Lewis Carroll in Through the Looking Glass pictured a fictional character Red Queen as an embodiment of cold and calm passion; she is “formal and strict, yet not unkindly”. Based on this character, evolutionary biologist Leigh Van Valen proposed the ‘Red Queen Hypothesis’, according to which species are in a constant race for survival, and if they wish to evolve they will have to find new ways of defending themselves. This hypothesis challenged the theory that evolution is driven by a need to adapt to the environment or habitat. It said that natural selection arises from interaction between species, rather than adaptation to the environment.

Hearts is a game of cards, and is commonly played by four people. There are no formal partnerships, but players in their own interest may help each other. Only the Queen of Spades and Hearts are worth penalty points. The winner is the player with the lowest score; each player tries to avoid accumulation of the penalty points.

Microbes also play the game of hearts. The researchers say that sometimes microbes lose the ability to perform a function that appears to be necessary for their survival. How then do these microbes endure and multiply? They say this is possible because they live in cooperative communities. In this community they find other microbes that are willing to help and fulfill their needs. The researchers called it ‘The Black Queen Hypothesis’.

The hypothesis says that some species depend upon other species to play their hand. According to the hypothesis, evolution pushes microbes to lose essential functions when there is another species around to perform them. It suggests that microbes can also survive by discarding genes. This is contrary to the common assumption about evolution, according to which genes are added, rather than discarded, as the evolution progresses. As the evolution moves forward, the complexity of the system increases. “But we know from analysis of microbial genomes that some lineages trend towards decreasing complexity, exhibiting a net loss of genes relative to their ancestor,” said the researchers.

A common plankton species in the open ocean, Prochlorococcus, was taken as a model organism to study the hypothesis. This species has a very small genome. This common photosynthetic organism, however, is extremely difficult to grow in pure culture. Researchers wanted to know how a species with so small a genome manages to exist in the absence of some important genes. They found that Prochlorococcus is “very sensitive to reactive oxygen species such as hydrogen peroxide and relies on other bacteria to protect them by breaking down these toxic substances for them”. They say that Prochlorococcus was once performing this function but found it to be too costly. The species thus abandoned this function as it evolved, like the Queen of Spades in the game of Hearts. The species outsourced the work to other organisms within the community. The Black Queen Hypothesis offers a new way of looking at complicated, inter-dependent communities of microorganisms; for example, how a complex natural community consisting of many different kinds of bacteria are evolved and formed.

Red and Black Queens can be the crucial game changers, as the above studies indicate.

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