164 EVOLUTION often live in the same lake, rather than in adjacent lakes, as might be expected if allopatric speciation had occurred (Figure 2) Although it seems intuitively obvious that populations that become physically dissimilar will eventually be unable to produce offspring, the genetic basis for this change can be demonstrated in the laboratory but not yet fully explained The fossil record can be used as a tool to help in the understanding of evolution and the formation of new species It may be that evolution progresses gradually for most of the time, an idea known as phyletic gradualism The classic fossil example of this slow continuous process of morphological change is the study by Peter Sheldon of Ordovician trilobites recovered from deep-water shales in central Wales Eight different genera of trilobite, including well-known forms such as Ogygiocarella, were found to exhibit incremental changes in rib number through the duration of one graptolite zone, which probably represents significantly less than Ma Gradual change is generally difficult to observe in the imperfect fossil record It could be argued that in a less continuous sedimentary record (or one sampled less finely) this sequence of events would appear as a series of abrupt changes Commonly, what is preserved is a long period where little or no change is observed followed by the abrupt appearance of a new form The theory of punctuated equilibrium attempts to explain this phenomenon not as the product of an imperfect fossil record but as a common pattern of evolutionary change This is done by applying the concept of allopatric speciation to the problem Eldridge and Gould, who developed the idea of punctuated equilibrium, argue that most species probably arise in small, geographically isolated areas and that they arise rapidly as they encounter new selection pressures At some later time the evolved offspring species may move back into areas where it encounters its parent species and may out-compete this form In most areas where this happens, the geological record will show one species – the parent – abruptly replaced by another – the offspring – with no intermediate steps The chance of the isolated population being represented in the fossil record during the short period of its evolution into a new species is very slim (Figure 3) It may be that Williamson, in a study of molluscs in Plio-Pleistocene sediments from Lake Turkana, found one such rare fossil example of punctuated equilibrium Species of gastropod and bivalve both appeared to remain static in shape for long periods of time, punctuated by brief periods when their shape changed abruptly Although some studies seem to show a punctuatedequilibrium style of evolution, others appear to show that evolution has progressed via phyletic gradualism, and a consensus has yet to emerge regarding these theories In practice, most evolution is probably the result of a mixture of punctuated and gradual periods of change, partly depending on the scale of Figure The difference between allopatric speciation and sympatric speciation, using the example of fishes living in lakes (A) Sympatric speciation occurs due to changes in behav iour or mode of life, in this case by a partitioning of the original population into limnetic and benthic groups Here, descendent species are most closely related to species living in the same lake (B) Allopatric speciation occurs following geographical sep aration of the populations, in this case caused by a fall in lake level Descendent species are most closely related to fishes living in adjacent lakes Figure The differences between phyletic gradualism and punctuated equilibrium models of speciation (A) In phyletic gradualism the shape change is gradual and populations are seen to move across morphospace continuously Periods of spe ciation are relatively long and can be recorded in the fossil record (B) In punctuated equilibrium the shape change is inter mittant, rapid, and related to geographical separation of a part of the population For most of the time the form of the population is static In most areas no speciation event is seen, and the fossil record shows abrupt changes of morphology with no intermediate stages