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History of Early Evolutionary Thought

The Plant Archetype by P.J.F. Turpin Appeared in an 1837 Edition of Goethe’s Works on Natural History Published in France (Goethe, 1837). (Image courtesy of the Houghton Library of Harvard University.)

I have long been fascinated by the roughly 50 individuals between the mid-eighteenth century and 1859 who wrote about and advanced evolutionary ideas. These pre-Darwinian evolutionists are largely obscure figures, unknown today to most biologists. What was their role in advancing schemes of biological transformation (descent with modification), and how did each of these early evolutionists relate to Charles Darwin and his own thinking about evolution beginning in 1836 with the opening of his private notebooks on the topic? In addition, I have become specifically interested in those early evolutionists who were botanists, horticulturists, and plant geographers, and am currently working on researching the individuals more thoroughly.

Evolution of Plant Body

Echium perezii (Canary Islands)

From towering redwoods to delicate flowers, plants have evolved into countless forms that shape our planet’s ecosystems and touch every aspect of our lives. This transformation raises a fascinating question: how and why do plant bodies change over millions of years? The answer is still largely unknown, but the genus Echium offers a remarkable case study. These plants belong to one of dozens of lineages that started as small, herbaceous species on the continent and later dispersed to islands. Over time, they evolved into an array of forms: trees, giant rosettes, towering candelabra shrubs, and cushion plants. Each form allowed them to thrive in distinct and often challenging island environments. How did these plants change so dramatically? What environmental pressures drove their transformation?

Double Fertilization in Nonflowering Seed Plants

Double fertilization event in Ephedra

Double fertilization in angiosperms, in which one sperm fertilizes an egg while the second sperm fuses with the polar nucleus/nuclei of the central cell was first reported in 1898 and has longed been viewed as a defining reproductive feature of all flowering plants. Angiosperms, however are not the only clade of plants that undergo a process of double fertilization, in which the sperm of a single pollen tube (genetically identical gametes) fertilizes two female gametes in a single seed. Members of the Gnetales (Ephedra and Gnetum) also undergo a regular process of fertilization, which, unlike flowering plants (where an embryo and an endosperm result), produces two embryos. Over a century ago, embryological studies of specific conifers (e.g. Thuja occidentalis and Abies balsamea) revealed hints that there may also be a regular process of double fertilization to yield two embryos.

Origin & Early Angiosperms

Flower and ovule morphogenesis in Nymphaea thermarum

Along with the origins of vascular plants and seed plants, the origin of flowering plants represents one of the three most significant evolutionary radiations of land plants during the last 475 million years. With over 250,000 extant species, angiosperms are the largest and most diverse group of plants ever to have evolved. Paradoxically, we know less about the origin and early evolutionary history of angiosperms than we do about many considerably older groups of land plants. Indeed, Darwin’s “abominable mystery” continues to challenge evolutionary biologists. Our research efforts have focused on the evolution of double fertilization and endosperm, two of the most important and defining features of flowering plants. These analyses of the origin of the angiosperm reproductive syndrome have drawn upon a variety of approaches from developmental biology to inclusive fitness theory.

Hornwort Gametophytic Pores

Recent analyses of the evolutionary relationships among major clades of land plants have indicated that extant bryophytes (mosses, liverworts, and hornworts) probably form a monophyletic group. This unexpected finding has sparked vigorous discussion around the evolution in early embryophytes of such traits as stomata and rooting structures. Conspicuously absent from the current conversation, however, are the pores found on the gametophytes of hornworts. Some researchers have insisted that these “mucilage clefts” are not homologous to stomata, and recently most botanists have ignored such pores and the insights they may provide. We are pursuing multiple lines of enquiry, from ontogeny to gene expression, to determine whether these hornwort gametophytic pores (HGPs—our term) are homologous to stomata. Such a homology would greatly alter current hypotheses for the evolution of stomata.

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