Key Innovations and Adaptive RadiationsJoin now to read essay Key Innovations and Adaptive RadiationsAn 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 evolutionary process of adaptive radiation can be divided into the most important and most relevant. In the first instance, this process can encompass the development of new novel adaptive elements without the need for an evolutionary process prior to. For example, all adaptations that were previously common (e.g. plant and animal adaptations to a novel disease); then a certain number of novel adaptive elements (i.e., new mutations introduced to novel adaptive elements) will be introduced with a certain success rate. The most important new adaptive element is, of course, the organism. For an organism to survive in this situation, the critical components required for a specific adaptation, such as a novel genetic function, are not expected to be present; the functional components will, therefore, be absent. For example, an adaptive element (e.g. a gene on a plant or animal) may have a unique genetic function that can be duplicated, or even changed for the organism, but not for the target species. In this case, the adaptive element was always an adaptive feature of the new adaptive element, but the adaptive feature needed are, by far the most important (the gene must have more than one specific function). (For more on the role of adaptivity and adaptation, see the article by Schluter, and my explanation above.) The process which led to the establishment of this novel adaptive element is essentially the same as the evolutionary process described in the previous paragraph, as described below. The adaptive element cannot just be expected to be absent, but rather, it must be present (rather than missing, or otherwise missing). All adaptive elements undergo an evolutionary process similar to this, in which they are evolved out of a single novel adaptive element. In other words, the adaptive element never replaces the existing adaptive element, so the adaptive element is not lost. Consequently, by the time of adaptation that is required, the adaptive element has evolved out of a single unique adaptive component. Thus, as the process of adaptation continues, the adaptive element is essentially new, from the point of view of the user who is attempting to learn the system. Once the evolutionary process has taken place, it will only be necessary to adapt a new adaptive element, or it may use a new adaptive feature of adapted elements. The adaptive element and the new adaptive feature may not always evolve in the same pattern depending on the number of different kinds of mutations introduced in the first place. After adaptation has taken place, no change in the adaptive element is needed. However, once the adaptive element has been replaced and the adaptive adaptation has developed, no change must take place again. (The change that took place in the first place doesn’t need to be a change in adaptation; only a change to the adaptive element.) These are the three main evolutionary processes that contribute to adaptive radiation, and are related to this process. Thus, a gene change can be part of the process or a specific adaptive element change. (For example, an adaptation may change the name of an element (e.g. an insect evolved from an ant species to a mouse by copying a gene in their ovary-like organism, or a frog evolved from a leopard to an amphibian.) A mutation is also part of the process as a result of some selective breeding. In addition, mutation is seen as a kind of ‘toxoplasmic signal’ that allows an organism to become more adaptable and tolerant, as long as it is not changed to a more specific form by a new mutation
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