February 1. Morphology

VARIATION WITHOUT EVOLUTION

• ONTOGENETIC VARIATION-occurs within an individual's lifespan-this is somatic variation WITHOUT genetic change-it exists because the development of an organism is affected by the varying parameters of its physical environment-rate of growth during ontogeny is a function of age, size, temperature, availability of food etc.
• POPULATION VARIATION-exists among individuals in the same population, i.e. among individuals that live side by side in a given place-this may not be due to genetic differences among individuals

ONTOGENETIC VARIATION

• changes can be either subtle and continuous, as with most mammals or abrupt and dramatic-the latter is referred to as METAMORPHOSIS and is known in many invertebrates and also amphibians-the presence of an exoskeleton makes possible more wholesale morphological changes; it can be shed entirely at the end of a particular developmental stage

TYPES OF GROWTH

• ACCRETION-hard shelled organisms, e.g. molluscs and corals-addition of discrete growth layers to increase size-powerful paleenvironmental tool; Wells' rugose coral astronomy
• ADDITION-new parts are added through life cycle-chambered organisms, e.g. foraminifera and cephalopods; echinoderms add new plates; trilobites add new segments
• MOLTING-hard exoskeleton does not grow; new soft one is built underneath and then the old one is shed-pretty much unique to arthropods-growth is rapid until new exoskeleton hardens
• MODIFICATION-existing skeletal elements are continuously expanded-most vertebrates
• most organisms use a mixture of the above strategies
• an organism maybe preserved to the fossil record at any ontogenetic stage-juvenile organisms have sometimes been classified as different species until either 1) an extraordinary fossil find (a 'nest', brood pouch, molting individual etc.) proved the connection between juvenile and adult or 2) the adult and juvenile morphologies could be shown to be related by an algorithm

MEASURING THIS KIND OF THING

• the above quandary is resolved organisms that molt or modify if the fossil record yield a GROWTH SERIES of a particular species
• 'dissection' of species that grow by accretion or addition can yield the juvenile form beneath layers or at the beginning of a series of chambers
• once a growth series is available, then salient characters that define shape should be selected and measured-these values can then be plotted against one another-what you are looking for is a relationship between increasing size and shape
• if shape remains the same with increasing size, then it is called ISOMETRIC GROWTH-the relationship between characters is defined by a straight line ( y = mx + b )
• if shape changes with increasing size, then it is called ALLOMETRIC GROWTH-the relationship between the characters is defined by a hyperbola (y = mxc , where c=the constant of exponential growth)
• the intensity of allometry is closely tied to rate of growth-allometric changes represent a differential rates of growth among different parts of the body-one part of the body changes at a constant exponential rate compared to the rest of the body
• isometric growth is very rare in nature; allometric growth is the rule
• PRINCIPLE OF SIMILITUDE-the mass of an organism increases by a power of 3, while its cross-sectional area increases only by the power of 2-therefore as animals grow larger, their limbs most become more robust in design, rather than just being larger versions of those of smaller animals-colonialism is one way to solve this problem-the collective mass increases (with all of the accompanying benefits) without any increase in the size of the individual organism

POPULATION VARIATION

• any changes that are caused by environmental factors that affect the progression of ontogeny are called ECOPHENOTYPIC VARIATION
• an organism may be stressed at any ontogenetic stage and this may lead to changes in its morphology without any genetic change at all-examples include responses to: nutrient shortages (stunting, delicacy of the skeleton), space shortages (bryozoans or sponges growing in crevices), excessive sedimentation or repeated sedimentation events in a clear water environment (rejuvenence in corals), parasites or infections ('burls' on mollusc, arthropod, cephalopod etc. shells)
• GEOGRAPHIC VARIATION-geographical or microgeographic variation in the form or morphology of an organism may not always have an adaptive explanation-in the fossil record it can be difficult to make the distinction between subspecies and species-in the absence of co-eval (i.e., contemporaneous) morphologically intermediate populations widely separated, but similar groups of organisms may be designated separate species

VARIATION WITH EVOLUTION

• what happens when mutation and natural selection arrest, accelerate or otherwise mess with ontogenetic processes?-this is a common way to evolve new species

RECAPITULATION

• Ernst Haeckel's famous late 19th century pronouncement that "Ontogeny recapitulates phylogeny" was originally intended as an early defense of Darwin's embattled theory of natural selection-he predicted that examination of the juvenile stages of descendant species would reveal the adult features of their ancestors-this was a time when scientists were desperate produce evidence that showed proof of "descent with modification" as predicted in Darwin's work-recapitulation was embraced, taken to extremes, debunked and nearly discarded by the professional community by the 1930s ... although it lived on in the classroom
• more recently all changes in the timing of the developmental process have been called HETEROCHRONY-no changes in the genes that control structural proteins are needed -all changes are in the genes that control regulatory proteins-new anatomical features are not developed-instead extant features are either retained or discarded earlier
• retention of juvenile features into sexual maturity is called PAEDO-MORPHOSIS-there are three different ways of going about this:
• PROGENESIS stops development at an early stage
• NEOTENY merely slows down development
• POSTDISPLACEMENT delays the start of development
• the opposite phenomenon, the addition of extra ontogenetic stages beyond the adult reproductive stage, is called PERAMORPHOSIS-again there are three ways of going about it:
• ACCELERATION adds more stages in a shorter time
• HYPERMORPHOSIS adds more stages in a longer time
• PREDISPLACEMENT starts development earlier
• all these processes can lead to more or less complex morphological features in the descendant form-example: sea urchins-variation in the number and size of spines changes through ontogeny-phylogenetic diversity in sea urchins has partly been achieved by variation in the number and size of spines-urchins with smaller, less complex spines are considered paedomorphic-urchins with larger, more complex spines are considered peramorphic
• in 1977 S.J. Gould's book _Ontogeny and Phylogeny_ showed that both recapitulation (which was replaced by the term "peramorphosis") and paeodomorphosis were about equally common and important in evolutionary history