Evolutionary developmental biology

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embryonic development and hence the form of adult bodies. These genes have been highly conserved through hundreds of millions of years of evolution
.

Evolutionary developmental biology (informally, evo-devo) is a field of

biological research that compares the developmental processes of different organisms to infer how developmental processes evolved
.

The field grew from 19th-century beginnings, where

embryonic development was controlled at the molecular level. Charles Darwin noted that having similar embryos implied common ancestry, but little progress was made until the 1970s. Then, recombinant DNA technology at last brought embryology together with molecular genetics. A key early discovery was of homeotic genes that regulate development in a wide range of eukaryotes
.

The field is composed of multiple core evolutionary concepts. One is

pax-6, from the evo-devo gene toolkit. These genes are ancient, being highly conserved among phyla; they generate the patterns in time and space which shape the embryo, and ultimately form the body plan of the organism. Another is that species do not differ much in their structural genes, such as those coding for enzymes; what does differ is the way that gene expression is regulated by the toolkit genes. These genes are reused, unchanged, many times in different parts of the embryo and at different stages of development, forming a complex cascade of control, switching other regulatory genes as well as structural genes on and off in a precise pattern. This multiple pleiotropic reuse explains why these genes are highly conserved, as any change would have many adverse consequences which natural selection
would oppose.

New

neo-Lamarckian theory that epigenetic changes are later consolidated at gene level
, something that may have been important early in the history of multicellular life.

History

Further information: History of evolutionary thought

Early theories

Further information: Aristotle's biology

Philosophers began to think about how animals acquired form in the

The Parts of Animals treatise, he argues that Empedocles' theory was wrong. In Aristotle's account, Empedocles stated that the vertebral column is divided into vertebrae because, as it happens, the embryo twists about and snaps the column into pieces. Aristotle argues instead that the process has a predefined goal: that the "seed" that develops into the embryo began with an inbuilt "potential" to become specific body parts, such as vertebrae. Further, each sort of animal gives rise to animals of its own kind: humans only have human babies.[1]

Recapitulation

Main article: Recapitulation theory
Embryology theories of Ernst Haeckel, who argued for recapitulation[2] of evolutionary development in the embryo, and Karl Ernst von Baer's epigenesis

A recapitulation theory of evolutionary development was proposed by Étienne Serres in 1824–26, echoing the 1808 ideas of Johann Friedrich Meckel. They argued that the embryos of 'higher' animals went through or recapitulated a series of stages, each of which resembled an animal lower down the great chain of being. For example, the brain of a human embryo looked first like that of a fish, then in turn like that of a reptile, bird, and mammal before becoming clearly human. The embryologist Karl Ernst von Baer opposed this, arguing in 1828 that there was no linear sequence as in the great chain of being, based on a single body plan, but a process of epigenesis in which structures differentiate. Von Baer instead recognized four distinct animal body plans: radiate, like starfish; molluscan, like clams; articulate, like lobsters; and vertebrate, like fish. Zoologists then largely abandoned recapitulation, though Ernst Haeckel revived it in 1866.[3][4][5][6][7]

Evolutionary morphology

Further information: Morphology (biology) and Body plan
A. Lancelet (a chordate), B. Larval tunicate, C. Adult tunicate. Kowalevsky saw that the notochord (1) and gill slit (5) are shared by tunicates and vertebrates.

From the early 19th century through most of the 20th century,

chordates.[9][11]

19th century zoology thus converted

Nauplius larva, identifying several parasitic species that had not been recognized as crustaceans. Müller also recognized that natural selection must act on larvae, just as it does on adults, giving the lie to recapitulation, which would require larval forms to be shielded from natural selection.[9] Two of Haeckel's other ideas about the evolution of development have fared better than recapitulation: he argued in the 1870s that changes in the timing (heterochrony) and changes in the positioning within the body (heterotopy) of aspects of embryonic development would drive evolution by changing the shape of a descendant's body compared to an ancestor's. It took a century before these ideas were shown to be correct.[12][13][14]

In 1917,

gastropod's spiral shell, can radically alter an animal's form, though he preferred a mechanical to evolutionary explanation.[15][16] But without molecular evidence, progress stalled.[9]

giant pufferfish
, may arise, without molecular evidence.

In 1952,

Boris Belousov had run experiments with similar results, but was unable to publish them because scientists thought at that time that creating visible order violated the second law of thermodynamics.[18]

The modern synthesis of the early 20th century

Main article: Modern synthesis (20th century)

In the so-called modern synthesis of the early 20th century, between 1918 and 1930 Ronald Fisher brought together Darwin's theory of evolution, with its insistence on natural selection, heredity, and variation, and Gregor Mendel's laws of genetics into a coherent structure for evolutionary biology. Biologists assumed that an organism was a straightforward reflection of its component genes: the genes coded for proteins, which built the organism's body. Biochemical pathways (and, they supposed, new species) evolved through mutations in these genes. It was a simple, clear and nearly comprehensive picture: but it did not explain embryology.[9][19] Sean B. Carroll has commented that had evo-devo's insights been available, embryology would certainly have played a central role in the synthesis.[20]

The evolutionary embryologist

fossil record, since embryos fossilise poorly. As the gaps in the fossil record had been used as an argument against Darwin's gradualist evolution, de Beer's explanation supported the Darwinian position.[23] However, despite de Beer, the modern synthesis largely ignored embryonic development to explain the form of organisms, since population genetics appeared to be an adequate explanation of how forms evolved.[24][25][a]

The lac operon

RNA Polymerase, 2: Repressor, 3: Promoter, 4: Operator, 5: Lactose, 6–8: protein-encoding genes
, controlled by the switch, that cause lactose to be digested
Main article: Lac operon

In 1961,

an enzyme that splits a sugar, lactose; and lactose itself was the stimulus that switched the genes on. This was a revelation, as it showed for the first time that genes, even in organisms as small as a bacterium, are subject to precise control. The implication was that many other genes were also elaborately regulated.[27]

The birth of evo-devo and a second synthesis

In 1977, a revolution in thinking about evolution and developmental biology began, with the arrival of

Stephen J. Gould and the paper "Evolution and Tinkering"[28] by François Jacob. Gould laid to rest Haeckel's interpretation of evolutionary embryology, while Jacob set out an alternative theory.[9]
This led to
arthropods, one of the major phyla of invertebrate animals.[33]
fungi such as yeasts, and in plants.[34][35] There were evidently strong similarities in the genes that controlled development across all the eukaryotes.[36]
In 1980,
Eric Wieschaus described gap genes which help to create the segmentation pattern in fruit fly embryos;[37][38] they and Lewis won a Nobel Prize for their work in 1995.[34][39]

Later, more specific similarities were discovered: for example, the

parapodia of marine annelid worms, the ampullae and siphons of tunicates, and the tube feet of sea urchins. It was evident that the gene must be ancient, dating back to the last common ancestor of bilateral animals (before the Ediacaran Period, which began some 635 million years ago). Evo-devo had started to uncover the ways that all animal bodies were built during development.[41][42]

The control of body structure

Deep homology

Further information: Deep homology

Roughly spherical eggs of different animals give rise to unique morphologies, from jellyfish to lobsters, butterflies to elephants. Many of these organisms share the same structural genes for body-building proteins like collagen and enzymes, but biologists had expected that each group of animals would have its own rules of development. The surprise of evo-devo is that the shaping of bodies is controlled by a rather small percentage of genes, and that these regulatory genes are ancient, shared by all animals. The giraffe does not have a gene for a long neck, any more than the elephant has a gene for a big body. Their bodies are patterned by a system of switching which causes development of different features to begin earlier or later, to occur in this or that part of the embryo, and to continue for more or less time.[8]

The puzzle of how embryonic development was controlled began to be solved using the fruit fly Drosophila melanogaster as a model organism. The step-by-step control of its embryogenesis was visualized by attaching fluorescent dyes of different colours to specific types of protein made by genes expressed in the embryo.[8] A dye such as green fluorescent protein, originally from a jellyfish, was typically attached to an antibody specific to a fruit fly protein, forming a precise indicator of where and when that protein appeared in the living embryo.[43]

pax-6
gene controls development of eyes of different types across the animal kingdom.

Using such a technique, in 1994

embryogenesis[47] and has caused a radical revision of the meaning of homology in evolutionary biology.[45][46][20]

Gene toolkit

Expression of homeobox (Hox) genes in the fruit fly
Main article: Evo-devo gene toolkit
Further information: Plant evolutionary developmental biology

A small fraction of the genes in an organism's genome control the organism's development. These genes are called the developmental-genetic toolkit. They are highly conserved among

morphogens that diffuse through the embryo. All of these help to define the fate of undifferentiated cells in the embryo. Together, they generate the patterns in time and space which shape the embryo, and ultimately form the body plan of the organism. Among the most important toolkit genes are the Hox genes. These transcription factors contain the homeobox protein-binding DNA motif, also found in other toolkit genes, and create the basic pattern of the body along its front-to-back axis.[20]
Hox genes determine where repeating parts, such as the many
bodyplan,[49] homeobox genes are also found in plants, implying they are common to all eukaryotes.[50][51][52]

The embryo's regulatory networks

A gene regulatory network
Further information: Regulation of gene expression and Transcriptional regulation

The protein products of the regulatory toolkit are reused not by duplication and modification, but by a complex mosaic of

eyeless cis-regulatory region contains 6 cis-regulatory elements in over 7000 base pairs.[20] The regulatory networks involved are often very large. Each regulatory protein controls "scores to hundreds" of cis-regulatory elements. For instance, 67 fruit fly transcription factors controlled on average 124 target genes each.[20] All this complexity enables genes involved in the development of the embryo to be switched on and off at exactly the right times and in exactly the right places. Some of these genes are structural, directly forming enzymes, tissues and organs of the embryo. But many others are themselves regulatory genes, so what is switched on is often a precisely-timed cascade of switching, involving turning on one developmental process after another in the developing embryo.[20]

Gene product distributions along the long axis of the early embryo of a fruit fly

Such a cascading regulatory network has been studied in detail in the

nanos genes are at high concentration near the posterior end, and give pattern to the hindmost abdominal segments. The effects of these genes interact; for instance, the Bicoid protein blocks the translation of caudal's messenger RNA, so the Caudal protein concentration becomes low at the anterior end. Caudal later switches on genes which create the fly's hindmost segments, but only at the posterior end where it is most concentrated.[53][54]

bicoid
, setting up stripes across the embryo which start to pattern the body's segments.

The Bicoid, Hunchback and Caudal proteins in turn regulate the transcription of gap genes such as giant, knirps, Krüppel, and tailless in a striped pattern, creating the first level of structures that will become segments.[37] The proteins from these in turn control the pair-rule genes, which in the next stage set up 7 bands across the embryo's long axis. Finally, the segment polarity genes such as engrailed split each of the 7 bands into two, creating 14 future segments.[53][54]

This process explains the accurate conservation of toolkit gene sequences, which has resulted in deep homology and functional equivalence of toolkit proteins in dissimilar animals (seen, for example, when a mouse protein controls fruit fly development). The interactions of transcription factors and cis-regulatory elements, or of signalling proteins and receptors, become locked in through multiple usages, making almost any mutation deleterious and hence eliminated by natural selection.[20]

The mechanism that sets up every

Myosin II molecular motors, which are not conserved across species. The process may have been started by movements of sea water in the environment, later replaced by the evolution of tissue movements in the embryo.[56][57]

The origins of novelty

See also: History of evolutionary thought § 21st century

Among the more surprising and, perhaps, counterintuitive (from a

gene regulation.[20][59][60][61]

Variations in the toolkit

developmental-genetic toolkit genes controlling wing pattern formation
.

Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's

in parallel. distal-less generates wing patterns in the butterflies Heliconius erato and Heliconius melpomene, which are Müllerian mimics. In so-called facilitated variation,[68] their wing patterns arose in different evolutionary events, but are controlled by the same genes.[69] Developmental changes can contribute directly to speciation.[70]

Consolidation of epigenetic changes

Main articles: Genetic assimilation and Transgenerational epigenetic inheritance
Further information: Extended evolutionary synthesis

Evolutionary innovation may sometimes begin

Gerd B. Müller have suggested that organisms early in the history of multicellular life were more susceptible to this second category of epigenetic determination than are modern organisms, providing a basis for early macroevolutionary changes.[73]

Developmental bias

Main article: Developmental bias
Among the centipedes, all members of the Geophilomorpha are constrained by a developmental bias to have an odd number of segments, whether as few as 27 or as many as 191.

Development in specific lineages can be biased either positively, towards a given trajectory or phenotype,

gastropods, the snail-type shell is always built as a tube that grows both in length and in diameter; selection has created a wide variety of shell shapes such as flat spirals, cowries and tall turret spirals within these constraints. Among the centipedes, the Lithobiomorpha always have 15 trunk segments as adults, probably the result of a developmental bias towards an odd number of trunk segments. Another centipede order, the Geophilomorpha, the number of segments varies in different species between 27 and 191, but the number is always odd, making this an absolute constraint; almost all the odd numbers in that range are occupied by one or another species.[74][76][77]

Ecological evolutionary developmental biology

Ecological evolutionary developmental biology

epigenetic inheritance, genetic assimilation, niche construction and symbiosis.[79][80]

See also

Notes

  1. ^ Though C. H. Waddington had called for embryology to be added to the synthesis in his 1953 paper "Epigenetics and Evolution".[26]
  2. ^ Positive bias is sometimes called developmental drive.[74]
  3. ^ Spanish Wikipedia has an article on the subject: eco-evo-devo.

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External links

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