Friday, 5 June 2015

Bird Sex And The Intelligent Design Hoax

Evolutionary analysis of the female-specific avian W chromosome

One of the things you need to believe, if you believe in an Intelligent Designer and know just a little bit of biology, is that the 'Intelligent Designer' isn't very intelligent. It's not very intelligent to keep on re-inventing different solutions to the same problem, for example.

Speak to any half decent professional computer programmer and they will tell you they use a modular approach to design. They have a library of tried and tested routines and functions for doing a specific task. No decent programmer is going to reinvent a way to send a document to a printer, request and handle a password input or send a MySQL string to a server.

But the 'Intelligent Designer', if we are to believe the evidence we can see all around us, and if we accept for the sake of argument that there is an Intelligent Designer, continually re-invents ways of doing the same thing. It has invented wings at least three times - insects, birds and bats - flight five or six times, including spiders flying on long cobweb threads, gliding at least three times at least, and controlled flight. Sight has been reinvented half a dozen times; ingestion and excretion at least twice; reproduction at least twice with several variations on these themes, etc, etc, etc.

But this paper deals with just one more example of this multiple reinvention - how gender is determined in sexually-reproducing species.

Just one of these ways is the mammalian XY system where if the individual has a Y chromosome, they develop as males because the Y chromosome has genes for making a male. If there is no Y chromosome the individual develops as a female. Humans, of course, have two of each normal chromosome and either one each of the X & Y chromosome or two X chromosomes. humans are either XX or XY. Males only get the Y chromosome from their father but females get one X from each parent.

Now, that seems a perfectly straightforward system and seems to work very well, give or take the occasional error to XYY or XXY, so what was the Intelligent Designer thinking of, if it was thinking at all, when it designed gender determination in birds?

In birds, gender is determined differently, by the Z-W system, particularly by how many Z chromosomes the individual inherits. In the reverse of mammals, an individual bird with two Z chromosomes is a male, i.e. the one which produces the motile sperms. Females only have one Z chromosome.

But it doesn't stop there. When the 'Intelligent Designer' re-invented this method for gender determination it had to allow for the fact that it had also, for reasons which remain a mystery, also designed some genes on the Z chromosomes so they only work if there are two copies, one on each chromosome. Oops!

So it also invented the W chromosome which only females have and on which there are the genes the bird would have had anyway had it had two X chromosomes.

This means that, in addition to the mitochondrial DNA, birds, like mammals, inherit only from their mothers, females also inherit some unique DNA on the W Chromosome, most of which doesn't do anything but some of which is needed to make the single Z chromosome work properly.

From a biologists point of view, this DNA is interesting because it mutates and evolves very much more slowly than the rest of the DNA because it isn't reproduced very often in the germ cells. This is because, unlike the billions of sperm cells that males produce, females produce a relatively tiny number of eggs so the opportunities for mutation are greatly reduced.

This has allowed geneticists to analyse how the W chromosome has degenerated over time, as the mammalian Y chromosome has done, by studying the DNA of four related species of black-and-white flycatchers of the Ficedula and that of chickens. They were able to show that the W chromosome is a degenerated Z chromosome, with which it once formed a recombining pair, just like all other paired chromosomes.

Abstract

The typically repetitive nature of the sex-limited chromosome means that it is often excluded from or poorly covered in genome assemblies, hindering studies of evolutionary and population genomic processes in non-recombining chromosomes. Here, we present a draft assembly of the non-recombining region of the collared flycatcher W chromosome, containing 46 genes without evidence of female-specific functional differentiation. Survival of genes during W chromosome degeneration has been highly non-random and expression data suggest that this can be attributed to selection for maintaining gene dose and ancestral expression levels of essential genes. Re-sequencing of large population samples revealed dramatically reduced levels of within-species diversity and elevated rates of between-species differentiation (lineage sorting), consistent with low effective population size. Concordance between W chromosome and mitochondrial DNA phylogenetic trees demonstrates evolutionary stable matrilineal inheritance of this nuclear–cytonuclear pair of chromosomes. Our results show both commonalities and differences between W chromosome and Y chromosome evolution.


Now, for Intelligent Design advocates that means there is something else to be explained or more likely, tuck away in the corner of their minds and forgotten about. It stands to reason that if the W and Z chromosomes once formed a pair, they could not have been used for sex determination, therefore there must once have been a different system.

Why on earth would an intelligent designer design one system, then replace it with another, and why didn't it use exactly the same tried and tested method it had used for mammals, fruit flies, and lots of other only very distantly related species?

And I haven't even started on species like the social insects and some beetles, where gender is determined by how many copies of the whole genome the individual has.

Yet despite these glaring examples of for an Intelligent Designer to have designed living things it would have had to be about as stupid as it's possible for a designer to be, sad little creationist dupes cling to the Intelligent Design Hoax because it's the only way they can feel important and maintain a belief in an imaginary friend who made the Universe just for them.

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