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Why Evolution is True Jerry A Coyne Professor in the Dept of Ecology and Evolution University of Chicago “On The Origin of Species” by Charles Darwin was published in 1859. It introduced the theory of evolution by natural selection. Although hotly disputed at the time, the weight of evidence found in its favour thereafter subsequently persuaded the scientific community of its validity, a mere 10 years after its publication. So why does it still provoke public controversy in contrast to any other widely-held scientific theory e.g. the theory of gravity, the germ theory of causation of some diseases. Evolution teaches that we are related to all other living creatures and are as much a product of blind and impersonal evolutionary forces as all other living things. This doesn’t sit well beside the teachings of certain religions that God created all living things and that humans are a special case, created in his image. Professor Coyne shows how the relatively new idea of “intelligent design” is just creationism re-badged. The central fight is between creationism and evolution. In the UK, a 2006 poll by the BBC asked 2000 people to describe their views on how life is formed and developed. 48% accepted evolution, 39% went for creationism or intelligent design and 13% didn’t know. The idea of the book is to document the weight of evidence in favour of evolution. The author seems to think that people don’t accept evolution because it is described as “just a theory” so he goes to great lengths to show that “theory” has a different meaning in the scientific world than it does in everyday parlance. A scientific theory is a statement of what are held to be the general laws, principles, or causes of something known or observed. It must be testable and make verifiable predictions. This is all well and good but he insists on saying that evolution is true (the title of the book and repeated in the text many times) so when he then comes out with - despite thousands of observations supporting Darwin, new data might show it to be wrong – scientists must be open to this possibility – he paves the way for non-scientists to be confused and creationists to crow. The title “evolution is true” is misleading because he is talking about a scientific “truth” rather than using the more common definition of truth as absolute. He should have defined “true” as well as “theory”. I think you can say – the theory of evolution by natural selection fits all the observations we have made so far. Nothing we have found out scientifically disproves the theory. Creationism has no such weight of evidence behind it. So what is the modern theory of evolution? Life on Earth evolved gradually beginning with one primitive species – perhaps a self-replicating molecule – that lived more than 3.5 billion years ago; it then branched out over time, throwing off many new and diverse species and the mechanism for most, if not all, of evolutionary change is natural selection. This consists of six components: evolution, gradualism, speciation, common ancestry, natural selection and nonselective mechanisms of evolutionary change. 1. Evolution – genetic changes over time due to mutations in the DNA (Darwin didn’t know about DNA but its discovery explained the mechanism 2. 3. 4. 5. 6. whereby evolution operates. Species don’t all evolve at the same rate. Gradualism – it takes many generations to produce a substantial evolutionary change. Speciation – splitting of one species into two Common ancestry Natural selection – a tinkerer rather than a master engineer Genetic drift – caused by different families having different numbers of progeny (i.e. evolution other than by natural selection) We can test these tenets of evolutionary theory in several ways – we should find evidence of evolutionary change in the fossil record evidence of one species dividing into two in the fossil record examples of species that link together the major groups thought to have common ancestry e.g. birds with reptiles, fish with amphibians genetic variation for many traits examples of imperfect design natural selection acting in the wild Chapter 2 goes through a fair amount of fossil evidence showing the evidence for evolution through this medium and how we know how old fossils are – including radioisotope dating and other techniques. Chapter 3 documents the vestigial organs and why their presence is good evidence for evolution. Vestigial organs no longer provide the function for which they evolved – for example, ostriches have wings because they evolved from birds that could – the ostrich uses its wings to balance itself and see off enemies, not to fly. Humans have an appendix (vestigial from when our ancestors ate leaves), a coccyx (vestigial tail) or goose bumps (vestigial muscles at the base of each hair which evolved to make fur stand on end for insulation if its cold or to make the animal look bigger to frighten off its enemies). Atavisms are anomalies that occur in a particular individual that look like an ancestral trait – e.g. a horse born with extra toes, a human baby with a tail. They are thought to be the reexpression of genes that were functional in ancestors but silenced by natural selection. The information is still in the genes so that when something goes awry in normal development it allows the ancestral trait to be expressed. Atavisms and vestigial traits show us that when a trait is no longer use, or becomes reduced, the genes that make it don’t instantly disappear from the genome: evolution stops their action by inactivating them, not by snipping them out of the DNA. From this we can make a prediction. We expect to find, in the genomes of many species, silenced or “dead” genes: genes that once were very useful but are no longer intact or expressed. In other words, there should be vestigial genes. In contrast, the idea that all species were created from scratch predicts that no such genes would exist, since there would be no common ancestors in which those genes were active. The function of a gene is to make a protein – a protein whose sequence of amino acids is determined by the sequence of nucleotide bases that make up the DNA. Humans have about 30,000 genes and 2000 or so are pseudogenes – genes that don’t function. These dead genes always point to a gene that was active in one of our ancestors. Evolution can encompass these facts but they don’t make any sense if you think that life was created by intelligent design. All vertebrates begin development as an embryo in the same way looking rather like an embryonic fish. Darwin realised that this was due to the common parent-form of each great class of animals. Why does the human embryo appear to go through many stages of its evolution from primitive life forms on its way to becoming a fetus? The probable answer is that development is a very conservative process. Many structures that appear later require biochemical cues from structures formed earlier so if you try to tinker with embryonic development to streamline the process, you might produce all sorts of adverse side effects. So the embryo doesn’t actually replay all the adult forms of its evolutionary history but it does go through many of the embryonic forms of its evolutionary history. Other stages of human development harking back to our ancestors – the downy hair of the fetus shed about a month before birth – the lanugo – and the grasping reflex of young babies which they share with newborn monkeys and apes but babies lose the ability after a few months. Examples of poor design – asymmetrical nature of flat fish, the recurrent laryngeal nerve, the urethra exiting through the penis in the male, the birth canal exiting through the pelvis in the female. These are all explained neatly by evolution – an intelligent designer would have had to be having a laugh or trying to fool people into believing in evolution if he designed these features! Chapter 4 The geography of life – this documents the evidence for evolution from biogeography or the distribution of species throughout the world. Small remote islands with no native species of amphibian, reptile or mammal are home to a larger collection of rare, exotic species than that found anyhere else in the world. The evidence points to these animals landing on the island thousands of years before and evolving in unusual ways because of the very different environment they found themselves in. Jerry Coyne says that he has never seen a creationist book, article or lecture that has tried to refute the biogeographic evidence for evolution. On page 97, he talks about the molecular clock for evolutionary change and says that, “.. as species diverge from their common ancestors, their DNA sequences change in roughly a straight-line fashion with time.” This is poorly worded – I had to look up Wikipedia to understand what he was saying and it seems at first sight to be at odds with the view that species may not evolve at all for thousands of years – as he said on page 4. I think I understand but he should have made this clearer. If creation was true, why would the creator produce on different continents but very similar environments, fundamentally different plants or animals that look and act very much alike e.g. cacti and euphorbs? If transplanted into the other’s environment, they both do well. Another example is (the placental) rabbits in Europe and the native Australian marsupial, the bilby, which look similar – this is a case of convergent evolution.