TRENDS IN SYSTEMATIC BOTANY
87
taxonomy in interesting ways. It has been established that some taxonomic characters previously considered of fundamental importance in
the genus, such as presence or absence of pappus, wooliness of pappus,
presence or absence of pubescence on the disk-akenes, presence or absence of ray-florets, and presence or absence of an outer-involucre (a
tribal character!) have a very simple, often a one-gene, or two-gene
Mendelian basis, and are neither so restricted nor so important as they
had been thought to be. On the other hand, some much less startling
characters from the morphological viewpoint were found to be of much
greater evolutionary value and hence of higher taxonomic significance
than had been previously recognized.
The situation in Layia is a little more complicated than average, but
probably rather typical of a great many genera, for example, Clarkia
(Lewis, 1953a), Aster (Avers, 1953a, b), Narcissus (Fernandes, 1951),
and Gilia (V. Grant, 1953, 1954). This pattern with modifications of
various sorts is repeated in its broad outlines in the other genera of
Madiinae, all of which have been used in extensive parallel experiments,
and the indications from experimental work on other genera from this
and other families indicate that the following conclusions drawn from
Layia have a wide application among sexually reproducing seed plants.
1. The genus is represented in the different environments it occupies
by groups of populations physiologically adapted to these environments.
2. These groups are arranged into species, which are separated by
genetic barriers of various distinctness. Such species are ecospecies (or
in a few cases, monotypic cenospecies) usually containing but one ecotype. (A few species contain two, but none more than two ecotypes in
Layia. In some genera ecotypes are considerably more common.)
3. The species can be recognized morphologically, but these characters merely serve as markers for the boundaries determined by genetic
methods. The genetic basis of the species is of more importance than the
particular combination of morphological characters it brings forth.
4. Those species that are able to produce at least partially fertile hybrids with others belong to a polytypic cenospecies, or species complex.
Each such ecospecies of the cenospecies occupies its own environment.
5. Ecospecies of the same cenospecies may form hybrid swarms where
their ranges overlap. The distinctness of such ecospecies is presumably
maintained by natural selection operating through physiological characters. When such selective power of the environment is no longer operative, it is apparent that the parental ecospecies would tend to lose their
individual identities through continued hybridization and introgression.
6. Ecospecies of different cenospecies may freely occupy the same
87
taxonomy in interesting ways. It has been established that some taxonomic characters previously considered of fundamental importance in
the genus, such as presence or absence of pappus, wooliness of pappus,
presence or absence of pubescence on the disk-akenes, presence or absence of ray-florets, and presence or absence of an outer-involucre (a
tribal character!) have a very simple, often a one-gene, or two-gene
Mendelian basis, and are neither so restricted nor so important as they
had been thought to be. On the other hand, some much less startling
characters from the morphological viewpoint were found to be of much
greater evolutionary value and hence of higher taxonomic significance
than had been previously recognized.
The situation in Layia is a little more complicated than average, but
probably rather typical of a great many genera, for example, Clarkia
(Lewis, 1953a), Aster (Avers, 1953a, b), Narcissus (Fernandes, 1951),
and Gilia (V. Grant, 1953, 1954). This pattern with modifications of
various sorts is repeated in its broad outlines in the other genera of
Madiinae, all of which have been used in extensive parallel experiments,
and the indications from experimental work on other genera from this
and other families indicate that the following conclusions drawn from
Layia have a wide application among sexually reproducing seed plants.
1. The genus is represented in the different environments it occupies
by groups of populations physiologically adapted to these environments.
2. These groups are arranged into species, which are separated by
genetic barriers of various distinctness. Such species are ecospecies (or
in a few cases, monotypic cenospecies) usually containing but one ecotype. (A few species contain two, but none more than two ecotypes in
Layia. In some genera ecotypes are considerably more common.)
3. The species can be recognized morphologically, but these characters merely serve as markers for the boundaries determined by genetic
methods. The genetic basis of the species is of more importance than the
particular combination of morphological characters it brings forth.
4. Those species that are able to produce at least partially fertile hybrids with others belong to a polytypic cenospecies, or species complex.
Each such ecospecies of the cenospecies occupies its own environment.
5. Ecospecies of the same cenospecies may form hybrid swarms where
their ranges overlap. The distinctness of such ecospecies is presumably
maintained by natural selection operating through physiological characters. When such selective power of the environment is no longer operative, it is apparent that the parental ecospecies would tend to lose their
individual identities through continued hybridization and introgression.
6. Ecospecies of different cenospecies may freely occupy the same