This is a small part of the southern side of an 18th century garden wall. My house is just outside the north-east corner.
It is impossible to look at this picture and not notice that this section didn't start life as a garden wall. We can clearly see the remains of a derelict house that has been incorporated into the garden. There is a 17th century engraving on what was once a window lintel. Other parts of the wall contain old door openings and even a mill stone.
When geneticists examine the genomes of living things they cannot help but observe the relics of previous forms among the active code. These genetic fossils are an impossible challenge for creationists.
Part 4 in this series explained that half of our 800 olfactory genes are broken relics of a nocturnal past, when the ability to smell food and predators was more important than colour vision. In part 5 we saw that humans still have all of the machinery to produce vitamin C but, just like in our primate cousins, the final gene in the process was broken long ago.
Part 7 described how we also have the gene to produce vitellogenin the key ingredient in egg yolk. We saw that the gene is broken in exactly the same way in humans and in our closest primate relatives and in part 26 we saw how trichromatic colour vision evolved through gene duplication and simple mutations.
In this post I want to look at the other side of the story and see how colour vision is discarded by natural selection when it is no longer needed.
At ocean depths of around 200m there is very little light available. It's not surprising then that aquatic mammals like dolphins and whales have no colour vision. Their rhodopsin that provides us with vision in low light, is tuned to make the best of what is available.
What is surprising however is that dolphins and whales all have the SWS opsin gene that should produce the protein to detect blue light. In every species it is broken indicating that it was first lost in a common ancestor of all cetaceans.
We know from genetic evidence that the closest relatives of cetaceans are cows and dolphins. By comparing the genetic codes for the SWS opsin in two species reveals exactly how the gene was lost in the bottlenose dolphin. The asterisks show where letters have been deleted by copying errors leaving the dolphin without a working blue colour photoreceptor.
TTT CTT CTG TTC AAG AAC ATC TCC TTG - Cow
TTT *TT CTG TTC AAG AAC AT* * * * TTG - Dolphin
This is a classic example of the rule, "use it or lose it". When something is no longer contributing to a species fitness there is no penalty for an individual who is born with a faulty gene for that function.
This rule is confirmed by looking at prosimians - the group of primates that are most distantly related to humans. This group includes lemurs, tarsiers, bush babies and the lorises. Some of these are nocturnal and others are diurnal. Each of the nocturnal species have a broken SWS opsin gene and the deleted code is identical in each case indicating that the copying error happened in a common ancestor of the loris and bush baby.
One last example concerns the nocturnal owl monkey, the only nocturnal species among higher primates. Just sixty letters into the code for its SWS opsin gene the letters "TGG" has been changed to "TGA" which is the codon that stops translation of a gene. All the diurnal relatives of the owl monkey have a functional SWS opsin gene indicating that it was the change to a nocturnal lifestyle that relaxed natural selection in this species.
These relics of genes that were once vital to ancestors of living species make perfect sense in the light of natural selection. They pose an impossible dilemma for creationism.
Evolution is a Fact - Index #1-20
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Evolution is a Fact #21 - Footprints in the Sand
Footprints at Laetoli show our Australopithecus afarensis ancestors were bipedal 3.6 million years ago.
Evolution is a Fact #22 - The Hillocks of Hiss
A vestigial feature if the human ear shared by 10% of the population demonstrates our evolutionary history.
Evolution is a Fact #23 - Faunal Succession
The consistent sequence of fossils found in the rocks can only be explained by evolution.
Evolution is a Fact #24 - The Origin of Your Inner Ear
How the bones that reptiles eat with became the bones that we hear with.
Evolution is a Fact #25 - Deep Time
Scottish geologist Andrew Hutton discovered the proof of earth's great antiquity.
Evolution is a Fact #26 - Colour Vision
How gene duplication - new "information" -and mutation equipped us with trichromatic vision.
Evolution is a Fact #27 - Monkeys, Typewriters, Shakespeare, 747s etc.
Evolution is a combination of random mutations and non-random selection.
Evolution is a Fact #28 - Something Darwin Didn't Say
A long term study of pigeons demonstrates how natural selection acts on a local population