p297 These gaps affect our knowledge of timing, and of changes in body shape and function. "In reality, there are long periods within nearly every lineage for which the fossil record remains unknown".
Records of timing are bedevilled by gaps in the fossil record, often at those crucial early stages when numbers are low and geographical distribution is severely restricted. All agree, though, that the speed of evolution does change, however it is measured. It is part of evolutionary theory that all processes take place at the level of populations. Others use terms like macroevolution, or even megaevolution, as if the processes are different from those which occur below species level. Robert L Carroll prefers to use the term major evolutionary transitions, though it turns out that all or most of these could also be described as adaptive radiations. The term was introduced and discussed by George Gaylord Simpson, the palaeontologist who contributed to the modern evolutionary synthesis. This is an evolutionary process driven by natural selection. In the relative absence of competition, groups diversify to fill available habitats and niches. The greatest radiation of all took place early in the Cambrian period, when most of our animal phyla evolved: see List of animal phyla. The Ediacaran biota were the result of an early metazoan radiation. This must have something to do with the availability of ecological niches and relative absence of competition. However, in the most striking cases, such as occurred in the Triassic after the greatest extinction event in Earth history, many lines underwent rapid radiation simultaneously.
Some definitions phrase it in terms of a single clade: "Adaptive radiation is the rapid proliferation of new taxa from a single ancestral group". It produces more new species, and those species live in a wider range of habitats. It is an increase in the number and diversity of species in each lineage. There are at least 9,000 living species of birds, far more than mammals.Īdaptive radiation is rapid evolutionary radiation. The evolution of bird beaks and feeding methods led to a great increase in the number of bird species.
0 kommentar(er)
