That requires some kind of selection pressure. In the wild, the pressure is that if you can't beat other lifeforms to the good resources, you starve to death/get eaten and don't reproduce. Artificial selection seems like a better model for AI, though. Animals in a factory farm don't have to compete for food, and aren't allowed to control their surroundings in any way. The selection pressure there is that you're only allowed to reproduce if you're tasty and marketable.

With AI and robots, I think the selection pressure is going to be ability to do stuff that (some) humans value, or think they value. The risk isn't that the robots might rebel, it's that the people in charge of them don't understand the consequences of what they're selecting for and/or don't have everyone's best interests at heart.

Natural selection in biology has a mechanism. And it took a few billion years to produce anything that wants. What's the mechanism for natural selection in robots?

None. They don't reproduce, so there's no opportunity to evolve characteristics. And it is not our best interests to make an evolving robot, any more than it would be to accelerate evolution amongst animals.

We want work performed in better cheaper ways. We don't want competition.

Well... what if the form of reproduction is a lot more subtle than you would recognize? Meat people producing baby meat people is an outcome of evolution, but robots building more physical robots is the wrong way to think about it. Potentially, its more like github: the designs go out into the world and experience iterative improvement to the software. Sometimes, it forks. Sometimes, it rewrites because it has hit local maxima (aka legacy).

Who is iteratively improving the software?

The human slaves.

>any more than it would be to accelerate evolution amongst animals.

I'm going to assume you've never been to a farm and seen huge fat chickens. Yes, selective breeding is an accelerated evolution to traits we want.

And it is not our best interests to make an evolving robot,

But that not the question, the question is, can it be done. And if that answer is yes, it will be done.

In a way they could reproduce. They might just need to convince a human to handle the physical part - assembling a new host computer, but that could be just about giving the human the right incentives (maybe $ or knowledge).