Tuesday, 18 July 2017

Oops! Another 'Non-existent' Beneficial Mutation!

The sex chromosome Z of male zebra finches comes in two variants. Males who possess a copy of these variant each have particular reproductive success.

© MPI f. Ornithology
Mutation speeds up sperm of zebra finches | Max Planck Society

For anyone who still imagines creationism is based on real-world facts and observable evidence, this little factoid should go some way to dispelling that myth.

It is yet another example of one of those beneficial mutations that creationists tell us don't and can't exist - usually followed by some garbled nonsense about information theory and thermodynamics that isn't recognisable as real science or even real information theory.

This example, published yesterday in Nature Ecology & Evolution was found by researchers at the Max Planck Institute for Ornithology, Seewiesen, Bavaria, Germany and colleagues from the Czech Republic. They found that a mutation in the z chromosome of the zebra finch gives males a considerable advantage over males with the normal chromosome.

The mutation is an inversion of one of the z chromosomes, one of the sex chromosomes in birds analogous to the x and y-chromosomes in mammals.

The sperm of male zebra finches consist of head, midpiece, and tail. Only males with an inverted and a regular variant of the Z-chromosome form sperm with a long midpiece which contains the motor for fast swimming.

© P. Opatova
Male reproductive success depends on the competitive ability of sperm to fertilize the ova, which should lead to strong selection on sperm characteristics. This raises the question of how heritable variation in sperm traits is maintained. Here we show that in zebra finches (Taeniopygia guttata) nearly half of the variance in sperm morphology is explained by an inversion on the Z chromosome with a 40% allele frequency in the wild. The sperm of males that are heterozygous for the inversion had the longest midpieces and the highest velocity. Furthermore, such males achieved the highest fertility and the highest siring success, both within-pair and extra-pair. Males homozygous for the derived allele show detrimental sperm characteristics and the lowest siring success. Our results suggest heterozygote advantage as the mechanism that maintains the inversion polymorphism and hence variance in sperm design and in fitness.

The interesting thing from a scientific point of view is that, although there was a very significant advantage to carriers of this mutation, the frequency of the mutant allele remains stable at about 40% of the population. It would normally be expected that such a strong advantage would progress very quickly to fixation in the population, replacing the normal allele completely. So why didn't this happen?

The researchers found that the mutant allele only expresses in the presence of the normal form. Chromosomes are inherited in pairs, one from each parent, so a male can inherit two normal chromosomes, two mutant chromosomes or one of each. It is only those that inherit one of each that enjoy the higher breeding success, but 50% if it's sperms will carry the normal chromosome which also benefit from this increased breeding vigour so 50% of its male offspring will have the normal z chromosome:

Every male possesses two Z chromosomes, one of which they inherited from the mother and one from the father. Z chromosomes exist in two versions, the regular one (A) and the inverted type (B). “Only if a male possesses both types, meaning that it is heterozygous AB instead of homozygous AA or BB, the male’s sperm show a long midpiece which contains the motor for fast swimming” says Ulrich Knief, first author of the study. Animals with two identical forms of the Z chromosome (i.e. the homozygous males) cannot produce the superior sperm and hence show lower fertilization rates. The study is based on paternity analyses of 435 males in the absence of sperm competition and another 482 males under competition with other males. The study clearly shows that heterozygous males had higher fertilizing success under both conditions.

Not only is this another example of a mutation giving an advantage in refutation of creationist claims that there aren't and can never be any, just as the existence of intermediate forms refute their claim that there aren't any, but it is also an example of how a basic understanding of genetic evolution makes this readily understandable while it is utterly incomprehensible as a supposed example of intelligent design.

And of course, it is an example of how not only is the Theory of Evolution not a theory in crisis but how firmly embedded in and fundamental to biology it is, making an otherwise inexplicable phenomenon entire understandable. Yet another example of a piece of scientific research quite incidentally, quietly and without fuss or effort, dismantling another creationist dogmas with observable evidence.

There is also a wider creationist principle that this piece of research refutes. Creationists dogma states that no new information can be created and that mutations only destroy information - hence being impossible, bizarrely, since they also claim that information, like energy, can never be destroyed. No; it doesn't make sense does it. It's creationism. So, how is the amount of information changed by inversion? How does taking a length of DNA and turning it round change the information in it? It doesn't of course, it merely changes what that information means in a biological context.

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