenetic standpoint, it doesn't matter what taxonomic rank is involved, the principles stay the same. Variability in species is variability within the limits set by the higher taxonomic orders that species is a member of, and those limitations are due to the fact that higher-order taxonomic rankings merely reflect ancestery. From a strictly taxonomic standpoint this may not be true--taxonomy was developed long before evolutionary theory--but from a phylogentic standpoint the only real difference between two species and two phyla is that the two species branched off later.

Hedges is claiming "the" rate at which new species arise. Were I doing this research, I would have described first what one species concept I was applying and by what criteria I would identify a new species. I would make sure I applied the same criteria in each case.

Why? As long as you're careful, switching concepts doesn't make much difference. And again, it's standard practice in situations where you're dealing with extant and extinct organisms. It introduces some heafty issues concerning how to translate one concept into the other, but such issues are not insurmountable. And to be blunt, if this analysis is worth the paper it's printed on we'll have to address these issues sometime--there are very, VERY few clades that extend to 2 ma and for which all organisms involved are extant, which means that Hedges' analysis requires both extant and extinct organisms. May as well face this issue head-on, since it will make or break the concept in terms of utility.

Beak length, depth, width, gonydeal angle - all are important in morphological distinctions among birds, but there's also a great deal of overlap in these characters among closely related species.

I recommend exploring PAST and PAUP, two multivariant statistical packages that are commonly used in paleontology. They are geared specifically for this type of analysis. I believe PAUP you need to pay for, but PAST is free.

 

[The Shrike]

Why?

I don't know; it just feels wrong I guess. That's not a knock on paleontology, it's just a concern about this analysis.

there are very, VERY few clades that extend to 2 ma and for which all organisms involved are extant, which means that Hedges' analysis requires both extant and extinct organisms.

Wait - maybe that's my hurdle, and a pox upon me for having not read the original paper yet. I had been working under the assumption that Hedges' team used all sorts of taxa in their analysis, including extant and extinct lineages. Those with extant species to which biological and/or genetic species concepts have been applied could yield different estimates on the rate of apparent speciation than those with no extant representatives for which the morphospecies concept is generally the only game in town and the source material so often fragmentary. By then averaging the rate across speciation events they arrived at the 2 million year estimate, but there might be more interesting patterns within that result.

Feel free to ignore my ramblings until I've read the paper . . .

 

[Dinwar]

I don't know; it just feels wrong I guess. That's not a knock on paleontology, it's just a concern about this analysis.

I understand the sentiment. I just don't think there's any way around it, not that allows for sufficent temporal depth.

Those with extant species to which biological and/or genetic species concepts have been applied could yield different estimates on the rate of apparent speciation than those with no extant representatives for which the morphospecies concept is generally the only game in town and the source material so often fragmentary.

It's not just that--we've been in the middle of a mass extinction event for hte past 12k years, and an ice age for a few million. It's pretty clear (Zachos et al., 2001, Figure 2 shows it pretty well) that ice ages are pretty unstable in regards to climate, compared to the generally more sedate climate shifts of warmer periods. That could play a role in things as well. When the species is undergoing significant climactic shifts every few million years, it's bound to have an affect on speciation rates.

This would be exacerbated in edge environments. That's why the Mediterranian Sea is so popular with paleoclimatologists: whether it's there or not depends on climate, and it's VERY sensitive because it's just on the edge between "rainfall replenishes everything that evaporates" and "evaporation is making it disapear". If they used organisms in an environment that's particularly sensitive to climactic changes, that could be a factor as well.

So I'm in the same boat as you...Probably need to read the paper.