The Germ Plasm Theory by August Weismann
August Weismann (1834–1914), and described in his 1892 book , The Germ Plasm: a theory of inheritance. His theory states that multicellular organisms consist of germ cells that contain and transmit heritable information, and somatic cells which carry out ordinary bodily functions. In the germ plasm theory, inheritance in a multicellular organism only takes place by means of the germ cells: the gametes, such as egg cells and sperm cells. Other cells of the body do not function as agents of heredity. The effect is one-way: germ cells produce somatic cells, and more germ cells; the germ cells are not affected by anything the somatic cells learn or any ability the body acquires during its life. Genetic information cannot pass from soma to germ plasm and on to the next generation. This is referred to as the Weismann barrier. This idea, if true, rules out the inheritance of acquired characteristics as proposed by Jean-Baptiste Lamarck, like others before him, and accepted by Charles Darwin both in On the Origin of Species and as part of his pangenesis theory of inheritance.
Contribution of Mendel:
Darwin laid down the foundation of evolution by hypothesizing that if a trait is advantageous it will increase in frequency in a population because the offspring with the trait will survive and reproduce better and will pass on that trait to their offspring. However, at that time no one could explain how those traits could be passed over to the future generations. The credit for discovering the mode of inheritance of traits goes to the Austrian monk J. Gregor Mendel (1822-1884) who, through his breeding experiments on garden pea plants developed a few simple rules of inheritance. Mendel, the father of Modern Genetics, published his findings on inheritance in 1866 but his work remained largely ignored until it was rediscovered in 1900 by Hugo de Vries and Carl Correns. Mendel was a pioneer who laid the foundation for the whole of modern genetics. Rediscovery of Mendel’s principles led to the rapid and explosive growth of the discipline of genetics and established the basis for unraveling the deep secrets of biological reproduction and heredity. Mendel’s experiments helped him realise a few simple rules of inheritance.
Mendel proposed that there were discrete “factors” of heredity that united during fertilisation and then separated again in the formation of sperm and egg. It is remarkable how correct Mendel was, particularly in view of the fact that he knew nothing about DNA, chromosomes or meiosis (even the term ‘gene’ was not introduced until 1909). He was convinced that organisms inherit two units of each ‘factor’, one from each parent. Now we understand these ‘factors’ as genes and that most complex organisms are diploid, that is, they can have two copies of a particular gene or two different alleles and that alleles are not blended.
On the basis of his experiments, Mendel proposed the ‘Law of segregation’ . According to this law, when the gametes are formed in the parents, the heritable factors (genes) separate from each other so that each sperm or egg gets one unit of each pair. Mendel was correct. Today we understand that sperm and egg are haploid, with only half the number of chromosomes and genes of the parents. He also proposed what came to be known as the ‘Law of independent assortment’, which states that the factors (genes) for various traits assort independently of each other during the formation of sperm or egg. Mendel was partially right in this respect. It is because approximately 25,000 genes of the human genome do not float independently in the nucleus of the cell. Each gene is part of a homologous pair of chromosomes, and normally in humans there are only 23 pairs of chromosomes. Only genes meant for different traits, which are located on different chromosomes always truly assort independently.
Mendel published his findings in 1866, just seven years after Darwin’s ‘Origin of Species’. These findings went unnoticed until 1900, when eventually the mechanism of inheritance could be combined to natural selection. Shortly thereafter, a theoretical evolutionary model known as the ‘Modern Synthesis’ or ‘Synthetic Theory’ was born. Once in 1953, James Watson, Francis Crick and Rosalind Franklin explained the model of DNA molecule, the basic genetic component of evolution was revealed.
Theory of Mutation by Hugo de Vries
The term ‘mutation’ was introduced by Hugo De Vries. The term was used by De Vries for large spontaneous inheritable changes, which occur suddenly in naturally reproducing populations.
Mutation Theory of De Vries:
De Vries Mutation theory was based upon his observations on evening primrose, Oenothera Lamarkiana growing wildly in a field near Amsterdam. De Vries studied 54343 plants of Oenothera in a period of eight years. Mutation theory established that “New species originate as a result of these large, discontinuous variations which appear suddenly and full-fledged and from the new species at once”.
The main features of mutation theory are:
- 1) Mutations arise from time to time amongst the individuals of a naturally breeding population or species. The individuals with mutations are known as mutants. These mutants are markedly distinct from their parents.
- 2 ) Mutations are inheritable and establish new forms or races or species.
- 3) Mutations are large and sudden and are totally different from fluctuating variations of Darwin, which are small and directional.
- 4) Mutations may occur in any direction.
- 5) Mutations are subjected to natural selection.
- 6) Mutants found unsuitable are likely to be destroyed by natural selection.
- 7) Since mutations appear full-fledged, there is no question of development of organs from the incipient stages.
Criticisms:
- 1) Darwinists contended that evolution resulted from gradual fluctuating inheritable differences over a Jong series of generations, whereas mutationists believed in sudden appearance of species differences.
- 2) B.A. Davis discredited mutation theory by claiming that Oenothera lamarkiana is of hybrid nature, which couid be produced by crossing two wild American species.
We may conclude by saying that De Vries mutation theory based on changes, that were not genic mutations, was fundamentally correct in stressing the significance of mutations in the evolutionary process. However, mutations alone cannot account for evolution, but these furnish the raw material on which other forces can act to bring about the evolutionary change.
Theory of Gene by Thomas Hunt Morgan
Morgan developed demonstration that each Mendelian gene could be assigned a specific position along a linear chromosome “map.” Further cytological work showed that these map positions could be identified with precise chromosome regions, thus providing definitive proof that Mendel’s factors had a physical basis in chromosome structure. A summary and presentation of the early phases of this work was published by Morgan, Sturtevant, Bridges, and Muller in 1915 as the influential book The Mechanism of Mendelian Heredity. To varying degrees Morgan also accepted the Darwinian theory by 1916.
Sewall Wright Genetic Drift and Evolution.
The theory of genetic drift was developed by geneticist Sewall Wright in 1930. It is also called Sewall Wright effect or Scattering of Variability. It refers to the ‘Random Fluctuations’ in the gene frequencies in a small population generation after generation purely by chance.
Features of Genetic drift:
- 1) Genetic drift is an evolutionary force operating in small populations.
- 2) Gene frequency in small populations changes by chance.
- 3) In small populations some genes may be lost or reduced and others may increase by sheer chance irrespective of their selective advantage or disadvantage.
- 4) By genetic drift a new mutation arising in a small population may either be fined or lost irrespective of its adaptive value.
- 5) In a small population hetero zygosity tends to change to homo zygosity by chance.
- 6) Genetic drift may fix some non-adaptive traits in small populations.
- 7) Genetic drift tends to preserve or eliminate genes without distinction.
- 8) Isolated small populations of a large population come to possess some unusual characteristics not shown by their large parental population.
Influence of genetic drift on evolution:
Genetic drift plays a considerable role in the evolution of organisms. In this regard,
- 1) Most breeding populations of organisms are usually small.
- 2) Even a widely ranging broad based population is isolated into small subgroups (Demes) either on account of ecological or geographic dis-continuities or on account of instinct or territoriality. The size of the small deans or subpopulations is such that they appear to be affected by chance events underlying genetic drift.
- 3) Seasonal, annual or cyclical fluctuations may be observed in population size. If the season or occasion favours large gathering of population, then genetic drift may not take place. On the other hand, even if such seasonal fluctuations in population size cannot make it a large one, even then genetic drift may operate in that case the gene pool of that generation may not be the representative of parental gene pool. All this is due to chance changes or genetic drift. The altered gene pools in due course of time may lead to new species.
Haldane, R.A. Fisher, Population Genetics