   Constraints are often understood as biases or limitations on potential evolutionary trajectories. How do we identify potential trajectories? This will depend on how biology provides counterfactual information. We cannot appeal solely to realized evolutionary history, since not all potential trajectories have occurred. Evo-devo research on novelty provides one of the best strategies for exploring potential evolutionary trajectories (Wagner 2000, 2001a,2001b, Muller and Wagner 2003, Calcott 2009). As the causal mechanisms underlying development are better understood, we will gain a better grasp on the nature of constraints and how they explain evolutionary outcomes.