Evolutionary biologists like laws, models, theorems or rules that explain biological observations and can be used to predict future observations. Thus, Hamilton's Rule (which we blogged about a few days ago) which explains the perplexing (to a strict Darwinist) behavior of altruism, and which invokes the kinship assessing all-seeing eye of natural selection lest a simple act of kindness go by insufficiently rewarded by the beneficiary. Likewise almost every trait of an organism from its color to its organ structures is given, or forced to have, a specific selectionistic explanation for its origin. And of course the theory of natural selection has been used, refined and re-refined to explain all manner of traits and behaviors of individuals and even societies since Darwin's time. But, despite their rigorous precision in principle, that resemble laws an Einstein would admire, none of these laws/models/rules can explain every instance of X, and often require some hand waving to apply at all.
We think this is because the most general rule, one that applies to all of life, is 'whatever works, works'. That is, evolution follows no single rules. As a result, life is a sloppy process, and when you think you've found a rule that works in one instance, life defies you to apply it again in another. It doesn't sound like proper science, but if the shoe fits.....
So, in writing our book and in other things we've written, we've come up with a list of general principles that we think apply very broadly, even given that life is a sloppy process, and even if the principles are very vague or generic by comparison with the formulas for chemical reactions or the action of gravity on the Moon. And you don't have to take our word for it. These general principles are used explicitly or implicitly every day in genetics and developmental biology laboratories around the world. If you are a biologist, you will recognize right away that some or all of them guide your work, even if in an informal way. We didn't invent these principles, we have simply compiled them -- and we discuss them at length in our book.
1. Inheritance with memory: life is one continuous history, from the beginning 4 billion years ago to now, and cells 'remember' what they've inherited and carry it forth (except for changes, such as mutations in DNA);
2. Modular organization; life is constructed in LEGO mode, with repeated units, like segments, hairs, leaves, and the like. We have referred in earlier posts to the importance of polymers, long molecules made of different subunits (modular units), whose arrangement contains the 'information' of life.
3. Sequestration: the modules in life are isolated from each other, at least in part. If life is a history of divergence from common origin, from the beginning, that was possible only because of local, isolated compartments that can be separated enough from other compartments to become different. Parts of DNA are isolated from each other, cells are, organs are....and you are an organism all your own for the same reason.
4. Coding and interaction; Life is organized as above, generally because of the signaling interactions among units within and between cells (and beyond). The key to all of this is combinations of molecules present together in time and place within an organism. Combinations represent one of many kinds of 'codes', of which the genetic code is only one example. The dance of such interactions is what causes differential organisms.
5: Contingency: what's here tomorrow works only from what's here today. This is the hierarchical nature of combinatorial interactions that we mentioned yesterday.
6. Chance: action without direction. It is fundamental to life that there is a major component of chance in most aspects of what happens. Mutation in DNA or the chance transmission of variants from parent to offspring ('Mendel's rules'), and the chance aspects of birth and death, or of winning and losing competition (such as natural selection) are examples at various levels.
7. Adaptability in the face of changing circumstances. DNA reacts to its environment (genes are used, or not, depending on whether the DNA is grabbed near them by proteins or other molecules), cells react to conditions via signals of various sorts that they are primed to detect, and so on, up to you, who react to your environment and decide what to eat, when to hold 'em and when to fold 'em, who to woo and who to fear, and so many other things. Your brain is a hyperactive environment-sensor and decision-maker, and we're all familiar with that (and why some of our closest friends can't ever seem to make up their minds!). But every cell in your body is doing it all the time, and so are structures within them.
8. Cooperation. The above phenomena are, as we use the term, examples of cooperation, that is, co-operation, working jointly together. Sometimes, as among social organisms, this is the kind of socially supportive interactions we usually use the word 'cooperation' for, but the myriad interactions that involve multiple partners (signal, signal detector, cells in organs, and so on) are examples of cooperation in the mechanical and more literal, rather than emotional sense of the term.Not the least aspect of life that is very general is the lack of precision in these general principles of life: no matter what might be 'read' in the genes, development makes mistakes -- DNA gets wrongly copied during cell replication, e.g, or during gene expression, or a gene is expressed at the wrong time or place ('wrong' according to what we think are the rules; e.g., A's always pair with T's, and G's with C's in nucleic acids, or this gene is 'for' that, and only expressed here, or there's only one way to build a given trait). From cell to cell there is variation if you look closely, and the only reason we tend to overlook that variation is what can be called the central tendency, of many cells of a given type, no matter that they vary, together generally produce an acceptable structure (or the individual dies aborning).
Work-arounds have evolved for some of these 'mistakes' -- some DNA copying mistakes are repaired, but not all by any means, which is a good thing for biodiversity. But others, that aren't repaired, are either fatal to an embryo or survivable, and of the survivable ones, eventually they can even come to look like good ideas; another good thing for biodiversity. Or, an embryo can survive the mistake, but that same mistake might well spell doom later on, in an unforgiving environment.
These principles provide a logic to explain why traits have come about. Yes, you might say wings are naturally selected, and maybe they are (that is, proto-birds had more offspring than animals that weren't airborne at all, and so wings got trendy), but that doesn't tell you anything about how wings came about. And anyway, maybe wings weren't advantageous at all, they just happened, so your theory of natural selection tells you nothing at all about wings in the end.
Now none of these generalizations are secret or unknown, they are as simple as can be, and they're rather obvious. There's so much to say about them to show the point that one could, well, write a whole book on the subject. That's what we did, but even the fact that these general ideas are all around us and easy to perceive, people persist in pushing hard for very precise and rigorous 'laws' that it is almost just as easy to show don't work that way. We are too wedded to that kind of Galileo/Newton/Einstein science, for historical if not other reasons.
Tomorrow -- er, Monday -- we will take this kind of slopistry from a somewhat different perspective.....