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In evolutionary biology a Key Innovation is a phenotypic trait that allows subsequent radiation and success of a taxonomic group. Typically they bring new abilities that allows the taxa to rapidly diversify and invade niches that were not previously available. The phenomenon helps to explain how some taxa are much more diverse and have many more species than their sister taxa. The term was first used in 1949 by Alden H. Miller who defined it as "key adjustments in the morphological and physiological mechanism which are essential to the origin of new major groups."

The theory of key innovations has come under attack because it is impossible to test in a scientific manner, but there is significant evidence to support the idea.

Mechanism
The mechanism by which a key innovation leads to taxonomic diversity is not certain but several hypotheses have been suggested :

Increasing individual fitness
A key innovation may, by increasing the fitness of individuals of the species, result in extinction becoming less likely and allow the taxa to expand and speciate

Novel niche invasion
A key innovation may allow a species to invade a new region or niche and thus be freed from competition, allowing subsequent speciation and radiation. A classic example of this is the fourth cusp of mammalian molars, the hypocone, which allowed early mammalian ancestors to effectively digest their generalised diet. The precursors to this, the triconodont teeth of reptiles, were adapted for gripping and slicing rather than chewing. The evolution of the hypocone and flat molars later allowed animals to adapt to a herbivorous diet as they could be used to break down tough plant matter through grinding. The evolution of this ability led to mammals being able to adapt to utilise a huge variety of food sources. This also allowed early mammals to invade novel niches through the evolution of specialised herbivores, which experienced relative success during the middle eocene. Specialising for a plant based diet offered early herbivores sufficient resources to radiate as energy was not lost to higher trophic levels and few competitors existed at the time.

Reproductive isolation


A key innovation may be a specialised adaptation such as the nectar spurs in Aquilegia, a genus of flowering plant. These spurs aid in pollination by making the nectar further from the stamen, ensuring that insect or bird pollinators pick up pollen as they access it. These lead to reproductive isolation as plants and their pollinators can become specialised to each other, i.e. a species of pollinator exclusively feeds from a species of plant. This is possible as the shape and size of the nectar spur can evolve in response to pollinator adaptations, developing a co-evolutionary relationship. Specialisation in this manner leads to rapid speciation due to insect and plant specificity, and thus the Aquilegia taxa is very diverse with over 50 species.

Criticisms
As an evolutionary theory key innovations has come under strutiny due to the fact that it cannot be tested, and it is possible to to 'cherry pick' examples that fit the hypothesis. In addition the retrospective idenfification of key innovations offers little in terms of understanding the processes and pressures that resulted in the adaptation, and may identify a very complex evolutionary process as a single event. An example of this is the evolution of avian flight, which was identified as a key innovation in 1963 by Ernst Mayr. However, seperate evolutionary changes had to occur throughout the physiology of the avian ancestor, including the enlargment of the cerebellum and the enlargement and ossification of the sternum. These adaptations arose seperately and millions of years apart, instead of as a single evolutionary stage.