Today, I was listening in on Genesis 30, in which Jacob had this weird deal with his uncle Laban. The deal worked as follows:

“Let me pass through all your flock today, removing from it every speckled and spotted sheep and every black lamb, and the spotted and speckled among the goats, and they shall be my wages. So my honesty will answer for me later, when you come to look into my wages with you. Every one that is not speckled and spotted among the goats and black among the lambs, if found with me, shall be counted stolen.” Laban said, “Good! Let it be as you have said.” But that day Laban removed the male goats that were striped and spotted, and all the female goats that were speckled and spotted, every one that had white on it, and every lamb that was black, and put them in the charge of his sons. And he set a distance of three days’ journey between himself and Jacob, and Jacob pastured the rest of Laban’s flock. (Genesis 30:32–36)

Then, Jacob did something with the spotted, striped, and speckled flocks:

Then Jacob took fresh sticks of poplar and almond and plane (or chestnut or sycamore) trees, and peeled white streaks in them, exposing the white of the sticks. He set the sticks that he had peeled in front of the flocks in the troughs, that is, the watering places, where the flocks came to drink. And since they bred when they came to drink, the flocks bred in front of the sticks and so the flocks brought forth striped, speckled, and spotted. And Jacob separated the lambs and set the faces of the flocks toward the striped and all the black in the flock of Laban. He put his own droves apart and did not put them with Laban’s flock. Whenever the stronger of the flock were breeding, Jacob would lay the sticks in the troughs before the eyes of the flock, that they might breed among the sticks, but for the feebler of the flock he would not lay them there. So the feebler would be Laban’s, and the stronger Jacob’s. Thus the man increased greatly and had large flocks, female servants and male servants, and camels and donkeys. (Genesis 30:37–43)

The end result was that Jacob basically made a large, large flock for himself out of his uncle's flock... and if you read on, you'll see that his own flock size soon surpassed his uncle's.

Is there an explanation for Jacob's success?

Now, I was curious, is there a mechanism we can describe to explain the discrepancy over multiple generations of flock breeding?

Having thought about the problem using my prior biological training, and doing a bit of cursory searching on the internet, there's two possible factors that can explain Jacob's success: folklore medicine actually working (I still remain somewhat skeptical of it), and genetics.

Genetics

As a prior, coat colour is likely a genetic trait. Jacob engaged in selective breeding:

He put his own droves apart and did not put them with Laban’s flock.

Let's start with a highly reasonable and generous assumption that coat colour is genetically determined, regardless of whether particular patterns are dominant or recessive. Then splitting the flock and selectively breeding them would result in an evolution of Jacob's flock towards patterned coats while Laban's flock would be dominantly pure coated. Multiple generations are needed though. (see Genetics influencing colouration of goats)

It's hard to imagine, though, how coat colour is correlated with the strength of a flock. After all, the sheep and goats are not subject to external pressure, apart from humans driving their evolution.

Folklore medicine

Looking again at the Biblical text...

Then Jacob took fresh sticks of poplar and almond and plane (or chestnut or sycamore) trees, and peeled white streaks in them, exposing the white of the sticks. He set the sticks that he had peeled in front of the flocks in the troughs, that is, the watering places, where the flocks came to drink. And since they bred when they came to drink, the flocks bred in front of the sticks and so the flocks brought forth striped, speckled, and spotted... Whenever the stronger of the flock were breeding, Jacob would lay the sticks in the troughs before the eyes of the flock, that they might breed among the sticks, but for the feebler of the flock he would not lay them there.

What's up with poplar, almond etc.? Is this a food supplement? A careful reading of the text shows that Jacob set these sticks inside the watering troughs, i.e. immersed them in the water that the flocks were drinking from... but only for the "stronger" of the flock. What are the effects of immersing these sticks of trees in water?

• Willow bark and leaves contain salicyclic acid
• Poplar supplementation increases lamb reproduction
• Poplar and willow can increase animal protein utilization - likely contributing to "strength" of a flock.
• Almond tree bark increases production of mothers milk and decrease birth pangs in livestock - also likely contributing to the strength of a flock.
• Plane bark might have some medicinal benefits for livestock

Caveats

Now, I must clarify where I am doing a bit of interpretation here. Ancient people did not write in the direct scientific style that we do, so a lot of detail is likely lost. (And we moderners must also admit: our writing does not necessarily convey 100% the detail necessary for replication!) Moreover, we are working with translations, so more detail is literally lost in translation. There is a bit of interpretive imagination required to reconstruct the full picture of what Jacob did when feeding the flock; after all, Genesis 30 isn't a "day-by-day, play-by-play" log of everything Jacob did with the flock.

In the long run, I would prefer a mechanism, because Mechanisms are better than population empirical evidence. But in the absence of a mechanism, population-level empirical evidence can be a good backup.

Conclusion

With that in mind, it seems to me that the colour of the flock is influenced by genetic selection, while the strength of the flock was done by a combination of observing and selecting the animals had the appearance of good strength (larger, more prior offspring, lineage traced from fitter ancestors) and by supplementing their "general health" with folklore medicines -- medicines that we Homo modernus are finally putting to test with rigorous study.