----Subject: Florida Meetings
Now that most of the activities at the Graves Museum Seminar ahve been recounted, I would like to turn to another aspect of the event. It was my good fortune to be in Ft Lauderdale as an antendee, to witness all the presentations, to have extended opportunitis for discussion with colleagues and, yes, to walk on the beach and wonder around local marineas. Theis meant unincumbered time to think and reflect whihc is something that emeritus professor do.
IMPRESSIONS OF THE FLORIDA MEETING
It is not a matter of who won or lost. The evidence that birds (Archaeopteryx and descendents) shared a common ancestor with a group that also gave rise to advanced theropod dinosaurs is now overwhelming. That part is done. The questions that remain are to identify the sister group within dinosaurs, to estimate when the relevant events occurred (establish a time dimension), and to understand the timing and nature of the innovations that occurred in all the subsequent lines.
Based on the available fossil evidence, cladistic analysis has provided strong hypotheses of the patterns produced by evolution. This has been a significant contribution. Fossils as old as Archaeopteryx, extant birds, Late Cretaceous dromeosaurs, and possibly others, are sister taxa (clades) that shared a common ancestor within the dinosaurs. Birds are dinosaur descendents, but that statement is not enough. What is lacking, and eventually may be even more intriguing, is understanding the processes under which the patterns emerged. The pattern of origins is ultimately complex, not a single event. At present we have considerable knowledge of processes common to all animals and the mechanism (e.g. natural selection) by which evolutionary change occurs. However, even for a group as well defined as living birds we have not figured out how the observed patterns of relatedness were generated. It is neither an easy nor a trivial problem.
What is lacking is adequate evidence to account for the complexities and innovations that have emerged over time and are so important biologically. Evolution does not cease following a branching event. Birds have changed so much from the stem group that they can only be considered a separate, cohesive, successful unit (e.g. a crown group). Rather than our being confined to reconstructing past scenarios from models based on living forms, it is cogent to understand and appreciate the mechanisms that underlay the processes involved. Presumably, the biological mechanisms are constant Innovations such as the evolution of flight or the appearance of feathers, involve reorganization of processes common to all, but with different consequences. There is little direct paleontological evidence for these events. However, contemporary comparative studies provide insights into how animal work; the causes and significance of genetic variation; cellular function and communication; the wide variety of functions performed by the different tissues; the relation of the nervous system to muscle and how posture and movement happen?., but none of these leave a direct fossil record. Further, genomic studies are revealing that astonishingly large number of genes are shared among even distantly related animals. However, we are reduced to speculation as to how the processes were partitioned in the various groups. Families of genes that affect mitosis & meiosis and proteins that control cell shape, that control mechanisms of protein synthesis and cell signaling, and a variety of regulatory genes that produce cell-cell and cell-substrate adhesion, and even processes that control cell death have been identified. However, we fall short in understanding how the activities of these genes determine the outcome of ontogeny and influence evolutionary pattern. Even when the embryos begin with strikingly similar structures! Further it is only from our studies of extant organisms that we comprehended the diffuse consequence of body size, the biological roles played by complex structures (e.g. associating brain size with intelligence is a human conceit) and how these processes are organized and reorganized over evolutionary time.
The most bird-like dinosaurs are the maniraptorian theropods. The dromeosaurids might be closest to an avian ancestor, and there is currently debate as to which species has the most ?avian? morphology. But these Late Cretaceous beasts did not give direct
rise to birds. They most likely shared a common ancestor, but that group existed before Archaeopteryx at 150 MYA. The stem group minimally evolved in at least two directions. One, that we have records of, towards birds with the evolution of feathers and eventually the appearance of flight. Another clade, remained cursorial, but evolved many bird-like features for reasons yet unfathomed. Most likely there were more but they are missing from the fossil record or contributed to one or the other of the successful line and disappeared. The anatomical range in each group certainly was large. The range of size and habits approached that of modern birds. The information necessary to produce this diversity in form and function was read from what began as a common genome, and employed the same basic mechanisms that exist today.
There are several points to consider. First, this split occurred in the early Jurassic or late Triassic. The identity of the stem group is unclear, as is the nature of the taxonomic distribution. Even if we were to witness the event, it is unlike that the future of any of the taxa could be predicted. Our interpretation is from hindsight based only on contemporary biases. The ancestor was probably small, bipedal, and without epidermal structures other than a variety of scales. The event was comparable to a speciation event, may have occurred numerous times, and the sister groups would have resembled each other rather closely.
Second, the earliest members of each subsequent clade were probably almost indistinguishable and could have potentially interbred. They were, for all intents and purposes, equivalent to subspecies and the rules of speciation certainly applied. More importantly, each (and there could have been a considerable number of then) had the capacity for subsequent change. Evolution did not stop at this juncture. Birds did not leap fully formed from a primitive theropod structure.
Third, for whatever ecological, ontogenetic, and selective reasons the maniraptors tended to become bird-like along several lines. Most of the subsequent change was parallel in nature as each population began with the same genetic and morphological capital and was constrained by the same forces. Nevertheless, different innovations ensued in the various lineages. For whatever the suite of reasons, one group thru a complex series of changes and relations gained the characteristics we recognize, a posteriori, as leading to modern birds. Some of these are found in the fossil record. The theropod radiation ended at the end of the Cretaceous. The avian radiation persisted to the present and continues as speciation continues.
The fossils, spectacular and informative as they are, provide only a very small and spotty sampling of what was. Consequently, the appearance of birds in the fossil record seems precipitous. Further, we have only vague clues regarding the nature of the stem group and even less information of the patterns of change over the 75 million or so years involved. The features present at the start are only inferred, certainly have changed, and are incomplete in light of subsequent evolutionary innovation
WHAT IS TO BE DONE
Where we go next may be the most important question to emerge at Ft Lauderdale. Cladistics has provided a pattern that is considered to be the results of decent from a common ancestor. The challenge is to understand why cladistics still does not resolve many problems (e.g. the relationships among the orders of birds). In addition to further cladistic analysis, and fortunate fossil finds, we need to unravel the mechanisms by which the novelties and innovations in each or any group evolved. This is not a systematics question but involves genomics, comparative biochemistry, ontogeny, and functional morphology and the other processes common to animals.
It is axiomatic that ancestral populations have features in common with their descendents. There is also a potential to evolve alternative or additional combinations and to lose, then perhaps re-evolve, others. Each divergence in the branching process begins essentially a speciation event. As such, all the existing machinery and features of the species are involved. It is almost impossible for observers to identify speciation when it occurs (although there are some notable contemporary exceptions in studies extending over several generations of flies and birds). The gene combinations that survive reside in populations that continue to change over time. They can be reassorted in subsequent events, changed due to mutations, duplications and divergence, etc. Simultaneously, innovative combinations, or their potential, can be produced early in the processes that profoundly influence subsequent developments. This is simply a reminder that one should expect parallel evolution as all the units begin with essentially the same instructions and materials.
Radiation?s are only recognized in retrospect and are, in actuality, an integrated series of speciation and extinction events, coupled with the introduction of innovations. New combinations of extant features lead to emergent features in taxa. Identifying the commonalties among taxa and what is new over time, is a human activity. It must be remembered that most groups within taxa have become extinct and leave no fossil record. This does not mean that they were poorly adapted, merely that they were replaced by something else as times changed. Every taxon has the potential for variation. While selection can appear inventive it is not. The appearance of progress in evolution and teleological interpretations is a human calculus. Many possibilities exist within every group. But the combinations are not infinite as constraints exist. One constraint is the basic design of the ancestral group. Another are the options that are impossible biologically (e.g. rotary motions around a joint (ball bearings).
Our sampling of this change (which may occur rapidly or not) is always incomplete. Fossils are only time-dependent samples of an ongoing process. The speciation process generates the observed change. Each species differs at some level from its sister. A species existence provides a pool of variation and information subject to selection. Change inevitably results. While most of the record is unavailable modern biology provides insights to all the mechanisms and many of the consequences. Because different combinations exist at different level in time we perceive a pattern of radiation.
Primitive species may be older (occur earlier in the record), but they are not less adapted than a derived (descendent) group. The success of any group many be assessed by many factors among which are diversity at a time level, persistence in the record, biogeographical complexity, etc., but these are human concepts based in large part on hindsight and a compulsion to quantify. In fact, the evidence that species exist or existed is itself a measure of success. The problem is that our sample through time is always sparse and therefore inadequate. They represent the past but are only part of history. That history must include the processes at all levels of organization and the processes by which they change over time. The consequences of the complexity are engaging and our interpretation biased. Understanding requires caution and curiosity.
Alan H Brush
21 April 2000
Alan H. Brush
M.J. Spring Brush
92 High St.
Mystic, CT 06355