Imagine a change that increases incidence of red hair. Sound familiar?
In Africa, high melanin production (which makes skin and hair darker) is strongly selected because it's so sunny: low melanin= sunburn and dehydration --> death. In Ireland, on the other hand, it's much less sunny. In other words, there's almost no selection for high melanin because it's fairly irrelevant to reproductive success there. So the gene changes over time to low melanin production, which also causes red (and blond) hair.
When the frequency of a trait changes by chance, often because it is under weak or no selective pressure, this is called genetic drift. It's not necessarily beneficial. Sometimes it's neutral. Sometimes it's helpful. Sometimes it's not: Ashkenazi Jews have drifted-and-inbred enough to be strong carriers of Tay-Sachs.
Why are somany people in Ireland red-headed? Because it's an island. The population is limited, which increases the incidence of drift. Especially if there's a smaller generation once or twice, the effect is increased, because the drift goes through a bottleneck of sorts.
Imagine, for example, that an Irish generation had only four people, and three were carriers for red hair. The incidence of red hair would shoot up in the next generations. Now imagine it happened again: even more red hair. As drift increases in this or other ways, alleles (gene variants) head for fixation, or 100% occurrence in the population.
"The Age of Darwin" (still not worth reading) also tells us:
Human beings, in our current understanding, are jerry-built creatures, in which new, sophisticated faculties are piled on top of primitive earlier ones. Our genes were formed during the vast stretches when people were hunters and gatherers, and we are now only semi-adapted to the age of nuclear weapons and fast food.This kind of determinism postulates that we are headed for an 'evolved end', a higher function. Note that this same theory impelled colonization and forcible conversion: White people are more evolved. (See: number of times the word 'primitive' is used in the Jesuit Relations.) Again: mutation is random.
'Our genes were formed during the vast stretches when people were hunters and gatherers.' Riiiight. No, our genes were formed while we were the one-celled last common ancestor, and then a bunch of stuff and then squidgy fish things and then small furry mammals with lots of teeth and then larger mammals, and then monkeys. Oh yes, and then eventually hunters and gatherers. Earliest hominid: 8 to 4 million years ago. We only diverged from REPTILES 360 million years ago.
Eukaryotes and prokaryotes diverged from the last common ancestor around 1.5 billion years ago. Our genes were being 'formed' the whole time. They're still being formed.
As for being poorly adapted to nuclear weapons and fast food: Cheeseburger-eating Americans live a lot longer than the average Paleolithic hominid. And what's the world population again?
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References:
1. Encyclopedia of Evolution. Mark Page, Ed. Oxford UP 2002
Genetic Drift: "Each genetic locus can exist in several versions that differ somewhat... Without selection, mutation, or migration, the relative frequencies of these alleles in a population are expected to remain the same from one generation to the next. However, because all populations are finite, allele frequencies can change by random chance, even when the frequency of alleles is expected to be the same on average. ... A good experimental example was provided by Buri (1956), who followed the frequency of eye color alleles in Drosophila populations of different sizes... In the extreme case, where only one allele copy by chance gives rise to all alleles in the next generation, the population will immediately become fixed (100%) for that allele and genetic drift will be at its maximum."
2. Classic Buri paper: "Gene Frequency in Small Populations of Mutant Drosophila",
Peter Buri. Evolution, Vol. 10, No. 4, 367-402. Dec., 1956. (Subscription through JSTOR, but if you want the article, email me.)
3. More on Drift: "Genetic drift additionally contributes to the evolutionary variance around expectations, under neutrality. Weak selection is difficult to distinguish against a background of genetic drift."
