374
The Biology of Sea Turtles, Vol. II
14.9.1 P RESERVING G ENETIC D IVERSITY
When discussing the conservation of genetic diversity in a population, geneticists
refer to the effective population size (Roughgarden, 1979). This number refers only
to adults, and is typically smaller than the total size of the adult population for
several reasons. First, if breeding is nonrandom, some individuals will contribute
more to the genetic makeup of the next generation than will others. In fact, some
adults may not breed at all, or very infrequently compared to others. This makes
the effective population size smaller than the actual adult population numbers,
because effectively fewer adults are contributing to the genetic makeup of the next
generation. Some populations undergo fluctuations in numbers, rising and falling
over time. In such cases, the effective population size over a long time period may
be nearer that of the small, bottleneck populations (when genetic diversity has been
reduced) than to the average or larger population sizes. As the species suffers a
population crash, genetic diversity declines. When the numbers increase again, there
almost certainly will be different allele and genotype frequencies in the larger
population than there would have been if the crash had never occurred. Similarly,
in populations with unequal sex ratios, the effective population size will be reduced
by the rarer sex. For these and other reasons, the effective population size is almost
always smaller than the actual population of adults. In addition, the actual total
population size of adults and juveniles will then be much larger than the effective
population size.
More than 20 years ago, Franklin (1980) and Soulé (1980) suggested that an
effective population size of 500 adults was sufficient to maintain genetic variability
in a species over the long term, and the so-called Franklin-Soulé number has been
used in conservation and management plans ever since. More recently, Lande
(1995) demonstrated that an effective population size of 5000 might be necessary
to ensure the maintenance of adaptive genetic variance in quantitative characteristics. He further suggested that this might require average actual population sizes
of wild species to be in the tens of thousands or even much higher to ensure their
long-term viability.
In the case of sea turtles, it may be difficult to determine effective population
size. We need information on the sex ratio of adults, on the proportion of females
(and possibly males) that are actively breeding, on whether breeding is random,
and if not, then on the specific breeding pattern. We also need information on
what exactly constitutes a population, as opposed to a nesting aggregation. Information from the analysis of mitochondrial DNA suggests that a nesting aggregation
may be genetically isolated from other rookeries because of female philopatry to
their natal beaches (Bowen, 1997; Francisco et al., in press). If this is true, then
we may be able to delineate populations geographically if we can assume that
male turtles born to mothers from a particular nesting aggregation breed only with
female turtles from the same natal beach. Some mitochondrial information indicates that male green turtles do display philopatry (FitzSimmons et al., 1997a).
However, recent evidence from nuclear DNA studies on green turtles (FitzSimmons
et al., 1997b) and loggerheads (Francisco-Pearce, 2001) indicates that males also
may be important vectors, moving genes between and among nesting aggregations.
1123 book.book Page 374 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
14.9.1 P RESERVING G ENETIC D IVERSITY
When discussing the conservation of genetic diversity in a population, geneticists
refer to the effective population size (Roughgarden, 1979). This number refers only
to adults, and is typically smaller than the total size of the adult population for
several reasons. First, if breeding is nonrandom, some individuals will contribute
more to the genetic makeup of the next generation than will others. In fact, some
adults may not breed at all, or very infrequently compared to others. This makes
the effective population size smaller than the actual adult population numbers,
because effectively fewer adults are contributing to the genetic makeup of the next
generation. Some populations undergo fluctuations in numbers, rising and falling
over time. In such cases, the effective population size over a long time period may
be nearer that of the small, bottleneck populations (when genetic diversity has been
reduced) than to the average or larger population sizes. As the species suffers a
population crash, genetic diversity declines. When the numbers increase again, there
almost certainly will be different allele and genotype frequencies in the larger
population than there would have been if the crash had never occurred. Similarly,
in populations with unequal sex ratios, the effective population size will be reduced
by the rarer sex. For these and other reasons, the effective population size is almost
always smaller than the actual population of adults. In addition, the actual total
population size of adults and juveniles will then be much larger than the effective
population size.
More than 20 years ago, Franklin (1980) and Soulé (1980) suggested that an
effective population size of 500 adults was sufficient to maintain genetic variability
in a species over the long term, and the so-called Franklin-Soulé number has been
used in conservation and management plans ever since. More recently, Lande
(1995) demonstrated that an effective population size of 5000 might be necessary
to ensure the maintenance of adaptive genetic variance in quantitative characteristics. He further suggested that this might require average actual population sizes
of wild species to be in the tens of thousands or even much higher to ensure their
long-term viability.
In the case of sea turtles, it may be difficult to determine effective population
size. We need information on the sex ratio of adults, on the proportion of females
(and possibly males) that are actively breeding, on whether breeding is random,
and if not, then on the specific breeding pattern. We also need information on
what exactly constitutes a population, as opposed to a nesting aggregation. Information from the analysis of mitochondrial DNA suggests that a nesting aggregation
may be genetically isolated from other rookeries because of female philopatry to
their natal beaches (Bowen, 1997; Francisco et al., in press). If this is true, then
we may be able to delineate populations geographically if we can assume that
male turtles born to mothers from a particular nesting aggregation breed only with
female turtles from the same natal beach. Some mitochondrial information indicates that male green turtles do display philopatry (FitzSimmons et al., 1997a).
However, recent evidence from nuclear DNA studies on green turtles (FitzSimmons
et al., 1997b) and loggerheads (Francisco-Pearce, 2001) indicates that males also
may be important vectors, moving genes between and among nesting aggregations.
1123 book.book Page 374 Monday, November 11, 2002 11:11 AM