PLB 449 1997

From Ray to Darwin

Chapter 2

I. A bit of review (see PLB 304 lecture notes here)

A. How did plant descriptions, naming, and classifications come about? All of these functions reflect the needs, level of knowledge, philosophical concepts and available technology of the historical periods.

B. Classification systems can be grouped into five categories:

1. Utilitarian. System based upon the uses of the plants (food, drug, fiber, construction, etc.). Examples: Dioscorides and the Herbalists such as Fuchs.

2. Form. System based upon the habit of plant, e.g. trees, shrubs, herbs. Examples: Theophrastus and Caesalpino.

3. Artificial. System based upon a few characters, e.g. number of stamens, flower color, etc. Examples: Tournefort and Linnaeus.

4. Natural. System based on many characters considered together (i.e., the overall resemblance). Examples: Bauhin, Ray, de Jussieu, de Candolle, and Bentham & Hooker.

5. Phylogenetic. System based on evolutionary relationships. Examples: Engler & Prantl, Bessey, Hutchinson, Cronquist, Takhtajan, Thorne, and Dahlgren.

This chapter does not cover early utilitarian and form-based classifications but begins with Ray whose concepts fall under the natural system. Discussion begins with early concepts that involved transmutation, i.e. direct conversion of one species into another.

II. Ray and the definition of species.

A. John Ray (1628-1705). A British botanist, his most famous work: Synopsis Methodica Stirpium Britannicarum the first British flora. He also published Methodus Plantarum Nova (1682) and Historia Plantarum (1696-1704). His natural system grouped together plants that looked alike, but was complicated in that it used many different characters and split natural groups and lumped together unnatural ones. He believed in "essential" characters. Last edition of Methodicum treated 18,000 species!

B. Ray was probably the first to seek a scientific definition of species. No transmutation. Definition involved plants breeding true:

"In order that an inventory of plants may be begun and a classification of them correctly established, we must try to discover criteria of some sort for distinguishing what are called 'species'. After a long and considerable investigation, no surer criterion for determining species has occurred to me than distinguishing features that perpetuate themselves in propagation from seed."

C. Recognized interspecific variation in many plants:

• Polemonium caeruleum - typical blue and atypical white flowers

• Digitalis purpurea - typical purple and atypical white flowers

• Verbascum blattaria- typical yellow and atypical white flowers

• Pedicularis palustris - typical red and atypical white flowers

• Geum rivale -- typical single flowers and atypical double flowers

• Geranium sanguineum - typical erect and atypical prostrate plants

D. "Accidents" should not be given species status; the latter influenced by concept of fixity of species (specially created by God at one time - Genesis).

III. Chain of Being (Scala Naturae)

A. A Platonic concept, where God is at the top, "brute beasts" at the bottom, and humans somewhere in between. Poor plants were at the bottom of the scale! Later this concept was transformed into biological evolution by scientists such as Ray.

B. These concepts are actually still with us. Note use of the terms "higher" and "lower" to refer to plants, primates, etc.

IV. Linnaeus

A. Carl von Linné or Carolus Linnaeus (Latinized form of his name). (1707-1778). Son of a Swedish parson, born in Rashult, Sweden, May 22, 1707.

B. His most important works were Systema Naturae, Genera Plantarum, and Species Plantarum. The latter, published May 1, 1753 is the starting point for botanical nomenclature and described 5,900 species. Species names given in the margin. Description were like the old polynomials. Developed the binomial system as a short way of referring to species. He was incredibly prolific naming 12,000 species (7,700 plants, 4,300 animals). 1,105 genera.

C. Early works championed the concept of fixity of species (see quote p. 10). Variants were recognized and called "monstrosities", "sports", or "abnormal forms", but these were considered transitory and did not affect species fixity (typological). Job of taxonomist was to recognize "elemental species" with natural variation being a distraction or illusion.

D. Linnaeus may have been one of the earliest to perform experiments testing the source of variation in species. Flower color changes with addition of acid. Transplantation studies with Ranunculus aquaticus showing the differences in leaf form in water vs. wet soil. Changes induced by cultivation:

• Martagon sylvaticum -- typically pubescent, glabrous in cultivation

• Lactuca, Sphondylium, Matricaria -- typically entire margin leaves, become crisped (curly) in cultivation.

E. Linnaeus acknowledged that Species Plantarum was an artificial system, but he never completed a natural system.

F. Linnaeus' species concept changed with time. In Philosophia botanica (1751) species were fixed but varieties induced by the environment or the gardener. He showed in his writing that he could not always distinguish species and actually alludes to evolutionary origins. Some examples:

• Beta vulgaris (cultivated beet) -- derived from Beta maritima (wild beet).

• Linaria vulgaris (typical zygomorphic flower) -- peloric variant (actinomorphic flower) [note Fig. 2.2a and 2.2b are reversed in your book]. Linaria purpurea zygomorphic, L. purpurea peloric mutant.

G. Linnaeus and hybridization. In Somnus plantarum he says that "mongrel" or "hybridus" plants are frequently produced and that "if not admitted as new species, are at least permanent varieties."He is credited with the first scientific production of an interspecific hybrid, between the goatsbeards Tragopogon pratensis (yellow flowers) and T. porrifolius (violet flowers). Awarded prize for the work in 1760. This complex becomes a classic case of studying plant hybridization by Ownbey in the 1950s and later by D. and P. Soltis, both groups at Washington State University (more on this later in the course! Figure). Linnaeus even got segregation in the F2 progeny, but of course would not have been able to interpret this genetically (Mendel work not recognized until 140 years later).

H. Fundamenta fructificationis (1762). Linnaeus ascribes an almost evolutionary origin to species, with genera arising first, species later (see quote pp. 16-17).

V. Buffon and Lamarck

A. The French naturalist Buffon (1707-1788) [technically Georges-Louis Leclerc, Comte de Buffon] believed in the fixity of species but his belief wavered later in his life. Cuvier believed in the Creation with disasters where the survivors repopulated the earth.

B. The French naturalist Lamarck (1744-1829) [technically Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck]. Most remembered today as the "fellow who got it wrong", i.e. inheritance of acquired traits, but one has to read a bit deeper to realize that he had many ideas that anticipated Darwin. He produced Flore Francoise which used identification keys similar to dichotomous keys of today. Was the first to use the term biology in the modern sense. The French still honor him as the "Founder of the doctrine of evolution."

C. In Philosophie zoologique he did not believe all species were created at the same time (touché !) and described how the environment could change species AND this change was heritable. Read quote (p. 19). This is essentially a description of ecotype which we will discuss later in the course. He also uses the Ranunculus aquaticus example, but in contrast to Linnaeus, believes the environment is the direct cause of the differences in leaf shape.

D. Was Lamarck completely wrong with his theory? Read recent literature about inheritance of acquired traits (Lamarckism) and you will see this topic revisited with regard to bacterial evolution (Cairns' work). More broadly, what about mutations in DNA acquired during an individual's lifetime (Neo-Lamarckism) that become incorporated in sex cells and are thus transferred to the next generation (more on this later in the course when we talk about the Weismann Doctrine, soma vs. germ line, etc.).

VI. Darwin

A. Charles Robert Darwin (1809-1882) was born in Shrewsbury England February 12, 1809 in a family of remarkable intellectual distinction. His father was a physician and his grandfather was Erasmus Darwin, the well-known author of "The Botanic Garden." Darwin "failed to profit" at either Shrewsbury or later at Edinburgh University where he studied medicine. In 1827 he entered Christ's College, Cambridge where he developed a close relationship with John Henslow, a professor of botany. Henslow is the one who got him on the expedition of the HMS Beagle which circled the globe (ending in 1836) and brought Darwin into contact with many habitats where he studied geology, zoology, botany and more.

B. Was Darwin the first to propose evolution by natural selection? Others before him had "partial anticipations". In fact, Darwin acknowledges many of these in an introductory part of his Origins. It is clear from this section that the scientific and even some of the theological world were ready for Darwin's ideas. It has been suggested that he delayed writing The Origin of Species because of Robert Chambers' (published anonymously) Vestiges of the Natural History of Creation, which because of its evolutionary interpretation of nature, received condemnation from religious sorts. This book was very popular, however, with a grand total of 14 editions!

C. Darwin was spurred into action (20 years after the voyage of the Beagle!) to write On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life [usually abbreviated as The Origin of Species or just Origins] when in 1858 he received a letter from Alfred Russel Wallace enunciating an evolutionary mechanism nearly identical to his own. Wallace's paper was read at the Linnaean Society meeting on July 1, 1858 as were Darwin's papers and letters to Asa Gray (from 1857, see below).

D. Both Darwin and Wallace were heavily influenced the work On Population by Thomas Malthus (1831). The main idea is that populations will increase geometrically until food and other environmental factors limit this growth.

E. Main concepts of evolution with natural selection as the mechanism:

• Individuals variation (not saltations) were present in all organisms and were important for evolutionary change.

• Natural checks would limit the geometric increase in population size (Malthus).

• As a consequence of the above, only some individuals survive and these are the ones that carry particular traits that influenced their survival.

• Because those individuals with adaptive traits survive, they pass these on to the next generation (in a genetic senses, they contribute more genes to the next generation).

• This selection process continues over many generations and in a changing environment the variants replace the original organisms.

F Criticisms of Darwin's views

1. Saltation. Asa Gray believed saltation WAS an important factor. What are the differences?

2. Mechanism of heredity. Blending (which Darwin believed) vs. the Mendelian particulate mode of inheritance would lead to dilution of any variant by backcrossing with "typical" forms.

3. Effect of chance. Variants too few to affect large populations (related to above concept).

4. Limits of variation. How far can natural selection go? Can it only act on existing structures? If so, how can novel organs be formed? Classic examples discussed are eyes and orchids.

5. Role of isolation. Can speciation occur without physical isolation or is this a requirement? This topic is still active today (cf. allopatric, parapatric, and sympatric speciation) and will be a topic of discussion later in this course!

6. Age of earth. Historically, has gotten progressively greater. Biblical (4004 years), Kelvin (400,000 years), modern (4.6 billion years). With Kelvin's value, thought that there was insufficient time to produce all the biotic diversity of today using Darwin's "slow" process.

G. Darwin's view of species and classifications

1. Differed from Ray and Linnaeus which were more typological. Darwin had trouble defining species and resorted to the classic definition "a species is what a competent taxonomist says it is." Gave examples in plants where species, subspecies, etc. are difficult to define such as Rubus (blackberries), Rosa (roses), Hieracium (hawkweeds), and Quercus (oaks). We will discuss these types of species later in the course with reference to apomixis and hybridization. See quotes by De Candolle and Darwin on p. 27. He also notes that hybrids are not universally sterile. The long quote on p. 28 expresses difficulty in defining species precisely, indicating one should treat species in the same manner as genera, i.e. as artificial constructs.

2. Darwin makes it clear that natural systems of classification are not reflecting the "plan of the Creator" but are founded on descent with modification, i.e. a genealogical or evolutionary explanation. Interestingly, the only figure present in Origins is a phylogenetic tree!

3. Was Darwin religious? To some degree, Darwin did waver in response to pressure from religious groups after the first edition of Origins. For example, the phrase in brackets in the last sentence of the book was added in the second edition:

"There is grandeur in this view of life, with its several powers, having been originally breathed [by the Creator] into a few forms or into one."

VII. Tests of Specific Difference

A. Can hybridization (or not) be used as the sole criterion to determine species boundaries? Godron (1863) had such a strict system:

a X a' --> fertile offspring, a and a' are subspecies

a X b --> sterile offspring, a and b are separate species

A X B --> no offspring, A and B are separate genera

B. Jordan (1864) looked at 53 "elementary species" of Erophila [Draba] verna which differed in many traits (Fig. 2.4). Others did this too, describing hundreds of "species" of violets, dandelions, and buttercups!

From Ray to Darwin

Chapter 2

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