It is the 146th anniversary of the initial publication of On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, by Charles Robert Darwin. The text is freely available here and it is, frankly, beautiful. The book is extremely accessible for a Victorian and a scientific work; the brilliance of Darwin evident in the manner by which he attacked and solved considerably difficult problems for empirical research.
Darwin was by no means the first to suggest that organisms changed over time, or even that all organisms on Earth descend from a common ancestor. But he did reject the metaphysics many scientists of the time still appealed to, and combined the different evolutionary hypotheses; he himself collecting a horrendous amount of data which in turn knocked down or shored up these different hypotheses. He developed a coherent mechanism by which evolution functioned; natural selection. In short, The Origin of Species turned descent with modification into a science.
There is a lot that he wrote that was wrong, as we must say about the work of every scientist after a century. He still supported the notion of Lamarckian inheritance in some instances; the notion that variability in the use and disuse of appendages would cause organs to be more fully developed, weakened, or lost in offspring. For example a Lamarckist could suggest that a giraffe's neck is particularly long because its ancestors had to stretch their necks to reach certain plants, and that subsequently each animal's neck became longer as each offspring inherited the longer neck of its ancestor.
He relied particularly heavily on Lamarckism when considering vestigial organs, such as the receded or useless eyes of cave fish, which Darwin could not fathom as being a burden to the fish, (although he accepted that they are useless in such an environment.) We now know that eyes are an extremely energy taxing organ, and since eyes serve no practical purpose in the dark of a cave, they are indeed selected against in such an environment. Lamarckism fell by the wayside, replaced by Darwinism after Darwin's death.
Darwin had no theory of genetics; he simply didn't know about the existence of genes. However, his evolutionary theory predicted and needed a mode of inheritance, that is to say a method by which parents passed on their variance to their offspring. Darwin constructed a hypothesis called pangenesis which served to explain the different traits seen in species, but which ultimately was replaced with genetics, which more parsimoniously explained the evidence. Pangenesis essentially held that every cell in an organism's body contributed a small part of itself (termed gemmules) to the creature's descendant. Meanwhile, Gregor Mendel was conducting his famous experiments with pea heredity which would have saved biologists decades had his results become widely known. He sent a letter about his work to Darwin which, unfortunately, was never opened.
It was not until the 1880s that DNA was isolated; a nucleic acid with a biological function that scientists could only guess at. Two decades later Mendel's work, and its importance, were rediscovered. Over the next forty years great strides were made in biology, evolution and genetics; chromosomes became recognised as the mode by which inheritance occurred, though how they conveyed information was still unknown. It became clear from fruit fly experiments that mutation and recombination were the primary sources of variability within a population. This period marked a reconciliation between biology, genetics and evolution, which became known as the neoDarwinian, or modern evolutionary synthesis. DNA's structure was not unraveled until Watson and Crick's Science paper in 1957, beginning our understanding about how chromosomes convey the information that evolution required to function.
The central ideas of evolution have only accumulated evidence supporting their validity. We know that there is variation within a given population. These variations are passed on from parents to offspring. The environment can only support a limited number of organisms, and so there is competition. This competition results in differential rates of survivability due to variance; aka natural selection, and differential rates of reproduction, as for example in sexual selection. Geology, astrophysics and palaeontology have supported an old age of the Earth (approximately 4.6 billion years,) and the organisms which have lived upon it (about 3.85 billion years.)
The fossil record supports common descent; the idea that all organisms we know of living on Earth are descended from a small population, or a single ancestor. It demonstrates with intermittent elegance the profound similarity of form; the homogeneity of organs and appendages in organisms, with transitional forms serving to blast apart the notion of Richard Owens' archetypes, as well as the notion that any organism is more perfect or more evolved than any other. These are the facts, the data of evolution.
Evolutionary biology is an ongoing science, one constantly adapting as new evidence is uncovered. The mechanisms by which evolution occurs, and the relative importance of each, are a constant source of debate within the scientific community. A 'theory', in scientific jargon, is neither guess nor speculation, but a powerfully evidenced explanation for a wide body of data. It is the most successful a hypothesis can get: the best explanation humanity can provide from our knowledge at a given time. The mechanisms by which evolution occurs are the theoretical explanation for the facts of evolution.
Darwin is not God, and he made mistakes, sometimes considerable mistakes, though they were quite understandable given the level of knowledge in his time. But this does nothing to diminish his importance and intelligence. Medicine, agriculture, and vaccination have been revolutionised or could only develop in the first place because of evolutionary theory, and their successes at saving millions of lives are amongst the strongest evidence for its predictive power, usefulness, and validity.
-The Rev. Schmitt.