Evolutionary biology is the scientific study of the CHANGE and DIVERSIFICATION of the biota.
Here are notes on the thematic content of this course which expand upon their brief mention in the main document. In essence, they simply constitute an expansion of the course description you may have read in the main document or in the Zoology department's course descriptions booklet. It represents the way that I (and many other evolutionary biologists) see the intellectual structure of evolutionary thought. There are other ways of structuring the material, and different emphases to be made. I imagine that the similarity of structure and emphasis of this outline to that of Ridley's book will be apparent to you.
While there is broad-scale agreement among a majority of evolutionary biologists about most of the disclipline, there are, as in most sciences, several 'schools of thought.' Today, as has long been the case, the primary argument is about the relative importance of chance and historical contingency on the one hand, and deterministic processes (natural selection) on the other, in influencing both the short- and long-term evolutionary dramas.
Some believe that the evidence is best interpreted as suggesting that chance and unique, contingent, history are the primary dictators of much of the evolutionary story - if one were to run the experiment "World" many times again under similar cosmological conditions, the outcome after 3.8 billion years would be different every time, -extinction of taxonomic groups is, they say, more a matter of bad luck than bad adaptation, and contingencies have a major impact on what else develops thereafter. Thus, for example, they say: there is nothing inevitable about vertebrate animals, still less about humans - maybe there's nothing inevitable about animals, or even eukaryotes, at all - perhaps the world might have continued happily for ever with a prokaryote biota................. after all, they managed the planet alone for about 2 billion years - more than half of life's tenure...........
Others are persuaded that, although chance events are indeed greatly influential over many historical details (such as the obliteration of the dinosaur lineages, perhaps), many prominent features of life on earth would reappear with each re-run of the "World" experiment, albeit in different phylogenetic guises (again, provided that the earth's physical environments showed similar trends in each re-run.) Thus, for example, they would say: photosynthesis, morphological complexity, heterotrophy, homeothermy and behavioural complexity would reliably evolve, no matter which lineages would actually be involved in giving rise to them. Perhaps, although there may be nothing guaranteeing the arrival of our actual, specific, human lineage (the one that invented Shakespeare) with a history going back through apes, Old-World monkeys, prosimians, early mammals etc.), maybe there is something almost inevitable about the arrival of a taxon of highly intelligent, highly social, behaviourally plastic creatures, with a complex symbolic language from some lineage. Perhaps, even though their skin be scaly, they would have invented geometry, discovered evolution and general relativity, and put one of their number on the moon.............................
Stephen Jay Gould and Niles Eldredge are prominent advocates of the 'cosmic roulette' view; Richard Dawkins and John Maynard Smith (and now, prominently in his new book "The Crucible of Creation" - an explicit response to Gould's "Wonderful Life" book, Simon Conway Morris) argue for versions of the 'predictability' view. You should be aware that prevailing attitudes to this fundamental question have swung widely from pervasive " contingency-based non-adaptationism" to "pan-selectionist perfectionism," and back, and forth.......... The fashionable view these days seems to be 'roulette' rather than 'predictability' (at least, this is so in N. America, largely due to the persuasiveness and pervasiveness of Gould's arguments, but not so in Britain; this difference perhaps illustrates the influence of societal and cultural factors in science). So I like to emphasise the other side.& Call it perversity or sheer bloody-mindedness, as you like; but the argument is there, and should be understood. So here's the outline:
What follows is a list of topics, arranged as developing sequences, and often posed or indicated in the style of a question or open-ended statement, with which the study of evolution is concerned. The slightly unconventional style of this list is intended to promote a questioning attitude of mind, rather than to intimidate by simply providing an catalogue of "things you must know". You will already be familiar with some of the terms and ideas, while some will be new; what will be new for almost all of you are the connections among the various fragments, and the manner of their connection. An appreciation and understanding of this intellectual structure is a central goal of this course. The study of evolution is as much concerned with the development and manipulation of concepts as with the aquisition of empirical information.
CHANGE implies the passage of time; therefore evolution is a science with a HISTORICAL dimension; events have unique histories, which may constrain possible futures.
CHANGE can be on a local space/time horizon - say population composition changing over 1 to 1000 generations; or it can involve large changes in evolutionary lineages over a global space/time horizon -say 1 million to 1 billion years.
-what are the patterns of change on these various spatial-temporal levels?
short-term local changes in population composition, involve a variety of agencies, including chance phenomena (e.g. drift) and deterministic mechanisms (e.g. natural selection operating on available variation).
Natural Selection as a process which inevitably follows given heritable variation in characters affecting FITNESS.
-what do we mean by FITNESS? -it may be defined in several ways............
-such heritable variation in characters affecting fitness appears to be ubiquitous and likely ever to have been, so should we not conclude that NATURAL SELECTION is a LAW of nature? Perhaps a Law which applies across the Cosmos.......................?
-selection leads to ADAPTATION; -adaptation implies heritability.
-what do we mean by ADAPTATION? -it may be defined in several ways.....
-to what entities does the selection process apply? -individuals? -populations? -species? -maybe the gene? -linkage groups?† which entities therefore show adaptations?
longer-term changes in lineages may involve speciation, which generates DIVERSIFICATION.
-what is the relationship between adaptation (selection) and speciation?
-does the calculus of the selection process necessarily imply the phenomenon of species and the process of speciation? i.e. does the existence of heritable variation in characters affecting fitness inevitably lead to speciation? how might this depend on the particular form that life takes (e.g. compare prokaryotes & eukaryotes)
-or do species and speciation have little or nothing to do with adaptation? is speciation itself, rather, a random process, analogous perhaps to mutation at the individual level?
-in short, are the processes going on at the local, species level (including adaptation) unconnected with the processes going on during the speciation process and beyond (including extinction)?
the longest-term changes inolve cycles of extinction & biotic turnover.† What causes such change? Are there agents which drive it or is it something that just happens? Bad genes or bad luck? Can we call it progress?
HOW IMPORTANT IS HISTORY? -meaning "to what extent is the array of present-day phenomena (characters, species, higher groups, biotas) constrained, influenced, or dictated by, the specifics of unique histories?
This historical question has two major components:
a) -phylogeny: all organisms have specific ANCESTRIES; these specific ancestries may constrain (e.g. through their given body-organization, or control of developmental sequences) changes that are possible within a lineage, regardless of the strength of selection's challenges; how strong are such constraints?
b) -contingent events: all lineages have been subject, throughout their histories, to a unique set of circumstances, peculiar to their lineage, including both external events (like population-fragmentation, glaciers, continental drift), and internal events (like what mutations have occurred, and the state of the rest of the genome when they occurred); how important are such vagaries of chance? example: how dependent (if at all) is the rise of mammals in the Cenozoic on: a) the demise of the dinosaurs, or b) the drying and cooling of the earth's climate?
With ANCESTRY and CONTINGENT CIRCUMSTANCE as the background, throughout evolutionary time lineages will have been subject to both chance and deterministic processes (drift, gene flow etc. and selection).
-in short, is there something deterministic in the selection process, and in the structure of living things, which makes the evolution of something like a mammal (hair, homeothermy, live-bearing, lactation etc.) almost inevitable under the physical/climatic conditions present on earth? (Remember, we now have good reason to suppose that late dinosaurs were possessed of quite "non-reptilian" characters like hair and a complex social organization) If that's so, then could something just like us have developed, albeit from the dinosaur reptile lineage, rather than from a group of long-extinct pelycosaur reptiles (our actual ancestors)......?
Can selection thus erase history? -or will the footprints of history (genealogy) always be detectable?
Can evolutionary convergence (an adaptive process generated by similar selective imperatives acting on lineages of different descent and history) lead to such similarity of form that the various different ancestries and histories be utterly obscured? In other words can analogies be mistaken for homologies?
-if this is possible, then could taxonomists have been misled by such close similarity into misclassification?
How do taxonomists do their job? -what characters can/should they consider?† -they need characters which betray ancestry (or, put another way, characters which defy selection), but what if most characters reflect adaptation........? -the search for "non-adaptive characters" -do they exist?
If we don't have a taxonomy which reflects true ancestry, then how can we properly use the comparative method?
The COMPARATIVE METHOD has been, and remains, one of the most powerful tools for discovering and analysing adaptations, so we are faced with the prospect of: finely-tuned adaptations, based on adequate and appropriate variation, driven by a highly-effective process of selection, masking the true relationships among organisms to such a degree that we do not recognise convergence as having occurred: rather, we (mis-)conclude similar ancestry. This would be a pity because the recognition of close convergence (requiring different ancestry) provides one of the main planks of evidence for demonstrating the power of selection...........
-we can be saved from this logical bind by the analysis of truly selectively neutral characters. Are there such things? Probably. Much electrophoretic variability is probably neutral, and the actual sequence-structure of DNA is, so far as we know, of no adaptive significance in itself. So we think that DNA-sequence similarities and allozymic genetic distances may finally provide us with unambiguous (albeit tricky) evidence of evolutionary lineage. Evidence of this kind is beginning to suggest that, there have been many cases of mis-classification.
Although the possible importance of historical and chance phenomena must be kept in mind in devising explanations of a given phenomenon, I feel one can only proceed using a frankly-stated Selection Paradigm, since it permits progress in analysis. Selection is, in many ways, the most parsimonious paradigm to hold (as opposed to one of randomness, say).
To quote a selection-minded biologist, Graham Bell (he's at McGill):
I shall base all my reasoning on an axiom of perfection, which, in the present context states that adaptation is extremely precise. I shall take for granted not merely that selection tends to favour more fit at the expense of less fit phenotypes, but that the phenotypes which are actually present are more fit than any alternative phenotypes; or, in a phrase, that selection alone is a sufficient explanation of organic diversity. Perhaps this is wrong, and the world is full of queer accidents that cannot be explained in the absence of a perfect knowledge of the past; in which case, biology is the study of miracles. It may be so; I do not insist upon the truth of the axiom; but I do insist that it is absolutely necessary to assume its truth if we are to make any progress in explanation. If its truth is denied, then whenever we are confronted with a problem whose solution is not self-evident, we shall be tempted to refer it to the operation of unique causes in an unknowable past. We shall become mythologists.
Graham Bell : The Masterpiece of Nature. 1982.
Similar sentiments are expressed by another enthusiast of natural selection, John Endler:
If we did not use natural selection, adaptation, or functional utility as an initial working hypothesis, we would never learn anything about the biology of organisms. If we assume that characters are non-adaptive and artefacts of history, constraints, or pleiotropy, then no investigations will be made........to the impoverishment of biology. The assumption of non-adaptation can easily give rise to the construction of very pausible nonadaptive fairy tales............ it is certainly correct that the usefulness of a trait must not be taken for granted, but equally, its uselessness cannot be taken for granted either!
John Endler : Natural Selection in the Wild. 1986.