Tuesday, December 29, 2009

Answering Creationist Claims (Part 5 – The Egg and the Sperm Disprove… What?!)

This time, Bible Life Ministries claim that the “Human Egg and Sperm Prove Evolution is Wrong”. I cannot understand what do the reproductive cells have to do with disproving evolution, but since they put it up, I’ll have to refute their funny claim.

Where in the Universe did you learn about sexual reproduction?

The human female like other mammals has XX sex chromosomes, and the male has XY sex chromosomes. The female egg contains the X-chromosome, and the male sperm contains either an X-chromosome for the reproduction of a female or a Y-chromosome for the reproduction of a male. The female eggs all develop within the ovaries while she is a baby (foetus) within her mother's womb.

So, the female has XX sex chromosomes like other mammals, while the males do not? It seems they think that all mammals have XX chromosomes except human males! Guess I’ll need to give them an elementary crash course on sex chromosomes (if you already understand it, just skip this section).

Sex-determination Systems

XX/XY System

In this system, females have XX chromosomes, and are known as the homogametic sex; males have XY chromosomes, and are called the heterogametic sex. This system is found in most mammals (including humans), some insects of the genus Drosophila, and some plants of the genus Gingko.

Drosophila XY sex-chromosomes.

Drosophila XY sex-chromosomes.

XX/X0 System

This system is similar to the XX/XY system above. Females have XX chromosomes, while males have X0 chromosomes (the 0 indicates none). This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea).

ZZ/ZW System

In this system, it is the ovum that determines the sex of the offspring, instead of the sperm as the XX/XY and the XX/X0 system. In this system, males are the homogametic sex with ZZ chromosomes; while females are heterogametic with ZW chromosomes. This is the system used in birds, some fish, and some insects (including butterflies and moths), and some reptiles, including Komodo dragons.

Haplodiploid System

This system is special in the sense that an offspring that is formed from the union of a sperm and an egg (fertilised) becomes a female; an unfertilised offspring becomes a male. Males have have only half the chromosome count of females, and are haploid; females are diploids. This system determines the sex of the offspring of many hymenopterans (bees, ants, and wasps), spider mites, coleopterans (bark beetles) and rotifers.


Haplodiploid diagram.

Temperature-dependent sex determination

The sex of the offspring of this system is determined by the temperature of the eggs. Instead of chromosomal sex determination systems, this is a environmental sex determination system. The eggs are affected by the temperature at which they are incubated during the middle one-third of embryonic development. This critical period of incubation is known as the thermo-sensitive period (TSP).

Temperature-dependent Sex Determination

Temperature-dependent Sex Determination

Polyphenic System

A sex-determining polyphenism allows a species to reproduce normally while permitting different sex ratios. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male, and blue wrasse fish are the reverse. If the dominant individual dies, another individuals will change its sex and replace it. This system ensures that there will always be a mating couple when two individuals of the same species are present.

Other Systems

Some species, such as some snails, practice sex change: adults start out male, then become female. In Bonellia viridis, larvae become males if they make physical contact with the female, and females if they end up on soil. In some arthropods, sex is determined by infection, as when bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.

So there you go. These are the main sex determination systems found. None of these systems pose any threat to evolution, and instead support it strongly.

Phenotypes Cannot Affect Genotypes

Evolutionists claim environmental factors cause small changes in the offspring in the evolutionary chain. However, the environmental experience of the female cannot change the chromosomes within her eggs and cannot have any effect upon her offspring. Her body cannot go into the eggs contained within her ovaries at her birth to make an intelligent genetic change. Females cannot be a part of the evolutionary theory for these reasons.

They got half of it right. The last sentence is just plain ignorance, though (the same thing is repeated for the male sperm). They foolishly think evolutionists say that evolution can be driven by direct effects of the environment on the genes! Perhaps they meant natural selection + genetic mutation. Here’s a short description of how it works:

Firstly, every time gene duplication/replication occurs, something goes wrong. The gene mutates. Most mutations are neutral as over 90% of our genes are useless. Of the mutations that have an effect on the phenotype, most are harmful. Those individuals with mutations that decrease their survivability will be eliminated from the gene pool. In some cases, though, the mutation is beneficial to the individual, and the individual will be better adapted to the environment, and thus pass on its genes.

It’s just that simple. Can’t Bible Life Ministries even understand this? In contrast to what Bible Life Ministries claim, chromosomal sex determination systems have been studied, and they do support the Theory of Evolution.

Multiple Independent Origins of Sex Chromosomes in Amniotes

The general consensus in the scientific community is that amniotes were sexually determined by environmental factors originally, and chromosomal sex determination systems appeared late on the scene.

Birds evolved the ZZ/ZW sex determination system, and snakes also evolved this system independently. On the other hand, mammals evolved the XX/XY system independently. The split of the mammals from the rest of the amniotes occurred around 315 million years ago, whereas the archosaurs (birds, crocs, dinosaurs, possibly turtles) diverged from the lepidosaurs (snakes, lizards) around 260 million years ago.

There are 2 ways to explain the presence of the ZZ/ZW system in both birds and snakes. The first model is that the ZZ/ZW system appeared before archosaurs and lepidosaurs diverged, and some of the archosaurs/lepidosaurs reversed to temperature-dependent sex determination later. This model has serious problems, as the regaining of a feature is considered extremely unlikely in the Theory of Evolution.

The second model predicts that snakes and birds developed the ZZ:ZW system independently. This system fits the Theory of Evolution nicely, and has been proven genetically. After all, the autosome being converted to sex chromosomes in birds is different from that in snakes.

Separate Origins of Chromosomal Sex Determination

Independent origins of sex chromosomes in birds, snakes, and mammals. In ancestral amniotes, which presumably used temperature-dependent sex determination, there were no sex chromosomes. Sex chromosomes then evolved from autosomes on three independent occasions in birds, snakes, and mammals. A different autosome was converted to sex chromosomes in each of these three lineages. The ZZ:ZW system emerged twice (once in birds and once in snakes), whereas the XX:XY system emerged once in mammals.


We have now established that Bible Life Ministries claim that the reproductive cells disprove evolution is nothing more but ignorance. You would’ve hoped they would at least do more research if they want to challenge the Theory of Evolution.

Their claim shows an even worse understanding of evolution, which I would take great pleasure in refuting. After all, DNA doesn’t have a mind, does it?


Eric J. Vallender, Bruce T. Lahn. (2006) Multiple independent origins of sex chromosomes in amniotes. PNAS vol. 103 no. 48 18031-18032.

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Next: Answering Creationist Claims (Part 6 – DNA Repair is Natural)