Comparing the genomes of people, plants, and protists reveals a startling
disconnect between our impressions of their complexity and the DNA that
underlies them. The human genome comprises some 3.2 billion base pairs
(bp). Humans are obviously more complex organisms than plants, let alone
protists. So it comes as a surprise that pine trees typically have about six times
more DNA than we do [65], while a single-celled amoeba with the wonder-
ful name Chaos chaos has 400 times more [18]. Another way we can quantify
the size of a genome is by the number of protein-coding genes it has. That
measure also reveals the unexpected. Homo sapiens has about 20,000 pro-
tein-coding genes. The nematode worm Caenorhabditis elegans has about
the same number, even though its body has only 1000 cells [28]. By contrast,
bread wheat (Triticum aestivum) has about five times as many protein-coding
genes as we do [10], even though it is unable to write music or program a
computer.
These examples highlight a strange fact about life on Earth. There is no simple
relation between our sense of the complexity of an organism and the size of
its genome, measured either by DNA content or number of genes (FIGURES
14.1 and 14.2). One might guess that the genome would be where adaptation
rises to the pinnacle of refinement. You will see shortly that the reality is quite
the opposite: genomes are the messy outcomes of conflict and cooperation, of
selection and random genetic drift, and of all the other evolutionary ingredients
that make life the beautiful tangle that we see in nature.The douc langur (Pygathrix nemaeus) is arguably the world's most beautiful
primate. It has an unusual adaptation to its diet of leaves: a duplicated gene that
allows it to efficiently digest the bacteria that ferment leaves in the monkey's gut.The Evolution of Dispersal
of Genes
Genomes large and small
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