recent reading: deep structure of biology

Since the primary purpose of this blog is to keep track of what I'm thinking about what I'm reading - some brief thoughts on things I read in the last couple months that I didn't have time to write up longer thoughts about (but which are part of longer rivers of reading which will undoubtedly entail later stops and longer tarries).

This entry ended up longer than anticipated after just the first book, so perhaps I'll do one entry per book, which also lets me post it earlier than I'd otherwise have time for. So.

These are basically my class notes, really, for a class I'm both constructing and attending all by myself, and I don't expect them to have the same value for you; that's just the nature of this blog.

Deep Structure of Biology --

A succinct summary of the phenomenon of convergent evolution:

"Beyond all reasonable doubt -- and here we can draw on embryology, comparative anatomy, histology, molecular biology, phylogeny, and the fossil record -- the common ancestor of the octopus and the blue whale could not possible have possessed a camera-eye. Each group has independently navigated to the same evolutionary solution, and it is one that not only works very well but has arisen at least five more times, in animals as diverse as snails and, more extraordinarily, jellyfish."

Another example, later:

"The cartilaginous fish and the bony fish both solved the physics of swimming back in the Silurian by evolving streamlined, fusiform morphologies. Some 230 million years later, a group of land-dwelling reptiles rediscovered this same morphology in their evolutionary return to the sea. And around 175 million years later, a group of land-dwelling mammals also rediscovered this same morphology in their own evolutionary return to the sea."

(The "return to the sea" here is a reference to the Devonian era, when the first land-dwelling animals evolved from sea-dwelling ancestors; before the Devonian, life was primarily a watery phenomenon. Today's aquatic reptiles and mammals have an evolutionary family tree that came up to land before returning to the sea.)

"The evolution of an ichthyosaur or porpoise morphology is not trivial. It can be correctly described as nothing less than astonishing that a group of land-dwelling tetrapods, complete with four legs and a tail, could devolve their appendages and their tails back into fins like those of a fish. Highly unlikely, if not impossible? Yet it happened twice, convergently in the reptiles and the mammals, two groups of animals that are not closely related. We have to go back in time as far as the Carboniferous to find a common ancestors for the mammals and the reptiles. Nonetheless, the ichthyosaur and the porpoise both have independently reevolved fins.

"Contrary to the dictum that 'biological evolution has no predictable destination,' I predict with absolute confidence that if any large, fast-swimming organisms exist in the oceans of the moon Europa -- far away in the orbit around Jupiter, swimming under the perpetual ice that covers their world -- then they will have streamlined, fusiform bodies; that is, they will look very similar to a porpoise, an ichthyosaur, a swordfish, or a shark."

Gould argued strongly against this kind of conclusion with his famous "rewinding of the tape" analogy: 

"Any replay of the tape would lead evolution down a pathway radically different from the road actually taken ... The diversity of possible itineraries does demonstrate that eventual results cannot be predicted at the outset. Each step proceeds for cause, but no finale can be specified at the start, and none would ever occur a second time in the same way, because any pathway proceeds through thousands of improbable stages."

The Gouldian argument is not disproven by convergent evolution -- that is, you do not have to deny that convergent evolution is a fact to support Gould's view of evolution. But I think it's important to remember that he constructed that view in the 1980s, and even for scientists it can often be easier to fit new evidence into an existing model -- especially when nothing is going to come along that actually proves it one way or the other, by the nature of the theory -- than it is to re-weigh the evidence every time the balance changes and decide all over again. It's always politically dangerous to talk about the fallibility of scientists because climate change and vaccination denialists pounce on it to "prove" that science is "lying" or "wrong" about those things, which is ridiculous: here we are talking about a very small subsection of a very small corner of a particular science, and how one school of thought may have differed given thirty years difference in the accumulation of evidence.

The phenomenon of convergent evolution raises many questions about:

a) extraterrestrial life, with reference to the Gouldian argument -- if the conditions necessary for life are or have been present elsewhere in the universe, has life evolved in a recognizably similar way? and especially, once simple life has evolved, is it inevitable (assuming that the necessary resources are available) that multicellular and more complex life will evolve eventually?

b) the evolution of intelligence: nevermind the "not all people are smart in the same way" discussions, which are interesting in of themselves. Are the differences between potentially evolvable intelligences enormous, even enormous enough to be mutually unrecognizable, or will intelligence evolve more or less convergently, the way sharks and swordfish are significantly different from one another but still evolved substantially similar structures of swimming?

On primate and corvid intelligence:

"It is certainly plausible to argue that surviving the trials and tribulations of a complex social world makes intellectual demands on many primates. Individuals need to know who is who, they need to keep track of who did what to whom, where and when, and to use this information to predict the actions and intentions of other individuals in their social network, as well as understanding how these relationships change over time. In short, the need for effective competition and cooperation with conspecifics may have provided the main selective advantage for the evolution of primate intelligence.

"That said, there is no reason to assume that intelligence is restricted to primates or that such abilities have evolved only once. Indeed, we shall argue that there is good reason to believe that complex mental characteristics have evolved several times and that the existence of intelligence in different, distantly related lineages must have arisen as a result of convergent evolution in species facing similar social and physical problems. ... perhaps the most dramatic case for convergent evolution of cognition comes from comparing primitive cognitive abilities with those of crows, given that the common ancestor of mammals and birds lived over 280 million years ago and that not all birds and mammals share the complex mental abilities found in crows and primates."

On social and cultural evolution in the ocean:

"Steele notes that, in the ocean, environmental noise (after removing predictable cyclical variation: diurnal, lunar, annual) is largely 'red' (greatest over large time and space scales), while, on land, it is more 'white' (roughly constant over all scales, up to a century or continent or so). The fundamental contrasts between the two habitats are illustrated by the methods of the scientists who study them: terrestrial landscape ecologists plot habitats using geographical information systems, while their oceanographic counterparts use the partial differential equations of fluid dynamics to describe the marine environment. 

"The environment is the stage for evolution's greatest play: organisms evolve to maximize their fitness given the environment. Two general environmental traits known greatly to influence evolution are connectedness and variability. In these respects, and many others, marine and terrestrial systems differ radically. And so, with these different sets, we might expect immense contrasts in both the action of the evolutionary play and its results, whether it occurs on land or in the ocean. And there are. Looking out my north-facing windows onto the land, most of the primary productivity I see is in large, long-lived, slowly reproducing spruce and maple trees. To the south, in the sea, the primary productivity is in microscopic diatoms and dinoflagellates. There are squid on one side, neutrally buoyant and forming large schools in an open three-dimensional habitat, and foxes on the other, negotiating trees and rocks and roads by themselves or in small groups, anchored by gravity. These are very different creatures in very different environments. Terrestrial and oceanic environments provide a tough challenge for convergence. When traits do converge, something remarkable has occurred.

"... as we move up the trophic web, convergences between oceanic and terrestrial animals begin to appear. The eyes of squid and foxes are an example. But at the trophic peaks and at the highest levels of biological organization, then the convergences between oceanic and terrestrial systems become particularly strong and provoking. A diatom and a spruce tree have little in common other than being autotrophs but, as I will try to show, the social structures and cultures of sperm and killer whales have much in common with those of elephants and humans.

"Phylogenetic constraints play a part in this. The diatom and maple are about as distantly related as any two organisms on Earth, whereas elephants, sperm whales, killer whales, and humans are all mammals and share all the constraints and advantages of the mammalian order, including backbones, air-breathing, warm blood, live birth, and lactation. But their common ancestor, perhaps one million years ago ... was small, likely socially and culturally primitive, certainly nothing like today's large and dominant mammals of land and ocean."

Later in same section:

"However, the most comprehensive convergence between marine and terrestrial mammals is between ... the sperm whale and the elephants. Termed 'the Colossal Convergence' by an editor at American Scientist, it features a wide range of traits in which elephants are more similar to sperm whales than they are to other terrestrial mammals and sperm whales are more similar to elephants than other marine mammals. In both species, females live in largely matrilineal social units of about elevan animals within which there is communal care for the young and communal defense against predators. These social units aggregate to form larger social structures, including groups of about twenty animals. Males leave their mothers' social units, segregate from the females, and grow to become much larger than their mothers, In their late twenties, the males return to the habitat of the females to mate, roving between the female units, competing with each other and being selected by the females.

"There are additional nonsocial parallels between sperm whales and elephants. For instance, the species have very similar life histories and are nonterritorial and quite mobile. And both are extreme in other respects. These include body size and brain size: the sperm whale has the larges brain of all species, and the elephant the largest among land animals. Another parallel is in ecological success. Elephants, due to their size, numbers, and feeding methods can restructure habitats while the world's sperm whale population, even though substantially reduced by whaling, currently removes about as much biomass from the oceans as all human marine fisheries combined. 

"This convergence between a terrestrial herbivore and a marine teuthivore (the sperm whale principally eats deep-water squid) is striking, especially given the radical contrasts in their habitat and food: quite a puzzle for the evolutionary biologist."

Then the author (Hal Whitehead) goes on to explain his theory, that the convergence begins with the nose, which is interesting but not as interesting, overall, as the Colossal Convergence itself.

Explaining convergence:

"Two prime candidate theories have been proposed:

(1) "One is that convergence is evidence for what might be called the limited scope of biological materials; favored by authors such as Stephen J Gould, this explanation rests largely on the idea that phylogenetic, developmental, and physical constraints restrict the forms that biological systems can take, and, therefore, change tends to be strongly limited to what is developmentally possible. This explanation has been considerably reinforced by the evidence for the deep conservation of regulatory genes across many lineages.

"However, there is also mounting evidence that the genome can be dynamic at this level. These same regulatory genes are capable of producing widely divergent phenotypes at a very fundamental level, and convergence at the very basic level of biological systems, such as the evolution of multicellularity, can occur there through entirely different genetic means.

(2) "An alternative view is that convergence is so common because the adaptive problems faced by organisms in the struggle to survive recur frequently, and so selection tends to favor the same solutions. In this case, it is not the nature of biological materials that is limited but the 'imagination' of the selective forces."

This is the view that would have us predicting the nature of sufficiently large swimming life on Europa, for instance. (Naturally the author points out that both views are probably right in various cases.)