Chapter 5
Characterizing Genetic Diversity:
Quantitative Variation
Quantitative (metric or polygenic) characters of
Most concern to conservation biology are those
Related to reproductive fitness such as:
Number of fertile offspring contributed by an
Individual that survive to reproductive age.
For endangered species, quantitative variation for
reproductive fitness is involved in the major
genetic concerns in conservation biology, namely:
Reduction in reproductive fitness due to inbreeding
(inbreeding depression)
Loss of evolutionary potential due to small
population sizes.
Impact of crossing between different populations
on fitness, whether beneficial (heterosis) or
deleterious (outbreeding depression).
Effects of translocating individuals from one
environment to another.
Correlations between molecular and quantitative
measures of genetic diversity are low. Therefore,
molecular measures of genetic variation provide,
at best, only a very imprecise indication of
evolutionary potential.
Quantitative characters typically have continuous,
approximately normal distributions and include
characters such as reproductive fitness, longevity,
height, weight, disease resistance, etc.
It is not possible to directly infer genotype from
observed phenotype for quantitative characters.
Individuals with the same genotype may have
different phenotypic values and individuals with
the same phenotypic values may have different
genotypes.
Underlying genetic basis to quantitative characters
is that they are affected by a number of loci,
each possessing alleles that add to or detract from
the magnitude of the character.
Loci affecting quantitative characters, individually,
show usual Mendelian properties of segregation
and linkage.
A major challenge in the study of quantitative
genetics is to determine how much of the observed
variation is due to genetics and how much is due
to environment.
One of the central concepts of quantitative genetics
is heritability.
Heritability is the proportion of the total
phenotypic variance in a population due to genetic
differences among individuals.
Algebraically, we can define the phenotypic value
Of an individual as the consequence of the alleles
It inherits together with environmental influences
As:
P = G + E
Where P = phenotype, G = Genotype, and
E = Environment.
The genetic component can be partitioned from
the environmental component as:
VP = VG + VE + 2CovGE
Where, CovGE is the covariance between genetic
and environmental effects.
The covariance for this component is expected
to be 0 if conditions for different genotypes
are equalized by randomly allocating individuals
across the range of environment, which is
difficult to achieve in wild populations.
For example, in territorial species of birds and
mammals, the genetically fittest parents may
obtain the best territories.
Offspring inheriting the best fitness genotypes
also inherit the best environments.
This results in a genotype X environment
correlation that increases phenotypic resemblance
among relatives.
Differences in performance of genotypes in
different environments is referred to as
Genotype X Environment Interactions.
These develop when populations adapt to particular
environmental conditions, and survive and
reproduce better in their native conditions than
in other environments.
Genotype X Environment Interactions are of major
significance to the genetic management of
endangered species as follows:
Reproductive fitness of translocated individuals
cannot be predicted if there are significant
Genotype X environment interactions.
Success of reintroduced populations may be
compromised by genetic adaptation to captivity.
For example, superior genotypess under captive
conditions may perform relatively poorly when
released to the wild.
Mixing of genetic material from fragment
populations may generate genotypes that do not
perform well under some, or all, conditions.
Knowledge of genotype X environment interactions
can strongly influence the choice of populations
for return to the wild.
Genotype X Environment interactions must be
distinguished from the genotype X environment
covariances and correlations.
Genotype X environment correlations occur when
genotypes are non-randomly distributed over
environments.
By contrast, genotype X environment interactions
are detected by comparing all genotypes in
several common garden environments; if their
relative performances differ in the different
environments there is a genotype X environment
interaction.
Likelihood of genotype X environment interaction
increases with the magnitude of both genetic
and environmental differences.
Thus, it is more likely to be detected in species
with wide geographic, ecological, or altitudinal
ranges.
Further, quantitative traits closely associated with
reproductive fitness appear to be more prone to
genotype X environment interactions than
characters more peripheral to fitness.
Quantitative genetic variation has contributions
from the average effects of loci VA, from their
dominance deviations VD, and from interactions
(epistatic) deviations among gene loci VI as:
VG = VA + VD + VI
These are referred to as additive genetic
variance (VA), dominance variance (VD), and
interaction variance (VI).
Each of these has major conservation implications
as follows:
VA and especially the ratio VA/VP (heritability)
reflect the adaptive evolutionary potential of the
population for the character under study.
VD reflects susceptibility to inbreeding depression.
VI influences the effects of outbreeding, whether
beneficial or deleterious.
Therefore,
VP = VG + VE = 2CovGE
More specifically,
VP = VA + VD + VI + VE + 2CovGE
Evolutionary Potential and Heritability
Conservation genetics is concerned with the
evolution of quantitative traits and how their ability
to adapt is affected by reduced population size,
fragmentation, and changes in the environment.
Immediate evolutionary potential of a population is
determined by the heritability which is defined
as the proportion of total phenotypic variation
due to additive genetic variation or h2 = VA/VP.
Heritabilities range from 0 to 1.
Heritabilities of 0 are found in highly inbred
populations with no genetic variation.
Heritabilities of 1 are expected for characters with
no environmental variance in an outbred population
if all genetic variance is additive.
Heritabilities are specific to particular populations
living under specific environmental conditions.
Heritability and VA are fundamentally measures of
how well quantitative traits are transmitted from
one generation to the next.
Unfortunately, very few heritability estimates
exist for endangered species and there clearly is
need for many more estimates of heritability in
threatened and endangered species.