Chapter 5Characterizing Genetic Diversity:

Quantitative Variation

Quantitative (metric or polygenic) characters ofMost concern to conservation biology are thoseRelated to reproductive fitness such as:

Number of fertile offspring contributed by anIndividual that survive to reproductive age.

For endangered species, quantitative variation forreproductive fitness is involved in the majorgenetic concerns in conservation biology, namely:

Reduction in reproductive fitness due to inbreeding(inbreeding depressioninbreeding depression)

Loss of evolutionary potential due to small population sizes.

Impact of crossing between different populationson fitness, whether beneficial (heterosisheterosis) ordeleterious (outbreeding depressionoutbreeding depression).

Effects of translocating individuals from oneenvironment to another.

Correlations between molecular and quantitativemeasures 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 includecharacters such as reproductive fitness, longevity,height, weight, disease resistance, etc.

It is not possible to directly infer genotype fromobserved phenotype for quantitative characters.

Individuals with the same genotype may have different phenotypic values and individuals withthe same phenotypic values may have different genotypes.

Underlying genetic basis to quantitative charactersis that they are affected by a number of loci,each possessing alleles that add to or detract fromthe magnitude of the character.

Loci affecting quantitative characters, individually,show usual Mendelian properties of segregationand linkage.

A major challenge in the study of quantitativegenetics is to determine how much of the observedvariation is due to genetics and how much is dueto environment.

One of the central concepts of quantitative geneticsis heritabilityheritability.

HeritabilityHeritability is the proportion of the totalphenotypic variance in a population due to geneticdifferences among individuals.

Algebraically, we can define the phenotypic valueOf an individual as the consequence of the allelesIt inherits together with environmental influencesAs:

P = G + EP = G + E

Where P = phenotype, G = Genotype, and E = Environment.

The genetic component can be partitioned fromthe environmental component as:

VVPP = V = VGG + V + VEE + 2Cov + 2CovGEGE

Where, CovGE is the covariance between geneticand environmental effects.

The covariance for this component is expectedto be 0 if conditions for different genotypesare equalized by randomly allocating individualsacross the range of environment, which is difficult to achieve in wild populations.

For example, in territorial species of birds andmammals, the genetically fittest parents mayobtain the best territories.

Offspring inheriting the best fitness genotypesalso inherit the best environments.

This results in a genotype X environmentcorrelation that increases phenotypic resemblanceamong relatives.

Differences in performance of genotypes in different environments is referred to asGenotype X Environment InteractionsGenotype X Environment Interactions.

These develop when populations adapt to particularenvironmental conditions, and survive andreproduce better in their native conditions thanin other environments.

Genotype X Environment InteractionsGenotype X Environment Interactions are of majorsignificance to the genetic management of endangered species as follows:

Reproductive fitness of translocated individualscannot be predicted if there are significantGenotype 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 notperform well under some, or all, conditions.

Knowledge of genotype X environment interactionscan strongly influence the choice of populationsfor return to the wild.

Genotype X Environment interactions must bedistinguished from the genotype X environmentcovariances and correlations.

Genotype X environment correlations occur whengenotypes are non-randomly distributed overenvironments.

By contrast, genotype X environment interactionsare detected by comparing all genotypes inseveral common garden environments; if theirrelative performances differ in the differentenvironments there is a genotype X environmentinteraction.

Likelihood of genotype X environment interactionincreases with the magnitude of both geneticand environmental differences.

Thus, it is more likely to be detected in specieswith wide geographic, ecological, or altitudinalranges.

Further, quantitative traits closely associated withreproductive fitness appear to be more prone togenotype X environment interactions than characters more peripheral to fitness.

Quantitative genetic variation has contributionsfrom the average effects of loci VA, from theirdominance deviations VD, and from interactions(epistatic) deviations among gene loci VI as:

VVGG = V = VAA + V + VDD + V + VII

These are referred to as additive genetic variance (VA), dominance variance (VD), andinteraction variance (VI).

Each of these has major conservation implicationsas follows:

VA and especially the ratio VA/VP (heritability)reflect the adaptive evolutionary potential of thepopulation for the character under study.

VD reflects susceptibility to inbreeding depression.

VI influences the effects of outbreeding, whetherbeneficial or deleterious.

Therefore, VVPP = V = VGG + V + VEE = 2Cov = 2CovGEGE

More specifically,

VVPP = V = VAA + V + VDD + V + VII + V + VEE + 2Cov + 2CovGEGE

Evolutionary Potential and HeritabilityEvolutionary Potential and Heritability

Conservation genetics is concerned with the evolution of quantitative traits and how their abilityto adapt is affected by reduced population size,fragmentation, and changes in the environment.

Immediate evolutionary potential of a population isdetermined by the heritabilityheritability which is definedas the proportion of total phenotypic variationdue to additive genetic variation or hh22 = V = VAA/V/VPP..

Heritabilities range from 0 to 1.

Heritabilities of 0 are found in highly inbredpopulations with no genetic variation.

Heritabilities of 1 are expected for characters withno environmental variance in an outbred populationif all genetic variance is additive.

Heritabilities are specific to particular populationsliving under specific environmental conditions.

Heritability and VA are fundamentally measures ofhow well quantitative traits are transmitted fromone generation to the next.

Unfortunately, very few heritability estimatesexist for endangered species and there clearly isneed for many more estimates of heritability inthreatened and endangered species.