If we do the proper genetic studies on horses, lots of them ( > 10^6) we will eventually be able to find variants that predict how large or how fast individual horses are, and ( dare I say it) we will eventually be able to tell if certain breeds of horses are genetically big or small, fast or slow. People have always wondered if Percherons are really larger than Shetland ponies – or if that’s just a stereotype. Some say that Thoroughbreds are faster than Clydesdales – wouldn’t it be nice to actually know for sure? Of course there are deep philosophical questions about what ‘ genetically different ‘ really means – and we wouldn’t really know that two breeds were different unless we also understood the mechanism of each and every common variant that boosted or reduced size or speed. Since quantitative selection boosts any variant that favors the trait under selection, and those variants work through many different biological pathways, we have some work to do. But we still won’t really know that my Prince (a Shetland) was inherently* slower than Secretariat unless we understand every one of those mechanisms. And their interactions. And their little dog, too! We also need to understand the historical reasons for any differences – what selective pressures drove those differences, how those pressures varied over space and time – or we don’t really know anything.
Admixture studies might be useful, but they’re icky.
And we need to motorize those fences at Wrigley Field.
* Maybe Prince was deprived due to my cousins shooting him with their BB guns. He caught one of them later, kneeled on her, and broke her collarbone. Depraved on account of being deprived?