Here Be Dragons

HERE BE DRAGONS
tions ago. If we focus entirely on this gene, we can construct an evolutionary
tree that links all the affected individuals to that immigrant,
who becomes their "last common ancestor." But this ignores the fact
that, thanks to sexual reproduction, each of us inherits one half of our
genes from each of two parents, one quarter from each of four grandparents,
and so on. Thus, if we select a different gene to study, that immigrant
will almost certainly not be the common ancestor of all the
affected individuals; in fact, he or she may not show up as an ancestor
to any of them.
Thanks to the development of rapid gene-sequencing techniques, it
has recently become possible to determine the sequences of many
genes across different species. In fact, as of this writing, researchers
have sequenced the complete genomes of fifteen organisms, including
bacteria, archaea, and two eukaryotes (a yeast and a microscopic
worm). It turns out that, indeed, evolutionary trees based on different
genes are different from each other. If one focuses on genes that play
a role in the copying of DNA into RNA and protein ("informational
genes"), for example, one gets trees in which eukaryotes are more
closely related to archaea than to bacteria, as in the i6s TRNA tree. If,
however, one focuses on the much more numerous genes that code
for enzymes involved in regular metabolism ("operational genes"),
one gets trees in which eukaryotes are more closely related to bacteria.
There can only be one true history of evolution. Therefore, the
metaphor of the "tree," while it may apply to single genes, seems not
to adequately describe the evolution of organisms. Rather, we have to
acknowledge that something analogous to sex happens, not just
within species, but between species. Individual species must sometimes
acquire their genetic endowment from more than one parent
species. Instead of always diverging, the paths of evolution also sometimes
rejoin, and the appropriate metaphor for evolutionary history
may not be a "tree" but a "braided channel."
Transfer of genes between different species is, in fact, a well-documented
phenomenon. Bacteria belonging to different species can
pass genes for antibiotic resistance among each other, for example.
One bacterium, Agrobacter tumefaciens, can insert foreign genes into
the genomes of crop plants—it is much used in genetic engineering.
And retroviruses such as HIV (human immunodeficiency virus) can
insert genes into the genome of human cells (although not into
germ-line cells). Thus, one way out of the paradox of the evolutionary
data is to say that the "last common ancestor" was not a single
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