Since 1986 there has been a divergent selection experiment running at Stellenbosch University, near Capetown in South Africa.
The two lines started from the same base population of Merino ewes and have been selected either against the Number of lambs weaned (the low line) or for the number of lambs weaned (the high line). It is definitely on the bucket list to go and spend some time there because there is often a lot to learn from selection lines like this and the chance of ever having this style of research again is very remote.
I was excited to discover that there are a couple of new papers based on these flocks. The attached graph is from one of those papers. It shows that in the line selected for a higher number of lambs weaned the lambs were way less susceptible to chill index (a combination of rain, wind, and temperature). So even on a lamb killing day, those that were selected for higher total reproductive output were pretty bomb-proof.