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Unconscious selection in plants under domestication
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duction of the plants and/or animals into the anthropogenic environment, numerous
adaptations vital to survival in the wild lost their fitness and broke down. New
traits were automatically selected for, resulting in the build-up of characteristic
“domestication syndromes” - each fitting the specific agricultural, horticultural or
husbandry system provided by the domesticator.
The role of unconscious selection in the evolution of crop plants has been evaluated
by several authors (DARLINGTON 1963, 1973, ZOHARY 1969, 1984, HARLAN et al. 1973,
HAMMER 1984, HANELT 1986, HEISER 1988, ZOHARY and HOPF 2000). It is now widely
accepted that this type of selection shaped many of the traits that characterise plants
under domestication, and distinguished them from their wild relatives. Indeed this approach
has already considerably assisted crop-plant evolutionists in their reconstruction
of the evolution of grain crops, vegetables, fruit trees, and tubers and corms.
This paper aims at an updated outline of this process. It traces some of the main
ecological shifts introduced by the transfer of plants into cultivation. It sketches the
evolutionary consequences that could have been brought about, in response to these
environmental changes.
Maintenance practices and their impact
Two principal modes of crop maintenance have been traditionally employed for handling
plants under cultivation. In the bulk of the grain crops, in numerous vegetables
and truck crops, and in some ornamentals, the grower continues to maintain his
plants by seed planting, i.e., in the same way their progenitors reproduce in the wild.
In contrast, in most fruit trees and corm and tuber crops, in numerous ornamentals,
and in some vegetables, domestication depends on a shift from reproduction by
seeds (in the wild) to vegetative propagation (under cultivation). The choice between
seed planting and vegetative propagation automatically sets into motion two
very different courses of evolution in plants under domestication:
With very few exceptions (such as nucellar seeds in several Citrus crops and in
mango) seed planting means sexual reproduction. In other words, seed planted
crops undergo a recombination-and-selection cycle every sowing. Consequently
such crops have had, under traditional farming, hundreds (or even thousands) generations
of selection. They were repeatedly moulded into
(i) clusters of inbred lines (in predominantly self-pollinated crops), or
(ii) distinct cultivated races (in cross-pollinated crops).
In numerous sexually reproducing crops, the results of such repeated cycles of selection
are indeed impressive. Under domestication, these crops diverged considerably
from their wild progenitors; and the cultivars are distinguished from their wild pro-