The meaning of the underlined parts

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The passage below is from the website as follows:
http://www.epubbud.com/read.php?g=H8AVF5J5&tocp=9

It was also somewhere near the Baltic or (close to the north shore of) the Black Sea between 6,000 and 10,000 years ago that a genetic mutation, substituting G for A in a control sequence upstream of a pigment gene called OCA2, gave adults blue eyes for the first time. ... Because it went with unusually pale skin, it probably helped those people who were trying to live on vitamin-D-deficient grain in sunless northern climates: sunlight enables the body to synthesise vitamin D. The gene’s frequency speaks of the fecundity of farmers.

? I can figure out all right with the literal meaning of these underlined parts.
But I cannot figure out why that mutation giving adults blue eyes helped people living on vitamin-D-deficient grain.

? I also don’t see the implicit meaning of last sentence.
Googling showed the meaning of the ‘gene’s frequency’ like this: Allele frequency, or gene frequency, is the proportion of a particular allele (variant of a gene) among all allele copies being considered. In population genetics, allele frequencies are used to depict the amount of genetic diversity at the individual, population, and species level.

But I don’t know why that ‘gene’s frequency’ speaks of the fecundity of farmers.

? Last, I also don’t see the meaning of ‘the fecundity of farmers.’
Does it mean farmers are good at growing crops?
Or does it mean farmers are having many babies?

Wish for your responses.
Thanks in advance.

Anonymous · Accepted Answer

Stenka25The gene’s frequency speaks of the fecundity of farmers.

The gene’s frequency is indicative of the fecundity of farmers.

Blue Jay · Answer

1) Vitamin D is produced by the skin when it is exposed to sunshine, and may also be obtained from food.There is evidence that light colored skin produces vitamin D more effectively than darker skin. The people in that region had a diet poor in vitamin D, and were exposed to little sunlight. A mutation that lightened their skin would help them produce more vitamin D in the little sunlight they got.
2) The gene is very common in northern Europe, so the people who had the gene originally now have many descendants.
3) I would say it means they had many babies.

Stenka25 · Answer

Thanks a lot for your detailed answer, Blue Jay.

Stenka25 · Answer

Thanks a lot, Anonymous.

AlpheccaStars · Answer

Stenka25But I cannot figure out why that mutation giving adults blue eyes helped people living on vitamin-D-deficient grain.

The mutation had two effects: first, pale skin, and second, blue eyes.
When the skin is dark, it cannot absorb the energy in the light from the sun. Dark-skinned people do not sunburn. But the energy from the sun helps the skin to produce natural Vitamin D, so lighter skin was a geneti advantage. In London during the early industrial age, all the smoke from burning coal made the skies cloudy and dark. Children suffered from a disease called rickets, caused by a lack of Vitamin D.
http://www.webmd.com/food-recipes/vitamin-d-deficiency

Stenka25But I don’t know why that ‘gene’s frequency’ speaks of the fecundity of farmers.

Farmers grew the grain, right? And this grain did not have much Vitamin D.
So evolution helped humans to avoid getting the diseases associated with a lack of Vitamin D by natural selection: the increased survival of the individuals with that light-kin + blue eyes mutation. People with that mutation are now extremely common in the northern climates. Thus this gene is very common. That implies that the Vitamin D deficient grains are also common. And that means that the farmers were extremely successful in growing the grain. That is the fecundity of farmers.

Stenka25 · Answer

Thanks for your elaborate answer, AlpheccaStars.

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