The only way to get from genotype to phenotype is via development. ~ American developmental biologist Scott Gilbert
Organismal development is an exercise in genetic control. Development is an interpretive process aligned with chemical conglomerations around DNA molecules. Development is also an ecological experience: cells interacting within an environmental context. From an evolutionary perspective, development (evo-devo) presents adaptive results from the experiences of prior generations.
Evolution does not produce novelties from scratch. It works on what already exists, either transforming a system to give it new functions or combining several systems to produce a more elaborate one. ~ François Jacob
Development transpires in a determined order and according to an exact schedule: a precise process of an organism transitioning from state to state. To differentiate “development” in early life from those experiences of “maturity” semantically obscures that living is always a series of state transitions. Development is defined by those youthful times when morphological changes are most pronounced.
Evolutionary innovations show themselves during development. Novelties may arise from deviations in the timing or position of gene expression (heterochrony and heterotopy), changes in growth rate (allometry), and/or alterations in other regulated events. Development derives from a coherent regulatory regime. Evolution emanates from tailoring the development regime.
Heterochrony is an evolutionary modification in the timing or rate of development events, leading to altered morphology. Neoteny – the retention of traits in adulthood of those only previously seen during development – is one kind of heterochrony. Animal domestication, such as seen in dogs, cats, and pigs, often results in neoteny.
Peramorphosis – delayed maturation with extended growth periods – is another type of heterochrony. The massive antlers of Irish elk resulted from peramorphosis.
The evolution of the giraffe is clear evidence of adaptation from a grazing to a browsing lifestyle. ~ Jon Erickson
Giraffe descent began ~25 MYA. Roaming Africa and Eurasia, the progenitor of giraffes looked like an antelope.
~8 MYA, Bohlinia, the direct ancestor of giraffes, appeared in southeastern Europe. Bohlinia had an elongated neck and legs, though not nearly as long as giraffes. Driven by climate change, the progeny of Bohlinia migrated to northern India and China. There the modern long-necked giraffe evolved 7.5 MYA.
7 MYA, giraffes entered Africa. The African giraffe would become the only one of its kind, as further climatic turmoil killed off the Asian giraffes 4 MYA. African giraffes radiated into several species, of which 8 are now extant.
Beginning 8 MYA, a drier climate in Africa thinned the tropical forests, replaced by woodlands, shrubs, and dry savanna. Responsive to the vegetation change, giraffes adopted the modern visually disruptive coat pattern, which acts as camouflage and assists heat regulation.
The giraffe now has only a single cousin: the okapi. Giraffes and okapi diverged 11.5 MYA. Okapi, giraffes, and humans all have the same number of neck bones: 7. Only giraffes undergo the peramorphosis which stretches their necks and legs.
By contrast to heterochrony, heterotopy is a spatial amendment in embryonic development. Like heterochrony, heterotopy changes bodily shapes and sizes.
German zoologist Ernst Haeckel introduced the ideas of heterochrony and heterotopy as facets of morphological innovation. Haeckel cited gonads – created by an evolutionary adjustment in the positioning of the germ layer – as an example of heterotopy. Haeckel’s encompassing idea behind heterochrony and heterotopy was his disproven hypothesis “ontogeny recapitulates phylogeny”: that embryonic development was a recapitulation of evolutionary stages from an animal’s remote ancestors (phylogeny) during gestation (ontogeny).
Allometry involves adaptive scaling in organisms and its commensurate effects. Dinosaurs were an upscaling from reptilian ancestors, whereas birds were a downsizing from dinosaurs.
Metamorphosis is the radical development plan of shedding one body for a different one altogether. The price of metamorphosis is high. Tearing apart an anatomy and building a new one burns a lot of calories. Metamorphosis is a complicated process which can go awry, resulting in defects. Metamorphosis takes time, leaving an animal vulnerable to predators and parasites.
Yet 80% of all animals experience metamorphosis: from jellyfish to butterflies to flatfish to frogs. This common life-history strategy arose only a few times. Once metamorphosis appears it rarely is abandoned for a different development cycle.
Metamorphosis is not easy to evolve but hard to lose. ~ Dutch evolutionary biologist Hanna ten Brink et al
Many foods are seasonal. Metamorphosis offers the distinct advantage of being able to change food source.
In some vertebrates, metamorphosis is camouflaged, but it is never lost. ~ French biologist Vincent Laudet
Our birth is a metamorphosis in more than swapping food supply. Leaving the womb sparks major tissue changes, governed by some of the same hormones that spark more amazing changes in other animals.
Metamorphosis also affords the means to change locomotion, and so expand range. Under a metamorphosis program, the extensive equipment necessary for mating need not develop until adulthood. Starting small and metamorphizing into a different vehicle offers an economy which cannot be achieved any other way.
Development is ultimately the story of cellular self-construction and interaction among cells. Changes in cell function and structure during development derive from the presence and activities of proteins. Proteins are the macromolecular expressions from a myriad of construals of genic material. Development is a crafting of organic artifacts from a set of symphonic communications which have as their score DNA and attendant markings.
Homeoboxes are regulatory genic sequences that steer morphogenesis in fungi, plants, and animals. Homeobox genes provide instructions for producing 60 amino acids, collectively known as the homeodomain. Most homeodomain-containing proteins act as transcription factors: controlling genetic interpretation during ribosomal protein production.
Often well-conserved, a certain homeobox protein may perform a selfsame function in distantly related organisms. Segmentation in annelids, insects, crustaceans, and vertebrates owes to homologous homeoboxes. Similar homologous homeobox performances are found in nerve and muscle cell development, and in many other developmental processes.
With homeoboxes conserved as evolutionary anchors, adaptive modifications are typically accomplished via tweaks in regulatory regimes. These are most often initiated epigenetically, with reflective genetic adjustments much later, after an adaptation is well proven in its utility.
DNA sequences are only the grossest manifestation of the intricacy of genetics.
Developmental innovations may be had by employing conserved regulatory genes in a novel way. Major biological changes commonly derive from significant editing of a specific genetic locus as contrasted to the culmination of minor changes in many genes.
One example of conserved gene regulation altered to major evolutionary effect is the specification of the ventro-dorsal axis in arthropods versus the dorso-ventral axis in vertebrates. In annelids and arthropods, the circulatory system is dorsal and the nerve cord ventral, whereas the locations of these systems are reversed in vertebrates. These fundamentally different patterns result from tweaks in just 2 complementary pairs of homologous genes.
The diversity of mammalian forms owes largely to selective editing of genetic expression.
The major differences between mammalian species lie not in the genes themselves, but where genes are switched on and off – that is, in gene regulation. ~ Spanish molecular biologist Diego Lozano
Beyond the interpretation of DNA, signal pathways define development. Depending upon the extracellular signal, phosphorylating enzymes activate different transcription proteins by following distinct routes from cell membrane to nucleus. In plants, common signals such as ethylene act in transduction pathways variously, affecting seed germination, cell development, floral blooming, and fruit ripening.
Many such pathways are strong conserved. Developmental evolution employs regulatory modifications to produce new functions by rearranging constituent activity, such as altering signals, pathways, and/or the targets of signaling.
Because development is based on dynamic processes, a cell’s responsiveness to signals changes with its history: the same signal pathway can express or inhibit different genes depending on a cell’s position in time and space. ~ Canadian biologist Brian Hall & Icelandic biologist Benedikt Hallgrímsson
The seeming simplicity of biological modularity is belied by it occurring via genic networks. A gene network may operate in several tissues and organs, and yet adaptively alter a tissue or organ very specifically.
The Mexican tetra has 2 varieties: a sighted one found in surface pools and a blind one in caves. The vestigial eyes of cavefish are compensated by expanded taste buds and heightened lateral line sensitivity. (The loss of sight in cave tetra saves ~15% in metabolic energy.) These 3 developmental modules are controlled by a single regulatory gene network.
Because development is hierarchical, a variety of entities can act as evo-devo modules. ~ Canadian biologist Gillian Gass & American biologist Jessica Bolker
Though the mammalian mandible is a single bone, it is composed via 6 modules, each of which arises from separate populations of cells, and each of which is subject to independent genetic control. Coherent adaptation in this complexly-constructed jawbone is illustrated by the tiny honey possum, a marsupial nectarivore. With no need to chew, the honey possum has the most reduced lower jaw and teeth of any mammal.
Modularity is an important property in biology because it helps a system ‘save its work’ while allowing further evolution. Modularity provides a basis to explore the space of biological possibility. ~ American biochemist Michael Deem et al
Modularity provides a readily adaptive means for independent-yet-integrated development, and for mosaic evolution. Biological modules evolve much like genes, doing so via: duplication, where the original is conserved and a copy effects a new trait; dissociation, in which altering a regulatory regime generates an adaptive novelty; and co-option, where a module is subsumed within another. These exemplary evolutionary techniques are neither exhaustive nor mutually exclusive.
There are of course constraints on evolutionary plasticity, but, beyond hand-waving theories about Nature’s ‘laws’, evolutionary biologists have been relegated to cataloging. Surprises, such as saltation, indicate that the wiles of coherence are beyond ken. Evolution appears alternately astonishing and predictable in hindsight.