Evolution within a species is known as microevolution. A species is defined by all of its potentially interbreeding organisms, and by their shared gene pool. However, in practice, species are made up of subunits, such as herds or flocks. While these populations may interbreed with adjacent groups, they are highly unlikely to breed with those animals living a long way away, and most breeding will go on within the group. This offers a method by which new species can arise, as groups are separated from each other, either by distance or by changes in behavior. In this view, microevolution is largely the study of how groups of organisms interact with and migrate through their environment. There appear to be two ways in which new species can emerge, by allopatric or sympatric speciation.
In allopatric speciation a group of organisms becomes physically separated from the rest of the population, perhaps by the appearance of a physical barrier between them, such as a new seaway. The isolated population will experience different environmental and biological stresses to those experienced by the parent, so that a different set of characteristics will be favored. Over time, these new characteristics will become so clearly differentiated from the parental set that the group ceases to be able to interbreed, even if the barrier to migration is removed. A new species has been created. The move towards a new species may be "kick started" simply because the isolated "sample" is not representative of the species as a whole.
In sympatric speciation, a species splits into two without part of the population becoming physically isolated. Instead, isolation may be achieved by changes in behavior, food, or habitat preference. Again, because the selection pressures acting on the two populations will be different, they will become physically different over time, and will eventually be incapable of interbreeding.
Species that remain as single geographic and behavioral units may still evolve as the conditions they encounter change. In this case, while no new species are created, shape changes will be observed in the fossil record of such a group. When studying such fossils, taxonomists eventually define a point where such change makes it appropriate for the species to be given a new name, on the basis of its new characteristics. The parent species appears to have become extinct, and the daughter species to have originated, but this is actually an artifact of the need to classify organisms into discrete groups. It is sometimes known as pseudoextinction.
The rate at which new species appear has been a major research topic in paleontology for many years. It was originally assumed, from a first reading of Darwin's ideas, that evolutionary change should be continuous, but slow and steady. Species were expected to change morphology continuously through time, so that eventually a new species, or several, would emerge from the parent population. This idea is known as phyletic gradualism.
However, this steady rate of change is rarely preserved in the fossil record. This may be an effect of the incompleteness of this record. In most environments the preserved sequence of fossils represents a series of "snap shots" of the population at widely separated intervals of time. These intermittent records tend to produce a "jerky" effect on any measurements of shape change. However, in those rare sequences of rocks where deposition is rapid and continuous, such as in some deep lakes, this "jerkiness" still seems to be recorded.
This observation has led to the development of an alternative hypothesis relating to the rate of evolution, known as punctuated equilibrium. This model suggests that species tend to be stable for considerable periods of time, and that daughter species evolve suddenly, in sharp bursts of morphological change. This model effectively implies that the selection pressure experienced by a species is not constant, but episodic. The physical separation of one population from the rest of the species would suddenly change selection pressures, as would a sudden change of climate.
There is much debate about the nature of evolution above the level of a species. This higher level evolution is what produced dinosaurs from thecodonts in the Triassic, or birds from theropod dinosaurs during the Jurassic period (Fig. 2.4). The appearance of higher taxonomic groups, or the appearance of a novel structure, say a bird's wing, could be explained simply be "scaling up" the explanation for the appearance of diversity within a species. Thus species could be thought of as the unit of selection for genera and higher taxonomic groups and their patterns of change interpreted entirely in the light of the environment they encounter. There is good evidence for such an argument, but also strong support for the idea that in such a complicated system there will be properties that emerge from the simple patterns that explain basic processes. These emergent properties are well known in other fields. For instance, many molecules of water have the emergent property of "wetness", while a single molecule of water does not possess this characteristic.
In order to explore these contradictory styles of explanation, it is helpful to look for patterns in the emergence of major new groups of organisms through geological time. The first observation is that higher taxonomic groups do not appear at a steady rate through time. There are clear bursts of evolutionary innovation at the Precambrian-Cambrian boundary, in the Ordovician, and after mass extinctions such as the ones at the end of the Permian or Cretaceous periods. No single reason can be invoked to explain these bursts, but their patchiness does suggest that high level taxonomic innovation is an intermittent process, which lends weight to the idea of it being emergent.
The majority of the major periods of innovation, the so-called evolutionary radiations, happen when ecological space
is created. Radiations following mass extinctions are a clear example of this. For example, the major radiation of flightless predatory birds during the early Palaeocene can be interpreted as this group evolving to fill ecospace left by the large predatory dinosaurs. Some authors explain the radiation of mammals in the same way, though there is growing evidence that many lines of mammals actually evolved while the dinosaurs were alive.
Ecospace can also be created by the migration of a group of animals into novel environments. To an extent, this explanation is circular, because it could be argued that a major evolutionary innovation is needed to facilitate this move in the first place. However, there is no doubt that more innovation follows the pioneers. Plants with woody tissue finally conquered the land and began to form forests during the late Devonian period, following millions of years of slow evolution of low-growing forms that lived near water. These forests were rapidly filled by animals, and the evolutionary innovations included flight, the evolution of insects from primitive arthropods, and the evolution of tetrapod vertebrates such as amphibians and reptiles.
Some of these arguments can also be applied to two much more problematic evolutionary radiations, at the Precambrian-Cambrian boundary and during the early Ordovician period. In the first case animals evolved mineralized hard parts, such as teeth and skeletons, over the course of a few million years. There followed a spectacular evolutionary radiation, which has remained dramatic even as the timescale over which we understand it to have happened has been increased from a geological instant to perhaps 30 million years. Although multicellular animals clearly evolved before the boundary, and had radiated into a range of niches and over much of the globe, most of the modern phyla we recognize appear during this radiation. Were they evolving into new ecospace, or was their evolution a consequence of the biological impact of biominerals? The answer is probably a mixture of the two causes, with new ecospace being made available by the functional potential of hard parts. With skeletons it is possible to be larger, faster, and more diverse in feeding strategy than without. Whether we see this as a biological driver for evolution or an ecospace driver depends on seeing one or other of two sides of the same argument.
The evolutionary radiation during the early Ordovician is possibly the most enigmatic of these events, and eludes a simple explanation despite much research being focused on it. The most plausible solution is that the diversity of planktic organisms increased, for reasons that may be related to changes in ocean circulation. As a consequence, bottom-dwelling animals had an abundant source of food and were able to radiate to utilize this resource. It is noticeable that many of the groups that evolved rapidly during this time were filter feeders, such as corals and crinoids.
Another example of a possible biological driver for macro-evolution is the appearance of new groups of predators during the early Mesozoic. This led to the evolution of new groups of animals that burrowed into the sea bed or into hard surfaces like rocks or wood for protection.
To summarize, it seems that macroevolution is in some sense emergent, in that it is episodic and appears to run by itself after being initiated. The initiation is usually the appearance of ecospace, either due to the extinction of an incumbent group or due to a single biological innovation which allows organisms to exploit resources in a new way. This rather general explanation focuses attention on how organisms can generate large-scale changes to their body plan in a short space of time. This has been partly solved by an increased understanding of how genes work. We now know that genes have a hierarchy of function, with some genes encoding the production of particular molecules and others controlling the timing and duration of activity of these genes. Changes to regulatory genes can lead to significant changes in morphology. A change in the timing of sexual maturity, for example, can change the whole shape of a reproducing organism, or a change in the instructions controlling the arrangement of segments in arthropods can radically alter the resulting animal.
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