Historical constraints

Natural selection proceeds in small, local steps; each change has to be advantageous in the short term. It is possible for natural selection to climb to a local optimum, where the population may be trapped because no local change is advantageous, although a large change could be. Selection itself - when considered in a fully multidimensional context - or neutral drift may lead the population away from local peaks, but also it may not. Some natural populations may now be imperfectly adapted because the accidents of history pointed their ancestors in what would later become the wrong direction.

Figure: Historical change in adaptive topography leaves a species stranded on a local peak. (a) The population (X) evolves to an optimum peak. (b) The environment changes over time, the species reaches the optimum. (c) The topography has changed, a new global peak has arisen. The species is stuck at the local peak because selection does not favor evolution to the global peak.