Fig. 1

The direction of morphological evolution. a Example of a two-trait morphospace considering, for example, limb length (X-axis) and width (Y-axis). Each point in such morphospace represents an individual limb morphology in a population. The gray point represents the population mean for both traits. b Example of evolution in a morphological direction in the limb morphospace The gray areas represent the distribution of the individuals of a population at different successive generations. The arrows show the direction of evolution between generations (a vector from each generation mean to the next generation mean). c Arrows show a sample of the directions of possible morphological variation under the isotropic expectation for a two-trait morphospace. d Example of a fitness landscape on the same two-trait limb morphospace. The contour lines show points, i.e. limb morphologies, with the same fitnes (the higher the fitness the thicker the line). In this example morphological variation is isotropic so the population can go from any point in the morphospace to any other nearby point. As a result one can deduce how morphology will change based on the fitness landscape: the population would evolve towards the steepest peak. The gray line shows that this is the evolutionary trajectory the population would follow. d As in C but in this case morphological variation is not isotropic. The gray regions show the morphologies, i.e. points in the morphospace, that are possible by changes in development (the darker the point the more likely it is to arise from changes in development). In this case, as shown by the trajectory in white, the population would not evolve towards the steepest fitness peak because there is no morphological variation in that direction