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Author Topic:   Evolution Occurs Faster at the Equator
Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 6 of 18 (309126)
05-04-2006 5:07 PM
Reply to: Message 1 by melatonin
05-01-2006 8:19 PM


Hi Melatonin,
Interesting article. Actually, I'm not sure I agree with the authors' conclusion, however. I'm going to try and dig up the original PNAS article and see what it says. I think the conclusion is erroneous from the simple fact that the highest biodiversity hotspot on the planet is the tropical Andes - 15,000 endemic species of plants, mostly located in cloud forests above 2000 m (representing 1/6 of all plant species in Earth in less than 1% of the surface area). I guaran-damn-tee you that heat is not the operative factor here (I live less than 30 min drive from three of the Ecuadorian biggies: Pasachoa, Antisana and Cayambe-Coca). The cloud forests are perpetually cool and often mist-shrouded. Their upper ends at 3800-4000 m are actually cold.
Omni was closer to the mark, I think. The rapidity of molecular change has more to do with levels of ionizing radiation than heat. I readily agree with their observation on the rate, but disagree with their causal explanation.

This message is a reply to:
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Replies to this message:
 Message 10 by Mespo, posted 05-05-2006 3:19 PM Quetzal has replied
 Message 12 by MangyTiger, posted 05-11-2006 9:40 PM Quetzal has replied
 Message 15 by fallacycop, posted 05-16-2006 12:29 AM Quetzal has replied

  
Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 7 of 18 (309160)
05-04-2006 6:07 PM
Reply to: Message 1 by melatonin
05-01-2006 8:19 PM


quote:
We selected a total of 45 tropical woody plant taxa with latitudinal/altitudinal ranges coinciding with the warmest sectors of the total climatic ranges of their respective genera.
Here's one problem with their study. Actually, it's not a "flaw" in the sense that it represents sloppy methodology or something, but rather provides a key to understanding why they got the results they did. It was a MUCH narrower study than the livescience article would lead you to believe. They deliberately chose samples from the "warmest sectors". Which is okay if all they were trying to show was the effect of relative warmth on the evolution of congeneric plants. Which, in fact, appears to be what they were trying to do based on the full abstract:
quote:
Using an appropriately designed and replicated study of a latitudinal influence on rates of evolution, we test the prediction by K. Rohde [(1992) Oikos 65, 514-527] that the tempo of molecular evolution in the tropics is greater than at higher latitudes.
In other words, although they demonstrated a higher-rate of molecular evolution in tropical vs. temperate closely-related plant genera, nowhere in their study did they allow for "other" factors in developing their conclusion. They wanted to find a heat difference, so the samples and methods used were predicated on heat difference. Hence the conclusion that heat difference was the cause. The logical flaw here is obvious, I would think. IOW, it's an interesting study, and adds to the literature on the latitudinal stratification of biodiversity, but doesn't support the interpretation in the cited article.
For the full text, see Wright, et al 2006, The Road from Santa Rosalia: A faster tempo of evolution in tropical climates, PNAS 103:7718-7722

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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 9 of 18 (309229)
05-04-2006 8:58 PM
Reply to: Message 8 by melatonin
05-04-2006 6:14 PM


I guess it's just one hypothesis as to why these areas have greater diversity than more temperate regions. Just thought it was interesting.
As did I. I just don't agree with them.

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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 11 of 18 (309485)
05-05-2006 6:05 PM
Reply to: Message 10 by Mespo
05-05-2006 3:19 PM


Re: Altitude vs. Latitude
Not necessarily. Insolation is only one factor, albeit a big one. Moisture is another. Topography, climate, geology, etc also all play a role. As does heat (they weren't wrong, I'm quibbling with their conclusion on relative strength of heat vs other factors). In any event, to directly address your point, most conservation organizations list the Colorado Plateau, for instance as a biodiversity hotspot due to the very high level of endemism. Only on the Olympic Penninsula and in the California Floristic Province (which is a global biodiversity hotspot) is there an equivalent level of biodiversity - which is interesting in its own right. The eastern seaboard (as far as I can determine - I could be wrong) has a lower level of biodiversity overall at the same latitudes. You might find this article: DT Rutledge, CA Lepczyk, J Xie and J Liu, 2001 "Spatiotemporal Dynamics of Endangered Species Hotspots in the United States", Conservation Biology 15:475-487. Although the paper doesn't deal with your question directly (don't just read the abstract, they were looking at something else), there is a very interesting graphic on page 480 that seems to support my contention (note the hotspots in the southern Rockies).
I admit number of endangered species doesn't directly translate to high biodiversity. OTOH, endangered species are an indicator. You'd have to find a floristic study that compared Rocky Mountain and Eastern Seaboard species numbers to be sure. I'm not sure that's been done. It would be an interesting comparison in any event.
I base my contention on a few, demonstrated facts:
1. The tropical Andes region contains some 30-35,000 species of plants with over 50% endemism, making it one of if not the highest biodiversity hotspots on the planet.
2. By comparison, the Atlantic coastal rainforest of Brazil (representing sort of the southern end of the latitudes encompassing the Tropical Andes), also a global biodiversity hotspot, contains only ~20,000 species of plants in spite of being substantially hotter and wetter than the Andes.
3. An additional comparison, with the Upper Amazon Basin (specifically, the Upper Rio Negro - which puts it smack in the middle of the Tropical Andean latitudes) contains only ~15,000 plant species. This area is also hotter and wetter than the Andes.
I'm open to a different explanation, of course. However, to me the simple comparison of species numbers is an indicator that heat, per se, although possibly a factor, is not the major criteria for biodiversity - in spite of what was published in the PNAS article.
edited 'cause I kant spel
This message has been edited by Quetzal, 05-05-2006 06:07 PM

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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 13 of 18 (311360)
05-12-2006 9:04 AM
Reply to: Message 12 by MangyTiger
05-11-2006 9:40 PM


You are equating current biodiversity in two or more locations with rate of evolution in those locations. Although this may well be valid it is not guaranteed to be.
Actually, the original article (which I linked to above) is the one that made the case that rate of evolution = biodiversity. I was arguing on their terms. I agree with you that rate does not necessarily equal biodiversity. In fact, that's at the heart of one of my key arguments against the conclusions in the paper (although I admit I didn't phrase it this directly). The paper (to recap) claims that heat = higher metabolism = higher rate of evolution = higher biodiversity. I completely agree with the first three terms. It's the equivalancy they make at the end that I disagree with. My counter examples were intended to show that this isn't necessarily the case. So you're right. Not bad for someone who hasn't taken biology since the '70s.
All other things being equal - which of course they never are - you could reasonably expect the second location which has half the rate of evolutionary change but has been doing it for ten times as long to have the higher current biodiversity.
Logically you'd be correct. However, as you noted, "All other things being equal" is the real kicker. This is a case that very rarely happens in the real world. For example, much of South America is fairly "young" - the Andes are ~3-5 my old in most places. However, the Guyanan Shield is part of old Gondwana and dates to around 300 my. This one fact alone has huge implications for the flora and fauna of the two regions due to nutrient availablility, etc. Compare the tepuis of the Shield to the equivalent kind of geologic formation in the Cordillera del Condor region of southern Ecuador/northern Peru. The older formation is actually biodiversity-impoverished compared to the "newer" parts of the upper Amazon basin which drains the much younger Andes. Heterogeneity of habitat, historical climatic fluctuations, topography, etc etc etc are also implicated in relative biodiversity. Islands also generally show a higher rate of evolutionary change, as evidenced by the proportion of endemics, with an almost invariant lower degree of overall biodiversity. Ergo, in my opinion, the paper was rather poor quality (although interesting), and doesn't reflect the main factors in biodiversity.
I toyed with the idea of sending in a comment to PNAS, but they probably wouldn't accept it because I'm not affiliated with any academic institution. Journals tend to be sticky about those kinds of things. Ah, well. Take it for what it's worth, and ignore the rest, I guess. It's not all bad, by any means.

This message is a reply to:
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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 16 of 18 (312371)
05-16-2006 9:35 AM
Reply to: Message 15 by fallacycop
05-16-2006 12:29 AM


The problem with the article, and indeed this discussion, is that the factors influencing biodiversity are extremely complex. As scoff noted, the geology of the region in question has a lot to do with biodiversity, as does the availability of niches (heterogenous vs. homogenous habitat, for instance). Observations have shown that niches seem to be more finely divided in the tropics than elsewhere, but the causal explanation is still wanting.
In the case you mention (the effect of genetic drift on small populations) rate of evolution may increase, but rate doesn't necessarily increase biodiversity - which is the heart of my quibble with the article. Take the case of the Galapagos as a for-instance, which scoff mentioned. It is actually a biodiversity-impoverished area overall. However, it has a very high level of endemism and both speciation and incipient speciation, indicative of a high rate of evolution. The islands are only about ~10 my old. Part of the explanation is geologic, some is topologic and climatic, some is ecological (dispersal ability, colonization sampling error, etc).
Mutation, of course, is the primary means of increasing inheritable variability in a population over time. It therefore stands to reason that whenever you see a high rate of endemism and/or speciation, the rate of mutation is also high. The cause of the mutation rate can be argued, as can the relative contribution of drift.
I think another issue we may be having trouble with is the use of the term "biodiversity". How you measure this is problematic, with a number of differing definitions leading to variant results. For example, in my work we usually use "total number of plant species" as the yardstick. This is consistent with the IUCN's definition, and is how global biodiversity hotspots are determined. Another widely used (and useful) definition is the rate of endemism. As you can plainly see, if we use the endemism scale the Galapagos show very high biodiversity, whereas if we use the species number definition, Galapagos is impoverished. I suppose as long as everybody knows which measure is being used, there isn't a problem.

This message is a reply to:
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Quetzal
Member (Idle past 5872 days)
Posts: 3228
Joined: 01-09-2002


Message 18 of 18 (312502)
05-16-2006 3:31 PM
Reply to: Message 17 by DrFrost
05-16-2006 2:08 PM


Re: biodiversity
It causes me to wonder if there could be another explanation for the biodiversity in these cloud top forests other than radiation.
I think that's a given. The article made a simplistic equivalency that falls apart when you look at the details (not the quality work I usually see in PNAS). Higher heat = higher metabolism = higher mutation rate = higher evolution rate = higher biodiversity. I don't think that is justified by the evidence. They used what appears to be a very biased sample that showed the effect they were looking for. What a surprise.

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