Species and Varieties, Their Origin by Mutation Informative Summary

Overview:

Hugo de Vries, a Dutch botanist, challenged the prevailing Darwinian theory of gradual evolution in his book, “Die Mutationstheorie.” De Vries proposed that new species and varieties arise through sudden, discontinuous jumps, which he termed “mutations.” This theory, known as mutationism, offered a different explanation for the process of evolution.

De Vries supported his theory with extensive experimental observations on the evening primrose (Oenothera lamarckiana), demonstrating that this plant consistently produced new types with distinct characteristics. These new forms were not simply variations within the original species, but rather new, stable entities, analogous to species. This book, presented as a series of lectures delivered at the University of California in 1904, expands on de Vries’ theory of mutationism, providing a wealth of examples from nature and horticulture to illustrate his arguments.

Key Findings:

  • Mutations are a significant force in evolution: De Vries argues that new species and varieties arise through sudden leaps, termed mutations, rather than gradual changes.
  • Mutations are not random: They often occur in groups, suggesting a specific underlying mechanism.
  • Mutations are inheritable: The new forms produced by mutations are stable and transmit their characteristics to subsequent generations.
  • Mutations occur in all directions: New species can be more robust, less robust, or exhibit a wide range of modifications, offering a variety of possibilities for natural selection.

Learning:

  • Understanding the difference between elementary species and varieties: De Vries distinguishes between elementary species, which are stable and distinct from their ancestors, and varieties, which are derived from their parent species through the loss or gain of a single characteristic.
  • Exploring the concept of latent characters: Some characteristics of a plant can be dormant or latent, only appearing under specific conditions or in rare instances. De Vries explains how latent characters can reappear through atavism.
  • Appreciating the role of vicinism in horticultural practices: Vicinism, or variation influenced by neighboring plants, is a significant factor in the instability of horticultural varieties. This concept challenges the traditional notion of atavism as an innate tendency.
  • The impact of nourishment on variability: The amount and quality of nourishment can significantly influence the expression of traits, both in terms of size and the development of anomalies.
  • The importance of understanding mutation periods: De Vries suggests that evolution occurs in periods of mutability, during which new species arise rapidly, followed by periods of stability.
  • The limitations of artificial selection: While artificial selection can be used to improve traits within a species, it cannot create new species. Furthermore, improvements achieved through artificial selection are often temporary and require continuous selection to maintain.

Historical Context:

This book was written in 1906, a period of intense debate surrounding Darwin’s theory of evolution. While the concept of evolution was generally accepted, the mechanisms by which it occurred were still widely debated. De Vries’ theory of mutationism offered a new and challenging perspective, proposing a different mechanism for the origin of species.

Facts:

  • Species are not static entities: They are comprised of smaller, stable units called elementary species and varieties, which can be distinguished and isolated.
  • Cultivated plants are often mixtures of different forms: These forms can be distinguished and isolated by variety-testing, a process that is crucial for improving cultivated plants.
  • Vicinism is a significant factor in horticultural practices: Bees and other pollinators frequently cross-pollinate plants, leading to the appearance of “atavistic” traits in cultivated varieties.
  • Characters can become latent: They can be reactivated through mutations, leading to the appearance of atavistic traits in plants.
  • Double adaptations are common in nature: Plants can develop different traits depending on their environment, such as aquatic vs. terrestrial forms.
  • Mutations often occur in groups: This observation supports the idea that mutations are not random but are triggered by a specific underlying mechanism.
  • Mutations can be replicated: The same mutation can occur in different individuals of a species under varying conditions, suggesting a universal underlying tendency.
  • Mutations are typically sudden and complete: New forms appear instantly with all the characteristics of the new type, without gradual transitions.
  • Mutations can occur in all directions: New species can be more robust, less robust, or exhibit a wide range of modifications, offering a variety of possibilities for natural selection.
  • Some mutations are more readily observed than others: Double flowers and variegated leaves are relatively common, while others, like the “five-leaved” clover, are more rare.
  • The frequency of mutations is influenced by environmental factors: Favorable conditions tend to increase the expression of anomalies, while unfavorable conditions can suppress or limit their appearance.
  • Double races (ever-sporting varieties) are often more variable than single varieties: They exhibit a range of variation between two opposing traits, such as striped vs. non-striped flowers.
  • Double races are often more sensitive to selection than single varieties: Continued selection in a specific direction can greatly influence the expression of the desired trait.
  • Mutations can occur in both sexual and asexual propagation: Bud sports, like those observed in the scarlet ribes, can be just as stable and inheritable as mutations arising from seeds.
  • Hybrids of elementary species can be stable: These hybrids inherit a combination of traits from both parents and often remain constant for multiple generations.
  • Hybrids of varieties typically split in subsequent generations: This splitting is governed by Mendel’s law, which explains the segregation of traits in the progeny.
  • Mendel’s law applies to balanced crosses, where traits are present in both parents: This law explains the re-appearance of parental traits in the offspring of a hybrid.
  • Many garden plants are hybrids: They were originally derived from crossing different species and varieties.
  • Many mutations are considered detrimental: They reduce the plant’s ability to compete in the wild, explaining why these forms are rarely seen in natural settings.
  • The influence of nourishment on variability is paramount: Stronger, better-nourished plants tend to produce more robust offspring and often exhibit more significant expressions of anomalies.
  • The concept of a “sensitive period” for mutations: During this period, external influences can significantly affect the expression of a trait.

Statistics:

  • Over 200 elementary species of Draba verna exist: This exemplifies the diverse range of variation possible within a single species.
  • The “Galland” wheat rapidly deteriorated when grown near local varieties: This experiment illustrates how vicinism can rapidly overwhelm the characteristics of a selected variety.
  • In experiments with the wild oat (Avena fatua), this weed rapidly outcompeted cultivated oats in unfavorable years: This demonstrates the powerful influence of natural selection.
  • The number of “five-leaved” clover leaves varied on a single plant: This shows the significant influence of environmental factors on the expression of anomalies.
  • The proportion of “five-leaved” clover seedlings varied depending on the parent plant: This highlights the importance of heredity in determining the expression of traits.
  • A hybrid between Oenothera biennis and O. muricata remained constant for several generations: This demonstrates that hybrids of elementary species can be stable and not simply a temporary blend of traits.
  • The “Nepaul-barley” produced a high percentage of abnormal spikes: This is a striking example of a persistent, stable anomaly that does not diminish over generations.
  • In a controlled experiment with the opium poppy, 22.5% of the hybrid offspring exhibited the recessive trait: This result supports Mendel’s law of inheritance.
  • The “Tuscarora” corn rapidly lost its characteristics when grown near European corn: This highlights how vicinism can lead to the rapid replacement of one variety by another.
  • The “Minnesota No. 169” wheat yielded 1-2 bushels more per acre than its parent variety: This demonstrates the significant impact of intraspecific selection.
  • The beach-plum (Prunus maritima) exhibits a wide range of variations in fruit characteristics: This indicates the existence of distinct races or elementary species within a single systematic species.
  • The wild Ipecac spurge (Euphorbia Ipecacuanha) shows significant variations in leaf shape and seed characteristics: This suggests the existence of numerous elementary species within the overall species.
  • In experiments with flax, seeds exhibiting a specific pattern yielded pure strains of differing cultural value: This highlights the importance of specific characteristics within a species.
  • The coconut has over 50 different varieties: This further demonstrates the diversity of variation possible within a single species.
  • The “Swan” variety of the opium poppy yielded a small percentage of single-flowered and red-flowered plants: This shows how vicinism can lead to impurities within a cultivated variety.
  • The percentage of tricotyls in a culture could be significantly influenced by selecting parent plants with high hereditary percentages: This demonstrates the effectiveness of choosing parents based on their breeding ability.
  • The “Washington” tomato variety arose suddenly from seeds of the “Acme” variety: This is an example of a mutation occurring in a stable, cultivated variety.

Terms:

  • Elementary species: A subdivision of a systematic species that is stable and distinct from its ancestors.
  • Retrograde varieties: Varieties that are derived from their parent species through the loss of a specific character.
  • Mutations: Sudden, discontinuous jumps in the genetic makeup of an organism that result in new, stable forms.
  • Unit-characters: Discrete, inheritable traits that can be combined and separated in sexual reproduction.
  • Latent characters: Characteristics that are dormant or inactive in an organism but can be reactivated under certain conditions.
  • Atavists: Individuals that exhibit traits of their ancestors, usually due to the re-activation of latent characters.
  • Vicinism: The phenomenon of variation in a variety under the influence of neighboring plants, caused by cross-pollination.
  • Fluctuating variability: The continuous variation of traits within a population, driven by environmental factors.
  • Individual fluctuation: Fluctuations in traits that are primarily determined by the environment during the embryonic stage.
  • Partial fluctuation: Fluctuations in traits that are primarily determined by the environment during the development of the organism.
  • Double races (ever-sporting varieties): Varieties that exhibit a continuous range of variation between two opposing traits.
  • Hereditary percentage: The proportion of offspring that exhibit a specific trait, used as a measure of breeding ability.
  • Tricotyls: Seedlings with three or more cotyledons, an anomaly that is often inheritable.
  • Syncotyls: Seedlings with their cotyledons fused, another common anomaly.
  • Peloria: A morphological anomaly in which a zygomorphic flower becomes regular or symmetrical, with five equal spurs.
  • Petalody: The transformation of stamens into petals, a common phenomenon in double flowers.
  • Sepalody: The transformation of petals into sepals.
  • Ascidia: Pitcher-like structures formed by the union of the margins of a leaf.
  • Peltate leaves: Leaves with the stalk attached to the center of the blade.
  • Double adaptations: The ability of an organism to develop two distinct forms depending on the environment.

Examples:

  • The white-flowered currant (Ribes sanguineum) occasionally reverts to the red-flowered type: This is an example of bud atavism, a rare phenomenon.
  • The “Mephisto” and “Danebrog” varieties of opium poppy when crossed produced a hybrid with the dominant black heart trait: This exemplifies Mendel’s law of inheritance.
  • The “Tuscarora” corn rapidly lost its characteristics when grown near European corn: This is a classic example of how vicinism can lead to the loss of a variety.
  • The “Wealthy” apple was produced by a single seedling: This is a case of a new variety arising through a mutation.
  • The water persicaria (Polygonum amphibium) exhibits two forms, aquatic and terrestrial: This is a striking example of double adaptation.
  • The wild camomile (Matricaria Chamomilla) has a form lacking ray florets: This is an example of a retrograde variety, or a species that has lost a specific trait.
  • The “five-leaved” clover exhibits a strong periodicity in the development of extra leaflets: This is an example of how the expression of anomalies can be influenced by developmental stage.
  • The opium poppy exhibits a strong sensitivity to external factors: This is a classic example of how the environment can influence the expression of traits.
  • The “Nepaul-barley” has an anomalous flower situated on the inner scale of the spikelet: This is a striking example of a stable, persisting monstrosity.
  • The laciniated variety of the greater celandine (Chelidonium majus) was discovered in a garden: This is a well-documented example of a mutation appearing in a cultivated plant.
  • The “Giant of Zuidwijk” strawberry, a tall variety, was produced from a single seedling: This is a clear case of a mutation arising in a cultivated plant.
  • The “Paradox” blackberry is an example of a highly desirable hybrid produced through the selective breeding of over 40,000 hybrids: This illustrates the effectiveness of hybridization in producing new varieties.
  • The “Madame Crozy” canna hybrid, with its large and bright flowers, is a testament to the power of selection: This demonstrates how selection can rapidly improve desired traits in a cultivated plant.
  • The “Washington” tomato, an upright variety, arose from a mutation in the “Acme” variety: This demonstrates the potential for mutations to occur even in highly stable varieties.
  • The “one-leaved” strawberry was discovered both in the wild and in a cultivated setting: This highlights the potential for the same mutation to arise independently under different conditions.
  • The “Nepaul-barley” has a supernumerary floret, considered a useless trait, yet is highly stable: This is a classic example of a mutation that persists despite not appearing to have any adaptive value.

Conclusion:

De Vries’ book, “Species and Varieties, Their Origin by Mutation,” presents a compelling argument for the significance of mutations in the evolutionary process. His research on the evening primrose demonstrates that new species can arise through sudden, inheritable jumps, rather than gradual changes. This concept challenges the prevailing Darwinian theory and opens a new avenue for understanding the mechanisms of evolution.

While de Vries emphasizes the role of mutations, he also recognizes the importance of selection in shaping the diversity of life. He highlights the importance of variety-testing, intraspecific selection, and the principle of breeding ability in shaping both wild and cultivated populations.

De Vries’ work, although published over a century ago, continues to inspire contemporary research in evolution and genetics. His insights into the nature of variability and the power of mutations provide a valuable framework for understanding the dynamic and complex process of evolution.

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