The Elements of Evolution (47-4) Development continued


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.