biology zoology blog benno meyer rochow selection detection

Selection Detection

The fittest survive, but what does that mean?

Even now there are still people who do not fully understand the tenet of the survival of the fittest and believe it has something to do with winners and losers of a combat and physical strength. However, that even the smallest individual could be the fittest individual perhaps under conditions of a food shortage or an availability of shelters best suitable for small individuals and that an individual which is weak, but smarter than others could become the “fittest” through the process of selection is something that needs to be emphasized. To explain natural selection, zoologists often use the example of the white and dark specimens of the peppered moth Biston betularia in England: the originally rather rare dark variety became increasingly more common as industrial pollution increased and the whitish tree trunks of birch trees turned grey with soot. When pollution levels subsided and the environment became cleaner again, it was the lighter coloured variety that gained the upper hand once more.

A nice example, but we do not need that example from England, for in New Zealand on our beaches we have one that is just as good: I am thinking of an approximately 7 mm, roundish beetle, which lives in the sand, usually under semi-dry, washed up heaps of kelp. On North Auckland sandy beaches with a bit of iron sand, ground up fragments of seashells, glittering silica grains and sediments from the land above, the majority of these beetles are mottled in coloration. There are a few black ones and some totally creamy white ones, too, but they represent a minority. What is the zoologist to make of this? We have one species, but individuals in that species vary from completely black (well actually more a dark brown) to completely white (a creamy kind of white): the species Chaerodes trachyscelides, a tenebrionid beetle, is said to be polymorph (sometimes also referred to as polyphenic), just like the moth mentioned at the beginning. Or, come to think of it, garden snails, goldfish, chickens, dogs and other domestic animals – and humans, of course.

Back to our little New Zealand beach beetles. When you look for them on a predominantly black sandy beach like that of Raglan, you will without exception find only dark coloured ones and no white ones. On white sandy beaches like Whangamata or Mt Maunganui the situation is reversed. One thing we could show in the lab was that the beetles from the white beaches were less affected by bright light but more susceptible to heat than the black specimens from the black sandy beaches. But there is more to it. A talented student of mine by the name of K.L. Teh was able to show that given a mixed population of white and dark beetles and introducing one predator, in our case a juvenile tuatara (a unique New Zealand reptile with a lineage right back to the dinosaurs), black beetles on black sand and white beetles on white sand stand a much better chance to survive and then of course to reproduce than black beetles on white sand and vice versa. Background matching is selected for by predation and only the most suitably adapted (and not necessarily the strongest) individuals, old Darwin’s “fittest”, survive and pass on their genes.

© Dr V.B. Meyer-Rochow and, 2018.
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