Evolution has never proceeded with a uniform speed over long periods but its rate has fluctuated due to environmental changes, geological upheavals, mutation rates and selection pressures operating differently in different situations.
Both internal as well as external factors determine the rate of evolution in organisms. In a non-changing environment, evolution is always slow as the population is adapted to the existing conditions and emergence of new types is not favoured by natural selection. A high rate of mutation upsets genetic equilibrium and modifies the gene pool.
Highly mutating genes not only provide raw ma But eventually it is the interaction of mutant genes with the environment that decides the speed and course of evolution. Bases on the degree of environmental changes and its interaction with mutation pressure and selection pressure, the speed of evolution can be slow, average or fast, which is denoted by the terms bradytely, horotely and tachytely respectively.
Bradytely is slow evolutionary rate or phyletic gradualism, when animals do not change markedly over long periods of time or evolve slowly. Darwin (1859) pointed out that the brachiopod genus Lingula has remained unchanged for about 500 million years, since Ordovician period of Palaeozoic, as it inhabits warm and shallow seas which provide the most stable conditions on earth.
When animals live in a more or less stable environmental condition, mutations producing extreme changes in the body are eliminated by stabilizing selection, thus preserving individuals having average characters. Such animals find themselves to be the fittest in the existing conditions and are selected in. The population is therefore prevent That is why the evolution of marine animals is generally slower as compared to the terrestrial ones, because more or less stable conditions generally prevail in the sea while large and rapid changes occur on land.
Higher animal classes on an average evolved faster and have lower phylogenetic age than lower animal classes. Xiphosura which includes king crabs have existed almost unchanged since Cambrian. The species of genus Limulus are very similar to the fossil species that existed 150 million years ago. There are nine orders of Crustacea for which phylogeny can be traced back to 400 million years in Palaeozoic era without demonstrating appreciable change.
Similarly 98 families of Prosobranchia (Mollusca) have phylogenetic age of about 125 million years. There are 30 genera of present Foraminifera (shell forming Protozoa) that have existed unchanged for about 200 million years. Their calcareous shells settle at the bottom and leave a more or less complete record of their evolution in the sedimentary rocks in seas. Cockroaches are excellent examples of Bradytely, which have changed little from Palaeodictyoptera that evolved in Pennsylvanian period. Termites and dragonflies have evolved very slowly since Carboniferous period.
Among terrestrial animals periods of rapid evolution are followed by long phases of slow evolution when adaptations are gradually perfected. Thus the bats of Eocene are not much different from the present ones. Majority of the present-day mammalian groups had evolved and diversified in Palaeocene epoch after the mass extinction of dinosaurs and have been evolving slowly since then. Thus ancestors of whales, ungulates, bats, insectivores, primates, carnivores can be identified in Eocene epoch. Opossum (Didelphis), a marsupial found in the Central and South America has not changed much since Cretaceous period.
While all other marsupials have been exterminated by the advent of placental mammals, opossum has survived owing to its omnivorous and arboreal habits. Monotremes have been very slow in their evolution since Cretaceous and among reptiles, Sphenodon, turtles and crocodiles have evolved very slowly since Mesozoic period.
Simpson (1953) used the term horotely to describe average rate of evolution. In stable or gradually changing environmental conditions, when selection pressure is minimal, the rate of evolution seems to be average even if mutation rate is high. In a not so fast changing environment, genetic changes might produce extreme characters but they do not find favour in the existing conditions and therefore are selected out.
The evolution thus produces gradual and continuous change over long periods, producing different adaptive types. Thus horotely allows species to perfect their adaptations over long periods once they enter new environmental zones owing to their preadaptations.
Thus horses entered grasslands and camels arid areas from forests on the strength of their preadaptations and survived and perfected their adaptations by horotely to give rise to modern species. Similar phenomenon can be seen in the phylogeny of elephants. When ancestors of elephants left swampy areas to inhabit forests and grasslands, modifications in their body were gradual but directional and constant to give rise to new adaptive types.
Horotely is a common phenomenon that occurs in great majority of organisms as generally environmental conditions neither stay unchanged or change suddenly but alter gradually over long periods and organisms show average and steady evolutionary speed.
Tachytely is sudden spurt in evolution or explosive radiation that gives rise to entirely new adaptive types that can invade a very different environmental zone and survive. Such event is also called megaevolution that in the past produced major phyla and classes, such as Amphibia, birds and mammals. Goldschmidt (1940) suggested that large “systemic mutations” were the cause of tachytely and the resulting megaevolution.
In plants new species can arise suddenly by poloyploidy. Tachytely is expected to occur when organisms face the demands of a new adaptive zone that is markedly different from the existing one and where selection pressure is extremely strong. Thus there appeared to be a rapid burst of evolution in Silurian-Devonian period when great changes in sea level occurred and mountains were formed, forcing plants and animals to invade land by undergoing large changes in their morphology and physiology.
Simpson (1964) termed this sudden change as quantum evolution, although he himself was not satisfied with the term since a quantum change is a small event while these evolutionary events were on large scale.
Mass extinctions provided ideal environment for tachytely as they wiped out a large portion of the flora and fauna from earth, leaving the surviving organisms free to occupy empty niches and evolve rapidly. Thus in Mollusca, 47 new genera arose suddenly in Ordovician period after the Cambrian mass extinction, 25 new families emerged in Triassic after the end-Permian extinction and 24 families of marine Prosobranchia arose in early Tertiary, immediately after the end-Cretaceous mass extinction.
Among insects, 11 orders arose suddenly in Jurassic after the Triassic mass extinction and 10 arose in early Tertiary after the Cretaceous extinction, when also due to the emergence of flowering plants, insects underwent an explosive adaptive radiation.
Birds evolved almost overnight from reptiles in Jurassic after the Triassic mass extinction and then diversified by producing 16 new orders in early Tertiary, immediately after the Cretaceous mass extinction. After the extinction of mighty dinosaurs, mammalian evolution flourished with such speed that 19 new orders and 102 new families emerged during Palaeocene-Eocene epochs alone. The primitive carnivores produced 26 genera within Palaeocene, immediately after extinction of dinosaurs.
As we have seen earlier, in tachytelic evolution new morphological forms appear suddenly in the fossil records and in majority of cases transitional forms do not exist or are rare. Darwin thought that incompleteness of fossil records is due to lack of explorations and that transitional forms would be discovered sooner or later.
However, Niles Eldredge and Stephen Jay Gould (1972) accepted the sudden appearance of new forms as a scientific fact and explained it with their theory of Punctuated Equilibrium. According to their proposal, rapid morphological changes take place in a lineage during the formation of new species when organisms invade new environment. Once established in the new environment, there are long periods of stasis or no change or gradual change due to stabilizing selection. The theory suggests that evolution is a slow and steady process punctuated here and there by events of rapid speciation.
Mayr (1963) thought that peripatric speciation can result in the emergence of new types. When a small population from the periphery gets isolated and undergoes rapid change due to ‘genetic revolution’, it is unlikely that the transitional stages will be fossilized from such small numbers but the new species will later expand and carry the lineage forward. Even genetic drift can cause rapid changes in gene frequencies that can lead to rapid morphological changes. Colonising species generally undergo rapid transformation, especially on islands as evident in honey creepers of Hawaii which have diversified into 22 species in 9 genera. Darwin’s finches in Galapagos Islands are another example of rapid adaptive radiation.
Sessile marine invertebrates belonging to the minor phylum Bryozoa have abundant fossil records of the past 100 million years and the present species are well documented and classified. Jackson & Cheetham (1994) studied fossils from the Caribbean starting from Miocene to the present living species. Their studies showed a pattern of rapid morphological changes, followed by long periods of stasis or no change. Bryozoans offer a typical example of punctuated equilibrium.