Adaptive radiations in evolution

Adaptive radiations in evolution

In the past 300 million years, an innumerable variety of vertebrate species have evolved from the bony fishes of the early Paleozoic.   At several points in evolutionary history, there have been periods of relatively rapid adaptation and expansion of organisms known as adaptive radiations.   These are limited only by a speciesO adaptive   potential and the adaptive opportunities of the environment.   There have been three major adaptive radiations of terrestrial vertibrates.
The first of these was the rapid diversification of reptilians leading up to the age of reptiles during the Mesozoic.   The predecessors of reptiles were amphibians, capable of producing a stronger and self-sufficient egg.   When they adapted to a fully terrestrial life cycle, reptiles had virtually no competition for ecological niches.   Over a relatively short period of time, reptiles became the dominant life forms on the planet.
Approximately 65 million years ago at the beginning of the Cenozoic, another major adaptive radiation took place.   With the decline of the dinosaurs, a wide variety of ecological niches were opened to mammals.   The first mammals are believed to have arisen from small mammal-like reptiles.   They were small and resembled modern rodents.   Possibly the most important characteristic of the mammals is their advanced learning abilities and behavioral flexibility.   A larger brain size developed in the evolving mammals with a larger percentage of brain mass being made up by the cerebrum and neocortex.
In addition to their neurological advantages mammals have many unique physical characteristics that have facilitated their evolutionary success.   Mammals are heterodontal, they possess specialized and differentiated teeth.   This, in conjunction with their jaw strength increased their dietary diversity.   Homoiothermy, a self-regulated body temperature, although not unique to mammals helped them settle in colder ecological niches and allowed for nocturnal activity in areas with colder...