14. Role of Genetics in Conservation Biology
227
with the population frequency of the most common allele being smaller than 0.99.
The proportion of polymorphic loci, in turn, is a measure of the number of
variable loci among all sampled loci within a population. Genetic variation may
also be described by quantitative variation in traits derived from the actions (and
interactions) of many genes, termed quantitative genetic variation.
The most frequently used measure of populationwide genetic diversity is the
amount of heterozygosity. Individual heterozygosity describes the observed proportion of heterozygous loci in an individual (Mitton and Pierce 1980), and
average heterozygosity reflects the proportion of heterozygous individuals in a
population measured across several loci (Hartl and Clark 1989). The theoretical
predictions based on the Hardy Weinberg Principle provide the framework within
which to evaluate the amount and distribution of genetic variation documented in
natural populations. Deviations from the expected genotype frequencies within a
population, for example, can be indicative of past bouts of strong selection or
inbreeding.
To evaluate how loss of genetic diversity affects population survival, it is
crucial to distinguish the significance of different types of genetic variation.
For example, single-locus diversity is measured by individual heterozygosity,
whereas diversity associated with polygenic quantitative traits is measured by
phenotypic variation. Empirical studies suggest that most phenotypic changes in a
population are the result of small alterations at numerous loci rather than a consequence of major mutations at a single locus (Lande 1981; Lande and Barrowclough 1987; Lande 1995). Consequently, the adaptive potential of a population
may depend more on variation of quantitative traits determined by multiple loci
than on single-locus polymorphisms (Lynch 1996).
Lande and Barrowclough (1987) contrast the adaptive importance of these
types of genetic variation and discuss their maintenance in the context of neutral
and stabilizing selection. They suggest that a population of several hundred individuals is necessary to maintain amounts of quantitative genetic variation necessary for evolution. Similarly, Lynch (1996) emphasizes that quantitative genetics
focuses on the evolutionary properties of morphological and behavioral traits and
therefore can provide insights into the effects of small population size on fitness
and ultimately extinction risks. He goes further than Lande and Barrowclough
(1987) in his suggestion that maintenance of the adaptive genetic variation of
populations requires more than 10,000 reproductive individuals and that current
conservation policies leave most endangered species at risk of losing genetic
integrity.
A general prediction from population genetics theory is that existing heterozygosity erodes by 50% within 1.39N e generations, where N e is the effective size
of a randomly mating population (Wright 1931; Hartl and Clark 1989; see definition below). Small populations run a greater risk of becoming genetically
depauperate and, therefore, are at the center of attention of conservation genetics.
In the following sections, I discuss the merits of genetic diversity, tools to assess
genetic variation and phylogenetic uniqueness, factors affecting genetic diversity,
and the question of what to preserve.
227
with the population frequency of the most common allele being smaller than 0.99.
The proportion of polymorphic loci, in turn, is a measure of the number of
variable loci among all sampled loci within a population. Genetic variation may
also be described by quantitative variation in traits derived from the actions (and
interactions) of many genes, termed quantitative genetic variation.
The most frequently used measure of populationwide genetic diversity is the
amount of heterozygosity. Individual heterozygosity describes the observed proportion of heterozygous loci in an individual (Mitton and Pierce 1980), and
average heterozygosity reflects the proportion of heterozygous individuals in a
population measured across several loci (Hartl and Clark 1989). The theoretical
predictions based on the Hardy Weinberg Principle provide the framework within
which to evaluate the amount and distribution of genetic variation documented in
natural populations. Deviations from the expected genotype frequencies within a
population, for example, can be indicative of past bouts of strong selection or
inbreeding.
To evaluate how loss of genetic diversity affects population survival, it is
crucial to distinguish the significance of different types of genetic variation.
For example, single-locus diversity is measured by individual heterozygosity,
whereas diversity associated with polygenic quantitative traits is measured by
phenotypic variation. Empirical studies suggest that most phenotypic changes in a
population are the result of small alterations at numerous loci rather than a consequence of major mutations at a single locus (Lande 1981; Lande and Barrowclough 1987; Lande 1995). Consequently, the adaptive potential of a population
may depend more on variation of quantitative traits determined by multiple loci
than on single-locus polymorphisms (Lynch 1996).
Lande and Barrowclough (1987) contrast the adaptive importance of these
types of genetic variation and discuss their maintenance in the context of neutral
and stabilizing selection. They suggest that a population of several hundred individuals is necessary to maintain amounts of quantitative genetic variation necessary for evolution. Similarly, Lynch (1996) emphasizes that quantitative genetics
focuses on the evolutionary properties of morphological and behavioral traits and
therefore can provide insights into the effects of small population size on fitness
and ultimately extinction risks. He goes further than Lande and Barrowclough
(1987) in his suggestion that maintenance of the adaptive genetic variation of
populations requires more than 10,000 reproductive individuals and that current
conservation policies leave most endangered species at risk of losing genetic
integrity.
A general prediction from population genetics theory is that existing heterozygosity erodes by 50% within 1.39N e generations, where N e is the effective size
of a randomly mating population (Wright 1931; Hartl and Clark 1989; see definition below). Small populations run a greater risk of becoming genetically
depauperate and, therefore, are at the center of attention of conservation genetics.
In the following sections, I discuss the merits of genetic diversity, tools to assess
genetic variation and phylogenetic uniqueness, factors affecting genetic diversity,
and the question of what to preserve.