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'Tiddlers' become evolutionary models

By Dave Armstrong - 16 Oct 2013 9:35:0 GMT
'Tiddlers' become evolutionary models

This is ridiculous! These Icelandic sticklebacks have been invading freshwater. Now they've invaded the land, too - this unfortunate individual was swept up on shore by a storm; Stickleback image; Credit: © Shutterstock

As we look at how evolution changed with theorising such as that of the currently-honoured W. D. Hamilton, it's uplifting to see the tiddler that amuses almost every kid in the northern hemisphere make good. Three-spined sticklebacks, Gasterosteus aculeatus aculeatus are introduced to us in local lakes and ponds in many countries. In fact, they belong originally, like the ten-spined stickleback, to the sea!

In Hamilton's enlightening modification of evolution theory, genetic kinship gave individuals, "inclusive fitness," or a kind of right to help others to reproduce, simply because they were related. Sticklebacks don't have kinship issues, but their right to real fame belongs to their regular habit of speciation into freshwater, leading in some Canadian island lakes to 2 distinct populations (these are ecotypes that do not interbreed, in most cases.) They occupy both a benthic and a surface-feeding niche. Functioning as different species, these nearby populations break the rules on evolution.

Beren W. Robinson, of the University of Guelph in Ontario, Canada, wrote this paper in the Proceedings of the Royal Society B. The paper investigates just how plastic a response the genotype (genetic formula) can have on an animal's traits as it rapidly adapts to a new environment.

Natural examples of this are almost unknown but the stickleback in Iceland has evolved a more rapid growth trait that takes advantage of low salinity. Often in sticklebacks, the freshwater varieties are smaller, but this interesting situation gives us a persistent plastic growth rate that is phenotypic and seems to be disconnected from an immediate genetic influence.

The accommodation by individuals and their offspring in both an ecological and an evolutionary reaction to the low salinity helps to explain exactly how evolutionary fitness is affected. How does the change in the phenotype avoid any stabilisation process by the genotype, for example? Genetic accommodation seems to provide the plasticity that makes the species so very adaptable to its many novel freshwater niches. This is the opposite of what is much more common, the genetic assimilation of a useful trait.

The evolutionarily-conservative marine population of sticklebacks do maintain their genotype and phenotype until they make excursions into the low-salinity environments they frequently find so inviting. Apart from body size, shape and coloration also change their appearance (phenotype.) This change is intriguingly rapid, as indicated by the 10,000 years elapsed since de-glaciation formed these freshwater habitats in Iceland. From an ancestral zooplanktivore, the fish has become a zoobenthivore throughout the lakes of the large island.

Why the marine species should have freshwater-adapted genes is a mystery. What is not mysterious is how these genes help the fish to survive overwinter temperatures with its larger size. It also matures earlier in order to reproduce in time for warm summer temperatures and better food resources, and it can reduce energy consumption by reducing the need for energy-expensive osmoregulation.

An environmentally sensitive phenotype is beneficial for the rate of evolution, as it seems to accelerate it. The plastic response guides the evolution of the phenotype in a new habitat. In other words, the higher growth rate and the other adaptations provide evidence of the evolution of strong plastic responses to freshwater (low salinity.) We need to test to see if altitude/temperature of lakes affects the size of sticklebacks, as lower temperatures also seem to create conditions for higher growth rate. One of the most interesting parts of this evolution is the way in which limited resources have caused many other freshwater sticklebacks to evolve lower growth rates, in both Iceland and North America However, where there are numerous Chironomid (midge) food resources in, for example, Iceland's Lake Myvatn, the "tiddler" grows very large, up to about 10cm.