We humans like to think we’re pretty complex – what with having invented the wheel, wars, New York, and so on – so we tend to forget that evolution doesn’t care about complexity. All that matters to natural selection is who makes the most babies, and sometimes complex adaptations can get in the way of that criterion. A study recently published on the always open-access PLoS ONE provides a good example of this principle in action – given the right selective pressures, photosynthetic organisms will give up on the whole photosynthesis thing.
Photosynthesis is clearly a complex adaptation, requiring specialized cellular structures and biochemical processes that can use light to power the synthesis of sugars. Complex enough for a whole additional organism, in fact, since the chloroplast, the cellular structure in which most eukaryotes conduct photosynthesis, probably originated as a symbiont that never left its host cell [$a]. (In some organisms, this process of becoming photosynthetic is still underway.) There are clear advantages to the ability to make your own food conferred by photosynthesis. Yet there are numerous examples of non-photosynthetic organisms with photosynthetic ancestors. For instance, plants as varied as the big, exotic Rafflesia or Monotropa, whose small white flowers are easy to spot in North American woods, have inactive chloroplasts and parasitize other plants. These cases are good reason to think that there may be selective conditions in which the cost of maintaining the mechanisms of photosynthesis outweighs the benefit of independent food production.
The new paper describes just such a set of selective conditions. The authors build a mathematical model of competition between microorganisms, such as flagellates, that can either be mixotrophs, able to conduct photosynthesis or capture prey to feed themselves, or heterotrophs, only able to sustain themselves by eating other critters. The model’s result hinges on two key facts of life for single-celled predators: (1) it turns out that the size of a flagellate cell determines what size of prey it is best able to capture [$a]; and (2) chloroplasts take up space in a cell, limiting the evolution of cell size.
The relative advantage of retaining photosynthesis, then, is directly related to the size range of available prey. Mixotrophs, whose cells are big enough to accommodate chloroplasts, are most efficient predators of larger prey; with no chloroplasts, heterotrophs can be small enough to take advantage of smaller prey. The question of which form wins out, then, relies on the distribution of available prey sizes and the light environment. If there’s lots of light for photosynthesis, mixotrophs can out-compete heterotrophs even if they don’t hunt very efficiently; but if there’s not much light and mostly small prey, the more efficient heterotrophs win.
The fact is, it’s rare for any given adaptation to be useful under all possible conditions. Biological structures or metabolic processes that become disused are no longer under selection for efficient performance of their original function – they are free to accumulate mutations that may make them degenerate into uselessness, or to be co-opted for entirely new functions. But if an adaptation is actually costly to maintain, then natural selection may eradicate it altogether.
References
de Castro, F., Gaedke, U., & Boenigk, J. (2009). Reverse evolution: Driving forces behind the loss of acquired photosynthetic traits. PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008465
Hansen, B., P. K. Bjornsen, & P. J. Hansen (1994). The size ratio between planktonic predators and their prey.
Limnology and Oceanography, 39, 395-403
McFadden, G. (2001). Chloroplast origin and integration Plant Physiology, 125 (1), 50-3 DOI: 10.1104/pp.125.1.50
