Atheistic evolutionists have been quick to criticize the Intelligent Design movement as not being falsifiable. ID proponents such as William Dembski have responded, saying simply that if the design in nature could be duplicated without external intervention, the theory would be falsified.
The principle of falsifiability is a useful tool in science. Theories are tested by trying to prove them wrong. If they can be falsified, the theory is rejected. Theories that hold up to strict attempts to disprove them are kept over time and used to further knowledge.
It appears that the accusation of a lack of falsifiability can cut both ways. In the modern practical applications of evolution, we are told that the biological world is the way it is because of natural selection. If a specie expresses a trait, it is because that trait helped survival in some way. Modern use of the theory is in fact unfalsifiable. If, for example, a human endangers himself to save another human, it is called herd instinct, and the altruism is explained as a natural trait that enhances survival of the individual by protecting the group. If the opposite happens and one human kills the other, we are told it enhances the survival of the individual.
If the principal of falsifiability is a measure of whether a theory is considered good science, then evolution would not appear to pass the test. Whatever state of affairs exists, then even if the opposite state of affairs were the case, we are told it is due to evolution.