Brunner, John. Total Eclipse

John Brunner, Total Eclipse (1974). Mess with eugenics and capitalism-like structures merged into one institution, y'all might wind up dead.

Here's the relevant bit (big spoiler alert):
“But that’s absurd,” Lucas said after a pause. “Going bankrupt— well, it could bring down a civilisation, but it couldn’t wipe out an entire species.”
“It could!” Ian insisted. “Look, it occurred to us to wonder whether the Draconians traded among themselves, and we decided yes, they must have, but it never occurred to any of us to ask what kind of currency they employed.”
Cathy jumped to her feet. “The printed crystals!” she burst out. “Those can’t have been money!”
Karen shouted. “You’d find money all over everywhere, not concentrated in great big storehouses—”
[like, an hour later] 
“Am I being obtuse?” Karen said. “Or have you not yet explained how going bankrupt killed them off?”
“I was just coming to the details of that. I think I already said— excuse me, but my head is buzzing insanely with all the implications— I think I said I started asking what an individual could accumulate by way of reward, or payment.”
There was a brief hush. Nadine ventured, “Promises that when he became she, there would be outstanding genetic lines reserved to— uh— to her?”
“That’s it. That’s what killed them.” Igor leapt to his feet and started pacing back and forth, thumping fist into palm. “I’ve almost got it,” he said. “You mean that without realising what they were doing, they restricted their genetic pool until it became dangerous, and then it was too late. Like fortunes being concentrated in the hands of a few ultra-powerful families? A sort of genetic capitalism?”