Theories of Biological Evolution - Modern Synthetic Theory of Evolution

Modern synthetic theory of organic evolution (Neo-Darwinism):

• Neo-Darwinism is a modified form of Darwinism along with recent research by Weismann, De Vries, Stebbins, Dobzhansky, Sewall Wright, Mayr etc.  
• According to this theory following factors are responsible for the formation of new species: 
  (i) Rapid multiplication  
  (ii) Limited food and space  
  (iii) Struggle for existence.
  (iv) Genetic variations  

1) Genetic variation: The change in gene and gene frequencies is called genetic variation. Genetic variations occur due to gene mutations, gene flow, genetic drift, chromosomal aberrations, and genetic recombination.

1. Gene mutations: Discontinuous source of variations.
2. Hybridization: It is the crossing of organisms which are genetically different in one or more traits. 
3. Gene recombination: New combinations of genes which are usually caused by the crossing over during gametogenesis. It is a continuous and common source of variation in a sexually reproducing population.
4. Gene migration and Gene flow: When the migration of a section of the population to another place and population occurs, gene frequencies change in the original as well as in the new population. New genes/ alleles are added to the new population and these are lost from the old population. There would be a gene flow if this gene migration, happens multiple times.  
5. Genetic drift: If the change in gene frequency occurs by chance, it is called genetic drift. Random change of gene/allelic frequencies in a population merely by chance is called genetic drift. It operates rapidly in small population. It is due to habitat fragmentation, isolation, natural calamities or any epidemics. Founder effect and bottleneck effect are two forms of genetic drift.  
   

   (i) Founder Effect: When a section of the population gets isolated or migrated or drifted from the original population than this section becomes genetically different from the original population due to a change in allelic frequency because the gene pool of this section may contain some alleles in a very low frequency or may lack a few alleles. Sometimes the change in allelic frequency is so different in the new sample of the population that they become a different species. The original drifted population becomes founders and the effect is called the founder effect.  
   
   (ii) Bottleneck effect: Bottlenecks are the natural calamities like earthquakes, volcanic eruptions, floods, storms etc. A sudden change in the environment may drastically reduce the size of a population and now this population may be genetically different from the original population. Certain alleles may have more frequency among the survivors, others may be less, and some may be absent altogether. If a population that has passed through a bottleneck ultimately recovers in size, it may have low levels of genetic variation for a long period of time and this may produce a new species. 

2. Natural Selection:  

Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave a greater number of progenies. The critical analysis makes us believe that variation results in a changed frequency of genes and alleles in the future generation. Coupled to enhance reproductive success, natural selection makes it look like a different population and leads to new species formation.  

Examples of Natural selection:  

• Industrial Melanism- This phenomenon was studied by Bernard Kettlewell in England. 
• In a collection of moths (Biston betularia) made in the 1850s, i.e., before industrialization set in, it was observed that there were more white-winged moths on trees than dark-winged or melanised moths.  
• However, in the collection carried out from the same area, but after industrialization, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.  
• The explanation put forth for this observation was that 'predators will spot a moth against a contrasting background'.  
• Before industrialization set in, thick growth of almost white-coloured lichen covered the trees - in that background the white-winged moth survived but the dark-coloured moth was picked out by predators.  
• Lichens can be used as industrial pollution indicators. They will not grow in areas that are polluted.  
• During the post-industrialization period, the tree trunks became dark due to industrial smoke and soot. Under this condition, the white-winged moth did not survive due to predators while dark-winged or melanised moth survived.  
• Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived.  
• This understanding is supported by the fact that in areas where industrialization did not occur e.g., in rural areas, the count of melanic moths was low.  
• This showed that in a mixed population, those that can better adapt, survive and increase in population size. Remember that no variant is completely wiped out.  
• Drug resistance: The drugs which eliminate pathogens become ineffective in the course of time because those individuals of pathogenic species which can tolerate them survive and flourish to produce a tolerant/ resistant population. 
• Sickle cell anaemia and Malaria:  
• Individuals, homozygous for sickle cell anaemia die at an early stage due to anaemia and the individuals in which heterozygous condition is present for this character, the RBC become sickle-shaped.  
• In this type of RBC, the malarial parasite can't have normal growth and individuals become resistant towards malaria.  
• The individuals with the heterozygous condition have better chances of survival, and hence are selected by nature.  
• Thus, the process of natural selection maintains the abnormal form of haemoglobin along with the normal form in a region where malaria is common. This type of selection is called Balancing selection. It means the preservation of genetic variability is maintained by the selection of heterozygotes which is called balanced polymorphism. But this kind of balancing selection is found very rarely in nature. 

Genetic basis adaptations/Natural selection:

•  The essence of Darwinian Theory about evolution is natural selection.  
• The rate of appearance of new forms is linked to the life cycle or the life span.  
• Microbes that divide fast have the ability to multiply and become millions of individuals within hours.  
• A colony of bacteria (say A) growing on a given medium has built-in variation in terms of the ability to utilise a feed component. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions.
• In due course of time, this variant population outgrows the others and appears as new species. This would happen within days. 
• For the same thing to happen in a fish or fowl would take millions of years as the life spans of these animals are in years. Here we say that the fitness of B is better than that of A under the new conditions. 
• Fitness or adaptive ability is based on characteristics which are inherited. It has a genetic basis. Hence, there must be a genetic basis for getting selected and evolving. 
• Microbial experiments show that pre-existing advantageous mutations when selected will result in the observation of new phenotypes. Over a few generations, this would result in Speciation. 

Lederberg's replica plate experiment: 

• Performed by Joshua Lederberg & Esther Lederberg.  
• They cultured the bacterial cells on an agar plate and obtained many bacterial colonies. This multi-colony agar plate is known as the master plate. 
• They prepared a replica of this master plate by gently pressing it on a velvet-covered wooden block.  
• Now they tried to prepare a replica on the agar plate which contains antibiotic penicillin. It was seen that some bacteria failed to grow on the penicillin agar plate while some bacteria were able to grow and developed new colonies.  
• It was concluded that the bacteria which survived were penicillin-resistant because they had a penicillin-resistant mutant gene which enabled them to survive in changing environments.  
• It means mutations are pre-adaptive and natural selection fixes them in a population over the generations. 

Types of Natural selection:  

1. Stabilizing selection: It favours the average or normal phenotype and eliminates the extreme variants. After this natural selection means value never changes. Peak gets higher and narrower because more individuals acquire mean character value. Always operates in a constant environment. e.g. Mortality in human babies: The optimum birth weight favoured by stabilizing selection is 7.3 pounds. Newborn infants less than 5.5 pounds and more than 10 pounds have the highest mortality rate.
   
   
2. Directional/ Progressive selection: It favours one extreme value and eliminates another extreme value and average value. After this natural selection mean value always changes. Peak shifts in one direction because more individuals acquire value other than the mean character value. Always operates in changing environment. e. g. (i) Industrial melanism, (ii) DDT resistance in pests.
   
   
3. Disruptive selection: In this natural selection members of both extremes are selected simultaneously and the average value gets rejected. After this natural selection, two peaks are formed because more individuals acquire peripheral character values at both ends of the distribution curve. e. g. Shell pattern in limpets: Shell patterns of limpets (marine molluscs) present a continuous, ranging from pure white to dark tan. The white or light-coloured limpets camouflaged with white barnacles and tanned ones are protected on the tanned-coloured rocks. Limpets of intermediate shell patterns, being conspicuous are preyed on by predatory shore birds, resulting in disruptive selection.  

3. Reproductive Isolation:

• Isolation: Isolation is a segregation of populations by some barriers which prevent interbreeding. The reproductive isolation between the populations due to certain barriers leads to the formation of new species.
• It is the prevention of interbreeding between the populations of two different or closely related species.  
• It maintains the characters of the species but can lead to the origin of new species.  
• This mechanism of reproductive isolation was explained by Stebbins in his book 'Process of Organic Evolution'. Two main subtypes of reproductive isolation are 

1. Prezygotic isolation - Prevention of mating and the formation of hybrid zygote.
   (i) Ecological isolation: Isolation due to different habitats of two species. For example, one may be living in fresh water and other in the sea. 
   (ii) Temporal isolation: Due to differences in breeding seasons or flowering times of two species.  
   (iii) Behavioral isolation: Due to difference in sexual or coitus behaviour of two species. (iv) Mechanical isolation: Due to incompatible external genital organs. (v) Gametic isolation: The sperms and ova of different species can't fuse due to differences in their surface chemicals. 

2. Postzygotic isolation - A hybrid zygote is formed but it may not develop into a viable fertile adult.
   (i) Hybrid inviability: Hybrid zygote fails to develop. In plants, embryos arising from the interspecific crosses are not viable. 
   (ii) Hybrid sterility: Hybrid adults are sterile and do not produce gametes. e. g. Mules and hinny 
   (iii) Hybrid breakdown: Sometimes inter specific mating produces a hybrid, which give rise to next hybrid by back cross but they have reduced vigour or fertility or both.