The theory fails to exclude one contingency, thus it cannot be falsified
Evolutionary theory does not rule out a single possible case. We see different creatures behaving in different ways. When asked why this is so, or why it is the way it is, evolutionary biologists have nothing to say but "different organisms try different things." This is a very poor answer.
The Darwinian picture of the history of biological life is far too simple. It does not, and simply cannot, account for all the uniqueness, peculiarity, and idiosyncrasy that one finds in every environment within the biosphere. Consider, for example, the male redback spider. This spider is frequently eaten in the midst of copulation. Biologists used to think that the female redback spider was just predacious and would overwhelm her male partner, but now they know that the male willfully offers himself up as food for his female partner. This bizarre act is unique to the male redback spider; no other spider of the Latrodectus species does this. This one peculiar example, like far too many others, requires a thorough explanation. But evolutionary theory does not provide particular principles that outline when sexual suicide should occur within the Latrodectus species and when it should not. And what would such principles in biology even look like? The problem becomes clearer once we consider the distinctive features and habits of a larger variety of organisms. David Berlinski asks a number of sincere questions: "Why is the Pitcher plant carnivorous, but not the thorn bush, and why does the Pacific salmon require fresh water to spawn, but not the Chilean sea bass? Why has the British thrush learned to hammer snails upon rocks, but not the British blackbird, which often starves to death in the midst of plenty? Why did the firefly discover bioluminescence, but not the wasp or the warrior ant; why do the bees do their dance, but not the spider or the flies; and why are women, but not cats, born without the sleek tails that would make them even more alluring than they already are?" When these kinds of questions were posed to Nobel Laureate George Wald, he could only answer by saying "various organisms try various things." The fact that various organisms try various things is plain to see. Such an answer says nothing about why things are the way they are. Berlinski's argument is that if the way things are in the biosphere was to suddenly change, such that thorn bushes were carnivorous but not pitcher plants, or that ants turned bioluminescent in the night but not fireflies, then evolutionary biologists would present the same unhelpful explanation: various organisms try various things. This shows that evolutionary theory can account for every possible scenario—for every contingency. It excludes nothing. Such a theory can never be falsified.