Intercontinental stickleback study sheds new light on the repeatability of evolution

  • Wednesday, December 2, 2020

A question that has intrigued biologists for decades is: to what extent is evolution repeatable? Only in science fiction could we wind back time to see if the same forms of organisms would emerge if evolution played out for a second time. In the real world, however, a powerful approach to begin to answer this question is to explore whether similar traits emerge in populations evolving in different places but under similar environmental conditions - a phenomenon known as parallelism.

Image - Intercontinental stickleback study sheds new light on the repeatability of evolution

A new study on stickleback fish evolving in populations across two continents provides striking new evidence of parallelism, and also affords new insights into the way that these animals’ DNA sequences and morphology, and the environment they colonize, are related during adaptation to new environments. Sticklebacks provide a powerful ‘natural experiment’ to test parallelism as they are originally marine fish (where they still occur today) that have since the last deglaciation period colonized and adapted to freshwater environments in most of the northern hemisphere.

Working in Alaska, British Columbia, Iceland and Scotland, the team of researchers - led by Dr Isabel Magalhaes, Dr James Whiting and Professor Andrew MacColl of the University of Nottingham - sampled over 1300 fish from 73 freshwater lake populations and four marine populations. They used a genetic technique known as RAD-sequencing to quantify the frequency of specific alleles, measured a range of the fishes’ physical characteristics (including body shape and degree of body armour), and also quantified various aspects of the physical environment in which they lived. They then explored the link between the genetic measures and morphological and environmental variation.

Analysis of this large dataset indicated that:

  1. parallelism in morphologies and environments can predict genomic parallelism;
  2. originating from the same pool of genetic variation is the biggest determinant for finding genomic parallelism even at large geographic scales (hence higher parallelism within than between continents);
  3. similarity in the environment can be a better indicator of parallelism in the genome of individuals from those populations than similarity in their morphology.

One of the lead authors of the study Dr Isabel Magalhaes, now a senior lecturer at the University of Roehampton, said “By connecting the evolution of parallelism more explicitly to environmental and morphological variation, our study has given us a better idea of the environmental and genetic backgrounds against which we would expect to find similar looking sticklebacks, even across large geographic scales. We learned for example that sticklebacks inhabiting water with pH 5 are likely to have similar body armour, and that this was achieved through the re-use of the same genomic regions, even if one population is in Alaska and the other in Scotland. This information could then be used to predict how populations colonizing new environments might look, which is particularly important as species are being displaced to new areas due to fast climate change”. Professor Andrew MacColl added “Studies of animals at the this scale (two continents) are rare, but ours makes clear that evolution follows similar paths in widely different locations and this helps us to understand the origins of biodiversity”.

The paper “Intercontinental genomic parallelism in multiple three-spined stickleback adaptive radiations” is published in Nature Ecology and Evolution.