Biology Reference
In-Depth Information
a clinal pattern. That is, rather than forming tightly bounded clusters, human genetic varia-
tion is continuous, and is patterned as a strong association with local geography: populations
in close geographic proximity will tend to be more similar to each other in their pattern of
genetic variation, and the further apart populations are, the less similar they become.
Formally, this is known as a pattern of
isolation by distance
, or IBD. Several scholars have
argued that this pattern has resulted from the tendency of people to find mates within close
geographic distance from each other, although some will find mates further afield (
Konigs-
berg, 1990
;
Relethford, 2004; Manica et al., 2005
; among others).
At the same time, others were suggesting that instead, the strong correlation between
geographic proximity and genetic similarity results from a serial fission process (
Ramachan-
dran et al., 2005
). Picture a single founder population that grows in size to the point that
a daughter population splits from it. The original population persists, while the daughter
population grows in size and itself splits. This serial fission process repeats itself, with
successive daughter populations spreading out from the original founding population.
This will create a
nested
pattern of variation, because each daughter population will consist
of only a subset of the variation found in its parent population.
Most recently, Hunley and colleagues (2009) demonstrated that both processes
d
local
mate exchange (leading to IBD) and serial fissions
d
account for the pattern. Specifically,
they demonstrate a nested pattern of population genetic structure that is consistent with
a history of serial population fissions, population discontinuities, as well as long-range
migrations associated with the peopling of major geographic regions, and gene flow between
local populations. This inferred historical process is different from what has been argued
previously, and cannot support the thesis that the human species consists of independently
evolving biological races.
No doubt, as more and more individuals are sampled, and as more of their genomes are
investigated, we will be hearing much more on the topic. Additionally, as our capabilities to
glean information from degraded skeletal remains improve, we will be able to obtain more
direct information about genetic variation in the past.
FUTURE RESEARCH OPPORTUNITIES
In the Pipeline
Advances in genetics and genomics are happening very quickly so that almost every
month a new breakthrough in technology and reduced cost occurs. Though most people refer
to the current moment as the “genomic era,” some would argue that we are transitioning to
a “post-genomic era.” The reasoning here is that the genomic era occurred during the devel-
opment of technologies that have allowed us to sequence entire genomes. That trend started
more than ten years ago, so that at this point, we have complete genomic sequences from
individual organisms from multiple species, and genomic sequencing efforts are now being
focused on increasing within-species sample sizes. For example, the “1000 genomes project”
is aiming to sequence in-depth 1000 genomes from geographically dispersed humans (see
www.1000genomes.org
)
. Further, the post-genomic era will introduce methods and software
to quickly and easily “annotate” genomes (i.e., find and tag locations of interest).
