Evidence for evolution

Well, not being a geologist, molecular biologist, or other of that ilk ( ), I find that the evidence from biogeography is the most compelling. It's the study of the distribution of both living and extinct organisms, and of related patterns of variation over the earth in the numbers and kinds of living things. Both Darwin and Wallace independently derived the theory of evolution via natural selection from examinations of biogeography. As one example, whereas a particular habitat type might exist in multiple places around the world (similar physical conditions, availability of resources, climate, etc), species are invariably more closely related to species in adjacent, albeit different, habitats than they are to species in identical habitats elsewhere. A related but even more compelling line of evidence revolves around endemism - an endemic species is one that is found in a restricted geographic area and nowhere else — regardless of whether the specific habitat type exists elsewhere. Patterns of endemism showing in-place evolution are some of the most persuasive examples of evolution and adaptation that exist.I'll be happy to provide specific examples if you'd like.

Ummm. Sorry, Robin, you've lost me. Could you please explain your question?

Weeeell, I'd say the fact that life has a history, and has changed over time is about as solid a fact as, say, the sun rising tomorrow. Descent with modification from a common ancestor is a little less a sure bet, primarily because it's very difficult to tell whether the fossil we're looking at is an ancestor or a cousin, so that bit although pretty darn sure is maybe not a total "fact". How all that played out in detail, however, remains an arguable question.

Thanks Nosey. That's what I get for trying to be clever...Any questions, Robin?

Good on you! Gakk. Well, yeah, it's important in one sense. However, your original question to me was about the best evidence for evolution. As you say, abiogenesis is a different topic from the ToE. I mentioned biogeography as a key line of evidence in favor of evolution writ large, and also for evolution-the-theory (the how of biodiversity). So what I meant was: do you have any questions concerning what I wrote up to this point? Not that I'm necessarily the best one to quibble about being on-topic, but I would prefer to complete one discussion on this before starting another.There are several good origin of life threads in the aptly named forum Origin of Life that would be a better place to persue abiogenesis discussions.

Right. Stipulated. Wrong on all counts, but a truism. However, as my last no-doubt-vain attempt to keep this discussion somewhere in the remote vicinity of the original topic, I'll reiterate: do you have any other questions concerning my statement that biogeography is one of the key lines of evidence supporting both the fact of evolution and the theory of how it all came about?

Go for it Ned. Mine's gonna have to be one of them loooong posts. Ya know, like having to build the tools to build the tools to build the machine to produce the item? There's such a whopping lot of ground to cover just to make it plain that any shorter version is welcome.

Well, yeah. You are mostly mistaken. I'm working on a reply that should cover the basics. It's a bit more than a one-liner answer that's required, here, unfortunately. Hopefully it won't be wasted effort.

As requested:

quote:

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The foregoing statements in regard to the inhabitants of oceanic islands, - namely, the fewness of the species, with a large proportion consisting of endemic forms — the members of certain groups, but not those of other groups in the same class, having been modified — the absence of certain whole orders, as of batrachians and of terrestrial mammals, notwithstanding the presence of aerial bats, - the singular proportions of certain orders of plants — herbaceous orders having been developed into trees, &c., - seem to me to accord better with the belief in the efficiency of occasional means of transport carried on during a long course of time Charles Darwin, Origin of Species, pg 335

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As I mentioned before, biogeography is the study of the distribution of both living and extinct organisms, and of related patterns of variation over the earth in the numbers and kinds of living things. It seeks to answer questions like: why are species distributed around the globe as they are? Why are closely related species found geographically near to each other? Why are continental islands full of terrestrial endemics that are different from anywhere else - but whose nearest relatives are on the adjacent continental masses? Why are oceanic islands filled with endemic birds, insects and plant life, but few if any mammals? In addition, biogeography explains why although a particular type of habitat might occur in several widely scattered places throughout the world, species in one habitat are more closely related to nearby species in other habitats than to species in the same habitat elsewhere. Finally, it explains why species distribution is not uniform around the globe — some 25 regions contain over 70% of all primate and carnivore species diversity, for instance. (Sechrest et al, 2002)One of the most fascinating aspects of biogeography is the existence of endemics — as Darwin recognized. An endemic species is one that is found in a restricted geographic area and nowhere else — regardless of whether the specific habitat type exists elsewhere. There are two forms of endemicity: neo-endemicity where a novel species has evolved in situ, and paleo-endemicity where the endemic is isolated simply because the continental species from which it arose is extinct. For our purposes, I’ll be dealing with neoendemics — forms that have newly arisen due to their isolation.The island of St Helena in the south Atlantic presents a beautiful example of neoendemicity. The island is located some 1900 km from Africa and 2900 km from South America. It is volcanic, formed fairly recently by geological timescales, and has never been connected to any continent (it formed from a volcano off the mid-Atlantic ridge). It is completely surrounded by deep sea (about 4200 m), and hence could never have been connected by dry land to either continent — no matter how much sea level fluctuated during a putative flood. Isolated islands such as St. Helena show the action of several of the postulated mechanisms for evolution: ecological release, as colonizers move into vacant habitat; natural selection operating on initial (often minimal) population variation as a myriad of selection pressures create novel forms; and the population ecology mechanisms of dispersal and extinction. When St. Helena was discovered in the 1500’s, there were an estimated 70 species of flowering plants on the island — 60 of which (in ten genera) were endemic only to the island (Cronk, 1989). Two of these plants are extraordinary examples of evolution in action: the gumwood tree (Commidendrum robustum) is a member of the family Asterecidae — sunflowers and daisies — and the cabbage tree (Lachanodes arborea), also Asterecidae, but completely different morphology and habitat than gumwood. Both of these flowers have occupied and adapted to the tree niche on the island in the absence of any other plant in that category. Until the arrival of humans, terrestrial vertebrates were limited to four endemic bird varieties, three of which are now extinct. No mammals or reptiles existed on the island prior to the arrival of humans.Another island, Madagascar, is considered one of the great evolutionary showcases. It is a large chunk of Africa that broke off from the mainland around 120-165 mya. The island is home to a whopping number of endemics, including unique primates (family Lemuridae), 115 endemic bird species (of 250 total species) including several huge flightless varieties — one of which was larger than an ostrich (but totally unrelated), - a single mammalian predator (the fossa) and a family of small insectivores that I consider to be the poster children of evolution: the Tenrecidae.Tenrecs are a fascinating case, not only because they are terminally cute, but because they show a vast degree of niche adaptation permitted by their relatively predator and competitor-free ride on the island. They quite plainly evolved on the island itself — their nearest living relatives are the golden moles of Africa (Chrysochloridae) — not even the same taxonomic family. Although the fossil evidence is a bit sparse (not unexpected with tiny mammals — the largest can sit on the palm of your hand), it is clear from genetic testing (18s and 12s rRNA, among others, see Douady, et al 2002) that the 23 species of tenrecs (in three genera) all diverged from their common ancestor with the Chrysochloridae some 55 mya. Today, these small creatures have radiated into a myriad of otherwise unoccupied niches, each with its own unique adaptations. There are burrowing tenrecs filling the mole niche, ground-dwelling (but not burrowers) tenrecs that are good runners, but poor climbers that hunt ground-dwelling insects, a swimming tenrec that eats crayfish, tenrecs that are fair climbers but poor runners and jumpers, tenrecs that are excellent climbers but not jumpers, and a tenrec that appears to be exploring the glider niche which one researcher described as a fuzzy superball whizzing through the treetops. The only conceivable explanation for this incredible variation is ecological release as novel forms evolved to take advantage of unoccupied habitat.Another aspect of biogeography that lends its strength to evolution is the existence of so-called ring species. It starts with the observation that there is a continuum of variation from one end of a species range to the other — organisms at the extremes often differ from one another in either subtle or very marked ways. The next observation is that some discrete populations at the extremes occasionally differ enough that they can justifiably be considered distinct species. Three of the most common examples are the Ensatina spp salamander complex of California, the greenish warblers ( Phylloscopus spp) of the Tibetan Plateau, and the globe spanning herring gull complex (Larus spp) that extends completely around the world in the northern hemisphere. Where these three examples meet themselves at the end of the ring, the populations cannot interbreed — there’s enough difference that we consider them distinct species. This lends very strong support to Darwin’s argument that the nearest relatives of a species are the adjacent species. Ring species’ populations are capable of interbreeding freely (free gene flow) between adjacent populations, but the ends of the rings aren’t. The only conceivable explanation for this is allopatric speciation, considered to be a major mechanism of evolution: gene flow from one end of the species’ ranges to another is interrupted or minimized to the point that the ends of the chain are distinct under the biological species complex.Hope this helps. Again, if you have any questions, feel free to ask.References:Cronk QCB, 1989, The past and present vegetation of St Helena J Biogeo 16:47-64Douady CJ, Catzeflis F, Kao DJ, Springer MS, Stanhope MJ, 2002 Molecular Evidence for the Monophyly of Tenrecidae (Mammalia) and the Timing of the Colonization of Madagascar by Malagasy Tenrecs, Molecular Phylogenetics and Evolution
22:357—363Sechrest W, Brooks TM, da Fonseca GAB, Konstant WR, Mittermeier RA, Purvis A, Rylands AB, Gittleman JL, 2002 Hotspots and the conservation of evolutionary history, PNAS 99:2067-2071

You're welcome. I hope you'll find it useful. There are literally thousands of examples that fall within the purview of biogepgraphy which are explained by evolution: from the distribution of Anole spp in the Caribbean to the extreme toxicity (for the species) of Bothrops insularis venom on Queimada Grande to why there are no predators on Barro Colorado to why tigers live on Bali but not Lompok (which are almost identical in size, shape, habitat, climate, etc, and are only 40 km away from each other). Even better, the mechanisms and theories have been tested in real-time (the Grants on the Galapagos, Ted Case in the Gulf of California, and dozens of other field ecologists, biologists and evolutionary ecologists etc). The evidence, taken in toto, is overwhelming.

Hi Trae.I'll try and clarify. A biodiversity "hotspot" is basically a spacially delimited area that contains a statistically significant higher percentage of species overall than a non-hotspot. Although a bit arbitrary, and there's no real specificity as to size, each of these areas is identified by the fact that they are exceptionally high in endemicity - i.e., they contain species found nowhere else on Earth. Let me see if I can post a graphic (from the Sechrest 2002 article I referenced above) that shows what I mean. Heh, it worked. The top map is primate, the bottom carnivore. As you can see, there is no real size correlation. Although most of the hotspots are clustered around the tropics (as would be expected), that is also not necessarily a criteria - there are plenty of tropical areas that are NOT hotspots, including all of central Africa. The formal definition of hotspot (and there are now 34 recognized regions rather than the 25 Sechrest noted, see Conservationists Name Nine New Hotspots from Feb 2005) relates to endemic plant varieties - to be a hotspot, the region must contain at least 0.5% of the world's plant diversity (not animals - plants represent the productivity base of an ecosystem, and thus are more representative of actual biodiversity than critters would be).Hope this helps. Let me know if you have further questions.