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why is evolution true? (if it is): part two, the problem of macroevolution

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Fig-8-9-Chimp-Skeletons

So, in Darwin’s day, there was a clear problem. Fossilised bones turning up everywhere, sometimes of gigantic creatures unlike anything on earth, sometimes of creatures very like those then living but not quite the same – in any case all indicating change, change, change. And there were many other oddities, some of them observed by Darwin himself on his Beagle voyage. Marine fossils embedded in landscapes way above sea level. Darwin had a great interest in geology, courtesy of Charles Lyell, whose landmark work, Principles of Geology, he carried with him on his great voyage. He was very interested in Lyell’s view, derived from Hutton, that landscapes changed slowly, with mountains rising from the sea, over periods of time much greater than the biblical account. So imagine his mind, full of Lyell’s speculations, when on March 4 1835 he was exploring the cliffs above Talcuhano Harbour, near Concepcion in Chile, shortly after the devastating earthquake, and found maases of seashells embedded in the rock. The Andes had risen from the sea, surely! Yet he might well have been in two minds – slow change, yes, perhaps, but the earthquake had also changed the physical landscape in an instant, bringing rocks dripping and oozing with marine life up several feet above the sea surface…

Meanwhile, dinosaurs. Of course the bones of these critters have been unearthed for millenia, but it was only in the early nineteenth century that they were treated scientifically. It was Richard Owen, later to become Darwin’s bête noir, who coined the term in 1842 (it’s from the Greek, roughly meaning ‘terrible lizard’ though dinos weren’t lizards, and they weren’t all terrible, or terribly large). These huge beasts (dinos come in all sizes, but large bones are more easily preserved than small ones, giving a false picture, and of course bigness grabs the public imagination) had clearly disappeared, but when? Why? How long ago? It all made the question of the earth’s actual age and history rather more urgent.

Darwin, back in England after a richly stimulating voyage in which he’d collected and ruminated over a vast number of exotic species, was exercised by a number of problems. Why did whole species disappear? Surely this had some connection with changes of landscape and habitat? He’d been making observations with regard to predators and prey, how species depended on other species, how individuals competed for mates. It seems that, unlike Wallace who came upon the insight of natural selection more or less in one fell swoop years later, Darwin was piecing things together painfully slowly, with hesitation, scepticism and uncertainty, but also with a dogged accumulation of evidence, so that when, finally, impelled by the famous letter from Wallace in the late 1850s to express his views, he was able to do so fulsomely, in spite of a lack of writerly ability. And the rest, as they say, is history.

The theory of natural selection is the most spectacularly successful and productive theory in biology, and is in fact its foundation stone. It has been reinforced by all that has been discovered since, especially in genetics and microbiology, fields that didn’t exist in Darwin’s time. The basis of the theory is quite simple, though it has been much misrepresented. Creatures reproduce, and generally the offspring are pretty well identical to the parents, but sometimes mutations occur. The offspring is in some way different. Usually the difference is ‘negative’, disadvantaging the offspring. The offspring is thus unable to reproduce and its line dies out. Sometimes the difference is ‘neutral’ and the line continues to reproduce, until or unless natural (environmental) conditions change and that line becomes either positive or negative within the context of those conditions. In other words it thrives compared to others or it dies out. Sometimes the difference is immediately positive, and this line outcompetes the others. The variation is random, but the natural environment ‘selects’ the best fit – the birds with the best beak for pecking out food; the worms with the best chemistry for thriving in a particular soil; in more recent times, the bacteria that can best resist the antibiotics we throw at them.

So the theory of natural selection describes incremental, gradual change. Its effect upon species is more difficult to explain, and it’s with this that creationists like to play, raising lots of dust and fog with respect to the species concept.

So what exactly is a species? The first more or less universally accepted classification of living things into groups was that of Linnaeus in his Systema naturae of 1735. It was a thoroughgoing system, from kingdom at the top, ranging down through phylum, class, order, family, genus and species. It’s still used today, of course, with various additions intercalated with these layers, but in the 20th century a new taxonomic system called cladistics, based on a more scientific understanding of descent from common ancestry, and so incorporating the new science of genetics, has won increasing favour.

One of the main reasons for this new development is that the term ‘species’ has historically been  frustratingly vague. Originally it was based on morphological characteristics – in other words, visible similarities. Nowadays, though, with the emergence of population genetics and genomics, we can be more rigorous about species and speciation. Basically, a species becomes separated from another when it no longer breeds with that other. More often than not, this is due to geographic separation. Early on in the separation interbreeding is still possible, but over time, with continued lack of opportunity, the two groups become increasingly distinct and unlike (and one or both groups may go extinct). This branching has of course occurred oodles of times, creating an evolutionary bush, each twig of which can be traced back to the original stem.

So far, so clear, I hope. So where do the creationist terms micro-evolution and macro-evolution come in? Well, off the top of my head, I think that, since creationists really really dislike the theory of natural selection as presented by Darwin, they have to account for obvious changes somehow without abandoning divine creation, especially of humans, as soul-blessed, dominion-holding, image-of-god types. So, they distinguish micro-evolution, changes within species (e.g. different breeds of dogs) from macro-evolution, transformations from one species to another, which they claim doesn’t exist. Presumably they think that every species was specially created by their god, though why he should have created so many and rendered the vast majority of them extinct before humans even came on the scene is a mystery. This points up a major problem for those who believe in directed evolution as well as creationism.

Okay, to be clear, micro-evolution and macro-evolution aren’t terms invented by creationists, though they’ve taken to them like babies to their mothers’ milk. The terms were first used by evolutionary biologists early in the 20th century to characterise not different processes but different scales of evolution. Micro-evolution plus time (in which minute changes accumulate) equals macro-evolution. Creationists, then, are reduced to claiming that, because we don’t ‘see’ speciation, it doesn’t exist. Presumably they can say the same for the big bang and black holes, but we can detect such objects and events through increasingly precise instrumentation, and we can pretty well map the relations between species, and the branchings-off, by examining genomes. They tell us, for example, that we share an ancestor with our closest living relatives, the chimps and bonobos, dating back between 5 and 7 million years ago. We are equally related to these two species because they branched off from each other later, between one and a half and two million years ago. Richard Dawkins, in his monumental work The Ancestors’ Tale, attempted to trace these nodes of connections between the ancestors of humans and other species, back to the first life forms. There are gaps in our knowledge of course, but they’re being filled in on an almost daily basis.

As Dawkins points out in another of his books, River out of Eden, the DNA ‘revolution’ that got underway as a result of Watson and Crick’s unravelling of the molecular structure of the gene, is a digital revolution. The genetic code is quaternary, with four nucleotide elements – adenine, thymine, cytosine and guanine which can be combined in specific ways. Therefore the difference in the coding for different proteins, leading on the large scale to all the variation we see, can be worked out mathematically. This allows us to define more precisely our cousinship to other species – which are the more distant cousins, horses or pigs? Or, how closely connected are bees and butterflies? We can illustrate these relations using cladograms:

663px-Cladogram-example1

 

The technology we now have at our disposal allows us to map whole genomes increasingly cheaply and efficiently, and so we’re finding some surprising relationships. For example, recent DNA analysis has revealed that falcons, previously thought for fairly obvious reasons to be closely related to other birds of prey such as eagles, are in fact more closely related to parrots, songbirds and passerines such as the humble sparrow – a significant shift in taxonomic placement.

The obvious connections between species, and the fact that we can draw the evolutionary bush with increasing confidence, makes a mockery of creationist claims against natural selection, which not only explains speciation but also extinction. We may not know exactly why the neanderthals, or the trilobites, or the Australian megafauna died out, but natural selection points us in the right direction for answers – climate change, food scarcity and the introduction of new predators into the environment being the obvious candidates. The creationist, on the other hand needs to answer the question – why would their god keep creating these species, endlessly, only to have them snuffed out? No answers about the opacity of their god’s intentions are acceptable. And of course that’s far from being the only question they can’t answer.

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Written by stewart henderson

June 13, 2015 at 5:14 pm

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