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3 Phylogeny, genetics and the New Systematics

3 Phylogeny, genetics and the New Systematics

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KNOWING PLANTS



Alphonse de Candolle and others

have shown that plants which have

very wide ranges generally present

varieties; and this might have been

expected, as they are exposed to

diverse physical conditions, and as

they come into competition (which,

as we shall hereafter see, is an

equally or more important

circumstance) with different sets of

organic beings.

Charles Darwin, On the Origin of

Species



on the Scala Naturae, that organisms were adapted to their particular

environment by the utility of Divine design. Nature was now seen to

be dynamic, and evolving by a materialistic process (natural selection)

that could be understood in everyday terms. Revolutions, by their very

nature, usually call for a total overthrow of the prevailing conditions

or modes of thought, and this was no less true for Darwinism. But it

would be unrealistic to believe that everything pre-revolution was outmoded or that the new paradigm would be a cure-all for archaic ways

of thinking and doing science. Lamarckism still held sway for many

people, and Darwin himself was still partly inclined towards the view

that inheritance of acquired characters played a supplementary role

in evolution. Plant classification in the immediate post-Darwinian

period did not alter profoundly, due to the stability of natural systems such as those of A. P. de Candolle, Bentham and Hooker, and

others.

As the furore mellowed, the gradualist evolution hypothesis of

Darwin, was accepted by many influential botanists such as Sir Joseph

Dalton Hooker and Asa Gray, although ‘Darwin’s bulldog’ T. H. Huxley, and Darwin’s cousin Francis Galton, believed that evolution occurs

by discrete leaps or saltations. Darwin’s explanation for the discontinuity among species and taxa was to postulate extinct hypothetical

ancestors or missing links. The lack of fossil angiosperms led him to

talk of their origin as an ‘abominable mystery’. Even today, the discrete macro-evolutionary gaps between taxa are commonly regarded

as entirely caused by the extinction of intermediate forms. But many

of the leading geneticists of the day adhered to a saltationist view

of evolution through mutations (the term ‘mutation’ was actually

introduced by Waagen in 1869) in the genetic material, especially de

Vries, who dismissed all alternative views, yet maintained that his

theory was a modification of Darwin’s. William Bateson (1861--1926),

who strongly influenced many of his contemporaries, was opposed

to gradualism, and denied that natural selection played a major role

in evolution. Because species are discontinuous, he hypothesised that

variation is also discontinuous, and has its origins, not in the environment nor in adaptation, but in the intrinsic nature of organisms

themselves (heterogenesis). Bateson gathered such voluminous data

on morphogenesis and variation (published in his Materials for the

Study of Variation in 1894) that it potentially provided the foundation

for a well-formulated science of rational morphology.

One of the areas that Bateson researched was teratology because,

like Goethe before him, he saw such aberrant forms as providing additional clues for the transformation of form. It is noteworthy that this

same approach has been used in the study of the genetic control of

floral development through the study of such mutants as apetala (see

Chapter 2). What the opponents of structuralism (who were mostly

advocates of a narrow interpretation of Darwin’s theory) failed to consider was development. They hypothesised about mutations, selection

and gradual evolution, but they did not consider the coordination



8.3 PHYLOGENY, GENETICS AND THE NEW SYSTEMATICS



of development during ontogeny, which was one area where

morphologists were qualified to contribute. In the absence of a theory

of transformation, they defaulted to gradualism, even to the extent

of rejecting Mendelian inheritance.

As the nineteenth century drew to a close, the momentum of the

new paradigm faltered for want of a mechanism of heredity. In 1896,

James Mark Baldwin published a paper that, while claiming that natural selection was sufficient to explain evolutionary change, intuitively

hypothesised a ‘new factor in evolution’. His hypothesis, subsequently

known as the ‘Baldwin Effect’, states that those individuals which are

more adaptable in their phenotypic response, and can accommodate

to the vicissitudes of their environment, are more likely to survive and

leave more progeny. This phenomenon may be accompanied or followed by a genotypically controlled response which has been coined

‘genetic-assimilation’ by Waddington. Waddington termed this superficially neo-Lamarckian response ‘canalisation’, a homeostatic process

that favours a particular ontogenetic trajectory. Thus, plasticity is

selected for, and builds ‘an epigenetic landscape, which in turn guides

the phenotypic effects of the mutations available’. It was a neat reinterpretation of the Darwinian model. Following on from this idea

was the proposal by Woltereck of the concept of the reaction norm, of

which W. Johannsen drew attention to its close similarity with that of

the genotype. Johannsen, who actually criticised Woltereck for misunderstanding the significance of environmental context, was, at that

time, already developing the concepts of genotype and phenotype. A

century later these ideas seem particularly relevant but the rediscovery of Mendel’s work pushed them into the shadows.



8.3.2 Gregor Mendel and the rise of genetics

Darwin’s theory of evolution was flawed because it lacked a theory

of inheritance of adaptive traits. He never understood how heritable changes occurred or resulted in variation. Although he made

some suggestions about how this might happen, the explanations

were not satisfactory. He thought that the features of the parents

were blended together in their progeny. Unfortunately this meant

that any evolutionary novelty was likely to be dissipated like a drop

of ink in a bucket of water and unavailable for selection. It was the

crossing experiments of Gregor Mendel (a correspondent of Nägeli),

carried out in his monastery garden at just about the same time as

On the Origin of Species was published that gave Darwinism credibility. Unknown to Darwin, Mendel had established the mechanism of

inheritance, thereby founding the science of genetics (Figure 8.14).

He made his discovery by a study of inheritance of characters such

as seed colour and texture and flower colour in peas. Most importantly he showed that differences were not blended in the progeny

but might be expressed in later generations. Mendel concentrated on

elucidating traits in organisms and his research on peas and other

plants eventually allowed him to hypothesise the existence of heredity



Figure 8.14. Gregor Mendel

established the mechanism of

inheritance.



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‘factors’, which we now know to be genes. Mendel’s work, first published in 1866, remained unappreciated until it was rediscovered in

1900. It was history repeating itself because, in a similar way, the earlier researches of Camerarius, upon which Mendel’s own work relied,

had remained neglected, until Koelreuter took them up half a century

after their first publication in 1705.



8.3.3 Genetics and Neo-Darwinism

In an ironic twist, when Mendel’s work was ‘rediscovered’ by de Vries,

Tschermak and Correns in 1900, it was interpreted as supporting a

discontinuous theory of variation and was thus cited to discredit

Darwin. Incidentally, long before this, Mendel’s work was known to

Nägeli, who actually published Mendel’s results in a book entitled The

Mechanical-Physiological Theory of Evolution (1884). Darwin certainly knew

of Mendel and, although he unfortunately never read his classic paper

of 1866, he had studied a paper by Focke that repeatedly cited Mendel.

Mendel, who was even mentioned in Encyclopaedia Britannica, also possessed a copy of On the Origin of Species. Meanwhile, in Cambridge,

Edith Saunders (1865--1945), in collaboration with William Bateson,

independently rediscovered some, at least, of Mendel’s laws before

his work became known to them.

With the realisation of the significance of Mendel’s work, and the

role of the nucleus as the carrier of heredity being recognised in

the 1870s by Oskar Hertwig and others, and the researches of August

Weismann (1834--1919), the science of genetics was born. Perhaps the

greatest contribution of Weismann was his clarification of chromosomal rearrangements or ‘crossing-over’, and its importance for evolution. When Weismann, in 1883--1884, proposed the complete and

permanent separation of soma and germ plasm, the so-called ‘Weismann Barrier’, the idea of the inheritance of acquired characters lost

all credibility. By then, however, Darwin, who saw natural selection

as only part of the evolutionary story, with inheritance as possibly

dual in nature, was already dead. Inheritance of acquired characters

was now firmly rejected by leading exponents of selection and gradualism, and was not to reappear until the somatic selection hypothesis

of Steele et al. in the 1980s reopened old wounds.

Weismann’s theory of the continuity of the germ-plasm had

become the cornerstone of early twentieth-century evolutionary biology. The efficacy of the Weismann Barrier for animal evolution was

assumed to apply equally to plants, if it was ever considered in the

context of plants at all. The Weismann Barrier is hardly applicable to

plants, which have several permanently embryonic regions (the meristems) derived from somatic tissue. Weismann’s insistence of the overriding importance of selection led Romanes, in 1896, to coin the term

‘Neo-Darwinism’ which is sometimes erroneously equated with the

‘synthetic theory of evolution’ or ‘modern synthesis’. From the turn of

the century until about the 1930s, the focus of evolutionary research

centred mainly on the cell and the mechanism of inheritance, as the

new science of genetics gained ascendancy. Simultaneously, research



8.3 PHYLOGENY, GENETICS AND THE NEW SYSTEMATICS



in ecology was developing, and eventually it all culminated in the

so-called ‘synthetic theory’, which became the dominant paradigm.



8.3.4 The ‘modern synthesis’ – new orthodoxy

In the west, during the 1930s, evolution was conceptualised as the

result of natural selection acting on variation within populations

rather than between individuals, and the scientific orthodoxy channelled research towards the study of changing gene frequencies in

populations. The Hardy--Weinberg equation demonstrated that gene

frequencies will remain unchanged in a large cross-breeding population unless there is a perturbation such as selection. Thanks to the

brilliant applications of mathematics by R. A. Fisher, Sewall Wright

and J. B. S. Haldane, this particular focus led to the development of

mathematical population genetics. Fisher had a conception of genetic

architecture based on genes as independent factors -- their effects

could be added together to produce the phenotype and even continuous traits. In contrast, Sewall Wright was impressed by the pleiotropic

effects of single genes and genetic drift, changing gene frequencies

as a result of random sampling in small populations.

During this period there were developments in ecology, spearheaded by G. Turesson, E. B. Ford, S. S. Tchetverikov, TimofeefRessovsky, and others, which were essentially field-based, an approach

that became known as ecological genetics or genecology (also known

as biosystematics). Tchetverikov rejected the idea of traits being determined by independent genes, but accepted the hypothesis that each

trait is determined by a whole complex of interacting genes. Thus,

the combination of ecology with genetics, and a powerful mathematical foundation, firmly established the importance of selection

and provided the bases for the new evolutionary synthesis. Paradoxically, adaptation became a problem because individuals in populations rested on adaptive peaks, and a transition to a different adaptive

peak required some individuals to cross valleys where they had less

fitness. It was Sewall Wright who introduced the metaphor of the

‘adaptive landscape’, which was later to become associated with the

ideas of Waddington.

The ‘modern synthesis’ was largely the outcome of the Jesup Lectures at Columbia University, and was completely biased towards zoology until Stebbins published his Variation and Evolution in Plants in

1950, apart from contributions by Gilmour (1940). Despite the fact

that pioneering efforts had been already developed in the field of

experimental genetics by Bauer, in phytosociology by Braun-Blanquet,

and in the demography of populations by several ecologists, the

efforts of plant biologists did not seem to contribute significantly

to the new synthesis, at least in the early stages. In part this was

because of the complicated nature of plant genetic systems, and the

inability of botanists at the time to formulate a uniform species concept applicable to plants. Population genetics demonstrated that only

small amounts of migration between populations could prevent their



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divergence re-emphasing the importance of reproductive isolation in

evolution.



8.3.5 Speciation



Figure 8.15. An example of

sympatric speciation following

hybridisation and polyploidy is the

evolution of Senecio cambrensis as

an allopolyploid derivative of a

hybrid between S. squalidus and

S. vulgaris.



Speciation, from Darwin onwards, was one of the most fundamental

problems of evolution. Although Darwin and Wallace never actually

subscribed to the idea that geographical isolation was a precursor to

reproductive isolation and speciation, Mayr declared that the ‘modern synthesis’ was clearly only the maturation of Darwin’s theory of

evolution. They obviously saw the importance of islands in the speciation process, but did not go as far as Mayr in elevating geographical

isolation as the major determining factor. By 1859 Darwin was ready

to accept sympatric speciation for many continental species due to ecological isolation. Reproductive isolation is achieved in plants by many

different mechanisms. Mayr clearly recognised that the actual mechanism of speciation ultimately resided in the genes and chromosomes.

Nowadays, the role played by internal factors such as polyploidy and

genetic turnover has to be addressed in any theory of plant evolution. Polyploidy, for example, is widespread in the angiosperms, and

current research is revealing that the plant genome is a dynamic

evolving system (see Figure 8.15). Several plant families such as the

Campanulaceae have massively rearranged chloroplast genomes in all

the major genera, the significance of which has yet to be realised.

Mayr’s Biological Species Concept (BSC) emphasises the reproductive isolation of species. The compelling attraction of the BSC is the

fact that it provides biological criteria for the recognition of some

of the discontinuity that exists in nature. The BSC places the species

as the basic unit of evolutionary biology, and as more natural than

higher categories. However, if time were somehow speeded up, we

might observe species becoming and disappearing in rapid succession, dissolving into one another, and into genera, just as we observe

the transient passage of individuals within generations. In a sense,

over evolutionary time, the species is no more real than any other

taxonomic category, and one can understand why the Nominalistic

Species Concept remained favourable among some botanists. However, functionally, the species has an importance not found in collective higher categories.

Mayr applied his BSC concept to a local flora in northeast United

States and concluded that an overwhelming majority of the species

could be embraced perfectly adequately within this concept. However,

in botany, the BSC has never been popular, and for most botanists,

particularly herbarium workers, a taxonomic (morphological or phenetic) species concept was preferred. This has had at least one unfortunate consequence for botany: diverse phenotypes, often of a single

species or ploidy level, and often based on a single collection, have

been given specific status, thus burdening the nomenclature with

superfluous names, and hindering subsequent investigation.

However, the most obvious reasons for the preference for a taxonomic species concept in botany are as follows.



8.3 PHYLOGENY, GENETICS AND THE NEW SYSTEMATICS



r Plants display a wide amplitude of variation with respect to their

environment

r Reproductive isolation in plant species is initially harder to prove

than in animals because plants lack behavioural traits which could

indicate some sort of reproductive barrier

r Plants are usually collected for study rather than observed in

the field and, therefore, morphological criteria are used to define

species boundaries

r Plants have several alternative reproductive strategies such as vegetative reproduction, and apomixis, which bypass sexual reproduction altogether

In practice, reproductive isolation in plants is frequently not absolute.

Even though they may differ morphologically and genetically, hybrids

may differ in degree of sterility, or be perfectly fertile. In addition variation is often reticulate and multi-dimensional, and is not amenable

to discrete recognition. Turesson (1922) saw the Linnaean nomenclatural system as limiting in its ability to conceptualise the variation in

plants in Nature, and therefore he developed his genecological terminology. He coined the term ecospecies for the Linnaean species from

an ecological perspective, and ecotype for the total phenotypic expression of a particular ecospecies within a more localised habitat. For

the complete aggregate of populations or indeed, species, which are

capable of hybridisation, Turesson used the term coenospecies. In complex cases the genecological approach is inadequate and the ‘deme’

terminology was devised as a solution. It avoids formal names. The

core of this terminology is the neutral suffix ‘-deme’, to which is

attached one or more prefixes that imply restricted applications of

the complete term. The prefixes used are based on standard terms

used in taxonomy and ecology. The suffix ‘-deme’ does not imply that

the plants in question form a population. The following is a list of

the major categories of demes and their subtypes.

Denoting an association with a specific locality and/or habitat

Topodeme: occurring in a specified geographical area

Ecodeme: occurring in a specified habitat

Denoting phenotypic and/or genotypic difference

Phenodeme: differs from others phenotypically

Genodeme: differs from others genotypically

Plastodeme: differs phenotypically but not genotypically

Denoting reproductive behaviour

Gamodeme: individuals that interbreed naturally

Autodeme: composed of predominantly autogamous individuals

Endodeme: composed of predominantly closely interbreeding

dioecious plants

Agamodeme: composed of predominantly apomictic plants

Denoting variational trends

Clinodeme: one of a series of demes, which collectively show a

specified trend, or cline



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In practice, depending upon

context, it is necessary to

recognise several different kinds of

species:

successional species

(palaeospecies),

microspecies (agamospecies),

biological species (genetical

species),

taxonomic species

(morphological species;

phenetic species),

biosystematic species

(ecospecies; coenospecies).



In 1926 I called attention to

another important similarity, which it

seems to me, greatly strengthens

the comparison between plant

community and organism – the

remarkable correspondence

between the species of a plant

community and the genes of an

organism, both aggregates owing

their ‘phenotypic’ expression to

development in the presence of all

the other members of the aggregate

and within a certain range of

environmental conditions.

A. G. Tansley, The use and abuse of

vegetational concepts and terms,

Ecology, 16: 3 (1935), 284–307.



Morphological similarities and differences that could be used to

delineate species are difficult to describe objectively. Plant species may

be genetically very similar yet reproductive barriers exist, preventing

hybridisation. Indeed, some taxa such as Musschia (Campanulaceae)

differ radically in morphology from their putative relatives, yet are

closely related genetically. This is an example of morphology being

out of phase with the plant’s genome.

Hybridisation between morphologically highly distinct entities is

particularly common in some groups such as the orchids. Allopolyploidy can lead to multiple origins of a new species (polytopic origin),

while, among asexual populations, distinct phenotypes may persist

indefinitely, for example Limonium (Plumbaginaceae). All these examples make it clear that the BSC is difficult to apply to plants. As with

animals, distinctive allopatric populations, especially if they also have

distinctive ecological requirements, are usually treated as species, but

the evidence is generally inferential. Transplant experiments such

as those done by Clausen, Keck and Hiesey cannot establish criteria for species boundaries although they can provide evidence for

affinities.

For practical classification and identification purposes it is expedient to use the Taxonomic Species Concept provided one bears in

mind that, like a phylogenetic tree, it only has an approximation

to reality, though the taxonomic species corresponds precisely with

the biological species. The development of molecular techniques has

provided a measure of genetic distance between species, but at what

percentage of difference in gene sequence or genetic markers should

the boundary be placed? Anyway, if we were to recognise and name

species on the basis of differences in DNA sequences, then the whole

edifice of classification would collapse. Nevertheless, the recognition of differences in plant populations at the genetic level is profoundly important for conservation purposes, so the student of plant

evolution has, simultaneously, to operate within several relatively

independent frameworks.



8.3.6 Plant ecology

Ecological studies were also being developed independently both at

the level of individual species (autecology) and the community (synecology). The description of vegetation as communities of organisms

was made by the American ecologist Clements (1874--1945), and the

English ecologist Tansley (1871--1955), although the development of

vegetation during this period was largely through the study of succession towards a climax vegetation.

Within an autecological perspective, and following on from

the pioneering efforts of Gaston Bonnier and C. Schroeter (1926),

Clements and the Danish botanist Turesson (1892--1970) developed the

study of botanical genecology. Through their studies of the adaptation

of plants to environments, they were able to demonstrate the distinction between the effects of inherited genotypic differences between

individuals, from plastic differences moulded by the environment.



8.3 PHYLOGENY, GENETICS AND THE NEW SYSTEMATICS



Population genetics rather ignored the relationship between the

genotype and the environment, but the analysis of variance and the

study of heritability of continuously varying traits, especially by plant

and animal breeders, re-emphasised the contextual nature of the phenotype. Each phenotypic trait is determined by the genotype and the

environment (including both the external and internal environments)

acting together. The same trait could have high heritability, and be

changed by selection, or it could have low heritability, and be highly

resistant to change.

From the 1930s to the 1950s, the nature and extent of variation

within and between plant populations was investigated by Clausen,

Keck and Hiesey. Their approach was orthodox in that they believed

that local populations are the units of evolutionary change but they

saw that variation is contextual to a given environment and not

limited to average differences among individuals. Despite the fact

that they regarded macro-evolutionary change as being in accord

with environmentally correlated gene expression, their views were,

somehow, out of step with the prevailing ‘modern synthesis’. Their

findings on genera such as Mimulus (Scrophulariaceae) and Viola (Violaceae) showed that speciation in plants is a much more complex

phenomenon than the Biological Species Concept (BSC) promoted by

Mayr and Dobzhansky.



8.3.7 Voices of dissent

The ‘modern synthesis’ became the orthodoxy but some botanists

maintained a different tradition. In the newly emerging Soviet Union,

N. I. Vavilov made voluminous observation on variation in plants

at different levels in the taxonomic hierarchy, but especially at and

below the species level in grasses of economic importance. Vavilov was

particularly struck by the parallel series of variations which occur

in diverse lineages of plants, particularly those which have closer

genealogical relationships. From these observations he was able to

formulate his famous ‘Law of Homologous Series in Variation’, and

to predict the presence or absence of particular traits in populations.

Vavilov was laying the foundation for population phenetics, which,

in a sense, was ahead of its time owing to the lack of genetic techniques. But he drew attention to the phenomenon of repetitious variation across diverse lineages that could not be explained adequately

by natural selection acting on random mutations.

Vavilov, who was influenced by William Bateson, was well aware

of the implications of Goethe’s pioneering efforts, but he never got

the opportunity to develop his ideas. Tragically, in 1940, he was incarcerated in Saratov prison for daring to criticise Lysenko, and he died

there three years later. Since his official rehabilitation, the extent of

Vavilov’s immense contributions to botany can be fully appreciated.

His book, Centers of the Origin of Cultivated Plants (1926) remains a classic

for the study of crop plants, but he is also recognised as the foremost

biogeographer of his time during which he participated in over one

hundred expeditions to almost every corner of the globe.



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. . . genera more or less nearly

related to each other are

characterized by similar series of

variation with such regularity that,

knowing a succession of varieties in

one genus . . . one can forecast the

existence of similar forms and even

similar genotypical differences in

other genera.

Vavilov, 1922



In the momentum of the new orthodoxy, research on reaction

norms and phenotypic plasticity was largely ignored since it was

viewed as of minor importance in evolution, and was thought to have

little, if any, genetic basis. Simultaneously, the rational morphologists were severely criticised or, at worst, ignored. However, botanical

morphologists such as Arber, Troll, Zimmermann and Willis, empirical saltationists such as Goldschmidt and Schmalhausen, and the

palaeontologist, Schindewolf, didn’t exactly disappear overnight, but

continued to hover on the periphery of the new orthodoxy. Richard

Goldschmidt felt that natural selection was only part of the evolutionary story and relevant mostly to micro-evolutionary events. The bulk

of his research focused on the causal factors of macro-evolution, the

origin of species and phenotypic novelty, and resulted in his book, The

Material Basis of Evolution (1940), from which the unfortunate phrase

‘hopeful monster’ originated.

There is more to Goldschmidt’s legacy than this hopeless caricature. His work, especially on plants, convincingly demonstrated that

plants can survive and reproduce, even when their morphology is

far from the norm. He saw that different environments will produce

different phenotypes, and he coined the term phenocopy for those

plants whose phenotypes resemble the effects produced by known

mutations. One of the effects of mutation on plant development that

Goldschmidt recognised was the phenomenon of homeosis, which we

discussed in Chapter 2. Many of Goldschmidt’s ideas have relevance

today in modern studies of ontogenetic contingency and phenotypic

plasticity. In a modified form (Neo-Goldschmidtian) his theories have

resurfaced in the writings of van Steenis and, more recently, those of

Bateman and Dimichelle.

Schmalhausen emphasised the importance of changes in the

ontogeny of organisms for the evolution of form involving, initially,

alterations in the norms of reactions by direct environmental influence, and later by the mediation of genes. Variations in critical environmental parameters may invoke morphological changes that may

be interpretable as adaptive or not. For example, the leaf morphology of many aquatic plants such as Ranunculus fluviatilis (Ranunculaceae) is correlated with their ability for gaseous exchange in air or in

water.

Schmalhausen also drew the important distinction between those

plants which display phenotypic plasticity and those which do not

(i.e. ‘normal’ phenotypes or wild-type reaction norms). He referred to

the latter as displaying the effects of stabilising selection. Unfortunately, he also used the term in another sense for a two-stage process

whereby a plant is able to utilise other parts of its range of reaction

norms to accommodate or harmonise itself to changed conditions,

and eventually becomes selected for a new stable state. Schmalhausen

hypothesised that the mechanism responsible for this reshuffling

of reaction norms is differential allelic sensitivity due simply to

the biochemistry or physiology of the plant. The overall effect is



8.3 PHYLOGENY, GENETICS AND THE NEW SYSTEMATICS



eventually incorporated into the more complex genetic regulatory

system, a process which is essentially the same as the ‘canalisation’

of Waddington. The difference between the two stages of the process

is that the first is a ‘reaction’ to the environment whereas the second

is ‘anticipatory’ of the environment



8.3.8 The natural philosophy of plant form

Outside of mainstream botany in the first half of the twentieth century there were several developments that had their origins from

pre-Darwinian times, and from the period immediately following the

publication of On the Origin of Species. A structuralist programme was

largely missing from the modern synthesis, but several individuals

may be singled out as having had a major influence on structural

botany, even though they are not always given due recognition for

their contributions.

Wilhelm Troll (1897--1978), was who was a student of K. von Goebel,

was an idealist and rigid typologist, and could be said to have adhered

most closely to the natural-theology of the previous century, even to

a Platonic world-view. He rejected common descent as the basic explanation for systematic categories because his types, which represented

the fundamental order of Nature, were invariant. This was probably

the greatest weakness in his whole research programme. Although

his approach was empirical, he only accepted evolution as occurring through major saltational changes in the types. For Troll, typology was the predominant and fundamental procedure of systematics,

whereas phylogeny merely provided a means of tracing genealogical

lineages.

Troll’s view was based on two static type categories: organisation,

which is a metaphysical idea, and form, which is the perceivable form

or phenomenology of the organism. He was inspired by Goethe’s belief

in the ‘unity behind diversity’, but he lacked a dynamic perspective.

His morphology also reflects Goethe’s dualistic view of a universal reference system (archetype) and its many manifestations (Gestalten). His

organisation type (or Bauplan) was equivalent to Goethe’s archetype,

but differed in being discrete from other organisation types. Intermediates were not recognised, each organisation type had to be one thing

or another, and within each organisation type, a variety of forms or

‘Gestalten’ could be recognised that differed only in proportions. This

was Troll’s ‘Principle of Variable Proportions’.

His concept of homology, which was the natural outcome of his

purely structuralist approach, was based on relative position rather

than identity through phylogenetic relationship. He used the term

‘Gestalt’ for the outer appearance of forms whether analogous or

homologous, and he held the view that Gestalten could not be subdivided into component parts without loss of identity. They are therefore beyond analysis. Because of the diversity of flowers and inflorescences and their repeated convergences and parallelisms across

unrelated groups, Troll believed that Gestalt is independent of the



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underlying archetype and functional constraints. From an evolutionary perspective it is easy to see the difficulty that Troll must have

experienced in explaining botanical phenomena. He hypothesised

an ‘Urge to Form’ in order to explain analogous similarities among

diverse plant groups, which could be interpreted as vitalism. Despite

the inevitable negative consequences of Troll’s rigid typological system, there were a few hidden gems. By carefully documenting in diagrammatic form the characters of a great wealth of plants, especially

their inflorescences, he introduced a reference system for all parts of

the plant thus providing the first general view of plant diversity and

a scientific procedure for abstracting general rules from individual

plants.

Walter Zimmermann (1892--1980), whose main interests being

plant phylogeny and evolution, rejected metaphysical influences,

and regarded archetypes as genealogically related natural groups

whose fluid nature was established through phylogenetic analyses.

He believed in a strict distinction between subject and object and in

this respect he could be said to be more in line with an empirical

scientific approach rather than the idealistic views of Troll, or even

the ‘gentle empiricism’ of Goethe. For Zimmermann, rational analysis was the preferred scientific procedure. He distinguished between

‘Natural Laws’, which are intellectual abstractions and the bases for

hypotheses, and ‘Natural Regularities’, which are observable phenomena. In contrast to Goethe’s Urpflanze, Zimmermann’s hypothetical

archetypes were potentially real plants of the past. Zimmermann had

a major impact on the thoughts of Willi Hennig and the subsequent

development of cladistics

The upsurge in phylogenetic emphasis after Darwin, in combination with a strong appreciation of biological form and evolution,

prompted a diversity of evolutionary morphological studies. In the

late nineteenth century zoologists such as Ernst Haeckel were promoting the famous ‘Ontogeny Recapitulates Phylogeny Hypothesis’,

while, in botany an associate of Haeckel in Jena, Eduard Strasburger,

was continuing the pioneering work of Hofmeister. In Britain, F. O.

Bower also did much to advance knowledge of alternation of generations, particularly in ferns. In his Origin of a Land Flora (1908), Bower

adopted an evolutionary-adaptive perspective on alternation.

Agnes Arber (1879--1960) was perhaps one of the greatest visionaries of the botanical world in the twentieth century. She became

a renowned plant morphologist and anatomist, historian of botany,

botanical bibliographer, philosopher of biology, the first woman

botanist to be elected as a Fellow of the Royal Society of London,

and the first woman to receive the Gold Medal of the Linnean Society of London. She made original contributions to botany that, like

her two main contemporaries in Germany (Wilhelm Troll and Walter

Zimmermann), have largely been bypassed by the rise of molecular

systematics and developmental genetics. Her approach to plant morphology was developmental and dynamic, but it also allowed the

nature of the investigation process to be revealed. She treated plant



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3 Phylogeny, genetics and the New Systematics

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