What fuels the creation of complex features in life? How does a camera-eye develop, or causes zebras’ stripes to melt into a dazzling camouflage? Natural selection, acting over many generations, ensures that these intricate traits are slowly refined over time to create these, and other, sublime forms.
Such traits are usually the product of many genes acting in concert, the proverbial genetic choir contributing different proteins that build on each other. Hence genomic scans for such traits tend to find a several ‘candidate genes’ underlying them. In some cases, research has uncovered a few key adaptations that are based on the effects of a single, or very few genes. Unearthing these examples is exciting, as they instantly offer elegant case studies and a wealth of information into the process of adaptation. Here, I present a scattering of examples that have found a simple basis for various adaptations. Each is categorised using a ‘just-so story’ style heading, which I found amusing for some reason.
Update 20th March: It’s worth adding that even if the main effect of a trait could be pinpointed to a single gene, then it does not mean that said gene causes that trait. The ultimate outcome is very much dependent on how these genes interact with others in the same genome, as well as the environment they reside in. (I was reminded to add this after seeing Graham Coop of UCDavis post a quote about this issue on Twitter.)
How Tibetans became adapted to their high homelands
Tibet is one of the most mountainous regions on Earth, with an average elevation of around 4,900 ft. Most people would struggle to live in this environment due to low oxygen levels at these heights. Native Tibetans are able to live freely, having an adapted blood-oxygen regulation system that can cope with the thin air.
In 2010, a study compared the genomes from 50 native Tibetans, and looked for what genes enabled adaptation to this environment. These genomes were compared to those from Han Chinese individuals in order to determine which Tibetan genes might have differed due to adaptation. Comparison of over 20,000 sites revealed several candidates, but a key outlier was the EPAS1 gene, which is known to affect red-blood cell and haemoglobin count. Considering that Tibetans diverged from the Han Chinese 2,750 years ago, the rapid divergence of EPAS1 appears to be one of the quickest adaptations yet discovered.
Reference: “Sequencing of 50 Human Exomes Reveals Adaptation to High Altitude”
Why the mice became bright
The Sand Hills of Nebraska are home to a type of deer mice with a much brighter coat than their neighbours. This lighter colour, matching the sand under their feet, enables these mice to better blend in with the native soils. Linnen and colleagues found that the Agouti gene was responsible for this different coat colouring. Specifically, the Agouti gene has a chunk of DNA lost from it compared to the variant carried in dark-haired mice. A statistical model that analysed the high degree of similarity between different forms of the Agouti gene (see my previous blog post for details) suggested that it appeared after the formation of the Sand Hills 8,000 years ago. Furthermore, it probably originated as a new mutation before rapidly spreading to descendants, due to the strong selective advantage in evading predation.
Reference: “On the Origin and Spread of an Adaptive Allele in Deer Mice”
How sticklebacks lost their armour plates and did so in different ways in different parts of the world
(That’s enough bad titles – Ed.)
Oceanic stickleback fish are known for their striking armour plating. Yet this protection has been partially or fully lost in freshwater sticklebacks as they migrated around the globe. This observation begs the question: was there a common ancestor to these unplated freshwater fish that appeared only once, or did this adaptation repeatedly arise?
High-resolution genetic mapping pinpointed that mutations at the Eda gene were associated with loss of armour plating in freshwater fish. However, different variants of this gene were responsible for the adaptation; while most of the plateless Eda variants were closely related to each other, one population from Nakagawa Creek in Japan exhibited greater similarity to fully plated fishes. Furthermore, even populations of oceanic fishes tended to carry the plateless gene at low frequency. This variant appears to be recessive, in the sense that an offspring needed to inherit the same copy from both parents in order to lose armour plates. If only one copy was inherited, then the ‘dominant’ armour plate gene would be expressed instead.
All this evidence points to the fact that, unlike the previous examples, variants of the Eda gene leading to armour-plate loss arose several times in different locations. The fact that it is generally present at a low frequency means that, when required, sticklebacks entering freshwater habitats will cause this gene to become rapidly selected for.
Reference: “Widespread Parallel Evolution in Sticklebacks by Repeated Fixation of Ectodysplasin Alleles”
If you have your own favourite examples of simple adaptation, why not share them in the comments below?