There is a highly controversial interpretation of history that says that this was done in the USA to black slaves, resulting in superior athletic performance in their descendants. I won't touch on the relative merits of such a statement, other than to say that given what we have done to other species, it's plausible that it can be done to humans as well.

Even the chicken industry admits there's a difference between chickens raised for eggs or meat purposes; some of the more extreme examples of this start to show significant deviations in their physiology, but I have yet to come across a scientific paper that addresses and quantifies this deviation.

The Merino sheep have lost the gene to turn off their wool production. These are sheep that if not regularly shawn, will die of heat exposure from an increasingly thick coat of wool. These were effectively bred into existence in Europe in the 13th or 14th century.

All the cows on Earth are allegedly descendants of a herd of aurochs that were domesticated many tens of thousands of years ago.

The point being, the only difference between chickens, sheep, cows and humans is the longevity of the human generational period. In other words, if you really wanted to selectively breed humans you could do so and 2000 years is ample time to do it in.

The only problem I foresee is that humans are notoriously uncooperative with such programs, and being the intelligent species we are (and you're going to want to breed for that as well) we'll find ways to both breed when we're not supposed to and not breed where we're expected to. In such a case, successfully breeding humans will involve about the same amount of management overhead as successfully herding cats.

Of course you could selectively breed humans for particular traits just the same as any other animal.

You do not need any of the scientific advances you list to do it though.

Deliberate & knowing selective breeding of crops, agricultural animals & hunting dogs predates the theory of evolution by a very very long time (many hundreds of years if not thousands).

So there really is no plausible reason your Romans couldn't do this other than.

1. The rather obvious difficulties of selectively breeding an unwilling human population.


2. The problem of keeping a breeding program on track with a species that the overseers of the project won't live long enough to see more than two or three generations of reach maturity.

Regarding point 2. very few organisations (companies & the like) survive much past a couple of hundred years (& that's not long enough by a very long chalk to do this), though the catholic church on the other hand has survived the best part of 2000 years in more or less it's current form.

You wouldn't need your subjects to be exclusive either with an individual or to limit their procreation to just those within the chosen breeding group, you could reward them for producing children with a desirable individual & still let them reproduce with or marry whoever else they want. All that's really important for a selective breeding project like this one is that you get the offspring you want, not that their shouldn't be any other offspring, so with the right incentives point 1. can perhaps be overcome.

All you need then is an organisation with adequate resources able to persist for the required length of time & remain on target without significant drift in it's goals from those originally set & it's all good.

Empires don't tend to persist for long enough to accomplish what you have in mind (the Roman empire only lasted around 500 years & the Ottoman empire only did a little better at around 600 years) so to achieve what you want you'd probably need a religion or some sort of religious order within one of the more persistent religions.

It's only taken since the 1800's to produce Belgian Blue cattle by selective breeding.

So people that without ever having done any resistance training will still look like Arnold did in 1974 (if that's all you're aiming for of course) are plausible with just 500 or 600 years.

Eugenics

After all, we do it with animals, why wouldn't it work with people?

The concept has been around since the late 1800s, and was popularly accepted as not all that bad an idea even by the high and mighty up until WWII.

In 1883, Sir Francis Galton, a respected British scholar and cousin of Charles Darwin, first used the term eugenics, meaning “well-born.” Galton believed that the human race could help direct its future by selectively breeding individuals who have “desired” traits. This idea was based on Galton’s study of upper class Britain. Following these studies, Galton concluded that an elite position in society was due to a good genetic makeup. While Galton’s plans to improve the human race through selective breeding never came to fruition in Britain, they eventually took sinister turns in other countries. - history of eugenics

It's a dirty word now after a certain chap by the name of Adolf Hitler put it into full production and people started to understand what it really meant.

It was mostly considered in racial and class terms. The underclass, the poor, and immigrants, were generally to be discouraged from reproducing in favour of those with money or acceptable racial traits. We now consider that approach to be abhorrent, but as soon as you start suggesting selective breeding of humans you have to understand that it's not a new idea and quite how dangerous the ground you're treading on can become.

You don't need knowledge of genetics or cells or anything similar: breeding to improve stock (in both plants and animals) was well-understood by that point, having been practiced for millennia, even if they didn't didn't know why it worked.

So there's nothing in principle stopping it, but you run into several major practical problems.

1. Generation time

The average cow can be ready for breeding at 13 months old. A pig, 6 months. Fowl, 6 months. A horse, 18 months.

At best, you're looking at 13-14 years old for a human. But since humans take several more years to reach physical maturity, and if you're wanting to breed a "superior" human, you want your subjects to be old enough to see if they express those traits, so really you need to wait until they're 17-20 and in their physical prime before you can decide which members you want to breed for the next generation.

Unless you get ridiculously lucky and stumble on a beneficial mutation, you're looking at a century or more before you might start seeing some results, and even then it's going to take time for you to spread it around.

2. Control

You want to control breeding of cattle, or dogs, or chickens, or horses, it's not that hard. It's fairly trivial to control breeding access, separating the herd so that you can breed those traits you want in at least semi-controlled conditions.

Humans, on the other hand...they tend to not take being locked up very well, and have the tools (intelligence, manipulative limbs, language) to make trying that sort of control a true pain in the ass. And don't underestimate the power of language. If you go into a pen to pull out the best hen you want to mate with a specific rooster, you don't have to worry about the hens conspiring to ambush you and take the keys.

3. Empathy

You want to breed cows, aside from a few animal-rights activists (which were presumably rare back in Roman times) no one is going to really care. You want to do that same sort of thing to people, that's a much harder row to hoe. People, for the most part, care for other people. And that means you're going to have opposition.

In the Robert A. Heinlein future history series the Howard Foundation bred humans for longevity. They listed young men and women who each had four living and healthy grandparents and offered them payment to marry persons on the list. And after a (scientifically surprisingly) few generations people were born who could live for a few hundred years.

I have read a suggestion that if an organization paid men born to fathers twice as old as the average father were induced to father sons when they were twice as old as the average father, and if this continued for a few generations of old fathers, it would produce persons with an average lifespan twice that of normal humans.

In modern society people have a pool of millions of potential future spouses, though of course they can only choose from among the thousands of persons that they actually happen to meet.

But for hundreds of thousands of years young people in endogamous bands only had a pool of few dozen potential future spouses to choose from, while young people in exogamous bands only that maybe half a dozen times that many people in neighboring bands to choose from.

So I suspect that it is psychologically quite acceptable for a young human to be limited to choosing a spouse from among a limited group of a few thousand members of a specific society, such as humans being bred for intelligence, or stupidity, or strength, or weakness, or longevity, or short lifespans, or whatever qualities. Possibly the persons who were taken out of the program because they didn't test with enough of the desired quality would be more disappointed, as well as persons in the program who might be in love with them and forbidden to be with them.

It is possible to embed "ideals" into our culture, religion and so on, and to somehow make people search for partners bearing those properties. For instance, the eternal bliss of afterlife, may be described as being achievable only after successfully challenging some obstacles. Those obstacles may demand athletic performance, solving hard puzzles, being a brave and die-hard soldier. The Vikings have embedded those combat skills in their "Valhaha". The Mesoamericans (Maya, Aztec, Incas) describe the road to the afterlife filled with sophisticated puzzles, both mentally and physically. (The Xibalba -place of fear- of the Maya). You may add that the skills one develops in his material life are reflected in his/her skills necessary to reach Valhaha, or whatever. Obviously, women will search for men with those skills in a hope to inherit those good genes to their children.

So yes, religion and culture may coax people to cooperate, at-least to some degree. Needless to say, that the time needed for humans to reproduce, cultural influences and our curiosity for alternatives may push the brakes on such processes sooner or later.

Selective breeding of humans is possible but there are obvious social dangers. Also to really get the most out of it you have to be pretty brutal and cold blooded about your culling process. And there is always that question about what traits you want to breed for. Science Fiction horror stories abound with super soldiers and utterly faithful slaves. Usually the discussion involves the issue of fitness and what you can breed into people and what requires training. There are always trade-offs. Breeding for long life would be a disaster if severe population control were not imposed. With modern technology, strength and stamina, while attractive, is not as important as the ability to process computer data. In a world of nukes and smart bombs do we need meaner more lethal soldiers or more clever and accommodating diplomats? European royalty attempted to preserve something called royal blood through a de-facto process of inbreeding which resulted in stunted limbs, idiocy, and hemophilia. Their arrogant attempt to improve their genetic ability to rule resulted in exactly the opposite. A very good fictional example of a society that uses selective breeding is Hellstrom's Hive by Frank Herbert. Though his fictional Hive is horrifying to our eyes, many of the techniques described would make a great deal of sense if you plan on breeding humans. And remember his Hive had existed for only about a hundred years. Imagine what could be done in the time frame of the Roman empire.