Heterodox Concepts In Modern Evolutionary Embryology, 1900 .
Electronic Journal of Biology, 2016, Vol.12(3): 309-313Heterodox Concepts in Modern Evolutionary Embryology,1900-1950Andres Galera*Centro de Ciencias Humanas y Sociales, IH, CSIC, Spain.*Corresponding author. Tel: ( 34) 916022462; E-mail: andres.galera@cchs.csic.esCitation: Galera A. Heterodox Concepts in Modern Evolutionary Embryology, 1900-1950. Electronic J Biol, 12:4Received: May 31, 2016; Accepted: June 24, 2016; Published: July 01, 2016Review ArticleAbstractWhatever the evolutionary model we adopt, in thecase of sexual reproduction, the process has anembryological significance because this is the wayto generate individuals and to perpetuate the life.The connection between evolution and embryologyis a necessary event. In this evolutionary context, thekey question is: how two species are formed fromthe same biological unit? During the first half of the20th century embryologists as Richard Goldschmidt,Conrad Waddington, and Walter Garstang answeredthe question from a heterodox point of view. Theyintroduced new concepts that changed the wayto thinking the evolution. This essay analyzes thisunorthodox thought and its scientific impact.Keywords: Epigenetic landscape; Evolution;Embryology; Garstang; Goldschmidt; Heterochrony;Hopeful monster; Mendelian heredity; Recapitulation;Waddington.IntroductionTaking things to the simplest level, we can say thatthe fundamental problem of organic evolution knowshow a living being is formed. Multi-cellular organismshabitually use sexual reproduction to multiply: thefusion of parental gametes makes up a cell withthe potential to develop into another individual. Ouranalysis will focus on this mode of reproduction. Insexual reproduction, evolution takes place wheninformation regarding morphological changes isintegrated into the reproductive mechanism. Theconnection between evolution and embryology isclear – it is a necessary occurrence. Embryologistsswiftly made this connection, back in the 19th century,when the theory of evolution was first formulated.Under this criterion, the question of how speciesevolve is directly linked to embryogenesis. Thepurpose is to understand how a new species isformed during the morphological sequence occurringin the ovum after fertilisation. In order to achieve this,it became necessary to understand the mechanismsthat control the reproductive process.Supported by compared embryology, the theory ofrecapitulation, i.e., the conception of consideringISSN 1860-3122embryogenesis as a telling of the evolutionaryhistory of a species, exploded, under different titles,during the early 1800s. The theory soon becamepart of embryological knowledge, but its strongestinvolvement in the evolutionary debate took place inthe 1860s. It is well known that most of the creditbelongs to the German zoologist Ernst Haeckel andhis book Generelle Morphologie der Organismen,published in 1866 [1]. Known as the biogenetic law,Haeckel’s theory states that the different embryonicstates represent the different adult forms adoptedby the species along its evolutionary path. In brief,ontogeny recapitulates phylogeny. This statementis as widespread as it is erroneous. Let us take thecase of the human species. During its development,the human embryo resembles, successively, a fish,an amphibian, a reptile, a mammal, a primate, untilfinally adopting the human morphology.Under recapitulation theory, evolution is a summaryof parts, each identifying a specific final product. Letus ask ourselves, how does this happen? Haeckeldivided inheritance into two categories. One groupcontained standard characteristics transmitted bythe parents. The second consisted of characteristicsacquired by the adult through adaptation to itsenvironment. This was a unique concession toLamarck. Acquired inheritance was the source ofevolutional variability, manifested in the final phase ofthe embryonic cycle and thus increasing the numberof stages. Under evaluation, the argument presentsa serious practical problem that did not go unnoticed.The continuous addition of evolutionary stageswould result in a physiological distortion of ontogeny,making it unfathomable. The biogenetic law wastherefore reformulated. Embryogenesis would nolonger constitute an absolute recapitulation, but acondensed repetition of a species’ past. By the 20thcentury there was little doubt as to the falsehood ofthe theory. Haeckel himself acknowledged havingmanipulated tests in order to facilitate comprehension,by simulating a common evolutionary sequencebetween the embryos of the different species hehad compared [2-4]. Embryology returned to thescientific logic expressed by the biologist Ernstvon Baer in the 1820s. A simple, common senseargument: the embryo only resembles membersof its own species, and over the course of its- 309 -
Electronic Journal of Biology, 2016, Vol.12(3): 309-313development progresses from a general state toa particular state, from amorphous to specific,gradually acquiring the anatomical characteristicsof the informative being contained in the ovum [5].Embryonic coincidences between different groupsare no more than a reflection of their common past.An embryo does not recapitulate its past; it partiallyrepeats the ontogeny of its ancestors. So, what is theevolutionary significance of reproduction? This is arelevant, necessary and cardinal matter, shaped overthe course of the 20th century via genetics and thebiology of development.An early answer to the question was obtainedin 1866, although it went unnoticed. This is thenotorious pea plant experiment carried out by GregorJohann Mendel at the Cistercian Abbey in Brno.Over the course of a decade this monk crossbredthousands of plants and examined their fruit. Hestudied their shape, size, colour and texture in orderto explain evolution in a precise context: discoveringwhat biological mechanism enables offspring toinherit parental traits. His hereditary theory laidthe foundations for genetics. In his mind, speciesdid not evolve either under the influence of theirenvironment or guided by natural selection. Chancewas responsible for mixing parental characteristicsduring fertilisation. Spontaneously at times, theresulting genetic combination would stabilise in theoffspring. In this case, descendants would suddenlyform a specific, reproductively constant group.Another species would emerge. Plainly speaking,evolution would be the consequence of a singularchromosomic recombination [6].In 1900, Mendel’s laws were rediscovered, andgenetics started its unstoppable biological ascent.First there was the formulation of chromosomic theory;then came the notion of genes: the chromosomicunity responsible for phenotypical expression.Using Mendel’s model, the Dutch botanist Hugo deVries, one of the fortunate rediscoverers, wrote Diemutationstheorie; two innovative volumes devoted tothe origin of species [7,8]. In summary, his mutationtheory ventured that evolution did not follow Darwin’sprinciples. Species were not formed via the slow,gradual accumulation of small organic changes, butthrough the reproductive manifestation of abrupttypological variations, spontaneous, stable, sudden,hereditary changes, known as mutations, thatimmediately altered the parental typology. This eventwould be collective and final; occurring in differentmorphological groups of descendants. Mutationreplaced natural selection as the presumed motorbehind evolution. It would now be the primary cause.Only a few years later, convinced by Mendelism,the embryologist Thomas Hunt Morgan, of theUniversity of Columbia, started his experiments withDrosophila melanogaster, better known as the fruitfly. This tiny, hairy insect with protruding, vermillioncoloured eyes was to revolutionise genetics. InMorgan’s laboratory, flies were not bred by chance.ISSN 1860-3122The idea was to explore whether progeny displayedthe spontaneous modifications established underthe theory. This objective was partly achieved. Aftera number of years, an individual with white eyeswas born. This fly proved the existence of naturalmutations, although its evolutionary significance wasnot as had been expected: it was not a new speciesof fly. Tens of experimental mutants would appearover the following years. Flies with no wings, or withtheir wings curled up, stunted or grooved, and flieswith brown, chestnut or peach-coloured eyes, areexamples of this amazing zoology resulting fromgenetic manipulation. With this glimpse into themodus operandi of the genome, some of pieces ofthe morphogenetic puzzle began to fall into place:the chromosome is the material container of thegene, the expression of which regulates embryonicdifferentiation. Published in 1915, The Mechanismof Mendelian Heredity contained the results of thiswork. The book established the foundations ofmodern genetics [9].Under the denomination synthetic theory or modernsynthesis, Neo-Darwinism easily assimilated thepattern of gene variation by applying a well-knownrecipe: cause and effect [10]. It was thought thatevolution was an exact science written in a geneticlanguage. Evolution would have an exclusivelygenic medium resulting from the expression of smallmutations, causing occasional modifications inpopulation typology – the group in which selectiontakes place. Repeated on a gradual, ongoing andaccumulative basis, the phenomenon would explainhow species diversify over time via the gradual andselective addition of mutations. Population geneticswould characterise Neo-Darwinism for decades.Hopeful monstersFrom 1880 onwards, embryology was a purelyexperimental discipline that had broken the bounds ofdescriptive procedure. The mechanics of embryonicdevelopment were under investigation. Manyquestions arose. The main question to be answeredwas how cellular, tissue, and organic differentiationinfluenced the construction of the individual. Thisbiological problem focused on discovering whatfactors determined the transformation of the embryo.Research progressed towards the definition ofthe event as a chain reaction, meaning that theorganisational structure induced in one stage wouldbe a triggering factor for the next, and so on. Nextcame the concept of the morphogenetic field: theembryo is organised into self-regulating areas,called fields, each acting to create a certain typeof anatomy. These fields are correlatively adaptedto the embryological stage, governing what takeplace at all times. This means that the processhas the necessary plasticity to reach the relevantorganisational level for each successive stage.This was the 1920s, 1930s and 1940s. Adding upall genetic theory, the embryological model was theproduct of a complicated physiological process of a- 310 -
Electronic Journal of Biology, 2016, Vol.12(3): 309-313genic origin; but the assumed equivalence betweenthe genome and morphology was insufficient toexplain to interactive embryonic framework. At thispoint, upon identifying evolution as a mutationalphenomenon, biologists asked themselves, howdoes genomic change affect ontogeny by tracingout new life? The challenge was to develop a unifiedtheory that could integrate cellular chromosomicinformation with the embryological process triggeredby fertilisation. Richard Goldschmidt, an unorthodoxGerman geneticist residing in the United States,a professor at the University of Berkley from1936, accepted the challenge. He addressed theissue openly, head on. His theory was publishedin 1940, in a book called The Material Basis ofEvolution [11]. This manual contained four hundredpages devoted to the consolidation of a bifocalembryogenetic outline, based on the concepts ofmacro and microevolution. The general hypothesisregarded evolution as an embryological event, withchromosomes as the fundamental ingredient. Twomechanisms would act to re-design populations. Onewas microevolution, the result of the appearanceof micro mutations, identified as morphologicalalterations in line with the anatomical structure of thespecies and, therefore, compatible with the existingembryonic schedule. This change would involve theadaptive improvement of a group to a specific regionwithin the distribution area of that species, creatingsub-species, breeds or varieties. Simply put, withoutlosing identity, typology is efficiently reshaped in orderto inhabit local environments. Micro mutation wouldbe an evolutionary dead end; it would constitute amechanism of specialisation incapable of producingnew ution. This notion is defined as a genomicreorganisation – called systemic mutation – to adegree that constitutes a new chromosomic pattern;another genetic system. The emergence of a differentinformation system would also lead to a differentontogenic process. This change would be the originof new organisms belonging to a new evolutionaryline, viable provided they find an environmental nichesuited to their innovative nature. The first bird hatchedfrom a reptile’s egg, explained Goldschmidt to illustratethe idea. This was the hopeful monster: the embryonicformation of anomalous beings, preadapted to adifferent environment. Evolution would take placeas a succession of evolutionary leaps in disharmonywith the gradual-selective pattern established undersynthetic theory. Given his rebelliousness anddifferent way of thinking, the German scientist wasignored, condemned, ridiculed and excluded fromevolutionist thought [12]. However, the validity of hisproposal is being reconsidered as it explains certainevolutionary episodes, for example as a formula forthe speciesisation of the botanical group of the orchids.Epigenetic landscapeIn 1924, Hans Spemann, professor of zoology at theISSN 1860-3122University of Friburg, and his student Hilde Mangold,published the results of their experiments with newtembryos, showing embryonic induction; a milestonein developmental biology. It is an easy conceptto explain; the difficulty lies in establishing how ithappens. Early on in its development, after the stageknown as gastrulation, an embryo differentiates anarea of tissue that takes control of embryogenesis,determining its future organisation. It is known asthe organiser, characterised by its multifunctionalvalidity: when a section of this tissue is grafted ontoanother embryo, this second organiser generates asecondary embryo using the cellular structure of therecipient. The developmental schedule is executedthrough chemical signals. Over sixty years would goby before we understood the molecular basis of thismechanism.Embryonic induction was the path taken by theBritish naturalist Conrad Hal Waddington as hetraced the supposed synthesis between embryology,genetics and evolution. The correct question was:How can the informative rigidity of the geneticcode, the conservational nature of the ontogenicsystem and evolutionary variability be compatible?During the 1930s Waddington investigated thesubject in mammals and birds. He carried out somesurprising experiments. One of the most famous ofthese was the transplantation of a rabbit organiserinto a chicken embryo, causing the formation of astandard secondary embryo. If nothing else, theinter-special nature of the experiment proved thatthe signal emitted by the organiser is the same indifferent species of vertebrates. The response doesnot depend on the composition of genes, but on theirexpression. There are of course genetic differences,and molecular similarities, chemical markers thatare recognisable to the cellular unit regardless ofthe origin of the organic matter. This is informationregarding the activation of the process, neveron the content of the morphological programme,which remains unaffected. Waddington’s solutiondefined the activity of the organiser as a morecomplex event than a mere response to a signal.Let us simplify. Waddington proposed that thegenes in question, known as homeotic genes, havea quantitative effect and act together. The chemicalmarkers resulting from genetic transcription establishconcentration gradients, behaving like beaconsthat channel the spatial distribution of cells andthereby the morphological identity to be constitutedsubsequently. The outline is known as the epigeneticlandscape, an intuitive ontogenic scenario where theset of cells is guided by chemical signals towards thedifferent paths that they need to follow. What is theevolutionary purpose of this model? The fundamentalintention is to integrate genetics and evolution aselements in a dynamic system, and place them atdifferent operational levels in order to prevent theintroduction of evolutionary changes from causing achromosomic reorganisation incompatible with theviability of the organism. The concept of the epigeneticlandscape associates the variability of an alteration- 311 -
Electronic Journal of Biology, 2016, Vol.12(3): 309-313of genetic expression with a full or partial inhibition ofgene function, causing a different cellular responseand leading to a different typology. Simultaneously,this system’s unique structure allows it to interactwith the external environment, making it viable toinherit acquired characteristics. Conrad Waddingtonwas one of the great 20th century theorists ondevelopmental embryology and evolution. He wasan indisputable reference point, even beyond the1950s. Organizers and genes, and The strategy ofthe genes, were fundamental pieces of work in thisfield [13,14].HeterochronyOntogeny does not recapitulate Phylogeny: itcreates it. This statement was made by the Britishzoologist Walter Garstang. It was published in1922, in his article The theory of recapitulation:a critical re-statement of the biogenetic law [15].Eight words, just eight, were enough to refutethe theory. These words were necessary in orderto interpret evolution by applying a revolutionaryembryological criterion. Which one? The idea wassimple, ingenious, and possible. The idea requireda conceptual turnaround to alter the order ofthings; necessary in order to ensure that the resultof reproduction is an end product different to thatestablished in the chromosomes. The strategyconsisted of following the path of evolution usingthe juvenile ontogenic forms. A specialist in marineinvertebrates, Garstang detected the evolutionaryimplications of the approach taken to external sexualreproduction; widespread in this zoological group.Fertilisation takes place in the water. The fertilisedegg gives way to a sequential larva transformationthat shapes the individual until it becomes anadult. Each stage in this life cycle represents aself-sufficient organic outlie, differentiated from theadult in its anatomical composition and reproductiveimmaturity. Here we find the heart of the matter.Why? We could categorise the larva stage as apotential hopeful monster adapted to the mediumof water that, incapable of independent living,continues the routine metamorphosis indicated onthe embryonic script. Considering the typologicalseparation found in adults, in order to create a newspecies it would be sufficient for juveniles to acquirethe ability to reproduce prematurely. All this takesplace across a time-space alteration. The processis direct, immediate and conservative – it doesnot require new structures to be created-, decisivecharacteristics for determining viability. This was thepattern proposed by Walter Garstang: consideringthe potency of juvenile states for manifesting a newevolutionary line. The conclusion was that ontogenycreates phylogeny. Early sexual maturity occurswhen there is an alteration in ontogenic synchronism(heterochrony): the development of the gonads isbrought forward, allowing incomplete specimensto reproduce – the axolotl is an existing exampleof this phenomenon. Garstang’s model took on fullISSN 1860-3122evolutionary significance by investigating the lifecycle of the ascidia. This colourful tunicate, tubeshaped and fragile-looking, populates the seabedafter maturing. However, its young proliferatefreely in the ocean, propelled by an unusual caudalappendix. This strange tail bears a dorsa
Whatever the evolutionary model we adopt, in the case of sexual reproduction, the process has an embryological significance because this is the way to generate individuals and to perpetuate the life. The connection between evolution and embryology is a necessary event. In this evolutionary context, the
evolutionary biology. From this point of view, some authors have tried to extend the Darwinian theory universally beyond the domain of evolutionary biology (Cf. Dawkins, 1983), using the three principles of evolutionary theory (inheritance or retention, variation, and adaptation) as a heuristic for evolutionary economic theorizing (Campbell, 1965).
least partial convergence, in the methodological approaches employed. Dow . is necessary for the survival of economics. Even a division based on mathe-matical modeling is thwarted by mathematical models being used by some . another potential avenue to resolving the quandary of distinguishing a substantive part of heterodox economics from
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1 THECONSTITUTION OFTHEREPUBLICOFKOREA Jul.17,1948 Amendedby Jul. 7,1952 Nov.29,1954 Jun.15,1960 Nov.29,1960 Dec.26,1962 Oct.21,1969 Dec.27,1972 Oct.27,1980
