The main postulate in Darwin's theory of evolution was that a species evolves because natural selection acts on small inheritable variations in the members of the species (cf. Part 31).
But it was argued by his opponents that, since a species is also characterized by interbreeding, such small variations should get averaged away. Darwin had no answer to counter this because the actual mechanism of inheritance was not known at that time.
The answer in fact had been provided in 1865 (i.e. during the lifetime of Darwin, but apparently unknown to him) by the work of Gregor Mendel, the founder of the subject of genetics. We now know that the 'genotype' or the genome of an organism is its genetic blueprint, and is present in the nucleus of every cell of the organism. The 'phenotype', on the other hand, is the end-product (the organism) which emerges through execution of the instructions carried by the genotype. It is the phenotype that is subjected to the battle for survival and natural selection, but it is the genotype which carries the accumulated evolutionary benefits to succeeding generations. The phenotypes compete, and the fittest among them have a higher chance of exchanging genes among themselves.
Mendel’s laws of genetics were rediscovered independently by quite a few workers. One of them was the Dutch botanist Hugo de Vries, who not only rediscovered Mendel’s laws for the inheritance of 'dominant' (or expressed)and 'recessive' (or suppressed) characteristics, but also discovered genetic mutations. These were sudden (unexplained) changes of form which were inherited by the offspring.
The present, post-Darwinian, picture is that the inherited characteristics of the progeny are carried by genes. In sexually reproducing organisms, each parent provides one complete set of genes to the offspring. Genes are portions of molecules of DNA, and their specificity is governed by the sequences in which their four bases (adenine (A), thymine (T), guanine (G), and cytosine (C)) are arranged. The double-helix structure of DNA, together with the restriction on the pairing of bases comprising the DNA molecule to only A-T and G-C, provides a mechanism for the exact replication of DNA molecules. And the DNA sequences on genes determine the sequence of amino acids in the specific proteins created by the live organism.
Genes programme embryos to develop into adults with certain characteristics, and these characteristics are not entirely identical among the individuals in the population. Genes of individuals with characteristics that enable them to reproduce successfully tend to survive in the gene pool, at the expense of genes that tend to fail. This feature of natural selection at the gene level has consequences which become manifest at the organism or phenotype level. Cumulative natural selection is NOT a random process.
If like begets like (through inheritance of characteristics), by what mechanism do slight differences arise in the gene pool of successive generations so that the species evolves towards evolutionary novelty? One mechanism is that of mutations. Mutations, brought about by radiation or by chemicals in the environment, or by any other agents causing replication errors, change the sequence of the four bases in the DNA molecules comprising the genes. Most mutations are deleterious and get weeded out by natural-selection processes, but those which happen to be beneficial to the population have a selection advantage and get further propagated in the population.
If all living beings have the same or only a few ancestors, how have the various species arisen? The Darwinistic answer lies in isolation and branching, aided by evolution. Migrations of populations also play a role in the evolutionary development of species. If there are barriers to interbreeding, geographical or otherwise, single populations can branch and evolve into distinct species over long enough periods of time. Each such branching event is a 'speciation': A population accidentally separates into two, and they evolve independently. When separate evolution has reached a stage that no interbreeding is possible even when there is no longer any geographical or other barrier, a new species is said to have originated.
The term neo-Darwinism essentially connotes a modification of the original ideas of Darwin in the light of later knowledge about the mechanism of transmittal of genetic information from one generation to the next. Margulis and Sagan (2002), who disagreed with this neo-Darwinistic view of the origin of species (I shall describe their work in the next post), summed up neo-Darwinism as follows:
‘All organisms derive from common ancestors by natural selection. Random mutations (heritable changes) appear in the genes, the DNA of organisms, and the best “mutants” (individuals bearing the mutations) in competition with the others, are naturally selected to survive and persist. The unsuited offspring die – they tend to be called “unfit” – with fitness, a technical term, referring to the relative numbers of offspring left by an individual to the next generation. The most fit, by definition, produce the largest number of offspring. The mutant variations then leave more offspring, and populations evolve; that is, they change through time. When the number of changes in the offspring accumulates to recognizable proportions, in geographically isolated populations, new species gradually emerge. When sufficient numbers of changes in offspring populations accumulate, higher (more inclusive) taxa gradually appear. Over geological periods of time new species and higher taxa (genera, families, orders, classes, phyla, and so on) are easily distinguished from their ancestors.’
As emphasized by Stuart Kauffman, evolution of biological complexity is determined by two factors: natural selection, and self-organization. Self-organization creates order in any complex system. Darwinian natural selection acts on this existing order and hones it further.
The phrase 'selfish gene' was introduced by Richard Dawkins: 'The most inspiring way of teaching evolution is to say that it's all about the genes. It's the genes that, for their own good, are manipulating the bodies they ride about in. The individual organism is a survival machine for its genes.'