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Key Innovations and Adaptive Radiations

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An adaptive radiation was defined by Schluter (2000), as "the evolution of ecological diversity, within a rapidly multiplying lineage". Species can go through an adaptive radiation (involving a diversification of that species), in response to having invaded a vacant ecological niche. It is thought the ability to do this, can be attributed to one or more key innovations developed by this species (i.e. the species has developed a new 'key innovation', which makes it possible to invade new niche, that was not possible before). A radiating clade, which is the original ancestral species and descendants, then has opportune to exploit the new resources that often come with this new vacant niche. (And perhaps not have to compete with other species present, for the same resources and space).

The role of a key innovation in an adaptive radiation can be thought of as a new feature which increases ecological opportunity (Schluter, 2000). This ecological opportunity, in turn, can be defined as "the wealth of different resource types under-utilized by species in other taxa" (Schluter, 2000). These two main ideas, partly make up the "ecological theory". This theory generally states that the differences in phenotype observed between populations and species, is caused directly by differences in the environment they inhabit and resources consumed (Schluter, 2000). This essay looks at some examples of adaptive radiations that have seen the arising of several new species from an ancestral group in a relatively short period of time. Each example attempts to explain how each radiating clade is the result of a key innovation. Classic examples of adaptive radiations include the Galбpagos finches, the Hawaiian silversword alliance, and cichlid fishes. These examples are well covered in previous literature and this essay instead examines others. Other more unknown examples discussed include diversity of insects that feed on vascular plants, and diversity of weevils. Firstly, a historical view of some major radiations will be looked at, and what has to be considered before concluding that a key innovation itself has been largely responsible for an adaptive radiation. Another theory, 'the environmental stimulus theory' is examined briefly also, which opposes the key innovation hypothesis.

Some do refute the importance of key innovations in adaptive radiations. Schluter (2000) discusses hypotheses concerning the possible causes for adaptive radiations, two are discussed in this essay. The 'environmental stimulus hypothesis' states the ancestral animal group had always a certain potential to diverge into different groups, given the right conditions. So, following a major facilitating change, such as the removal of a environmental constraint (extreme temperatures, low nutrient or oxygen levels). This supposedly would stimulate the divergence of different groups.

The second hypothesis concerns whether or not key innovations are the major instigator of an adaptive radiation. The 'key innovation hypothesis' argues that instead it is the acquiring of a biological feature that allows the divergence of animal groups (as aforementioned). An innovation can be anything from a developmental gene to an external structure (specialised limb for example). The key innovation would supposedly arise in response to a selection pressure, or could even arise from the appearance of a mutant which may have had a reproductive/survival advantage (Hunter, 1998).

The certainty that the species was able to invade a new niche, directly because of new key innovations, is debatable. Obviously, other confounding factors must be considered first, to rule out that they themselves have prompted the adaptive radiation. Simpson (1953) suggests two steps are necessary to conclude the key innovation itself is responsible. Klak et al. seem to have covered both aspects thoroughly. Firstly, the correlation between the appearance of a key character and diversification must be confirmed. Secondly, the ecological opportunity gained from the key innovation must be separated from other mechanisms operating at the time. Klak et al. (2004) have considered confounding factors and concluded that neither climatic or ecological factors can explain the huge speciation burst observed in semi-desert ice plants with a range of certain morphological features (key innovations). It was also observed that those without these certain morphological features were contained within clades that were species-poor. This was further backed by research by Gianoli (2004), in which sister groups of climbing plants that themselves did not have a climbing habit, were found to have less species (in 38 out of 48 sister pairs compared).

The most important innovations that have led to practically all life on earth today, occurred in a very short period of time (relatively short compared to Earths age). During 40 million years, in the Cambrian explosion, all major phyla's body plans that exist today were formed, plus several other body plans that went extinct (Mayr, 2001).

Some of the major morphological innovations that appeared during the Cambrian explosion included the first coelomate animals (animals with a true body cavity) and the origin of bilateral symmetry. Segmented body plans, first external skeletons and appearance of appendages and notochord were also key in further radiations occurring later in history. These innovations made it possible for different ways of making a living (and therefore, fill new niches previously unavailable) (Mayr, 2001).

Moving much later on, towards the Silurian period, when fish were present, another acquired innovation set about a huge proliferation of species. During the Silurian, evolution of the jaws in fish resulted in the placoderms group, which diversified (arguably successfully because of this feature) largely and dominated until going extinct at end of the Devonian. The jaw, first present in the placoderms was observed in all 'fish' groups following its extinction; including the hugely diverse ray-finned fishes, lobe-finned fishes, and sharks/rays/cartilaginous fish groups. The evolution of jaws expanded the range of food edible in these groups, and they went on to diverge into a huge array of different forms. This innovation may have been the cause of the proliferation of several groups of species.

During the Devonian, colonisation of land was possible because of another innovation. The formation of limbs able to support ones weight out of water probably evolved from muscular fins observed in some lobe-finned fish. Along with this development, the ability to breathe air was also a precursor to terrestrial life (most probably developed as a response to low oxygen levels in stagnant waters). This movement



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