80
DAVID D. KECK
developed fertility barriers between them (Clausen et al. 1942; Clausen,
1951).
Another isolation mechanism that is playing an only partially appreciated role in evolution is the remarkable flower constancy of many
insect pollinators (V. Grant, 1949, 1950b). For example, the Geum
species studied by Gajewski (1954), which are sympatric, maintain their
specific distinctness in spite of high compatibility, hybrid vigor, and
fertility, and natural hybrids do not tend to swamp them. The principal
barriers between them are geographical and ecological and the differences in flowering time and insect pollinators. Gajewski has concluded
from these studies that these factors in nature are of more importance
for specific isolation than is intersterility.
Studies on several species of the section Peltanthera of the genus Penstemon in California have shown that specific insect pollinators are the
rule, and that certain interspecific F x hybrids resemble other preexisting
species. Despite rather frequent hybridizations, there is no tendency for
distinct species of Penstemon to be swamped. If this is due to flower
constancy of the pollinators, it is possible that a rare "adoption" of such
a hybrid by an insect that is not a regular pollinator of either parental
species could be the mechanism by which a new species could arise
without either geographic or strong genetic isolation being necessary
(Straw, 1955).
Heredity versus Environment.
To the taxonomist it is important to be
aware of the effect of the environment upon the populations of plants
that he studies (Constance, 1953). A long-term series of varied-environment experiments was conducted in a series of gardens along a transect
across central California from near the coast and sea level to near
timberline and the crest of the Sierra Nevada at an elevation of 3050
meters. The objective was "to discover principles that govern the distribution of plants and their organization into natural units." The relative significance of heredity and environment was constantly evaluated
during this program, and cytogenetic experiments went hand in hand
with these transplant experiments. Many principles of taxonomic interest
were discovered or confirmed (Clausen et al. 1940a, 1948a).
The results obtained from these California experiments were interpreted in accord with Turesson's (1922, 1929) classification of plants
into cenospecies, ecospecies, and ecotypes, because the principles established by Turesson were found to apply as well in California as they had
in northern Europe.
The categories cenospecies, ecospecies, and ecotype are genetic-ecologic and to a considerable extent they may be as difficult to define
DAVID D. KECK
developed fertility barriers between them (Clausen et al. 1942; Clausen,
1951).
Another isolation mechanism that is playing an only partially appreciated role in evolution is the remarkable flower constancy of many
insect pollinators (V. Grant, 1949, 1950b). For example, the Geum
species studied by Gajewski (1954), which are sympatric, maintain their
specific distinctness in spite of high compatibility, hybrid vigor, and
fertility, and natural hybrids do not tend to swamp them. The principal
barriers between them are geographical and ecological and the differences in flowering time and insect pollinators. Gajewski has concluded
from these studies that these factors in nature are of more importance
for specific isolation than is intersterility.
Studies on several species of the section Peltanthera of the genus Penstemon in California have shown that specific insect pollinators are the
rule, and that certain interspecific F x hybrids resemble other preexisting
species. Despite rather frequent hybridizations, there is no tendency for
distinct species of Penstemon to be swamped. If this is due to flower
constancy of the pollinators, it is possible that a rare "adoption" of such
a hybrid by an insect that is not a regular pollinator of either parental
species could be the mechanism by which a new species could arise
without either geographic or strong genetic isolation being necessary
(Straw, 1955).
Heredity versus Environment.
To the taxonomist it is important to be
aware of the effect of the environment upon the populations of plants
that he studies (Constance, 1953). A long-term series of varied-environment experiments was conducted in a series of gardens along a transect
across central California from near the coast and sea level to near
timberline and the crest of the Sierra Nevada at an elevation of 3050
meters. The objective was "to discover principles that govern the distribution of plants and their organization into natural units." The relative significance of heredity and environment was constantly evaluated
during this program, and cytogenetic experiments went hand in hand
with these transplant experiments. Many principles of taxonomic interest
were discovered or confirmed (Clausen et al. 1940a, 1948a).
The results obtained from these California experiments were interpreted in accord with Turesson's (1922, 1929) classification of plants
into cenospecies, ecospecies, and ecotypes, because the principles established by Turesson were found to apply as well in California as they had
in northern Europe.
The categories cenospecies, ecospecies, and ecotype are genetic-ecologic and to a considerable extent they may be as difficult to define