For a primeval swamp, New England isn't a bad place to be in mid-October: the trees are trumpeting their extraordinary colours against a bright blue sky.

But where healthcare is concerned, New England - like most of the US - really is a primeval swamp, where Darwin's theory of evolution may not only be applied in all its vicious glory, but actually tested and validated.

Darwin suggested that as environmental challenges appeared, the random changes that natural variation produced would help some creatures survive better than others. Those which had produced advantageous variations thrived, while the rest did less well or died.

Those which had inadvertently gained a natural advantage were better placed for survival and 'prosperity', and their kind flourished until the next challenge. Depending on that new stimulus, the previous gains might be helpful or work against them.

In American healthcare, the challenges facing the competing species tend to be financial pressures and consumer choice.

In a fee-based system, the payers (usually insurance companies), faced with pressure from their insured clients and competitors to keep prices down, have tried to pass some of the financial risk to their hospital providers. Instead of paying for every transaction carried out in a hospital, 'managed care' (or 'managed cost' to the more cynical) means the insurer passes to the provider a certain sum to cover all the activities likely to be required for a client or (more usually) group of clients. The providers carry the risk of providing a reasonable service within the defined cost envelope.

The pressure to keep costs down has been a powerful stimulus for natural selection to create imaginative ways of providing more service for less money.

But healthcare consumers seem determined to maintain their choice of what treatment they get, and where and when. The consequences for the community do not exercise them unduly. This pressure for choice is often at odds with the pressure on costs.

As these environmental challenges grow stronger, so weird new species develop. The pressure for choice and increasing 'market share' has led to a diversity of providers that apparently supply choice in every aspect of care: full-scale mammography units covering populations of 35,000 or less; instant admission for any illness requiring inpatient care; even MRI scans for patients with suspected sinusitis.

On the other hand, some larger providers have responded to managed care by forming giant cartels that maximise their leverage with the payers. Most of Boston's large, eminent, well-endowed teaching hospitals have formed a partnership to discuss their managed care arrangements with the various insurance companies. This '800lb gorilla', as it has been dubbed, can throw its weight into the negotiating process knowing the smaller creatures in the undergrowth cannot resist its loud cry and powerful muscles. Ironically, true choice is an early victim in such a monolithic response. So, too, is risk-sharing: when one party determines the terms of the risk, sharing is not the word that comes to mind.

The Darwinian metaphor highlights how organisations, like species, respond to the immediate stimuli with little thought to longer-term issues. As hospital cartels get bigger, they get more implacable and inflexible, making change less feasible, and allowing new, smaller breeds of providers to emerge, nimble, quick on their feet and able to change direction in a blink of an eye. Once they start eating more of the grass, and nesting in all the niches the giant hospitals could never fill, the cartels might find life more threatening.

This illustrates another evolutionary theory, the 'hunger theory of sex'. When conditions are good and food is plentiful, nature grows and grows; when money is plentiful, hospitals just get larger and busier, and fees for service inflate and expand.

Variation and innovation aren't important: they're not needed to increase market share. It's only when times get tough that new, enterprising ideas will be needed to ensure survival. That's when flexibility becomes important, and complacency or size work against you. Serendipity and happenstance drive change in the evolutionary environment, not design. Evolution produces change and fills niches; it doesn't produce rational variation or consider the 'bigger picture'.

So what does this mean for British healthcare? Apart from the fact that throwing new money at old problems may not be the best solution (as can already be seen in the way increasing NHS allocations are producing predictable non-results), the US story tells us more about the importance of the public nature of our arrangements than the value (or otherwise) of a capitalist model.

A public service looks at the needs of its population rather than its providers or a particular client group. That function is missing from most aspects of the US system, but has always been present in the NHS. With the partial exception of the internal market (it was never allowed to act as a proper market), we have always had an egalitarian ethos, where the greatest good for the greatest number drove things.

At its most conceptual, the public service ethos is not about providing services or financing healthcare; it's about public planning, linking rights and responsibilities so planners and users are aware that actions have consequences. Our public and our politicians are infinitely more sophisticated than those living in the fevered evolutionary soup of the US. We all realise our short-term interests cannot always be fully met if a system is to remain viable, but that to maximise the chances of longterm health, we must occasionally give way to the greater need.

Maybe that should become our definition of a public service and we should stop bickering about who pays for what. Within this definition, it may not matter.