In my back garden in Oxfordshire, England, I have a rather dilapidated horseshoe crab shell, a souvenir of a trip to Cape Cod a few years ago which we brought back to show our grandchildren. (It was very dead and there were literally thousands of them along the high-tide line on the bay side just south of Wellfleet, I hasten to assure any wildlife conservationists!) The long spike 'tail' is a particularly good for clearing out the feeding holes in bird feeders when they get clogged.I sometime wonder what future palaeontologists would make of it if it had the great good fortune to become fossilised and then to be discovered again in a couple of million years. Horseshoe crabs in central England!
And that leads me to the main topic here - so-called 'living fossils', of which the horseshoe crabs (plural because there are four different species) are frequently cited as an example.
The rather annoying oxymoronic term 'living fossil' need not apply (and rarely does) to a species:
A living fossil is a living species (or clade) of organism which appears to be the same as a species otherwise only known from fossils and which has no close living relatives. These species have all survived major extinction events, and generally retain low taxonomic diversities. A species which successfully radiates (forming many new species after a possible genetic bottleneck) has become too successful to be considered a "living fossil".Creationists love these, and especially love the term 'living fossil' without bothering too much about what it means. What they think it means is that some species show no signs of having evolved, which, in their desperate search for supporting evidence for their daft notion, is taken as proof that evolution doesn't happen. Never mind all the other millions of species; the 99.9% of the evidence. Lets go with the, perhaps 0.1% of things which seem to support us and, in the best Creationist tradition, ignore all the rest.Source: Wikipedia - Living fossil
But is this comparatively tiny number of 'living fossils' actually the evidence that Creationists crave?
Of course not.
In fact, as I mentioned earlier, there is not one horseshoe crab but four different species, all of which are known to have evolved from a common ancestor in the last few million years, just as humans and the other Great Apes have, just as horses, donkeys and zebras have and just as all the different birds have. (See B.Y. Kamaruzzaman, B. Akbar John, K. Zaleha and K.C.A. Jalal, 2011. Molecular Phylogeny of Horseshoe Crab. Asian Journal of Biotechnology, 3: 302-309. DOI: 10.3923/ajbkr.2011.302.309).
The mistake Creationists make is in assuming that evolution is all about morphology. Just because a living species bears a close morphological similarity to its fossil ancestors does not mean they haven't evolved. Evolution occurs in the genes and might well express in the details of proteins. It need have nothing to do with external appearances. What genetic sequencing is now allowing scientists to do is to look at the similarities and differences between living species and so map their relationship to one another and make a good estimate of the rates of diversification and how long ago they diversified. With the horseshoe crab we find evolution has been progressing exactly as we would expect it to.
Another oft-quoted example of a living fossil is the coelacanth, a lobe-finned fish which is similar to the common ancestor of the earliest land vertebrates and so is more closely related to amphibians, reptiles and mammals than to the ray-finned fish or the cartilaginous fish (sharks, skates and rays). One source of confusion here in in the ill-defined term 'fish' which is generally used for all non-mammalian, non-reptilian marine vertebrates.Coelacanth... is a rare order of fish that includes two extant species in the genus Latimeria: West Indian Ocean coelacanth (Latimeria chalumnae) and the Indonesian coelacanth (Latimeria menadoensis). They follow the oldest known living lineage of Sarcopterygii (lobe-finned fish and tetrapods), which means they are more closely related to lungfish, reptiles and mammals than to the common ray-finned fishes. They are found along the coastlines of the Indian Ocean and Indonesia. Since there are only two species of coelacanth and both are threatened, it is the most endangered order of animals in the world. The West Indian Ocean coelacanth is a critically endangered species.Things to note here are:
Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish that are related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms, rhizodonts, and Panderichthys. Coelacanths were thought to have gone extinct in the Late Cretaceous, but were rediscovered in 1938 off the coast of South Africa. Traditionally, the coelacanth was considered a “living fossil” due to its apparent lack of significant evolution over the past millions of years; and the coelacanth was thought to have evolved into roughly its current form approximately 400 million years ago. However, several recent studies have shown that coelacanth body shapes are much more diverse than is generally said. In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased on the a priori hypothesis that these species are ‘living fossils’.Source: Wikipedia - Coelacanth
- It was the order which was believed to have gone extinct, not a particular species. This was based solely on the circumstantial 'evidence' of a lack of discovered fossils which post-dated the Late Cretaceous era and a lack of authentically recorded living specimens known to modern science.
- The claimed evidence of morphological conservatism (i.e., lack of evolutionary morphological change) is false and was based on circular reasoning.
- Colacanths are much more diversified than was previously thought.
- Any claimed lack of evolution was, as with the horseshoe crabs, based purely on morphology, not on genetic evidence.
A third example, and one which illustrates another misunderstanding, either deliberate or made through ignorance is the trapdoor spider (Bothriocyrtum) which, because it has retained some ancestral characteristics which show its relatedness to the scorpions is spoken of as a living fossil.There is of course no requirement for an evolving species to lose all it's the features which the ancestor it shares in common with other related orders had. There is no sense in which trapdoor spiders are less evolved and other spiders are more evolved. When scientists speak of primitive features they mean features that were present in common ancestors, not features which are less adaptive. After all, we have far more similarities with our earlier proto-human and African Ape ancestors than we have differences. A decent surgeon could perform the same surgery on them as he or she can on us. A ENT specialist, an ophthalmologist or a decent proctologist could probably equally well treat an Australopithecine as a Homo sapiens. The point is that retention of earlier features is not a sign of a lack of evolution per se.
My last example is the maidenhair tree or Ginko biloba which is the sole surviving member of an entire class of plants which evolved before flowering plants. In fact, the fossil record shows not just one but at least eleven different species. Most of these seem to have gone extinct along with many ferns, horsetails and tree-ferns when flowering plants ousted them from their niches, leaving only G. biloba which seems to have occupied a highly specialised niche, growing in disturbed soil adjacent to streams. G. biloba is very long-lived and resilient. Some specimens are over 2,500 years old and two are known to have survived the atomic bombing of Hiroshima. They have a very slow reproductive rate.In addition, from having once been widespread they were extinct in the wild apart from two small area of China. In one of these sites the narrow genetic diversity suggests the population may have come from a small introduced population which may have been introduced and preserved by Buddhist monks over 1000 years ago. The other site has more genetic diversity. Some specimens may be older than the surrounding habitation and may be derived from specimens which survived the last ice age in sheltered valley refuges.
This longevity, resilience and slow reproduction, coupled with a slowly contracting distribution are thought to have contributed to their morphological stability. Additionally, they seem to be fortunate survivors of a nearly extinct species having survived a bottleneck of only a few specimens, hence the narrow genetic range of the existing population. This tells us nothing about any evolved genetic diversity which preceded this near extinction.
So, again there is no sense in which G. biloba can be regarded as having failed to evolve, only that it succeeded in surviving an extinction of this entire class of plants - just - and all surviving specimens, which are now only surviving because of human protection, are derived from a handful of fortunate survivors, otherwise the Ginkgophyta would be just another extinct class of plants which evolved long ago, had their day and then were replace by more successful plants in the struggle for resources.
It is, of course, nonsense to talk of any living species as being less evolved or in any sense less well adapted than any other living species. All living species have been evolving by the same evolutionary process for exactly the same length of time.
So, when you hear a Creationist telling you that 'living fossils' are proof that evolution doesn't work you know that they either:
- Don't understand Evolution and are lying to you about their expertise.
- Do understand Evolution and are lying to you about 'living fossils'.
| Twitter Tweet | StumbleUpon | Reddit |
0 comments:
Post a Comment