The theory of evolution, as has been widely observed, is a remarkable theory which seems to account very neatly for the wide variety of life on earth, for its excellent adaptation to its environment and for the fact that men have nipples¹. It also accounts for the fact that we all behave like weirdos every spring. Yes, propagating your DNA is the order of the month.

The ones in between

One argument which has persisted in the debate about evolution is about intermediate forms. Although the benefits of wings and eyes are obvious, the benefits of half an eye or half a wing are less easy to see (very difficult to see indeed, if you have only half an eye). What led the first animals to stick their arms out while leaping off the ground? Did tiny flaps of skin hanging off the arms really confer enough of an advantage in those who had them to make them prosper over those who did not²? How do you explain things which are almost entirely useless until completed?

The answer usually given is that the almost-entirely-useless thing was originally used for something else we don't know about, or just happened to be hanging around (in the case of harmless weirdnesses) and then suddenly turned out to be quite useful for something else, giving rise to an evolutionary pressure which perfected it.

Monstrosities

Another, and bizarre, answer, which was once advanced to me as evolutionary orthodoxy by a friend, is the so-called "hopeful monster" theory. It goes like this.

Occasionally a very large series of mutations will be produced in a species. Most will die. One, or perhaps two, will turn out to be beneficial and be preserved. This accounts for the more extreme and inexplicable things, like the lure on top of an anglerfish.

In reality, this theory solves nothing. For a start, among the billions of possibilities for a strand of DNA to encode, the majority do not even produce an organism. Among the minority of real, living things, most are horribly asymmetrical and hopeless. Among the symmetrical ones many have huge, unwieldy teeth protruding from their wrists and antlers which neuter them whenever they try to walk³. Of the tiny mutations usually experienced within a population, most are non-adaptive and confer a disadvantage on the organism. Only a very few are beneficial.

Given this fact, the larger any individual mutation is, the more likely it is to be harmful, and in some cases, lethal. The hopeful monster is a hopeless monster.

This is why I am rather hesitant to advance the following, to my mind unavoidable, exceptions.

The Great Leap Forward

The first exception is multicellular organisms. I shall explain what I mean.

Multicellular Organisms

In a multicellular organism, large numbers of cells fit together to accomplish a purpose impossible for a single cell. For this purpose, huge slabs of identical bacteria do not count as a multicellular organism. Now while huge slabs of identical bacteria, or something like this, were probably the testing ground for this mutation, and while the first multicellular organism probably didn't do anything that special, nonetheless, it must have reproduced as a multicellular organism. Even if only bicellular, this is still a big leap. However, this may prove comparatively easy to solve, whereas it leads on to a point which seems far from trivial.

Everything you ever wanted to know about sex (but were afraid to ask)

It is an oft-observed fact that sexual reproduction is of immense advantage in an evolving species. It allows, say, two pine trees to have random mutations in their saplines⁴, one of which has been preserved because it makes the needles slightly richer in chlorophyll (in Northern climes this has obvious advantages) and one of which makes the needles slightly longer. A child tree can then have longer, greener needles, which without sexual reproduction would require the same tree sapline to undergo both mutations consequently -- far more unlikely. In this way the rate of accumulation of favourable traits is multiplied many times, and this is a Good Thing.

Sexual reproduction, in fact, is usually represented as being an evolutionary advantage in and of itself.

Now here comes the tricky part. What is the intermediate form between an asexual organism and a sexual one? There doesn't seem to be a middle-ground which is even vaguely conceivable.

The science bit ...

Backing up a little, there are two cellular processes to consider here, one of which is the basic mechanism for almost all cellular reproduction and one of which is specific to sexual reproduction and does all the cool DNA mixing we're all so proud of.

Parting ...

The first process is called mitosis⁵.

The DNA in a cell's nucleus splits into two half-strands, each of which is formed into a full strand of DNA. Each half is pulled towards one side of the cell, giving two complete copies of the cell DNA, and the cell then splits in two, producing two identical copies. Thus:

a double DNA strand

  splits in two

  each strand redoubles

 <--->  separates into its half of the cell

 <-  |  ->  and the cell itself splits.

In bacteria⁶ and other monocellular organisms this is the mode of reproduction. In multicellular organisms this is a method of growth, and also ensures renewal of cells e.g. at the skin surface. Chemical differences in the environments of the cells can cause them to change function, called cell differentiation.

... and meeting

The second process is called meiosis, and is considerably more complex. The chromosomes in sexually-reproducing organisms are paired, and these pairs align. The DNA strands split -- as in mitosis -- but one of the half-strands of DNA from each chromosome is set aside and paired with that from the other chromosome, and DNA is swapped between them at apparent random. These split up, form complete strands of DNA and eventually gives rise to four gametes, specialised cells containing half the DNA of a normal cell and designed to pair up with another gamete from the same species and give rise to a new organism. So:

two chromosomes align:

they split into half-strands of DNA

a strand of DNA from one chromosome and a strand from the other align and swap DNA

    and they all double, split and form gametes, that is, cells containing exactly half of a pair of chromosomes.

Two of the gametes contain near-perfect copies of chromosomes within the source individual -- two contain merged copies with DNA from both chromosomes⁷.

It is when two of these gametes recombine that sexual reproduction takes place, and a great deal of fun it can be, too.

And now for something completely different

Sexual reproduction is a wonderful thing and allows many, many advantages in an evolving organism. However, if it's so great, why don't bacteria do it?

Well, it's probably for the same reason cheetahs don't photosynthesize. Making your own food from air is of course a fantastically useful thing to be able to do, but the apparatus for doing it is fantastically heavy -- so heavy that any organism capable of producing the energy to move a human around would weigh as much as a large tree. No organism can produce enough energy under photosynthesis to lift its own weight. Any cheetah with even the smallest amount of foliage would find the extra poundage far outweighed the possible benefit.

And so we come to single-celled organisms. When a single-celled organism splits, it produces two "children" automatically. When a cell splits into four gametes, each gamete relies on finding another in order to make a complete organism. The chances of producing two "children" are, inevitably, less than certain -- and if one cell is all you have to invest in such a gamble, potential evolutionary benefits down the line aren't going to be enough to start the ball rolling. The costs outweigh the benefits -- besides which, the reproduction cycle is often so fast that an organism can evolve quite rapidly just by accumulating mutations.

This is where I run out of momentum because Lucinda (et al) have just told me I'm oh, so very wrong. Or something. So I have to do more research, mostly in pubs.