The formation of a key evolutionary innovation is a very effective way in which an anagenetic change can increase the probability and thus the frequency of speciation; this consists in a trait that enables its carriers to occupy a new adaptive zone, i.e. utilize a certain set of niches that were not accessible to them until this time. If this set of niches is very extensive, adaptive radiation can occur in the particular line. During adaptive radiation, the relevant line very frequently splits into a large number of different species, each of which can lead to the formation of an independent phylogenetic line (Fig. XXVI.4). These species divide the newly available niches up amongst themselves. If the new niches include an important resource that is simultaneously part of the niches of an already-existing species, the new species can have a substantial negative impact on the species utilizing these original niches. They can reduce the area of their occurrence, utilize their ecological niches and thus reduce the sizes of their populations or can even cause their extinction. Homoiothermy in mammals and active flight in birds are apparently examples of anagenetic changes leading to adaptive radiation.
Adaptive radiation can occur through a quite different mechanism. If a member of a certain species enters a territory where there are no members of a great many taxa, for example if it gets to a newly formed volcanic island far from the mainland, it can rapidly undergo series of speciations and its descendants can occupy niches that are already occupied by other species on the mainland. The phylogenetically highly related but ecologically very diversified species of Darwin’s finches on the Galapagos and the Drepanididae on the Hawaiian Islands evolved in this way (Craddock 2000) (see Fig. XXVII.3). The king of the hill effect is very frequently active in both macroevolution and microevolution (see XXII.5.5). As soon as an ecological niche (or even a habitat) is already occupied by a certain species (for a habitat, a certain population), this species cannot be simply forced out by a species (population) whose members penetrated there at a later time. The factor favoring the current “king of the hill” consists in the greater number of the original species, which enables it to better resist any random fluctuations in the size of the population, and also its momentarily better adaptation to the local conditions. The newly arrived species (population) might, in the future, be able to adapt to the given conditions just as well or even better; however, competition from the “king of the hill” provides no scope for this.