Today is Darwin Day, marking the birthday of Charles Darwin. As the world looks back on the achievements of the great man, you are likely to see many “icons of evolution” triumphantly displayed. These famous, yet often flawed, success stories of Darwinian theory are held up as reasons to believe that the neo-Darwinian synthesis and everything it entails — scientifically and philosophically — has vanquished all legitimate challenges. But that is not so.

One of the most famous such icons is a small group of birds, an inspiration for Darwin’s On the Origin of Species, that populates a remote cluster of islands in the equatorial Pacific. The Galápagos finches, with their different beak sizes, are brandished as one of the clearest examples of evolution at work.

However, that is true up to only a very limited extent. These birds are, indeed, a clear example of micro-evolution. They are closely related to each other and their beaks have obviously been adapted through natural selection to the different food sources on the various islands. However, the finches also show what is required in order to expand the mechanism of natural selection to the larger or macro scale.

The Galápagos finches put on display the two strict requirements that must be present in order for natural selection to work its magic. If these two factors are not present, natural selection is impotent to change any creature at all, much less create a new species.

First, the finches’ beaks are clearly adaptive. Each distinct variation gives the lucky individual a definitive leg-up in its specific environment. There is an obvious, practical reason why the differentiation is helpful to the species in question. This is absolutely essential in order for natural selection to pick between variations in species. Natural selection can only “see” those variations that are adaptive — causing one individual to live, and carry on its genes, and another to die and not leave offspring. If a variation is neutral or does not somehow increase fitness in the specific environment the creature lives in, Darwin’s mechanism cannot select it.

Second, there is a functional continuum among the finches’ beaks. That is, between a finch with a tiny beak and a finch with a large beak, there are tiny, step-by-step changes, and each change makes the creature slightly more fit in its environment. This is also essential for natural selection to work.

The problem for Darwinian theory comes in explaining evolutionary change where, unlike the case of Darwin’s finches, these requirements are absent. First, there may not be a continuum. That is, natural selection cannot make large jumps or drastic changes. There must be small steps. Secondly, each single step must be beneficial to the individual. It is not enough for the first and last versions of the adaptation to be helpful — all the intervening steps must increase fitness as well.

There are examples of creatures throughout the biological world that break one or both of these rules. Many creatures just don’t fit the natural selection story like the Galápagos finches do.

For example, what is the adaptive significance of the many examples of geometric or abstract forms we see in the world, such as the shapes of leaves or the concentric whorls of flowers? Or consider the case of the enucleated red blood cell in mammals, which was the subject of my postdoctoral work. Not only have we found no obvious reason that such features increase fitness, there is no plausible continuum leading from a blood cell that keeps its nucleus to one that ejects it.

There are no such intermediate forms in nature, and it is impossible to plausibly imagine intermediates that are even stable, much less adaptive. I document many more examples in my new book, Evolution: Still a Theory in Crisis.

Without workable explanations for these many anomalies, Darwinian evolution may just go the way of Newtonian physics — applicable to a small area where specific rules apply, but unable to make universal statements about the world in general.

So when you see the media promoting the Galápagos finches as evidence for Darwinian evolution this Darwin Day, take it with a grain of salt. Not every species in the world is as obliging to the requirements of Darwinism as the famous finches. And this is just the beginning of life’s richness and complexity that cannot be reduced to Darwinian biology.