Monday, 23 November 2009

Darwin's Five Laws of Evolution

When Charles Darwin first proposed his hypotheses a century and a half ago, he saw them as one conjoined notion. With the supporting evidence he provided, he and others could describe that notion as a theory, Darwin's Theory of Evolution.

As Ernst Mayr pointed out 1, Darwin's notion has five parts, only one of which was accepted by all the evolutionists of his time: that part was the conclusion that the world is neither constant nor recently created, nor does it pass through cycles which repeat, but that it changes and that entities that live on it change, too.

Darwin's colleagues rejects various other components of his theory, either because they flew in the face of cultural beliefs or because of lack of conclusive evidence, or because of some combination of factors. However, in the time since Darwin first proposed his hypotheses, all five components have been proved, in simulations, observations, and experiment.

Hence rather than call them a theory, or theories, it is better, and more conventional, to call them laws. They are, after all, as well established as Newton's Laws, which we all know, are broken under certain circumstances, but which hold well enough.

Darwin's Five Laws are:


1.Evolution as such
This is the understanding that the world is not constant, nor recently created, nor cycling, but is changing; and that the types of entities that live on it also change.

This contradicts the `common sense' notion, prevalent in Western society since the ancient Greeks, that different animals and plants each had its own `essence' and, as a consequence, could not change from one kind to another any more than a triangle could change to a square.

You only had to look at a cat and a dog and ask how one could change into the other. Nowadays, we do not think of a cat changing into a dog, but ask about a common ancestor of both, from a time long before cats and dogs appeared.

2.Common descent
This is the understanding that every group of living entities that we know of on this planet descended from a common ancestor.

Common descent occurs because one lineage would, willy-nilly, be a little more prolific than another, and would thereby wipe out the other. So only one survives.

(Prions are an exception. These are proteins of a particular shape that catalyze other proteins to take on the same shape. Prions are not life in the usual sense of the word although they are self-replicating. Similarly, no one thinks of an economy as living in the usual sense of the word, even though an economy can reproduce it itself, too.)

Rather than ask how a cat could change into a dog, we ask how a previous ancestor of both could give birth to an animal that is slightly more suited to the wolf way of making a living than to the feline profession. (Ecologists call plants' and animals' ways of surviving and reproducing in the world their `niches'. These are what we humans call our `professions'.)

The answer is straight forward, one animal's blue prints, what some people in the latter 19th century called its germ plasm, and what modern people call its DNA, varied one from another. That change enabled an adult who was born with that variation to live more like a wolf or modern dog than its parents.

This understanding, by the way, answers the age-old question, `which came first, the chicken or the egg?' The egg came first, because it contains the part that changed. The egg was laid by a non- or pre-chicken entity; the egg grew up to be a chicken.

Only if you think that adult animals and plants can change their nature, and pass on that change to their children, will you think that perhaps the chicken came first. This latter form of change is called Lamarckianism. Human culture is invented by grown people and passed on by parents to their children. It is Lamarckian. But the looks and actions of animals, at least those without culture of their own, are passed on genetically. A parent's action does not influence the looks and actions of the child. Only changes in the egg change the child.

At the time Darwin wrote, many evolutionists still thought of animals and plants as being like humans. They asked whether an adult proto-giraffe could stretch its neck to reach higher leaves, and pass on a longer neck to its children, much as human parents pass on a language to their children.

3.Multiplication of species

This is the understanding that species either split into or bud off other species, often through the geographical isolation of a founder species.

Because different ecological niches provide different ways for an animal or plant to live — provide different `professions' — and because blueprints do not copy perfectly, different plants and come to fill different niches, with different shapes and behaviors.

4.Gradualism
This is the understanding that changes take place through the gradual change of population rather than the sudden production of new individuals.

`Gradual' is a relative word. In discussions of `punctuated equilibria', I have heard people talk of one species replacing another in the `blink of an eye'. What they meant was a time period that is many many times as long as written human history. The `blink' might last 100,000 years. In human terms, this is a long time. But in geological terms, 100,000 years is short. Hence the use of the phrase. But to humans, a change over 100,000, or over merely 10,000 years, seems gradual.

Put another way, gradualists claim that it is unlikely that starting tomorrow at 9 am, all humans born would possess green skins and lay large, hard shelled eggs.

5.Natural selection
This is the understanding that individuals in every generation are different from one another, or, at least some of them are. In every generation some individuals survive and reproduce better than others. Their genes multiply.

This is the key idea: natural reproduction is not perfect.

People make considerable efforts to ensure that human copies are perfect. Inexact copying indicates a failure of the scribe or bug in the program.

In natural reproduction, children may be similar to their parent if they bud from that one parent and if no stray cosmic ray changes their DNA and no DNA enters their cell from another.

More sophisticated plants and animals have two parents. These are species with sexual reproduction. In this circumstance, children come from mixtures of the two parents' genetic material. This mixing induces variation among the children.

Some of those children will do a better job at one or other niche open to it, and consequently will be more likely to survive and propagate whatever enables it to survive and propagate better.

The environment in which plants and animals reproduce is defined by the world around each plant or animal: and part of that world consists of other entities of the same type. Put another way, plants and animals must survive and reproduce in a world with others of their own type. The others will be the same species, with similar skills and talents.

This means that selection occurs within a species even when the rest of the environment does not change. (Incidentally, when characteristics like perceived health influence sexual choice, the process is called `sexual selection'.)

Increased intelligence sometimes increases survival and reproduction. David Brin suggested this in 1982 2. Doubtless, others enjoyed the same insight earlier.

For example, a female peahen will have more healthy offspring (probabilistically speaking) if she is able to identify a peacock that is more healthy than its peers. This task requires more intelligence than not being able to make the identification.

A side effect of this process is that some lineages should gain certain features, such as intelligence, even without changes in the non-lineage part of the environment. Not all lineages will gain these, since alternative ways to survive and reproduce also exist. But some lineages will.

This means that if the dinosaurs had survived, we humans might not exist. Instead dinosaur-descended beings might exist in our stead and these beings might also communicate symbolically, as we humans do in language.

Interestingly, Ernst Mayr 3, among others, does not accept this line of reasoning, and therefore argues that high level intelligence is a happenstance rather than an outcome that may well occur on any living world on which complex life survives for long enough.

Well, to be more precise, I think Mayr does accept this reasoning, but in his writing he focuses primarily on a different argument, that involving `purpose'. Evolution lacks purpose, but many people think otherwise, either because humans act according to purposes, or because their beliefs suggest it.

The requirement for lengthy survival poses barriers. As Peter Douglas Ward and Donald Brownlee point out 4, planets endure catastrophes that are frequent over the eons. A stable sun, like ours, grows brighter as it ages. The inward side of a solar system's `habitable zone' moves outwards. This makes a `runaway greenhouse' as on Venus more likely. If the planet starts out closer to the inward side of its habitable zone than earth, the planet may die before complex life has time to evolve.

Or microbes may consume so much carbon dioxide and other `greenhouse' gases that rather than overheat, the planet may freeze. The freeze may kill every living being on it before volcanic eruptions increase the supply of greenhouse gases which warm the planet.

Or major volcanic eruptions may poison the land and sea, or asteroids may strike.

As a practical matter, complex life may be rare, even if simple life is common http://www.rattlesnake.com/notions/evolution.html

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