The genesis of species, as hypothesised by Darwin, sparked much debate. Darwin's hypothesis lacks contemporary genetics and molecular biology concepts. He could not explain how characteristics emerge and persist in a population.
J. Gregor Mendel (1822-1884) researched the Inheritance of acquired traits, but it was not recognised until 1900. When Mendel's work on "Inheritance of Acquired Characters" was found in 1900, the notion of evolutionary change was more grasped. Only after the rediscovery of Mendel's ideas was the foundation of biological reproduction and heredity established. The field of genetics grew rapidly and explosively in the 1900s.
The entire development of the chromosomal theory of inheritance unavoidably resulted in a better understanding of the role of genes as determinants of heritable features. This progress was greatly facilitated by linking genetics with another nascent discipline: biochemistry. Thus, biochemical genetics was created to understand gene functions in biochemical terms.
The most important contribution of the new discipline is, without a doubt, the one gene-one enzyme idea proposed by Beadle and Tatum based on their pioneering work with the bread mould Neurospora crassa. Because not all proteins are enzymes, this idea has been renamed the 'one-gene-one-polypeptide' theory. This topic has complexities that we cannot discuss here. However, a troubling implication is that the immediate phenotypic result of gene expression is a protein.
What should appeal to you right away, as flowing from the concept, is that just as anatomical and embryological relationships and differences have been exploited to document organic evolution and also erect phylogenetic trees, it should be possible to compare the structure of proteins - i.e. look at the ordering of their constituent amino acids-serving the same function in different organisms to seek evidence for their 'evolution'. This has been a very active topic of study.
As indicated above, studying proteins' amino acid sequences - the fundamental structure - gave rise to molecular evolution. With the discovery of the gene's structure and subsequent tools for sequencing DNA molecules, it became feasible to investigate the architecture of genes and their evolution rather than the results of their expression, mainly proteins. This is a considerable advancement in more ways than one.
For one reason, studying the evolution of genes is not the same as studying the evolution of proteins. The rationale is straightforward. Proteins are the result of structural genes being expressed. Furthermore, structural genes represent a modest proportion of overall DNA content, particularly in higher species. Thus, studying the evolution of DNA sequences is a worthwhile endeavour in and of itself, and significant breakthroughs in our understanding of biological evolution have emerged from such research.
Thomas Hunt Morgan (1866-11945) thought Darwin's theory of natural selection was flawed because it was difficult for him to understand that extensive adaptations arose in an organism merely by accumulating minor random changes. Darwin offered no mechanism to explain the genesis or inheritance of variants. Morgan dismissed Mendel's hypothesis as entirely speculative. He acknowledged that Mendel's theory could explain and predict breeding outcomes, but Mendel could not explain the inheritance mechanism.
He contended that no one chromosome could be responsible for unique hereditary features. He conducted experiments on Drosophila and presented his renowned "Chromosome Theory of Heredity." Morgan proposed the sex-limited (now termed sex-linked) theory based on his research on Drosophila. He claimed that females' X-chromosomes carried these sex-limited characteristics, which held several separate heredity units. He referred to these units of inheritance as genes (the term gene was coined by Danish botanist Wilhelm Johannsen in 1909). He believes genes are connected in series on the chromosome and responsible for inherited features. Morgan was a pioneer in the study of genetics. Boveri-Sutton recognised chromosomes as the carriers of genetic material.
Hugo de Vries stated that natural selection is not required to emerge new species. He claims quick, extreme modifications can reorganise the whole genome, resulting in new phenotypes. These abrupt alterations were referred to as mutations or saltations by him. His mutation theory was based on multiple generations of studies on Evening Primrose (Oenothera lamarckiana).
He was amazed by the sudden development of seven new types of Evening Primrose in his garden, and he discovered that all of these varieties were heritable. He noticed that the unexpected appearance of new hereditary features in Evening Primrose offered no sign of their presence in earlier generations. He referred to these mutant Evening Primrose people as mutants and the rapid alterations as saltatory mutations.
Hugo de Vries defines mutations as "rapid, large heritable changes." On the contrary, Darwin emphasised minor cumulative alterations. According to Hugo de-Vries, mutations are random and can arise in any direction. Mutations can occur in different individuals of the same species, giving rise to many new related species; they can be beneficial or destructive when exposed to natural selection, nature favours and selects favourable mutations while eliminating detrimental ones. (death of the mutants). Less damaging mutations, on the other hand, can survive in the progeny.
Though rare, mutations are widespread; mutations may accumulate in natural populations and contain enough genetic variety to develop fast when environmental conditions change. The mathematical theory of population genetics, established by Ronald A. Fisher and John B. S. Haldane in England and Sewall Wright in the United States, demonstrates that natural selection and mutation work together to promote adaptive evolution.
Mendelian Genetics and Molecular Evolution are two important contributions to the field of genetics in the 1900s, with the one gene-one enzyme idea proposed by Beadle and Tatum being the most important. Morgan proposed the Chromosome Theory of Heredity, Boveri-Sutton recognised chromosomes as the carriers of genetic material, and Hugo de Vries proposed the Theory of Saltation. Mutations are rapid, large heritable changes that can be beneficial or destructive, and natural selection and mutation work together to promote adaptive evolution.
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