The influences of progenitor filtering, domestication selection and the boundaries of nature on the domestication of grain crops

1. Domestication generally involves two sequential processes: initial
identification of wild species with desirable characteristics (“progenitor
filtering”); and subsequent artificial and natural selection that
respectively improve features preferred by humans and adapt species to
cultivation/captivity (“domestication selection”). Consequently,
domesticated species can differ from wild species and may share
characteristics owing to convergent evolution (“domestication syndrome”).
Baring evolutionary constraints, domestication selection may generate
extreme phenotypes that transcend the “boundaries of nature” evident for
wild species. Despite evidence of domestication syndromes in some clades,
broader contributions of progenitor filtering and domestication selection
to characteristics of contemporary domesticated species have received
limited attention. 2. Using comparative analysis of 49 grain-crop and 87
wild annual plant species from 15 families, we: (1) addressed whether
plants of crop and wild species differ for mean seed number, per-seed mass
and total seed-mass investment; (2) assessed contributions of a)
progenitor filtering and b) domestication selection to these differences;
(3) evaluated whether crop characteristics exceed the boundaries of
nature; and (4) assessed whether seed-production characteristics of grain
crops constitute components of a generic domestication syndrome. 3. On
average, grain-crop plants produce heavier seeds and greater total seed
mass than wild species, but seed number per plant does not differ.
Comparison of wild species between genera with or without crop species
found no evidence of progenitor filtering. In contrast, crop species
differed from congeneric wild species for the mass traits, but not for
seed number. Greater seed investment by crops is consistent with
artificial selection for enhanced seed yield (mass per harvested area),
whereas heavier individual seeds suggest selection for improved
nutritional quality and (or) adaptation to cultivation environments. 4.
Seed number-size characteristics of grain-crop species lie within the
bivariate variation among wild species and so do not exceed the boundaries
of nature. Seed number and size varied similarly between species types and
generally aligned with seed-investment isoclines, suggesting an upper
investment limit. 5. Despite greater average investment in seed production
and individual seeds by grain-crop species, seed-production
characteristics did not vary less among crop species than among wild
species, which is inconsistent with a common domestication syndrome.