Western and Eastern Gorillas: Estimates of the Genetic Distance
In 1929 Harold Coolidge revised the genus Gorilla
to comprise a single species, despite clear differences in morphology
between the western and eastern populations. Today the single-species
classification is still the most widely used, although increasingly researchers
are advocating a two-species taxonomy, with the West African gorillas
belonging to Gorilla gorilla (with the subspecies Gorilla gorilla
gorilla and Gorilla gorilla diehli) and the eastern gorillas
to Gorilla beringei (with subspecies Gorilla beringei beringei
and Gorilla beringei graueri).
The most widely stated criterion for species-level
separation of two populations is whether or not they interbreed. However,
since western and eastern gorillas never meet in the wild, we must rely
on other methods. One way is to compare the amount of difference between
two populations relative to that seen in other closely related species,
in an effort to achieve consistency within a larger taxonomic group. For
gorillas, this most commonly means comparing the amount of difference
between western and eastern gorillas relative to that between common chimpanzees
(Pan troglodytes) and pygmy chimpanzees, or bonobos (Pan paniscus).
Genetic Distance Studies
Some of the recent discussion of revising gorilla taxonomy
has been motivated by results from studies examining the amount of genetic
distance between gorilla populations. Maryellen Ruvolo and co-workers
sequenced a portion of mitochondrial DNA (mtDNA) and found that the difference
between eastern and western gorillas was about the same as that seen between
common chimpanzees and bonobos. Since chimps and bonobos are considered
different species, it was suggested that perhaps we should recognize two
species of gorillas. Likewise, Karen Garner and Oliver Ryder found the
same picture when they sequenced a different part of mtDNA.
Recently, Michael Jensen-Seaman and Ken Kidd examined
yet another region of mtDNA in western and eastern gorillas. They found
very similar results as had previous studies of mtDNA. That is, that the
amount of difference between eastern and western gorillas was about as
large as that seen between chimpanzees and bonobos. This suggests that
western and eastern gorillas have been reproductively isolated for perhaps
as long as 2 or 3 million years.
However, these same researchers also examined DNA
sequence variation in the nuclear genome of these same gorillas and chimpanzees,
but found somewhat different results. Specifically, they found that the
eastern and western gorillas were not nearly as different from each other
as were chimps and bonobos. This pattern was consistent across multiple
nuclear genetic loci, and suggests that perhaps western and eastern gorillas
have been in reproductive contact much more recently than have chimps
and bonobos.
It is difficult to explain the discrepancy between
the mtDNA and the nuclear DNA findings. In general, we expect the differences
between the two genomes to be roughly similar, except for species in which
females do not transfer from their natal group. Since female gorillas
do transfer, the different patterns observed cannot be due to any simple
sex-biased dispersal pattern.
Implications for Biogeography and Taxonomy
If eastern and western gorillas have experienced much more recent gene
flow than have chimpanzees and bonobos, then either the initial split
between these gorillas occurred more recently than the chimp/bonobo split,
or there has been periodic gene flow between western and eastern gorillas
during the last 2 million years or so.
Periodically during the last million years, Africa was warmer and wetter
than today. During these times, the extent of the tropical forest would
have been greater than at present, providing a possible corridor for migration
between western and eastern gorillas north of the Congo River/Ubangi River.
Along these lines, it is tempting to speculate that the population of
gorillas which might have existed near Bondo in northern Democratic Republic
of Congo in the earlier part of the 20th century may reflect a relict
population from these intermittent forest connections. In contrast, after
bonobos were split off from chimpanzees by the Congo River, there would
seemingly be no possibility of gene flow between what are now the two
species of Pan.
Others have made the argument that since chimpanzees and bonobos are definitely
considered separate species, and since western and eastern gorillas initially
appeared to be as genetically different as chimps and bonobos, then we
should divide gorillas into two species. If we now believe that western
and eastern gorillas may not be as different as we thought, does that
mean that we should not separate western and eastern gorillas into separate
species? No. Taxonomy is more complex than sequencing a little bit of
DNA. There is no objective standard of how much genetic difference, or
how much time since separation, is required to consider two populations
different species. The use of the "chimp-bonobo" standard may
provide some comparative measure of divergence, but there is no reason
to believe that the separation between chimps and bonobos represents any
sort of minimum level of divergence required for species level status.
The relationship between genetic distance and reproductive isolation is
not at all understood.
Given the discrepancy between the results observed from the mitochondrial
genome and those from the nuclear genome, more data are clearly needed
to resolve the issue of exactly how genetically different are western
and eastern gorillas. Once this rather simple question is better understood,
we will need much more theoretical discussion of how to relate genetic
differences to taxonomic differences.
Michael Jensen-Seaman
Dr. Michael Jensen-Seaman recently graduated from Yale University. His dissertation research was on the evolutionary genetics of gorillas. He is currently a post-doctoral researcher at the University of Chicago.
In the IUCN's Red List, 2 gorilla species with 4 subspecies are listed:
Species
Eastern Gorilla (Gorilla beringei) - endangered
Subspecies
Mountain Gorilla (Gorilla beringei beringei), Virungas and
Bwindi - critically endangered
Grauer's Gorilla (Gorilla beringei graueri), Eastern Congo - endangered
Species
Western Gorilla (Gorilla gorilla) - endangered
(since 2007 critically endangered)
Subspecies
Western Lowland Gorilla (Gorilla gorilla gorilla) - endangered
(since 2007 critically endangered)
Cross River Gorilla (Gorilla gorilla diehli), Nigeria/Cameroon
border - critically endangered
