If evolution is a scientific theory worth its salt, then there must be some conceivable observations that could show it to be wrong.  I just wanted to put down, for the record, what some of those observations might be. First, let’s reprise what I see as the major components of the theory of evolution.

  1. Evolution occurs, that is, there is gene frequency change in populations over generations.
  2. Significant evolution takes time—that is, it usually (though not always) requires hundreds to thousands of generations to occur. It is not instantaneous, and it is the population and species rather than the individual that evolves.
  3. Lineages of organisms split, or speciate, so that the single lineage that gave rise to life 3.5 billion years ago has undergone numerous splitting events to produce the millions of species alive today (and also the even more millions that went extinct).
  4. The converse of #3: any pair of living species has a common ancestral species some time in the past.  That is, if you trace any pair of twigs on the tree of life, you will find a node where the line from the trunk bifurcates to produce them.
  5. The process producing the appearance of design in organisms is blind, purposeless natural selection. (There are, of course, evolutionary forces other than selection, including genetic drift, but they don’t produce the marvelous design that was once seen as the prime evidence for the hand of God.)
These parts of the theory don’t all stand together. For example, you could have evolution without branching: that would mean that only one long-evolved species would be alive today. Or you could have evolution, but not occurring by natural selection. (The complete absence of natural selection is hard to imagine, though, given that organisms replicate their genetic material, and some genes will replicate better than others in different environments. Lamarckism and teleologically-driven evolution, however, were once seen as the main drivers of adaptation.)  Finally, you could have evolution but not gradualism: every population could experience great saltational leaps in one generation.
In my general talk on the evidence for evolution, I give a list of seven observations that, if repeated and confirmed, would disprove parts of the theory of evolution described above. This shows that it is a scientific theory in the Popperian sense of being falsifiable.