In a previous post I wrote:
Some years ago there was a study done in which a population of Thermus Aquaticus bacteria were split into two. Those two populations were split up once again, and the splitting went on until 16 different populations had been created. Unsurprisingly, a genetic analysis revealed that populations that had split apart recently were genetically more similar (more closely related) to one another than to populations that had been apart split longer ago.* The scientists who studied them had even been able to predict the time when these groups had been split apart “with very small error” based solely on the genetic data. This experiment represents a miniature version of the evolution of all life; Just as the experiment begins with one population and spits populations as they change over time, Darwin had postulated the same explanation for all existing species. According to him, in the past there had been a single species of a single, common population which split apart, that later populations had split apart from the first split-offs, and so on, and throughout the split-offs life had changed genetically (just as it changed in the experiment).
* See discussion and references in Douglas Theobald, Phylogenetics Primer.
Many changes that occur in genes are neither beneficial to or harmful for survival. They are neutral. When two populations split apart, they begin to accumulate different mutations. The end result of the “descent with modification” scenario discussed above is that you get a regular and consistent pattern of similarity and dissimilarity. Talk.Origins houses a useful excerpt from Charles Darwin’s sixth edition of Origin of Species that explains:
“It is a truly wonderful fact–the wonder of which we are apt to overlook from familiarity–that all animals and all plants throughout all time and space should be related to each other in groups, subordinate to groups, in the manner which we everywhere behold–namely, varieties of the same species most closely related, species of the same genus less closely and unequally related, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single file, but seem clustered round points, and these round other points, and so on in almost endless cycles. If species had been independently created, no explanation would have been possible of this kind of classification; but it is explained through inheritance and the complex action of natural selection, entailing extinction and divergence of character, as we have seen illustrated in the diagram.
“The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during former years may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have at all times overmastered other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was young, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups.”
David Penny wrote a wonderful article in which he argued that Darwin’s tree of life metaphor has been greatly misunderstood, and I think I agree with him. Darwin probably did not mean the same thing that modern biologists mean when they use the “tree of life” metaphor. In addition, recent discoveries in biology have shown that there are some exceptions to the rule, that life does not work nearly as simply as we once thought it did. Nonetheless, it is still possible to sift through the genome and uncover the true relationships that organisms have with one another, especially between those that are not very closely related. Douglas Theobald has published papers on this very issue and has shown, by examining sixteen proteins shared by all life, that there is a consistent pattern of similarity/dissimiliarity, exactly as the theory of common ancestry predicts. His work is all the greater in virtue of the fact that he took his sequences from a wide variety of living things, and so his conclusion applies to every known species on planet earth.*
The tree-like pattern Theoblad found is a necessary consequence of common ancestry. On the other hand, if common ancestry is false, there is no known reason why such a pattern exists, and probably cannot be, because no creationist has ever come up with a non-contrived way of explaining it. So, the tree of life is 100% probable (or very nearly 100%) under the thesis of common ancestry, whereas under the alternative it is at best only 50%. The figure of fifty-percent comes from treating the two possibilities (a tree will exist or it will not) as the same. Of course, this is a generous overestimate, since there is certainly more than one way that a tree-like pattern could be totally absent. Tree-like patterns very typically do not exist in collections of man-made objects or naturally occuring things aside from life (we don’t find a tree like pattern in snow-flakes, for example),
* Unless we discover, for example, some organism that deviates wildly in its protein structure from its close relatives, which as far as I know has never been found and probably never will be