Category: Health & Medicine
[Skip to list of Topics for this Category →]
Convergence is apparent in various situations where organisms have evolved to deal with dangerous threats (e.g. toxins, predators, pathogens) that affect optimal bodily function, or what can broadly be termed 'health'.
Among the examples of health-related convergence presented on the Map of Life, the production of insect-repelling chemicals stands out as one case of special interest. Numerous plants and produce organic compounds to deter insect predators, and in a similar way certain millipedes synthesise benzoquinones that are distasteful to birds and other predators. Mosquitoes can pose a serious threat to the animals that they feed on, and a number of monkeys, Madagascan black lemurs and one bird, the strong-billed woodpecker have independently responded by producing mosquito-repelling chemicals. Humans frequently apply insecticides to protect crops from insect damage, with the predictable result that specific insect proteins targeted evolve to acquire resistance. Interestingly, molecular convergence is frequently observed, whereby the same amino acid substitutions allow proteins in different populations to continue functioning. Immunity from toxic effects also occurs by a convergent molecular mechanism in those animals that are resistant to the lethal effects of saxitoxin and tetrodotoxin. Production of saxitoxin and tetrodotoxin for defence or predation a convergent feature of many organisms (mostly marine) and resistance to toxicity is primarily conferred by precise yet independently acquired amino acid substitutions in sodium voltage-gated channel proteins. Another example of molecular convergence in the area of health relates to alcohol dehydrogenase, which renders alcohol less toxic to the diverse organisms that are exposed to it through eating rotting fruit (or deliberate consumption as in humans). The process of inebriation in fruit flies and mammals show that 'drunken' behaviour is intriguingly parallel in the two groups, and that fruit fly research might yield useful insights into any genetic basis of alcoholism in humans.
Among bacteria that cause disease in humans we find Shigella and Salmonella, two pathogenic strains related to the famous experimental bacterium E. coli. Interestingly, striking convergences have been noted in features such as genome reduction in these pathogenic bacteria. Well known for their troublesome effects on human health, two distantly related species of fungi, Malassezia and Candida have independently evolved to tolerate human skin chemistry. Malassezia is found on the scalp and is correlated with dandruff while Candida can cause systemic irritation and serious problems in those with compromised immune systems.
Innate immune systems themselves appear to have evolved independently in animals and plants, providing a powerful protection against pathogens. Adaptive immune systems are highly specialised, storing 'memories' of previous immune threats and raising targeted defences when specific threats are encountered again. It may seem surprising that such complex immune co-ordination and flexibility could have evolved more than once, but indeed it seems that adaptive immunity has evolved independently in jawless fish, jawed vertebrates and to a degree in some insects.
A final noteworthy case closely linked to optimal health is the phenomenon of sleep, which appears to be a convergent feature of animals that demonstrate complex behaviour, learning and memory. Animals that show true sleep include mammals, birds, octopus and even certain arthropods, such as crayfish and bees.