Paraphyletic groups in phylogenetic analysis: Progymnospermopsida and Préphanérogames in alternative views of seed plant relationships

Abstract

Summary.-Prompted by Patterson's (1981) criticism of fossils, the notion of «paraphyletic group», often used by paleontologists in phylogenetic studies, is investigated here in a cladistic con· text. Two groups are treated in detail as explicit historical hypotheses : fossil Progymnospermopsida and living and fossil Préphanérogames. Farris's formai definition of paraphyly is adopted but irnplemented using named transitions in cladistic characters as delirniters of each group. In addition, it is suggested that a «candidate for ancestor» of each derived «outside» taxon be recognized within each hypothesis of paraphyly. Using this approach, the paraphyletic status of both Progymnospermopsida and Phréphanérogames is assessed under the phylogenetic views of Beek, Rothwell, Meyen, and Doyle and Donoghue. Although there are major differences in the «candidates for anceston, paraphyly is supported for the Progymnospermopsida in each case. Paraphyletic status for Préphanéro-games, however, is only possible under the Rothwell and Doyle and Donoghue phylogenetic view· points, and seems vulnerable to likely changes in the interpretation of certain Mesozoic «pterido-sperm» taxa as they become better known. Beyond simple cladistic description, however, there remains the question whether the Progymnospermopsida is an «objectively real» or «biologically meaningfub group. We suggest that the combination of features «typically gymnospermous secondary growth» and «pteridophytic reproduction» indicates an evolutionary significance for the group.

Heterospory: The Most Iterative Key Innovation in the Evolutionary History of the Plant Kingdom

“…T h e Progymnospermopsida are thought to have evolved from the Trimerophytopsida (Beck: 1970(Beck: , 1976(Beck: , 1981Stewart, 1981;Stein, 1987;Stein & Beck, 1987;Beck & Wight, 1988;Stewart & Rothwell, I 993), which somewhat surprisingly appears uniformly homosporous. Five progymnospermopsid groups have been given ordinal status ( Fig.…”

“…Moreover, the phylogenetic relationships of the orders have not been satisfactorily resolved, though there is general agreement that the Aneurophytales is the most primitive (e.g. Doyle & Donoghue, 1986;Stein, 1987;Stein & Beck, 1987;Stewart & Rothwell, 1993). Life histories have been inferred in only one species each of the Protopityales (Walton, 1957;Smith, 1962a) and the Cecropsidales (Stubblefield & Rothwell, 1989); both were regarded as heterosporous.…”

“…Lyon, 1904;Pincher, 1935;Manton, 1950;Sussex, 1966;Andrews, Gensel & Forbes, 1974;Bell, 1979Bell, , 1989Duckett & Pang, 1984; Turnau & Karczewska, 1 9 8 7 )~ reviewers have tended to view heterospory primarily as a precursor to the seed habit rather than a profound innovation in itself (e.g. Coulter, 1898; Thomson, 1927Thomson, , 1934Walton, 1953;Andrews, 1963;Meeuse, 1963;Smith, 1964;Long, 1966;Pettitt, 1970;Jonker, 1977; Chaloner & Sheerin, 1 9 8 1 ; Steeves, 1983;Crane, 1985a, b ;Doyle & Donoghue, 1986;Chaloner & Pettitt, 1987;Stein & Beck, 1987;Beck & Wight, 1988;Rothwell & Scheckler, 1988; "...bearing spores of distinctly different types". (Jones, 1987: 41 I ) " .…”

Is Ginkgo biloba (Ginkgoaceae) really an oviparous plant?

“…Favre-Duchartre (1958) used the term ''oviparous'' in reference to the primitive features of the reproductive biology of Ginkgo biloba. This developmental scenario is widely believed to separate Ginkgo from all other living seed plants with the possible exception of some members of the Cycadales (Stein and Beck, 1987;Gifford and Foster, 1989;Stevenson, 1990). Germination is reported to occur the following spring or early summer after full development of the embryo on the ground.…”

“…It has a number of reproductive characters that are considered to be ancestral among living seed plant species (e.g., Doyle and Donoghue, 1992), including substantial investment into nutritive tissue (megagametophyte) before fertilization, haustorial pollen tubes, and zooidogamous fertilization (Hirase, 1896;Friedman, 1993). Most notably, Ginkgo is considered to be a préphanérogame, which are defined by Emberger (1942), as ''vascular plants, both living and fossil, whose ovules develop embryos only after release from the parent sporophyte'' (Stein and Beck, 1987). Favre-Duchartre (1958) used the term ''oviparous'' in reference to the primitive features of the reproductive biology of Ginkgo biloba.…”