172 CHAPTER 7only a single gene copy (or single individual) present then. There were nine other
copies in that generation, and it is only by chance that they left no descendants in
the long run.
If we could trace the gene tree of all human mitochondrial genomes backward
in time, we would eventually arrive at a single genome, which was their most
recent common ancestor. Since mitochondria are inherited maternally, the per-
son carrying that ancestral mitochondria would necessarily be a woman. Analysis
of variation in mitochondrial DNA among living humans suggests that she lived
about 125,000 years ago [33]. She is sometimes called the “Mitochondrial Eve.”
That name is misleading, however, because many other females were also alive at
that time, and they contributed other genes to modern humans. Likewise, tracing
the ancestry of all Y chromosomes would lead back to a single male [15].
We can be certain that the ancestor of all mitochondria in living humans was
carried by a female and that the ancestor of all Y chromosomes was carried by a
male. Thus different genes have different genealogies. They have different com-
mon ancestors that lived in different places and at different times. This is a general
principle that applies throughout the genome of all sexually reproducing species:
each part of the genome has a different genealogy.
This genealogical perspective also extends across species. The mitochondria
that each of us carry and the ones carried by all living chimpanzees are the descen-
dants of a mitochondrion that lived perhaps 8 million years ago (Mya) in an ances-
tor of humans and chimps, before those two lineages split apart. Going back yet
further, the common ancestor of the mitochondria in every individual of every
eukaryotic species now alive existed some 2 billion years ago. That single mito-
chondrion left an extraordinary legacy: the DNA that it passed on is now carried
by every animal, plant, fungus, and alga on Earth.How Strong Is genetic drift?
Genetic drift is a random process that is always at work. While it is stronger in small
populations than in large populations, only an infinitely large population would be
immune from drift. The most numerous organism on Earth is a bacterium called
Pelagibacter ubique that has a population size of 10^28 individuals [27], a number that
is more than one million times larger than the number of stars in the universe. But it
is still a finite number, which means that even Pelagibacter experiences genetic drift.
The strength of random genetic drift in a population is measured by the effec-
tive population size, represented with the symbol Ne. This number provides a way
to compare the strength of drift in different populations. We’ve already seen that
the size of a population affects drift—it is stronger in smaller populations. Many
other factors also affect drift. If most individuals in a population are too young or
too old to reproduce, then drift is stronger than it would be if all the individuals
were reproductive. Drift is also affected by changes in population size and unequal
numbers of reproducing males and females (see Figure 7.1). To account for all these
factors, we imagine an idealized hermaphroditic population of constant size in
which all individuals have an equal chance of leaving offspring (just as in the two
simulations). The effective population size is the number of individuals that would
give this idealized population the same strength of random drift as the actual
population of interest. A small value of Ne means that drift is strong, while a large
value means that drift is weak.
The value of Ne tells us several useful things about a population. We saw earlier
that any two copies of a gene share a common ancestor at some point in the past.
If the gene is evolving neutrally (that is, with no selection) in a diploid organism,
the mathematics of probability tell us that the average time back to this common07_EVOL4E_CH07.indd 172 3/23/17 9:09 AM