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Forest Management. Does selective logging degrade the genetic quality of succeeding generations through dysgenic selection?. Jonathan Cornelius. La Mosquitia , Honduras. “Mahogany bush”. “Mahogany bush--a dark mass of vegetation appearing without prelude on the savannas”.
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Forest Management Does selective logging degrade the genetic quality of succeeding generations through dysgenic selection? Jonathan Cornelius
“Mahogany bush” “Mahogany bush--a dark mass of vegetation appearing without prelude on the savannas”
Regeneration assured? • “…more than 40 per cent of the trees are infested with a termite, the Coptotermes crassus, which penetrates to the heart of the tree…as the government taxes the mahogany concessionaires twelve dollars for each cut tree…care is taken to allow the termite-infested trees to stand…so that a method of natural reforestation is thus ensured—not by man but in spite of him” (Von Hagen 1940)
Dysgenic selection • Dysgenic selection is selection that leads to an undesirable directional change in genetic quality over one or more generations
Claims of dysgenic selection and variation patterns “...another noteworthy feature is the lack of distinct variation patterns in relation to latitude and longitude; this may be due to the fact that the natural forests around the Mediterranean have been disrupted by man and subject for centuries to dysgenic selection” (Palmberg 1975)
Claims of dysgenic selection and selective logging “...in most areas this once famous tree now occurs as little more than a much-branched bush or small tree, a prime example of extreme genetic erosion due to past over exploitation of the best genotypes” (Pennington et al. 1981)
Claims of dysgenic selection and selective logging “...selective logging may promote dysgenic selection as a result of the continuous exploitation of large, superior individuals” (Lemes et al. 2007)
Dysgenic selection: the mirror image of genetic improvement Distribution of dbh in a commercial plantation of Vochysia guatemalensis (Sarapiquí, Costa Rica)
Positive phenotypic selection aiming at genetic improvement best trees selected
Negative phenotypic selection, possibly leading to dysgenic selection worst trees selected
There are no documented examples of dysgenic selection in forest trees • Poor form and slow growth are common, but can be caused by many factors
The breeders’ equation • …allows us to predict the effect of phenotypic selection, whether positive (improvement) or negative (dysgenic) Response to selection (R) = selection differential (S) x heritability (h2)
The breeders’ equation:Response to selection (R) = selection differential (S) x heritability (h2) • For example, if trees grown from improved seed grow 10% more quickly than those grown from unimproved seed, then the response to selection is 10% Response to selection = the improvement or decline in performance for a given characteristic from one generation to the next
The breeders’ equation:Response to selection (R) = selection differential (S) x heritability (h2) • Selection differential is a within-generation measure of phenotypic superiority and will usually reflect environmental differences as well as genetic differences Selection differential = the difference between the mean of the selected individuals and the mean of the population If plus-tree mean dbh is 40 cm and population mean dbh is 20 cm, then S = 20 cm
The breeders’ equation:Response to selection (R) = selection differential (S) x heritability (h2) • The value of heritability varies from zero to one Heritability = the degree to which the superiority measured in the selection differential is passed on to the progeny of the selected individuals
The breeders’ equation:Response to selection (R) = selection differential (S) x heritability (h2) R = Sh2 = 20 cm x 0.2 = 4 cm
Marajoara before logging Marajoara after logging based on Grogan et al. 2008
“Mahogany bush” “Mahogany bush--a dark mass of vegetation appearing without prelude on the savannas”
