Darwin's Dangerous Idea - Evolution and the Meaning of Life

90 THE TREE OF LIFE Color-coding a Species on the Tree 91
Until sexual reproduction is invented, almost all the branches we observe,
at any zoom level, diverge. The exceptions are remarkable, however. At the
time of the eukaryotic revolution, if we look in just the right place, we will
see a bacterium entering the rudimentary body of some other prokaryote to
create the first eukaryote. Its progeny all have a dual inheritance—they
contain two entirely independent DNA sequences, one for the host cell and
another for the "parasite," sharing its fate with its host's, and linking the fate
of all its descendants (now on their way to becoming benign resident
mitochondria) to the fate of the cells they will inhabit, the descendants of the
cell first invaded. It's an amazing feature of the microscopic geometry of the
Tree of Life: whole lineages of mitochondria, tiny living things in their own
right, with their own DNA, living their entire lives within the walls of the
cells of larger organisms that compose other lineages. In principle it only has
to have happened once, but we may suppose that many experiments in such
radical symbiosis occurred (Margulis 1981; for accessible summaries, see
Margulis and Sagan 1986, 1987).
Once sexual reproduction becomes established many millions of years
later, up in the fronds of our Tree (and sex has apparently evolved many
times, though there is disagreement on this score ), if we zoom in and look
closely at the trajectories of individual organisms, we find a different sort of
juncture between individuals—matings—with starbursts of offspring resulting.
Zooming in and "looking through the microscope," we can see in figure
4.4 that, unlike the coming together that created eukaryotes, in which both
DNA sequences are preserved whole and kept distinct within the bodies of
the progeny, in sexual matings each offspring gets its own unique DNA
sequence, knit together by a process that draws 50 percent from one parent's
DNA and 50 percent from the other's. Of course each offspring's cells
also contain mitochondria, and these always come from one parent only, the
female. (If you are a male, all the mitochondria in your cells are in an
evolutionary cul-de-sac; they will not get passed on to any offspring of yours,
who will get all their mitochondria from their mother.) Now step back a pace
from our close-up of matings-with-offspring and notice (in figure 4.4) that
most of those offspring's trajectories terminate without mating, or at least
without offspring of their own. This is the Malthusian crunch. Everywhere
we look, the branches and twigs are covered with the short, terminal fuzz of
birth-death without further issue.
It would be impossible to see at one time all the branch points and
junctions in the whole Tree of Life, extending over 3.5 billion years, but if
we backed way off from the details and looked for some large-scale shapes,
we could recognize a few familiar landmarks. Early in the multicellular fanout
that began about 700 million years ago, we could see the forks that
created two large branches—the kingdoms of plants and animals—and another
for the fungi, departing from the trunk of the single-celled organisms.
And if we looked closely, we would see that, once they become separated by
some distance, no matings reunite any of the trajectories of their individual
members. By this time, the groups had become reproductively isolated, and
the gap grew wider and wider. 1 Further forks created the multicellular phyla,
orders, classes, families, genera, and species.
2. COLOR-CODING A SPECIES ON THE TREE
What does a species look like in this Tree? Since the questions of what a
species is, and how a species starts, continue to generate controversy, we can
take advantage of the God's-eye perspective we have temporarily adopted to
look closely at the whole Tree of Life and see what would happen if we tried
to color-code a single species in it. One thing can be sure: whatever region
we color in will be a single, connected region. No separated blobs of
organisms, no matter how similar in appearance or morphology, could count
as composed of members of a single species, which must be united by
descent. The next point to make is that until sexual reproduction arrives on
the scene, the hallmark of reproductive isolation can have no bearing at all.
This handy boundary-making condition has no definition in the asexual
world. In those ancient and contemporary strands in the Tree
FIGURE 4.4
1. There have been some remarkable symbiotic reunions, however, of organisms that
belong to different kingdoms. The flatworm Convoluta roscoffensis has no mouth and
never needs to eat, since it is filled with algae that photosynthesize its nourishment
(Margulis and Sagan 1986)!