94
DAVID D. KECK
that the most primitive species in Crepis are robust, leafy perennials,
with few large flower heads in which the many large involucral bracts
remain unchanged as the fruits ripen, and the comparatively large fruits
themselves are not highly specialized for wind distribution. In contrast,
the most advanced species are slender annuals, with a basal rosette of
leaves and nearly naked stems bearing numerous very small heads in
which the few small involucral bracts become thickened and reflex as
the small fruits, highly specialized for wind distribution, ripen. Valuable
evidence of relationships was obtained from the chromosomes. There
are definite tendencies for the more advanced species to have shorter
and more asymmetrical chromosomes. The majority of the most primitive
species have 6 pairs of chromosomes, the great majority of the most advanced have 4 or 3 pairs. Species of those sections that are intermediate
morphologicaly are also intermediate cytologically, with 5 or 4 pairs of
chromosomes. Evidence from genetics and cytogenetics is also consistent with all the other evidence. Intrasectional crosses were made
comparatively easily, and most of the resulting hybrids were vigorous.
Intersectional hybrids, on the other hand, were made with much more
difficulty, and the hybrids obtained were much less vigorous. These
data supported the sectional scheme of classification worked out on a
morphological basis. The cytogenetic evidence demonstrated how a 3chromosome species descended from a 4-chromosome one, and how a
certain 4-chromosome species descended from a 5-chromosome ancestor.
The Crepis story, as summarized by Babcock (1944a), shows that
evolution in this genus has required three conditions: (1) plenty of
time (20 to 30 million years); (2) plenty of environmental changes with
the passage of time; and (3) isolation of populations through migration.
It has also required three vital processes: (1) the creation of isolation
through changes in chromosome numbers and associated genetic
changes; (2) differentiation of species within the several chromosome
number groups by means of gene mutations; and (3) along with differentiation, adaptation through gene mutation and natural selection.
From his own studies of them, Stebbins (1953) is led to believe that
"the general phylogenetic trends in external morphology and in the
character of the chromosomes which Babcock has demonstrated to have
occurred in Crepis have also probably been characteristic of the tribe
Cichorieae as a whole/' Certainly the species of this tribe do offer excellent chromosomal characters for classification purposes. Their chromosomes are relatively large and easily observed, of distinguishable morphology, and their numbers are relatively low. In this group it has been
clearly demonstrated that taxonomic relationship is usually accompanied
DAVID D. KECK
that the most primitive species in Crepis are robust, leafy perennials,
with few large flower heads in which the many large involucral bracts
remain unchanged as the fruits ripen, and the comparatively large fruits
themselves are not highly specialized for wind distribution. In contrast,
the most advanced species are slender annuals, with a basal rosette of
leaves and nearly naked stems bearing numerous very small heads in
which the few small involucral bracts become thickened and reflex as
the small fruits, highly specialized for wind distribution, ripen. Valuable
evidence of relationships was obtained from the chromosomes. There
are definite tendencies for the more advanced species to have shorter
and more asymmetrical chromosomes. The majority of the most primitive
species have 6 pairs of chromosomes, the great majority of the most advanced have 4 or 3 pairs. Species of those sections that are intermediate
morphologicaly are also intermediate cytologically, with 5 or 4 pairs of
chromosomes. Evidence from genetics and cytogenetics is also consistent with all the other evidence. Intrasectional crosses were made
comparatively easily, and most of the resulting hybrids were vigorous.
Intersectional hybrids, on the other hand, were made with much more
difficulty, and the hybrids obtained were much less vigorous. These
data supported the sectional scheme of classification worked out on a
morphological basis. The cytogenetic evidence demonstrated how a 3chromosome species descended from a 4-chromosome one, and how a
certain 4-chromosome species descended from a 5-chromosome ancestor.
The Crepis story, as summarized by Babcock (1944a), shows that
evolution in this genus has required three conditions: (1) plenty of
time (20 to 30 million years); (2) plenty of environmental changes with
the passage of time; and (3) isolation of populations through migration.
It has also required three vital processes: (1) the creation of isolation
through changes in chromosome numbers and associated genetic
changes; (2) differentiation of species within the several chromosome
number groups by means of gene mutations; and (3) along with differentiation, adaptation through gene mutation and natural selection.
From his own studies of them, Stebbins (1953) is led to believe that
"the general phylogenetic trends in external morphology and in the
character of the chromosomes which Babcock has demonstrated to have
occurred in Crepis have also probably been characteristic of the tribe
Cichorieae as a whole/' Certainly the species of this tribe do offer excellent chromosomal characters for classification purposes. Their chromosomes are relatively large and easily observed, of distinguishable morphology, and their numbers are relatively low. In this group it has been
clearly demonstrated that taxonomic relationship is usually accompanied