All of the above is actually beside the point. Forming germ cells is high-risk event. You see, normal cell division does its best to make an exact copy of all 46 chromosomes to go into the new cell. But when making a germ cell, there are “no holds barred” because sections of chromosomes fly around and mix. Mother’s chromosome 10 and father’s chromosome 10, just to pick a number, do not waddle politely off to the left or the right. Instead what happens is that they become more like a soup that reassembles into two brand new chromosome 10s. In this way the child gets exactly 50% of each grandparent’s half, or 25% on the nose even when you can’t have 25% of 46.
The process of making germ cells didn’t arise out of some arithmetic sense of fairness. Instead it evolved (guessing here, I’m no scientist) into the freer process of re-stirring the pot in each germ cell. What advantage drove that? One in particular – when there is a ‘bad’ gene, say a recessive that is bad news when both parents contribute that one – but also some novel gene that is better than the standard, and they’re both on the same chromosome, the recessive is only likely to haunt the good one half the time.
ANNND when it’s all over and done with, errors happen. Here’s a very simple example. The inherently dark skin found in African parentage is due to the huge number of genes that produce melanin in the skin. They appear on just about every chromosome. How did they spread like that? Guess – making germ cells lets them show up in more than one place. They evidently shoe-horned themselves into new chromosomes, and the benefit of having more melanin turned out to be a great survival advantage. (The flip side of the coin – many genes are only needed ONCE – when they double up, bad things happen. Life is risky.)
Let one such produce a child with someone from the north, where having any melanin is counter-productive. (Why? because melanin interferes with the skin’s ability to produce vitamin D from sunlight. Northern climates have less sunlight and a far higher demand to cover the skin.) Does the child get white skin or black? It gets half the African parent’s level of melanin. This explains southerners’ careful, scientific understanding of how many white grandparents a child may nave – quadroon says three, for instance. The difference was plain to see – one quarter African, three quarters European. At the same time, parents with both heritages are likely to have odd mixes of melanin levels on each chromosome. In short, their children can receive very different amounts of melanin genes.
Back to the topic! Individual genes tend to stay put. But across a thousand generations, the possibility exists for ‘good’ genes to proliferate, and ‘bad’ genes to fade out, and all because the process of making germ cells is chaotic and (very slightly) error prone.
IN SHORT, ERRORS ARE GOOD. Evolution thrives on mistakes.
Oh, and Another Thing 