Adam and Eve could be real?: Genes’ introns and exons tell different stories here. Who to believe?
|June 21, 2012||Posted by News under Human evolution|
We’ve been looking at a new book, Science and Human Origins, by Ann Gauger, Doug Axe, and Casey Luskin (Discovery Institute Press, 2012), in which Gauger takes issue with Templeton winner Francisco Ayala’s claim that genetics shows that there cannot have been a literal Adam and Eve.
“Cannot” is a big word, one that prompts investigation.
See also: Breaking: Adam and Eve are scientifically possible
Adam and Eve possible?: Ayala’s contrary claim built in favourable assumptions
Here’s another brief excerpt, not enough to justify being a noviewer (not reading the book, but still holding forth on it):
Ayala created his phylogenetic tree based on exon 2 sequences of the HLA-DRB1 genes, while Bergström et al. used intron 2 sequences. A third study by Doxiadis et al. examined the phylogenetic histories of chimp, macaque and human HLA-DRB1 genes again, but this time using sequences taken either from exon 2 or introns 1-4. Surprisingly, the tree alignments using exon 2 or using introns 1-4 give markedly different pictures of the gene’s phylogenetic history, even though both sets of sequences come from the very same genes. There is a substantial difference in the phylogenetic relationships. Exon 2 comparisons typically showed cross-species associations, while intron comparisons showed within-species associations.
It is clear that the intron sequences group according to species, whereas exon 2 sequences show no species-dependent relationships.
[Explanatory figure follows]
This should be surprising. Although trees based on gene comparisons sometimes do not show the same phylogenetic relationships as the species themselves do, as is the case for the exon 2 sequences, when this happens it indicates something unusual is going on.
It’s even more unusual that trees drawn from adjacent segments of the same gene disagree with one another. It’s not that exon 2 is highly variable and the introns are more conserved, because this is not the case. Intron lineages can differ quite a bit from one another. Rather, the intron lineages group together according to species, while the exon 2 lineages do not.
Some evolutionary biologists try to explain this discordance between the HLA-DRB1 trees by arguing that this proves that these genes have their origin in deep time, before the lineages of chimps, humans and macaques separated, and that it is the exon 2 data that defines the gene’s history. Others think that there has been cross-species shuffling of ancient peptide-binding motifs between different exon 2 sequences over time, but leaving the intron lineages unchanged. It is not clear, however, how such a patchwork cross-species assortment of exon 2 sequences could have been acquired without disrupting the species-specific introns. Furthermore, this would require that the incipient species’ populations intermingled for a prolonged period of time. The intermingling is highly unlikely to have lasted for thirty million years, though, which is the last time macaques, chimps, and humans supposedly shared a common ancestor. And the fact that the intron sequences do associate by species, with branch lengths as long or longer than the exon branch lengths, argues that many of these intronic lineages have been evolving independently for quite a while, indeed some as long as thirty to forty million years. Therefore this phylogenetic discordance is something that cannot be explained by common ancestry, especially when one considers an additional piece of information: The HLA-DRB1 region of chromosome six shows little or no signs of recombination.
Note: David Klinghoffer formally introduces the book here.