Modern concept of Natural Selection (or) Modern synthetic theory of Evolution.
Modern development in biological fields such as Cell biology, Genetics and Populations genetics helped in the development of modern synthetic theory of evolution. It was caused due to contributions made by eminent scientists such as Th. Dobzhansky, S.Wright, H.J.Muller, J.S. Huxley, R.A.Fisher, Ernst Mayr, G.L.Stebbins and others.
The basic concept of modern synthetic theory was provided by Th. Dobzhansky in his book titled 'Genetics and the Origin of species' (1937) G.L.Stebbins in his book 'Process of organic evolution' (1971) suggests five basic processes essential for evolution. They are gene mutations, chromosomal aberrations, genetic recombinations, natural selection and reproductive isolation. Contributions made by others provided additional factors such as Hardy-Weinberg equilibrium, Genetic drift and Polymorphism.
Since proteins perform several functions, they determine many of the characteristics of organisms. The role played by a protein is largely determined by its primary structure. The primary structure is due to the sequence of amino acids in its molecule. This structure in turn determines the three dimensional protein molecule. The conformation determines the protein function.
The basic sequence of amino acids in proteins is precisely regulated by the genetic code. Any change in the code due to gene mutation will result in the production of abnormal proteins. The abnormal proteins thus formed may be either beneficial or harmful. A gene producing a beneficial protein confers an advantage on its possessor. Gradually its frequency increases in a population. Individuals having harmful mutations may not survive to reproductive age. So, such mutated genes are not passed to the next generation.
The mutations are considered as the 'raw materials' for evolution. They help to create and provide variations in a population along with genetic recombinations. The collection of genes in a population is referred to as the 'gene pool'. Mutations enrich the gene pool with new modified genes. A large scale accumulation of such genes will lead to evolutionary modifications.
During the process of meiosis one or more chromosomes may break. Such broken fragments of chromosomes may be subjected to several modified organizations :
a) a small broken fragment may become lost (deletion)
b) the broken fragment may become attached to the end of another chromosome (translocation)
c) the fragment may become turned around and rejoin the chromosome (inversion)
d) they may become inserted into another chromosome (duplication)
All the above mentioned changes may alter the genomes of gametes.
Sometimes a pair of homologous chromosomes may fail to separate in meiosis. It will result in gametes with one chromosome less or one chromosome more, than normal. The progeny formed from such gametes are called polysomics. They will have fewer or more chromosomes than normal.
In certain cases, whole set of homologous chromosomes do not separate in meiosis. It results in diploid gametes. Fusion of such gametes with a normal haploid gamete gives rise to progeny with a triploid chromosome number. This condition is called polyploidy. It is commonly observed in plants. Polyploids are usually more vigorous. Such forms can give rise to new species.
While recombinations provide regular variations, mutations enrich such variations. Phenomena such as chromosomal abberrations, polysomics and polyploidy while, found advantageous to the organisms, provide new directions for speciation and further evolution.
During meiosis, due to crossing over of chromosomes, genic arrangements get altered. Such alterations cause reshuffling of gene combinations. Such recombinations are regular events in gametogenesis. Due to such events new allelic formations happen and after fertilization variations result in the progeny.
A population is defined as an assemblage of living beings showing a closely interacting system. A population comprising of sexually interbreeding organisms is termed as the genetic population or Mendelian population. A genetic population may be defined as 'a community of similar individuals living within a limited circumscribed area at a given time and capable of interbreeding'. The genes of all the individuals of such a Mendelian population will constitute the gene pool. A gene pool comprises a diverse forms of a gene combining and recombining by the process of sexual reproduction. The frequency of genes and genotypes in a population had been worked out by mathematical formulations.
The gene frequency refers to the proportion of an allele in the gene pool as compared with other alleles at the same locus. Hence the gene frequency can be calculated by substracting the number of a particu-lar gene in question from the total number of genes present on that locus in the population.
If the frequency of gene 'A' is represented by 'P' and that of gene a by 'q' and at gene equilibrium condition their total frequency is represented by 1, then at equilibrium
P+q = 1 or p = 1-q or q = 1-p
A mathematical interpretation for the distribution of gene and genotype frequencies in the population was developed by R.A. Fisher (England) and Sewall Wright (United States). A fundamental idea in the form of a law to understand population genetics was provided by G.H. Hardy of England and W.Weinberg of Germany in 1908. The law proposed by them is known as Hardy-Weinberg's law . It is the foundation of population genet-ics and of modern evolutionary theory. According to this law ' the relativefrequencies of various kinds of genes in a large and randomly mating sexual population tend to remain constant from generation to genera-tion in the absence of mutation, selection and gene flow or migration.
This law concerns a theoretical situation for a population not under-going any evolutionary change. Thus according to the law the normal mendelian genic frequencies are maintained under certain conditions only. If such conditions are not followed, the gene frequency will change leading to deviations and cause variations, such variation will be the sources for future evolution.
This theory was developed by Sewall Wright in 1930. It is concerned with the gene frequency of a reproducing small population. In a small population not all the alleles which are representatives of that species may be present. Thus the process of inheritance is in violation of Hardy-Weinberg law. In such a small population a chance event may increase the frequency of a char-acter that has little adaptive value. Thus the genetic drift may remain a signifi-cant factor in the origin of new species on islands and other isolated popula-tions. Due to loss of alleles having low frequency, amount of genetic variation may get reduced in small populations. Further, continual mating within such populations may cause decrease in the proportion of heterozygotes and in-crease in the number of homozygotes. However the small population as a whole may develop characters different from that found in the main popula-tion. Such deviations may even lead to speciation or formation of a new species.
When a small group of individuals due to genetic drift become founders of a new population the phenomenon is termed as 'founder principle'. The new population often has genotype frequencies different from the parent population.
Sometimes genotypic frequencies may get changed in a small popu-lation isolated temporarily due to natural calamities. When the population regains its original size the members of the small population may have di-verged genetically from the original parental population. Hence interbreeding between members of small and larger populations may not be possible. The small population might have evolved into a new species. This type of genetic drift is referred to as bottleneck effect.
In the modern or synthetic theory of evolution natural selection is considered as a population related genetic phenomenon. It leads to changes in allele frequencies and favours or promotes adaptation as a product of evolution.
When the population size of animals or plants in specific locality increases certain environmental factors such as availability of food may become limiting factors. Those organisms exhibiting characteristics which give them a competitive advantage may survive. Thus population size and environmental limiting factor operate together to produce a selective pressure. The selection pressure may increase or decrease the spreading of an allele in a gene pool depending on its adaptive value. This inturn will lead to evolutionary changes.
There are three types of selection processes in operation. They are stabilizing, directional and disruptive selections.
In stabilizing selection competition in nature is not severe. The phenotypic features coincide with normal environmental situations. However this selection may eliminate characters that are abnormal and harmful and it tends to maintain the phenotypic stability within population for successive generations.
The directional selection operates in response to gradual changes in the environment. It operates within the phenotypic range available within the population. The selection gradually changes the phenotypic character towards a possible extreme condition found suitable for the changed envi-ronmental situation. This selection will increase the frequency of desirable phenotypic character within the population. Thus it results in gradual evolutionary change.
In disruptive selection the selection pressure may favour the existence of more than one phenotype in a population. It may even split a population into two sub-populations. If gene flow between such sub-popu-lations is prevented a new species or a sub specie may evolve. When a disruptive selection produces more than one phenotype within a population the phenomenon is known aspolymorphism.
It is the 'the existence in a natural population of two or more alleles in frequencies too large to be explained by recurrent mutation'.
Thus a polymorphic population will have several alleles of a gene as a permanent feature of the species. The varied alleles are favoured and maintained in the population by genetical mechanisms.
A classical example for such a polymorphism could be the existence of a genetic disorder in humans, namely sickle-cell anaemia. This disease reduces the oxygen-carrying capacity of the blood and affects blood supply to various organs. This disorder is inherited as a Mendelian recessive. It is more frequent among American blacks than American whites. In spite of its harmful nature the allelic gene responsible for the disorder is maintained in
the black population. According to the work of Allison (1955, 61) it was shown that in Africa the same allelic gene conferred an advantage, that is it protected the inheritors of such gene from malaria. Thus the connection between sickle-cell anaemia and malaria was estabilished. Hence selection has encouraged the existence of such a polymorphic allele in the population.
A species may be defined as 'a group of organisms that are reproductively isolated from other such groups'. Thus the maintenance of a species as a dintinct group is due to several isolating mechanisms. They are
1. Geographical isolation
It is a common type of isolation. The isolation between populations is caused due to geographical barriers such as mountains, rivers, oceans, for-ests or deserts. These natural barriers prevent interbreeding between them. Thus mutations formed in one population will lead to the formation of new species. The existence of closely related species of frogs in Southern India and Srilanka is a classical example. These fresh water animals are prevented from interbreeding due to a narrow sea namely Gulf of Mannar. Because of isolation for a fairly long time they have evolved into distinct species.
2.Premating isolations - such mechanisms prevent interspecific crosses.
a) Ecological isolation - Members of the populations occur in different habitates in the same general region.
b) Seasonal or Temporal isolation - Mating or flowering periods occur at different seasons.
c) Sexual, Psychological or Ethological isolation - It is a behavioural isolation where males and females of the same species get attracted to each other.
d) Mechanical isolation - Physical non-correspondence of the genitalia or floral parts.
e) Gametic isolation - Spermatozoa, or pollen tubes of one species are not attracted to the eggs or ovules of another species.
3. Postmating or postzygotic isolations - These isolating mechanisms while allowing fertilization may prevent the hybrid zygote from further development.
a)Hybrid inviability - The hybrid zyotes are inviable.
b) Hybrid sterility - The hybrids develop but they remain sterile. They are incapable of producing a normal complement of functional sex cells.
c) Hybrid breakdown - F1 hybrids are normal and fertile, but F2 contains many weak or sterile individuals.
A species is a natural, biological unit. Among the various taxa, a species is not man made. It is a natural reality. The process of evolution operates at the species level only. It is because of these reasons, in evolution much importance is given to the 'Origin of Species'. There are several types of species.
Allopatric species - Species occupying different geographical areas. Ex : species of frogs in India and Srilanka. The two land areas are separated by the Gulf of Mannar. Sympatric species - closely related species living together in one common locality, yet maintain their species identity Ex: Rana hexadactyla, R.tigrina and R.cyanophlictis living together in a pond.