Wednesday, 3 August 2016

Unintelligent Design - Evolution Of Colour-Blindness

Seeing you in black and white.
Recruitment of Rod Photoreceptors from Short-Wavelength-Sensitive Cones during the Evolution of Nocturnal Vision in Mammals - Developmental Cell

Why do dogs (and most mammals) not have colour vision? The answer seems to be that they lost it so they could better compete with predatory dinosaurs!

So how did that work? You would expect colour vision to be advantageous so if early mammals needed to compete with dinosaurs - which probably had colour vision, wouldn't having better colour vision be the way to go? But of course, evolution doesn't think and plan. Maybe if it did and used human intuition that's what it might have tried. However, the dinosaurs would also have been evolving and responding to the challenge of these better-equipped mammals and there is no guarantee that the mammals would have won that particular arms race.

Instead, what this paper shows is that mammals exploited the fact that dinosaurs were probably mostly diurnal - hence their need for colour vision - leaving the night relatively safe for the early mammals, and colour vision is no use during darkness. During darkness, what is needed mostly is good black and white vision and the ability to discern differences in low light conditions!

Most mammals have lots of rods, which are good at detecting low levels of light but at the expense of resolution, and relatively very few cones, which are good at detecting colour. In most other vertebrates however, this situation is reversed leaving most mammals as the exception in lacking colour vision. Primates, which includes humans, re-acquired colour vision later, probably to recognise and exploit ripe fruit, as I explained in a blog post some time ago.

The researchers have shown by genetic analysis that these early mammals probably started off with colour vision but then lost it by a chance mutation that transformed cones into rods. Unfortunately the copyright holders want to charge £22.44 to post the abstract here! So hopefully I'll managed to summarise it.

By comparing the genetic and epigenetic basis for retinal development in zebrafish and mice (as representative primitive vertebrates and mammals respectively) the team showed that there were genetic and epigenetic remnants in mice of the zebrafish genes which gave rise to cones but which had now been recruited to give rise to rods. However, the cones in mice did not derive genetically from the cones in zebrafish! The implication being that the original cones genesis in vertebrates had been coopted to produce more rods and then the genesis of the fewer cones had evolved independently.

What seems to have happened is that the dynamic balance between good night vision and good day vision was shifted in the early mammals under selection pressure from predatory dinosaurs. Part of this adaptation for nocturnal existence could also have included the evolution of fur and better thermoregulatory and heat retention mechanisms. Mammals probably existed for several million years as small, nocturnal species until the catastrophic extinction of the dinosaurs opened up available niches. The adaptation to a nocturnal existence could well have helped them survive the permanent winter conditions that probably lasted for many years following the meteor strike at the end of the Cretaceous.

So, we come to why humans and our relatives on the primate branch of the evolutionary tree now have colour vision not derived from the same genetic mechanisms as the colour vision in fish and most other vertebrates. This occurred because a chance duplication in one of our fructivore, diurnal ancestors of the gene for producing opsin - the protein which detects colour. This allowed the re-evolution of colour vision so our ancestors could exploit the change in colour which the fruit of flowering plants had evolved as a signal to the birds (with their colour vision) that the berries were ready to be dispersed via their gut.

Now, because I know how creationists love this sort of question, perhaps we could have an explanation of why an intelligent (sic) designer who supposedly created all species as they are today, gave mammals the remnants of the genetic basis for colour vision in other vertebrates and then re-evolved colour vision in primates to mimic that of birds. It would also be good to have an explanation of why a couple of chance mutations gave rise to advantageous genes when they claim this is impossible and has never been recorded. The two mutations were, of course, the one which improved our remote ancestors' night vision so enabling them to live alongside predatory dinosaurs, then the one in our early primate ancestor which gave rise to colour vision again.

Where is the intelligence in giving a species the remnants of an earlier and perfectly good mechanism but breaking it so it so it doesn't work any more, then designing a new way of doing the same thing but not doing it any better the second time around?

Could it be that regarding this sort of inept design process as intelligent gives us a clue as to why intelligent (sic) design advocates see this as evidence of intelligence in the first place? Is the intelligent (sic) designer created in the intellectual image of creationists, perhaps?

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