This is my unauthorised summary of the talk that Jerry Coyne gave to the Glasgow Skeptics on Monday, November 26. It is no substitute for reading his outstanding book, Why Evolution is True, but I for one found it useful to see the arguments collated so succinctly.
Jerry discussed the distinction between the common and the scientific use of the terms “true” and “theory”. True, he suggested, means so well supported by evidence that it would be perverse to deny it. In principle all scientific truths are provisional because revisable, but IMO that is no reason for scientists to sound tentative all the time, since the same can be said of all our claims to knowledge about the world. Theories in science, as he very clearly explained, are the conceptual frameworks that we set up to make sense of facts, but in contrast to the common use of the word carry no suggestion of uncertainty. Examples are atomic theory, and the germ theory of disease, as well as the theory of evolution. He then went on to say that evolution had moved from theory to fact. This I think is a philosophical category mistake, though I am not sure that this matters to anyone except philosophers. I prefer to distinguish between the facts of evolution (e.g. the historical facts of shared ancestry) and the theory (referring to a historical context, or else meaning specifically the overarching conceptual framework).
The theory of evolution has five components:
- Evolution, i.e. change in populations, happens.
- The process is gradual, taking hundreds or thousands of years, or longer.
- Evolution leads to speciation, i.e. to the separation of an ancestral species into two descendants.
- As a corollary, distinct species share common ancestors, and indeed all life on Earth shares a common ancestry.
- Natural selection is a major driver of evolution, and is the sole process that causes adaptation and the appearance of design.
(I would note here that there is room for fruitful disagreement, since other reputable biologists come up with different definitions. I also wonder whether adaptation, like speciation, is much more easily recognised after the fact, and whether, given the existence of non-adaptive drift, the linking of adaptation to selection is tautologous.)
This theory makes a number of predictions:
- The first life forms should be simple, with complexity increasing with time in the fossil record.
- Lineages should change and split. Example, split in Rhizosolenia, an indicator species diatom whose changes can be followed in ocean core samples
- Common ancestry implies the existence of transitional forms, such as feathered dinosaurs. These need not necessarily be at the actual point of speciation, but can be identified by their possession of some of the traits that became specific to their descendants. Example, Sinornithosaurus, a flightless feathered dinosaur, intermediate between theropods and birds.
- These transitional forms should appear at the right time in the fossil record. Example, 10 major intermediate forms in the correct order over a 48 million year window between a terrestrial ancestor, identified by details of ear structure, and present-day whales. Another example; the discovery exactly where predicted, in Devonian strata, of lobefish-tetrapod intermediates [see Your Inner Fish, by Jerry’s colleague Neil Shubin]
- The course of evolution should leave bad designs in place. Examples, the male human prostate which unnecessarily surrounds the urethra and presses up against the bladder; ear wiggling muscles which only some of us can operate
- We should be able to see evolution occurring, as in Lenski’s work. We have even observed 300 cases, in a relatively short time that we have been carrying out observations. Under strong selection pressure, we can see changes within a single generation, as with the Galapagos finches faced with changes in the kinds of food available.
There are also examples of retrodictions, things that are not strictly predictions, but make sense only in the light of evolution. These include:
- The appearance in embryos of ancestral features not present in the organism itself. Examples include hindlimb buds on embryo dolphins, and body hair and tail on human embryos.
- The presence of vestigial organs, such as hindlimb and pelvis bones in the grey whale, unconnected to the rest of the skeleton.
- The presence of “dead” (identifiable but non-functioning) genes corresponding to capacities present in ancestors but now lost, including, in humans, genes involved in vitamin C synthesis and some of those involved in the development of olfactory receptors. Human embryos at four weeks include a yolk sac, and humans have dead genes for yolk production.
- Biogeography, including in particular the difference between continental islands, whose fauna relate to that of the landmass to which they were once joined, and oceanic islands, whose endemic fauna are restricted to species such as birds and insects capable of migrating across the open ocean, although other species can flourish and even become plague species when introduced by humans. This is an argument that the creationists have not even attempted to answer.
- Bad design, as evolution is constrained by its past.
One significant prediction is that we should be able, to the extent that our short observation time permits, to see evolution in action, and this is what happens.
The question of the origin of new information came up in questioning. Jerry gave as an example what happens after gene duplication; the two copies can become selected for different functions, as in the variations of haem proteins. (My own favourite example is the duplication and reduplication giving rise to Photosystems I and II, each of which contain two distinct but related subsystems.)
If evolution is a scientific theory, it must be open to falsification. (Here Jerry follows Popper, whose views I think are now regarded as too simple. Theories of merit are generally modified, or subsumed within larger theories, rather than being simply rejected in the face of counterexamples. I would argue that the theory of evolution has been modified, to include such factors as neutral drift, much as atomic theory has been modified to take account of transmutation, but that such modifications are in no way a sign of weakness.) Possible falsifiers include:
- Fossils in the wrong place.
- Adaptations within a species that benefited only some other species, or, more generally, that conveyed no advantage to their possessors or their relatives, or that could not be achieved step-by-step.
- Absence of genetic variation.
Again, I see room for fruitful disagreement, in particular about what would falsify, and what would merely require minor tinkering. A number of questions come to mind. Would group selection require rejection of the theory, as I think Jerry was close to suggesting?