evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life fo ...

, a key innovation, also known as an adaptive breakthrough or key adaptation, is a novel

phenotypic In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

trait that allows subsequent

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

and success of a taxonomic group. Typically they bring new abilities that allows the

taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

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 In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

. 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", although a broader, contemporary definition holds that "a key innovation is an evolutionary change in individual traits that is causally linked to an increased diversification rate in the resulting clade". The theory of key innovations has come under attack because it is hard to test in a scientific manner, but there is 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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

becoming less likely and allow the taxa to expand and

speciate Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

Latex Latex is an emulsion (stable dispersion) of polymer microparticles in water. Latexes are found in nature, but synthetic latexes are common as well. In nature, latex is found as a milky fluid found in 10% of all flowering plants (angiosperms ...

and

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on natu ...

canals in plants are used to deter predators by releasing a sticky secretion when punctured which can immobilise insects and some contain toxic or foul tasting substances. They have evolved independently approximately 40 times and are considered a key innovation. By increasing the plant's resistance to predation the canals increase the species fitness and allow them to escape being eaten, at least until the predator evolves an ability to overcome the defence. During the period of resistance the plants are less likely to become extinct and can diversify and speciate, and as such taxa with latex and resin canals are more diverse than their canal lacking sister taxa.

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 Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

and

. A classic example of this is the fourth

cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifurc ...

of mammalian

molars The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

, the

hypocone A cusp is a pointed, projecting, or elevated feature. In animals, it is usually used to refer to raised points on the crowns of teeth. The concept is also used with regard to the leaflets of the four heart valves. The mitral valve, which has two ...

, 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, and allowed early mammals to invade novel niches through the evolution of specialised herbivores, which experienced relative success during the middle

eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''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 result in reproductive isolation, whereby those individuals with the innovation no longer breed with those without. This can lead to rapid speciation as the two populations separate and accumulate mutations. The nectar spurs in ''

Aquilegia ''Aquilegia'' (common names: granny's bonnet, columbine) is a genus of about 60–70 species of perennial plants that are found in meadows, woodlands, and at higher altitudes throughout the Northern Hemisphere, known for the spurred petals Pu ...

'', a diverse genus of flowering plant, are considered a key innovation because of this. Nectar spurs aid in pollination by making the

nectar Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists ...

further from the stamen, ensuring that insect or bird

pollinators A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the maj ...

pick up

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

as they access it. These led rapid speciation within the genus as plants and their pollinators can become specialised to each other i.e. a species of pollinator exclusively feeds from a species of plant, and thus plant populations could easily become reproductively isolated from one another. In addition the shape and size of the nectar spur can evolve in response to pollinator adaptations, developing a co-evolutionary relationship. The genus ''Aquilegia'' has over 50 species.

Criticisms

As an evolutionary theory, key innovations has come under critical scrutiny due to the fact that it is hard to test. Identification depends on finding correlation between the innovation and increased diversity by comparing sister taxa, but this does not prove causality or isolate other causes of diversity such as stochasticity or habitat, and it is possible to 'cherry pick' examples that fit the hypothesis. In addition the retrospective identification 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 Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists. He was also a renowned Taxonomy (biology), taxonomist, tropical explorer, ornithologist, Philosophy of biology, philosopher o ...

. However, separate evolutionary changes had to occur throughout the physiology of the avian ancestor, including the enlargement of the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

and the enlargement and

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

of the

sternum The sternum or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major blood vessels from injury. Sh ...

. These adaptations arose separately, and millions of years apart, not in one step.

References