Botanical Variation, Varieties, and Cultivars (Redbud example)

Pink and White Flower Variation in Redbud

• The transcript opens with a classic example: a pink-flowered redbud (Cercis canadensis) is the common, prevalent form, while a white-flowered form appears as a rare variation.
• Distribution in the story: pink flowers are universal among offspring; white flowers show up only rarely in a seedling.
  • Quantitatively, pink is 99.9% of the time; variation to white occurs in less than 1% of cases. $99.9\%$ vs <1\%".
• A key practical question arises: if pink flowers fall off and a white-flowered seedling grows, does that change the classification of the group? The instructor uses this to motivate the idea of self-perpetuating variation and the concept of a botanical variety.

Core concepts: self-perpetuating variation, varieties, and variants

• Self-perpetuating: a characteristic that, when present, is inherited and appears in future generations without human intervention.
  • If a variant trait does not perpetuate through seeds and generations, it cannot justify a botanical variety.
• Botanical variety: a variation within a species that displays a unique and inheritable characteristic and yields seeds that grow into offspring that look like the parent, across generations.
  • Definition (summarized): a group within a species with unique, inheritable traits, and the trait is self-perpetuating across generations.
• If a variant is not self-perpetuating, it is called a variant plant (not a botanical variety).
• The key questions to distinguish a variety from a simple variant:
  • Is the trait self-perpetuating across generations? Does it reliably show up in all future generations?
  • Does the variant come true from seed? If yes, it supports classification as a botanical variety; if not, it may be just a variant or non-perpetuating variation.
• In the redbud example, the white-flowered form is treated as a botanical variety when it is self-perpetuating and comes true from seed; if one individual shows a different color but the trait doesn’t persist in offspring, it undermines the variety status.
• The instructor emphasizes there’s a spectrum of gray areas in taxonomy: deciding whether a given variation constitutes a separate species, a variety, or something else can be subjective and dependent on expert judgment.

Terminology and binomial nomenclature: varieties vs. varieties’ epithets

• Binomial naming framework: Genus species (and, for varieties, an additional epithet).
• Within a species, a botanical variety is expressed with a varietal epithet attached to the species, signaling the within-species variation.
• Example discussed: the white-flowered form of Cercis canadensis (redbud).
  • Genus: Cercis
  • Species: candidensis (note the correct specific epithet is actually canadensis in this example)
  • Varietal epithet: alba (meaning white)
• Stand-alone vs. formal botanical formatting:
  • Botanical convention: Cercis canadensis var. alba (the varietal epithet alba is in lowercase and often abbreviated as var. alba).
  • In horticulture, the term “variety” is used more loosely and the abbreviation var. alba is sometimes omitted in casual usage, but botanists typically prefer including the varietal marker (var.) to signal the rank.
• In the classroom convention presented:
  • The varietal epithet alba is in the third position (Cercis canadensis alba). This third position signals the varietal epithet rather than the species epithet.
  • The specific epithet is canadensis (the second word in the binomial). If one only has Cercis canadensis alba (without var.), it is treated as a standalone botanical variety within this teaching context.
• Test-taking guidance provided:
  • If the binomial points to Cercis (the genus), choose the genus.
  • If it points to canadensis (the species epithet), choose the species.
  • If it points to alba (the varietal epithet), choose the varietal epithet (not the specific epithet).
• Nomenclature rules highlighted in the lecture:
  • Varietal epithet is lowercase.
  • In botanical writing, the varietal epithet is often written as a separate word with an abbreviation like “var.” (e.g., Cercis canadensis var. alba).
  • Some horticulturists treat “var.” as redundant in casual usage, but botanists consider it important for rank designation.
• Summary of the naming conventions shown:
  • Botanical variety name (stand-alone): Cercis canadensis alba (in teaching terms, stand-alone variety) vs. the more formal Cercis canadensis var. alba.
  • The element alba serves as the varietal epithet (third position) and denotes whiteness of the flowers.
  • The specific epithet canadensis designates the species; genus Cercis designates the genus.

Case study: Cercis canadensis alba (white redbud) and related concepts

• The pink form is treated as the “straight species” with pink flowers and a high probability of passing pink color to offspring.
• The white form is introduced as a botanical variety within Cercis canadensis because it:
  • displays a unique and inheritable trait (white flowers),
  • the trait is self-perpetuating (produces white offspring in future generations), and
  • comes true from seed (seedlings are white when produced by white-flowered parents).
• The white form is labeled as a botanical variety (Cercis canadensis alba) with the varietal epithet alba.
• The common name discussed: “white redbud” (though a common name like white bud is mentioned; the lecture notes that common names can be less precise).
• If a white-flowered tree is found naturally but a second generation does not consistently stay white from seed, the group cannot be a botanical variety and would be treated as a variant or a non-permanent variation.

When variation is not self-perpetuating: the non-variety scenario

• A natural white occurrence might not persist as a variety if it cannot be reproduced true from seed.
• If vegetative propagation (cloning via cuttings) is used to create a population of blue-flowered trees from a single blue specimen, this creates a cultivar rather than a botanical variety.
  • Vegetative propagation: taking plant parts (e.g., cuttings) to generate new, genetically identical individuals.
  • In the blue-flowered case, all propagated plants are genetically identical and blue, but because the trait did not arise through natural seed-perpetuating variation, the group is not a botanical variety.
  • The result is a cultivated variety, or cultivar, maintained through human intervention.
• The taxonomy implication: even if the blue color could be selected in the wild, creating a stable, seed-propagating blue population would still be a cultivar if humans maintain it via cultivation.
• The instructor emphasizes: most variations that occur naturally do not come true from seed; cultivation is often required to maintain a desirable trait.

Cultivar concept: human involvement and market relevance

• Cultivar definition (as presented): a cultivated variety developed and maintained by humans through selection and propagation, not by natural seed-perpetuating variation.
• The blue-flowered group arising from human-driven propagation is a cultivar, not a botanical variety, because:
  • it required human intervention to propagate and stabilize,
  • the trait is maintained via vegetative propagation rather than seed-based inheritance.
• Nomenclature note: in horticulture, the term cultivar is widely used to designate such cultivated varieties; in this class, cultivars are discussed as a distinct concept from botanical varieties.
• Practical takeaway: in modern horticulture, cultivars dominate plant production (the lecturer notes that well over 80% of cultivated plants are cultivars).
• Monday’s plan: a deeper dive into cultivars, how to write cultivar names, and related conventions.

Ethnicity, ethics, and practical implications of inducing variation

• The discussion includes several ways plant variation can arise:
  • Natural mutations (mutations can occur spontaneously).
  • Mutagenesis, including induced mutations via chemicals or radiation, to create variation quickly for potential commercial gain.
• Ethical and practical concerns raised:
  • Inducing mutations via radiation or chemicals may not guarantee beneficial or desirable traits.
  • There are ethical questions about manipulating genomes and plant characteristics for profit.
  • The speaker notes that some scientists even propose radiating plants to generate variations, which can be controversial.
• The key takeaways: while mutations and inductions can create new traits, not all such traits are beneficial, and the implications (ethical, ecological, economic) require careful consideration.

Pedagogical digressions and memory aids used in the lecture

• Sesame Street anecdote:
  • The lecturer uses a humorous Sesame Street clip about letters (p, l, a, y) coming together to spell and teach reading, illustrating how people learn and remember concepts.
  • An anecdote about the author’s family and the Martians from Sesame Street, underscoring how people respond to familiar culture when learning new topics.
• Climatology anecdote:
  • A professor demonstrates that the color of the sky is not as fixed as it seems, encouraging skepticism and the scientific habit of questioning authority and testing observations.
  • The moral: critical thinking and verification are essential in science.
• These digressions illustrate how science can be taught through narrative, humor, and relatable examples, and they reinforce the idea that classification and interpretation in biology require careful reasoning rather than rote memorization.

Key takeaways: definitions, rules, and practical naming tips

• Key definitions:
  • Botanical variety: a within-species group with unique, inheritable, self-perpetuating characteristics that come true from seed.
  • Variant plant: a non-self-perpetuating variation within the species; not sufficient to define a variety.
  • Cultivar (cultivated variety): a human-maintained, cultivation-based group with a stable trait, often propagated vegetatively to maintain uniformity.
• Rules of nomenclature (as discussed in class):
  • Genus name: capitalized (e.g., Cercis)
  • Species epithet: lowercase (e.g., canadensis)
  • Varietal epithet: lowercase and in the third position when written as a variety (e.g., alba), often accompanied by the rank marker var. (Cercis canadensis var. alba).
  • In practice for horticulture, many people simply say “the variety alba” and use “alba” as the varietal epithet, but botanists prefer the explicit var. marker.
• Practical implications for plant papers and exams:
  • If the question asks for the part of the binomial corresponding to the varietal epithet, alba is the varietal epithet when presented as Cercis canadensis alba (or var. alba when written in botanical form).
  • If the question asks for the species epithet, canadensis is the specific epithet.
  • If the question asks for the genus, Cercis is the genus.
• Example recap: Cercis canadensis alba
  • Genus: Cercis
  • Species: canadensis
  • Varietal epithet: alba
  • Botanical form (preferred by botanists): Cercis canadensis var. alba
  • Horticultural shorthand (common in practice): Cercis canadensis alba (stand-alone variety) or simply “the alba variety” in casual usage.
• Practical note on content density:
  • The pink form is the baseline (the straight species) with high fidelity to pink across generations.
  • The white form is a case study in how variation can be classified, named, and managed in horticulture.
  • The emergence of a blue-flowered variant demonstrates how human cultivation can produce a cultivar, not a true botanical variety.
  • The majority of cultivated plants are cultivars managed by human cultivation rather than true naturally stable varieties.

Quick reference: summary checks for exams

• If a variation is self-perpetuating and comes true from seed: treat as botanical variety (name as Cercis canadensis var. alba; or Cercis canadensis alba in horticultural shorthand).
• If a variation is not self-perpetuating or does not come true from seed, it is a variant plant (not a variety).
• If humans must propagate vegetatively to maintain the trait (e.g., blue flowers via cuttings), this is a cultivar (cultivated variety).
• Expect test questions to ask you to identify genus, species, or varietal epithet from a binomial, so recall: genus (Cercis), species/canadensis, varietal epithet alba (and possibly the rank var.).
• The broader lesson: classification in botany involves judgment, evidence of inheritance, and the method of propagation. There are gray areas, and ethics in mutation and cultivation are important considerations beyond mere labels.