#galaxiid

You get a Banded Kōkopu

Divergent evolution

If you’ve gotten through a biology class you have probably seen an example of evolution triggered by the appearance of a mountain range. A species that has a population across a large area is separated by a growing mountain range, after which time it diverges into distinct populations. This transparent fish is a juvenile Galaxiid found in the streams of New Zealand, and its very existence is owed to exactly that type of speciation.

The Southern Alps on South Island are a rapidly developing mountain range that is still growing today. A few million years ago during the Pliocene, the geologic epoch that ended 2.6 million years ago, New Zealand had been above the waves for about 20 million years and freshwater fish had colonized its streams, but the Southern Alps hadn’t reached their current heights.

Today there are several dozen species of Galaxiids in New Zealand streams, but genetic evidence shows that during the Pliocene there were far fewer. The populations of fish exchanged across South Island, showing that the Southern Alps had yet to be such a strong boundary. Much of the divergence in these fish happened starting about the end of the Pliocene.

Just published research shows that other species, including insects, began to diverge into northern and southern populations shortly thereafter, during the Pleistocene. This divergence is thought to relate to the onset of glaciation on the island as there are occasional genetic exchanges between populations measured in those species.

These organisms therefore help date the uplift of the Southern Alps into a topographic divide. 5 million years ago these fish could exchange from stream to stream, but by 2.5 million years ago the processes of plate tectonics separated them into smaller populations.

-JBB

Image credit: http://bit.ly/2uZWehd