Speciation usually occurs over a long evolutionary time scale, during which the species may be isolated or continue to interact. If two emerging species start to interbreed, reproductive barriers may be weak, and gene flow can occur again. At this point, the selection of hybrids across the two populations may either stabilize the newly mixed group into a single population or reinforce the distinction between them as new species. Speciation may occur gradually or rapidly, and in some cases is punctuated between long periods without change followed by rapid rates of speciation.
In cases of speciation where two or more populations have become isolated for some time, they may reconnect. For example, in long periods of drought or climate change, large lakes can be split into many smaller lakes, isolating the inhabitants. The vast species diversity of African cichlid fish was fueled, in part, by periods of such population fragmentation. When the conditions changed, and fragmented lakes merged again, isolated populations got back into contact.
When reconnection occurs, if pre-zygotic reproductive barriers are weak, individuals from the two different populations may begin to reproduce. If the fitness of the hybrid offspring is higher or unchanged compared to the parents, the populations can integrate and merge. This process is referred to as stability. However, if the hybrid offspring are less fit than non-mixed offspring from the parent populations or pre-zygotic barriers to reproduction are strengthened with time, the two populations will continue to diverge even in sympatry—a process known as reinforcement. In the cichlid fishes, many new lineages and species were likely generated in this way.
Species may evolve at different rates depending on generation time, the strength of selection pressures and specific environmental conditions. Usually, change happens slowly, with alterations occurring in small increments over time until a new species emerges that no longer interbreeds with other species. This concept is known as phyletic gradualism. For example, if birds with slightly longer beaks can dig deeper into trees for grubs, the entire population may skew towards longer beaks over time, and eventually, become distinct from their short-beaked relatives.
However, it is also possible for species to change relatively rapidly. This ties into the theory of punctuated equilibrium that states that species may undergo spurts of rapid evolutionary change, followed by long periods of remaining relatively unchanged. Support for the theory comes from the observation that some fossil lineages appear to change little for long periods of time, then show rapid change in the fossil record.
The butterfly genus Heliconius shows strong selection to preserve color pattern due to selection for mimicry, and this generally keeps species stable even in sympatry with closely related sister species. However, rapid speciation can occur in the event of mutation or hybridization which produces a novel “fit” phenotype. Overall evolution and speciation can proceed in various ways and different time scales.
