The basics of biological evolution, part I: The Modern Synthesis
For a long time since its birth, the “Modern Synthesis,” or Neo-Darwinism, remained the dominant paradigm for understanding biological evolution. Despite some lingering conceptual difficulties (e.g., what a gene is, what adaption is) and some thorny philosophical issues (e.g., replicator versus interactor, units versus levels of selection),18 the core components of the Modern Synthesis are standard knowledge in any typical biology textbook (e.g., Futuyma 1998). Thus, although recent new discoveries in evolutionary biology have made it abundantly clear that the Modern Synthesis does not fully capture the complexities or wonders of biological evolution (see Section V), I shall introduce the central tenets of the Modern Synthesis first.
According to the Modern Synthesis, at the micro-level, biological evolution proceeds in three distinctive stages sequentially: diversity generation (i.e., variation or mutation of genetic material), natural selection (i.e., eliminating and retaining some phenotypes and thus genotypes along with them), and inheritance or inheritance (i.e., replication and spreading of the selected genotypes and thus phenotypes). This central mechanism ofVSI was first formulated by Darwin (1859) and is thus referred to as “Darwinian.”19 (As becomes clear later, the sequence ofVSI is also a critical component of the Modern Synthesis or Neo-Darwinism.)
In a natural setting, genetic mutations are generated in an unguided manner and mostly randomly, and there are no artificial forces involved. These genetic mutations serve as the ultimate basis for the selection process, in that expression of the information encoded in genetic materials gives birth to diversity in phenotypes, and it is these different phenotypes that are the direct targets of selection during the phase of selection (Mayr 1997).20 In other words, selection operates directly on phenotype, but only indirectly on genes. This is so because only phenotype—but not genotype—is visible for selection pressure in the environment.
The physical environment determines whether a particular phenotype is advantageous, disadvantageous, or neutral for an organism’s “inclusive fitness,” roughly measured as the number of offspring left behind by an organism, ceteris paribus. An organism with an advantageous phenotype will be more likely to win the competition versus another organism with a disadvantageous phenotype, ceteris paribus. By selecting phenotypes, the environment ends up indirectly selecting genotypes, and a genotype that underpins an advantageous phenotype will be more likely to survive and spread via the individual organism’s reproduction, while a genotype that underpins a disadvantageous phenotype will be less likely to spread and may eventually disappear completely. Hence, while mutation in biological evolution is generated in an unguided manner and essentially randomly, biological evolution via natural selection is not random because selection and inheritance are not random. This is a very important point to bear in mind.
Mendel (1822-1884) provided us with the first glimpse into the genetic foundation of biological evolution in 1865-1866. Unfortunately, his work remained essentially unknown for almost half a century until it was finally rediscovered in the 1900s independently by De Vires and Correns. In the 1910-20s, Thomas Morgan and his students provided a firmer foundation for Mendelian genetics by locating genes onto chromosomes.21 In 1937,Dobzhansky synthesized evolutionary biology with genetics into the “Modern Synthesis” or “Neo-Darwinism” as the integrative (or unified) theory of biological evolution.22 Inspired by Dobzhansky’s work, Mayr, George Simpson, and G. Ledyard Stebbins contributed further ideas to and refined the Modern Synthesis (Futuyma 1998,24-29).
In 1953, Watson and Crick discovered the key molecular foundation of biological evolution by revealing the famous “double helix” structure of DNA, the most important form of genetic material in the bio-system. Genetic materials usually take the form of double-stranded DNA, but sometimes also the form of single-strand or double-stranded ribonucleic acid (RNA) in some RNA viruses or retroviruses.23
The ultimate basis for the origins of species lies in genetic mutation. Genetic materials, by interacting with the environment through development, give rise to phenotypes. More often than not, there is no simple one-to-one correlation between genes and phenotypes: Some phenotypes (e.g., eye color) are controlled by several genes, and many genes have multiple copies and can give rise to different phenotypes when combined with other genes.
The presence of a barrier between the nuclei and the cytoplasma of a cell, discovered by August Weismann (1834-1914), nullifies the possibility of DIP-WGM in most circumstances. The “Weismann Barrier” dictates that genotypes give rise to phenotypes, but phenotypes cannot directly give rise to genotypes. The Weismann Barrier thus nullifies a key aspect of Lamarck’s theory of evolution: To allow DIP-WGM, the Weismann Barrier must be broken.
At the molecular level, the Weismann Barrier means that the information flow in biological evolution is unidirectional: from genetic material (DNA or RNA, which are often located in the nucleus of a cell) to protein, but not the other way around. More specifically, genetic information can be translated into proteins, but information stored in proteins cannot be translated back into genetic information. This unidirectionality of information flow, which precludes the possibility ofDIP-WGM in biological evolution and thus ensures only a Neo-Darwinian evolution, is the central dogma of molecular biology (Crick 1970).