Evolution__3rd_Edition

564 PART 5 / Macroevolution
(a) Gene loss and mistaken inference of gene transfer
Bacteria Worm Fruitfly Human
Gene loss
Gene present in ancestor
(b) Additional evidence may support hypothesis of gene transfer
Bacteria Plant Coral Worm Starfish Mouse Human
Genes can transfer between
species ...
. . . but phylogenetic evidence is
needed
Figure 19.3
Extensive evidence of the phylogenetic distribution of a
gene is needed to test hypotheses of gene transfer. (a) Certain
“bacterial” genes are known in the human genome, and have
been hypothesized to have originated by recent transfer from
bacteria to humans. However, the DNA of few species have been
sequenced. The observations can equally be explained by gene
loss in the lineage leading to the worm and the fruitfly. (b) If we
knew that the gene was lacking in many of the branches between
bacteria and humans, gene loss would be less plausible, and the
hypothesis of gene transfer better supported. Currently we lack
the information in (b), and the origin of the “bacterial” genes in
the human genome remains uncertain.
be overcome in a successful transfer. Transfers of mitochondrial genes to the nucleus
have contributed to the evolutionary expansion of the nuclear genome, and evolutionary
reduction of the mitochondrial genome. Gene transfer is a further mechanism, in
addition to duplication and deletion, by which genomes can change in size.
Genomes also evolve by horizontal gene transfer. Horizontal gene transfer (also
called lateral gene transfer) occurs when a gene is copied from the genome of one
species into that of another species. It is rare event, but sequencing projects have shown
it occurs at a non-trivial frequency over evolutionary time. It is probably most frequent
in bacteria. Genes are even known to have transferred between Archaea and bacteria.
Genes probably also occasionally transfer from bacteria into multicellular eukaryotes,
but at present it is difficult to be sure that any apparently bacterial genes in a plant or
animal genome are examples of horizontal gene transfer.
The reason for the current uncertainty is that we need a phylogeny with evidence for
several species before we can identify examples of horizontal gene transfer. While we
have evidence for only a few species, we cannot rule out the alternative hypothesis of
gene loss (Figure 19.3). To see the problem, consider the human genome. The human
genome contains about 100 genes that resemble bacterial genes but are not found in
other animals. One interpretation is that these 100 genes have recently been transferred
from bacteria to ancestral humans. The problem is that we do not have much evidence
for other animals. The only complete sequences are for “the worm” and “the fly.” The
genes might have been present in the common ancestor of all life, and have been lost in
the branch leading to worms and flies. We only need hypothesize one loss event to
explain the facts. What is needed is evidence from more species, and from species that
do not fall in the worm–fly branch (Figure 19.3b). If the genes were present in humans
and bacteria, and absent from corals, flies, starfish, and mice, we could conclude more
confidently that genes had recently moved horizontally from bacteria to us. As that
information is not yet available, it is currently difficult to show that a human gene originated
by horizontal gene transfer. Gene loss is at least as plausible an interpretation.
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