American Bison - Buffalo Field Campaign

Chapter 4 Genetics
As a science, population genetics is concerned with the origin,
nature, amount, distribution and fate of genetic variation present
in populations through time and space. Genetic variation
constitutes the fundamental basis of evolutionary change and
provides the foundation for species to adapt and survive in
response to changing intrinsic and extrinsic stressors. Therefore,
loss of genetic diversity is generally considered detrimental
to long-term species survival. In the short-term, populations
with low levels of genetic diversity may suffer from inbreeding
depression, which can increase their probability of extirpation
and reduce fitness. Plains and wood bison experienced severe
and well-documented population declines in the 19th Century
that reduced the census size of this species by over 99.99%.
The spectacular recovery to around 430,000 animals today
(Chapter 7) is a testament to their genetic constitution, and
represents one of the most significant accomplishments in
modern conservation biology. American bison have, however,
undergone artificial hybridisation with domestic cattle, been
subjected to domestication and artificial selection, and been
separated into many relatively small isolated populations
occupying tiny fractions of their original range. As well, all wood
bison populations contain some level of plains bison genetic
material due to artificial hybridisation between the subspecies.
All of these factors have had an effect on the current levels of
genetic diversity and on the integrity of the bison genome. As
a result, preservation of bison genetic diversity is a key long-
term conservation consideration. The following sections discuss
some of the major issues that are important for the genetic
management of this species into the future.
4.1 Reduction of Genetic Diversity
Within species, genetic diversity provides the mechanism for
evolutionary change and adaptation (Allendorf and Leary 1986;
Chambers 1998; Meffe and Carroll 1994; Mitton and Grant
1984). Reduction in genetic diversity can result in reduced
fitness, diminished growth, increased mortality of individuals,
and reduced evolutionary flexibility (Allendorf and Leary 1986;
Ballou and Ralls 1982; Franklin 1980; Frankham et al. 1999;
Mitton and Grant 1984;). There are four interrelated mechanisms
that can reduce genetic diversity (heterozygosity and number of
alleles): demographic bottlenecks, founder effects, genetic drift,
and inbreeding (Meffe and Carroll 1994). Unfortunately, over the
last two centuries, bison in North America have, to some degree,
experienced all of these mechanisms.
Lead Authors: Delaney P. Boyd, Gregory A. Wilson, James N. Derr, and Natalie D. Halbert
As American bison approached extinction in the late 1800s,
they experienced a severe demographic bottleneck, leading to
a concern that extant bison populations may have lower genetic
diversity than pre-decline populations. The consequences of
a genetic bottleneck depend on the pre-bottleneck genetic
diversity within a species, the severity of the decline, and how
quickly the population rebounds after the bottleneck (Meffe and
Carroll 1994; Nei et al. 1975). The decline of bison was severe,
with a reduction from millions to fewer than 1,000 individuals.
Recovery efforts, however, enabled bison populations to grow
quickly, more than doubling between 1888 and 1902 (Coder
1975). Although the effects of the bottleneck on the genetic
diversity of the species are not clear (Wilson 2001), there are
several possible repercussions. First, after a severe reduction in
population size, average heterozygosity is expected to decline
(Allendorf 1986; Nei et al. 1975). Heterozygosity is a measure of
genetic variation that is a direct reflection of the past breeding
history of a population. Heterozygosity values are expressed as
the frequency of heterozygotes (i.e., genes with dissimilar alleles)
expected at a given locus (Griffiths et al. 1993). A reduction in
the level of heterozygosity can result in inbreeding effects. At
the same time, a loss of alleles may limit a population’s ability
to respond to natural selection forces and reduce the adaptive
potential of a population (Allendorf 1986; Meffe and Carroll 1994;
Nei et al. 1975; Robertson 1960).
After the demographic crash, several small bison herds
remained in North America, many of which were derived from
very few animals. Overall levels of genetic variation in current
populations can, in theory, vary directly with the number of
original founders (Meffe and Carroll 1994; Wilson and Strobeck
1999). Remnant populations may not have been representative
of the original gene pool and, consequently, suffered reduced
genetic variability. Through time, the detrimental effects of
genetic drift may have compounded the effects of the earlier
bottleneck. Genetic drift involves the random change in gene
frequencies and leads to the loss of alleles over time. The rate
of this loss, or fixation of alleles, is roughly inversely proportional
to the population size (Allendorf 1986; Meffe and Carroll
1994). However, the actual count of breeding individuals in a
population is not appropriate for determining the rate of genetic
drift because factors such as unequal sex ratios, differential
reproductive success, overlapping generations, and non-random
mating result in the “effective” population size always being less
than the census size. For bison, the ratio of effective population
size (N e ) to the census population size (N) has most commonly
American Bison: Status Survey and Conservation Guidelines 2010 19