Evolutionary developmental biology, or "evo-devo" for short, has become an active area in biology research over the last few decades.
One of its first eminent advocates was Ernst Haeckel (1834-1919), well-known for his principle that "ontogeny recapitulates phylogeny", or in simpler words, "growth reruns evolution". That has turned out to be a rather grotesque oversimplification, and his most famous diagram has outright errors - it makes early vertebrate embryos look too similar.
Before him, Karl Ernst von Baer (1792-1876) had noted these principles of development, though he did not accept evolution:
- General characters appear earlier in development than specialized characters.
- Less general character appear later (and build on) the general framework of earlier stages.
- The embryo of any organism, rather than passing through the stages of other forms, tends to progressively differentiate itself from them.
- The embryo of one animal form never resembles the adult of another, but only its embryo.
In the early to mid 20th cy., biologists discovered several "homeotic mutations". They make one body part develop like another:
- Antennapedia - makes insect antennae become walking legs
- Proboscipedia - like antennapedia, but for mouthparts
- Bithorax - flies' halteres become wings
In the 1980's, biologists found some genes that make Drosophila melanogaster fruit flies grow segments and that give those segments their identities. The segment identity genes are better-known as homeobox or Hox genes. Some of them are named after the homeotic effects of some associated mutations, like antennapedia. Hox-associated mutations can have homeotic effects because they make Hox genes develop in the wrong places, or else change the effects of the Hox proteins. Since then, genes homologous to these genes have been found across the animal kingdom, specifying nose-to-tail identity in the same order in most species.
Plants have a somewhat similar mechanism for specifying flower parts, though with MADS-box genes instead of homeobox genes expressed along their stem-length axes.
There are also numerous homologies among genes specifying dorsoventral or back-to-belly identity, reviving an old hypothesis that vertebrates are flipped over relative to most of the rest of the animal kingdom.
- Invertebrates: ventral, vertebrates: dorsal
- Central nervous system
- Invertebrates: dorsal, vertebrates: ventral
The homologies extend to various other sorts of genes, like genes involved in making eyes. This has given rise to the concept of "deep homology", that many important features are homologous across much of the animal kingdom, even if they often look very different.
PZ Myers has blogged numerous times on evo-devo and related discoveries.
- Maternal effect genes - like bicoid, whose protein gets added to fruit-fly eggs
- Basics: Gastrulation - in frogs and amniotes
- Basics: Gastrulation, invertebrate style - in sea urchins and fruit flies
- The evolution of deuterostome gastrulation
- Basics: Neurulation - how the vertebrate spinal cord forms, by folding in
- Neurulation in zebrafish - easy to observe
- Return of the Son of the Bride of Haeckel - has a nice illustration of early vertebrate embryos
- A Devonian lamprey, Priscomyzon
- Where do the hagfish fit in?
- Hagfish embryos!
- Amphioxus and the evolution of the chordate genome
- How to make a tadpole - a sea-squirt larva
- Ascidian evo-devo
- Evolution of direct development in echinoderms - numerous times
- Acoelomorph flatworms and precambrian evolution - how bilaterians could have evolved from planula-like ancestors
- Reinventing the worm - Buddenbrockia, a cnidarian worm
- The heartbreaking beauty of development - from gametes to sand dollars
- Organismal size over evolutionary time is a constrained stochastic property - its maximum increases because that's what's available for change
- The Ubiquity of Exaptation - reuse of old features for different purposes
Main body axis: rostral-caudal / cranial-caudal / nose-to-tail
- A brief overview of Hox genes
- Hox complexity
- Hox genesis
- Hox cluster disintegration
- The Hox code - effects of vertebrate-Hox mutations
- Vertebral variation, Hox genes, development, and cancer
- Wells’ flagrantly false commentary on Hox complex structure
- Jellyfish lack true Hox genes!
- Axis formation in spider embryos - spider and grasshopper embryos both look like stubby-legged worms
- Snake segmentation - they can have 300 vertebrae
- How to make a snake - by modifying a lizard's genes
- How was the vertebrate/arthropod LCA segmented? - beetles grow their segments halfway between vertebrate and fly segment-growth mechanisms
- Swimming in the Cambrian - about fish muscle-block development and evolution
- Cephalopod development and evolution
- The greatest science paper ever published in the history of humankind - on cephalopod evolution
- Squid Hox genes
- Mother of all squid!
Other body axes: dorsoventral / back-to-belly and left-right
- We have the brains of worms - dorsoventral inversion in vertebrates, arthropods, and now annelids
- Patterning the nervous system with Bmp
- Ancient rules for Bilaterian development
- Bilateral symmetry in a sea anemone - the same mechanism as in bilaterians?
- Generating right-left asymmetries
- Symmetry breaking and genetic assimilation - on how asymmetries evolved
- Snails have nodal! - which makes left-right asymmetry, like in vertebrates
- Spiral cleavage - mollusk and annelid early embryos have that pattern of cell division
- Chirality in Euhadra - a land snail; many land snails have asymmetric shell coiling which can come in mirror images
- Polar lobes and trefoil embryos in the Precambrian
- Why are flounder funny looking?
Eyes and other sense organs
- The eye as a contingent, diverse, complex product of evolutionary processes
- Rhabdomeric and ciliary eyes - photoreceptors across the animal kingdom
- Evolution of vertebrate eyes
- How many genes does it take to make a squid eye?
- Brachiopods: another piece in the puzzle of eye evolution
- Epistasis and pathways in fly eye pigmentation | Pharyngula
- Modular gene networks as agents of evolutionary novelty - mainly about eye evolution
- The eyes of Anomalocaris
- Complex eyes in the Cambrian - before their owner was identified as Anomalocaris
- Evolution of sensory signaling - solute sensing (chemical and related) vs. solvent sensing (pressure and related)
Skeletal features, limbs, and internal organs
- Brains and beaks - embryo to adult for crocodilian, dinosaur, and bird skulls
- Evolution of the jaw
- Development, medicine, and evolution of the neck and shoulder
- Generic bumps and recycled genetic cascades - how vertebrate limbs form, including a well-known sexual limb
- Digit numbering and limb development - describes the frame-shift theory of birds' hands' digits: they lost the outer digits, making II-III-IV, but the first one develops like a thumb, making I-II-III
- Limusaurus inextricabilis - a dinosaur with hands in the middle of birds' frame shift
- Embryonic similarities in the structure of vertebrate brains
- Simple rules for folding a gut
- The basics of building a kidney - we go through three kinds of kidney as embryos
- Animations of urogenital development
- Evolution of the mammalian vagina
Genes and genomes
- The curse of the gingers - why the evolution of the redhead pigment pheomelanin?
- Evolving proteins in snakes - how snakes can become resistant to toxins in their prey
- Gene regulatory networks and conserved noncoding elements
- A tiny bit of knowledge is a dangerous thing - on some misunderstandings of the evolution of the human, chimpanzee, and gorilla genomes
- The platypus genome
- Basics: Synteny - conserved gene arrangements
- Basics: How can chromosome numbers change?
- Pufferfish and ancestral genomes - human-pufferfish synteny
- Reproductive history writ in the genome - what happened to mammalian yolk-protein genes
- Tandem repeats and morphological variation - in dog breeds
- A little cis story - gene-regulation evolution in fruit flies
- Basics: Sonic Hedgehog - a gene involved in a lot of development
- The sea urchin genome
- The molecular foundation of the phylotypic stage
- A complex regulatory network in a diploblast - sea anemones share some development-control genes with bilaterians
- Cnidarian molecular/genetic Entwicklungsmechanik - development mechanics
- Common elements of eumetazoan gene organization in an anemone - vertebrates often have less organization change than fruit flies
- Spongeworthy genes - eumetazoans and sponges share a lot of genes involved in multicellularity, genes that fungi and slime molds don't have
- Sponges have synapses?
- The choanoflagellate genome and metazoan evolution - those protists are animals' closest relatives
- How to afford a big sloppy genome
- The true story of the Archaean genetic expansion
- PZ Myers’ Own Original, Cosmic, and Eccentric Analogy for How the Genome Works -OR- High Geekology
Conferences and broader issues
- How I spent my morning at SICB
- The morning session at the Oregon evo-devo symposium
- The Sunday morning session at the Oregon evo-devo symposium
- The problem with evo devo - some of his criticisms of the field
- Those disreputable evo-deviants and their bigotry against the single-celled - about criticism of evo-devo biologists for animal chauvinism
- Evo-devo is not the whole of biology
- and there may be more in the Development category at Pharyngula.