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Ontogeny and Phylogeny In biology, epigenesis has at least two distinct meanings: the unfolding development of an organism, and in particular the development of a plant or animal from an egg or spore through a sequence of steps in which cells differentiate and organs form; the theory that plants and animals develop in this way, in contrast to theories of preformation. From Wikepedia More than this! Epigenesis implies that, at every time point, development is a consequence of the confluence of genes and prior experience, which in turn alters the subsequent expression of genes and their interactions with the environment Rousseau Preformationist; adult determined in egg, simply unfolds Locke Tabula rasa; blank slate upon which experience writes Preformationism was the answer to the difficult question of how a complex adult can arise from the simple gametes; the preformed embryo was complete and a direct result of evolution. Closely associated with the preformationist view is the notion that ontogeny recapitulates phylogeny. What does this mean? Ernst Haeckel was principal advocate of the principle that evolution occurred through changes introduced into the adult; ontogenies were simply the reflection of prior evolutionary change, which became compressed in time with evolution thus, evolution was the cause of ontogeny served to curtail research into developmental biology Laws of von Baer (1828) 1. During development, general characters appear before special characters; e.g., those of phylum before those of class (e.g. notochord before limbs). 2. From the more general characters develop the less general and finally the special characters (e.g., differentiation of wings and forelimbs from primitive limb buds) 3. Animals of different species become increasingly different as they develop. 4. Young stages do not resemble the adult stages of ancestors, but resemble the young of those ancestors Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis—addition of characters in youthful stages e.g., amnion of vertbrates Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation—new characters expressed in adulthood e.g. species-specific plumage in birds after final molt Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation—developmental alterations persist in adults e.g. gill slitsgills in fish, gill slitsEustacian tubes in mammals Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction—developmental loss of a character e.g. tails of tadpoles, human fetuses Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction 5. retardation—delay in the development of a character e.g. “wisdom teeth” of humans Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction 5. retardation 6. neoteny—retention of immature characteristics e.g. vertical face of humans; also plumage of flightless birds (e.g. ostrich) Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction 5. retardation 6. neoteny 7. acceleration—characters develop at earlier stages e.g. development of heart in avian embryos; calluses on the knees of ostriches Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction 5. retardation 6. neoteny 7. acceleration 8. hypermorphosis—prolongation of developmental period ontogeny gets longer as special characters are added during evolution. Eight ways altered ontogeny can produce phylogenetic change (deBeer) 1. caenogenesis 2. adult variation 3. deviation 4. reduction 5. retardation 6. neoteny 7. acceleration 8. hypermorphosis deBeer, G. (1958) Embryos and Ancestors. Oxford: Clarendon Press. Eight Principles of de Beer Simplified to Two by Stephen Jay Gould 1. Acceleration 2. Retardation Changes in the relative timing of developmental events more generally known as heterochrony. What is meant by homology? a character in two species is homologous if that SAME character is present in a common ancestor analogy? a character in two species is analogous if it serves a similar function but was not present in a common ancestor arises from parallel or convergent evolution These structures are homologous as forelimbs, as are bird wings (not shown). However, bird wings and bat wings are NOT homologous as wings. Evolutionary changes in ontogenetic timing os capable of producing morphological evolution i.e., by changing the timing of tissue induction, the number and quality of cells induced may be modified How can one determine homology of brains and parts of brains? Problem: No fossil record of brains Similarity of nuclear groups—cell types Physical resemblance Biochemical similarity—e.g. neurotransmitter used Similarity of connnections—afferent and efferent Functional similarity Similar ontogenetic development • minuteness of detail in similarities important • continuance through species of intermediate classes Related problem: How does one distinguish between primitive and derived characters? Primitive: of two characters, the one that has evolutionary precedence is the primitive character Derived: of two characters, the one appearing later is the derived character Out-group analysis A A or B? B B A or B? Leftmost in each diagram: out-group Center in each diagram: sister group Rightmost in each diagram: target Which is the primitive condition, A or B? A Out-group analysis B A B B A B A Left-most in each diagram: out-group Center in each diagram: sister group Rightmost in each diagram: target A Out-group Analysis Applied to Development some species: AB other species: A (throughout life) Terminal Addition or Paedomorphosis i.e., is B added in one species, or lost in the other? A A A terminal addition AB AB AB AB paedomorphosis A Encephalization as an Ontogenetic and Phylogenetic Principle How do brains evolve? Evolution is a consequence of natural selection. Selection acts on the phenotype, not the genotype, although transmission is primarily genetic. Natural selection selects against characters, not for them. How do brains evolve? What are the sources of selection pressures on the brain? Natural selection acts principally on behavior, not directly on the brain How do brains evolve? Two general mechanisms Addition of new projections and targets (invasion hypothesis) Progressive differentiation of previously undifferentiated tissue (parcellation hypothesis). Effects of periphery on brain organization. Motor systems Sensory systems Finally, why do most long connections in the nervous system decussate? What about Behavior? If natural selection acts on brains by selection of behavior, then does it make sense to talk about behavioral homologies? If so, how does one determine behavioral homology?