Evolutionary pressures on planktonic production of atmospheric sulphur
Key finding. Calculations of relative evolutionary pressure under both individual-selection and group-selection models indicate that neither climate modulation nor altruism could have been the primary factor in the evolution of mid-ocean dimethylsulphide production, which is instead best explained by selection on local interactions such as osmoregulation.
What question did this research address?
Dimethylsulphide from marine phytoplankton is the major source of cloud condensation nuclei over the remote ocean, and so a genuine influence on global climate. That led to a striking proposal: that plankton evolved to produce it *because* of the climate it creates, with altruism among neighbouring organisms sustaining the trait.
This paper asked whether that pathway survives contact with evolutionary theory — whether selection could actually favour an organism paying a cost to modulate the climate.
What did we find?
The argument turns on stability rather than plausibility. A non-altruistic neighbour of an altruist reproduces faster, so altruistic dimethylsulphide production is not an evolutionarily stable strategy — it is open to invasion by non-producers.
Group selection does not rescue it either. Models in which altruist-rich groups grow faster require intergroup mixing to be quicker than within-group ecological turnover, a condition the mid-ocean does not meet.
A benefit that accrues directly to the producer is a different matter, and would not count as altruism at all under the definition used, even if neighbours also gain.
Osmoregulation is such a benefit — dimethylsulphide has a role in managing salt stress, which pays the individual cell that makes it.
The conclusion does not deny the climatic effect. A dimethylsulphide-climate feedback loop may well modulate fluctuations in Earth's climate; what it cannot do is explain why the trait evolved.
Why does it matter?
It separates a mechanism from its supposed purpose. Plankton do influence cloud formation and climate, and that influence is real whether or not it was selected for — the error is inferring the evolutionary cause from the planetary consequence.
The reasoning is a specific check on Gaian arguments generally. Any claim that an organism evolved a trait for the good of the biosphere has to survive the invasion problem, and most do not.
What remains is arguably more interesting: a globally significant climate feedback that arises as a side effect of cells managing their own salt balance.
Citation
Ken Caldeira (1989). Evolutionary pressures on planktonic production of atmospheric sulphur. Nature 337, 732-734.