Theories of Evolution (Part-III)

Hugo de Vries’s theory: The theories of Lemarck or Darwin is based on the population study but both theories could not be able to explain the origin of variation and their mode of transmission from one generation ot next generation. To understand the gap, dutch botanist, Hugo de Vries has put forward mutation theory in 1901. According to the De Vries mutation theory state that new species arise from the pre-existing one in a single generation by sudden appearance of new features through a genetic variation known as mutations. Contrast to earlier theories (Lemarck or Darwin), De Vries proposed that evolution is a sudden, discontinuous and jerky process rather than continuous and gradual. He termed the process as Salation ie single step large mutation. In addition, natural selection works on mutation, preserve the mutations found useful and eliminates the mutants with harmful mutation. But he didn’t support the struggle between the organism considered the co-existenance of them with the parent species (non-mutated species).

De Vries Experiment: To test the proposed mechanism of evolution, Huge De Vries conducted experiment on plant called as Evening Primrose (Oenothera lamarckiana). The outline of the experiment is given in Figure 91. He observed the subtle but significant difference between different wild type varients. He took normal plant and collected the seeds through self pollination. He found that majorty of seeds were normal like their parents except few one. These seeds were quite different from there parents. The plants with variants were true to give rise to plants with similar characterstics. In 2nd generation, it breeds to give plants with majority of plants similar to their parents but few plants with different varients. Hence, he observed that in each generation, majority of the offsprings are similar to their parents but it gives few off springs with varients. In addition, he found that mutations appeared suddenly and were inherited by offsprings. De Vries found 4 different types of plants:

(i) Progressive, with newer traits.

(ii) Retrogressive, that had reduced or lost traits compared to their parents.

(iii) Degressive, plants with weak with low survival.

 
(iv) Inconstant, these plants are unstable and they resembles parents as well as at times produced variants.