Natural Selection and Evolution

Key Concepts

• Evolution

• Neo-Darwin ism

• Genetic variations and their causes

• Natural selection

• Isolation of species

Introduction:

Evolution can be defined as a slow, continuous, gradual, and irreversible process through which the present-day complex forms of life developed from their simpler pre-existing forms. In simple terms, evolution can be defined as change over time. 

Various scientists and biologists have put forward their theories of evolution in their time. Some of these theories are as follows: 

1. Lamarckism  

2. Darwinism 

3. Mutation theory 

4. Neo-Darwinism

Explanation:

Neo-Darwinism:  

Neo-Darwinism is the modified version of Charles Darwin’s theory of evolution by natural selection. It is the integration of Darwinism, Mendelian genetics, and mathematical population genetics. 

The term Neo-Darwinism was first used by August Weismann, a German naturalist, who combined his theory of the germplasm with Darwin’s theory in the 1880s. 

Neo-Darwinism is also known as the modern or synthetic theory of evolution. It is the most modern theory of evolution and has been constantly revised during the 20th century by the contribution of the scientists such as R.A. Fischer, J.B.S. Haldane, Ernst Mayr, Julian Huxley, T. Dobzhansky, H.J. Muller, H. DeVries, G.L. Stebbins, G.H. Hardy, W. Weinberg, Sewall Wright, etc. 

The following aspects are responsible for the formation of new species according to  

Neo-Darwinism.  

• Variations  

• Mutations  
• Natural selection  
• Genetic drift  
• Isolation of species  

These five main factors are broadly classified into three main concepts which are as follows:   

1. Genetic variation caused due to various aspects of mutation, recombination, and migration 

2. Natural selection 

3. Isolation 

1. Genetic variations: 

Genetic variations are the changes occurring in genes and gene frequencies. Genetic variability is an opposing force to heredity and is necessary for evolution as the variations provide the raw material for evolution. As per the studies, it is observed that the units of both heredity and mutations are genes that are located in a linear way on the chromosomes.  

Various factors contribute to genetic variations, some of which are discussed further.  

1. Gene mutation: A sudden large, permanent, and inheritable change in the genetic material is called a mutation. Mutations can occur in the gene, in the chromosome, and in chromosome number.  

A mutation that occurs within a single gene is called point mutation or gene mutation. Point mutations can cause a change in the phenotype of the organism, causing what is called variation. 

Gene mutations are of three types: 

1. Deletion: It involves the loss of one or more nucleotide pairs. 

2. Addition: Involves the gain of one or more nucleotide pairs. 

3. Substitution: It involves the replacement of one or more nucleotide pairs by other base pairs. These may be transition or transversion types. 

2. Genetic recombination: When gametes are formed in sexually reproducing organisms, the exchange of genetic material occurs between non-sister chromatids of homologous chromosomes. This is called crossing over. It creates new genetic combinations which result in variation.  

Fertilization between opposite mating gametes leads to various recombination resulting in phenotypic variations causing a change in the frequencies of alleles. 

3. Gene flow: The passage of genes into or out of a population is termed gene flow. This movement of genes may be in the form of migration of organisms, gametes (dispersal of pollens), segments of DNA (transformation).  

Gene flow also changes gene frequency causing evolutionary changes. 

4. Genetic drift: Genetic drift is any random fluctuation or alteration in the allele frequency occurring in a natural population by pure chance.  

For example, when the size of a population is drastically reduced due to natural disasters like earthquakes, floods, fires, etc., it causes the deletion of particular alleles from a population. Smaller populations have a greater chance for genetic drift. It will result in a change in the gene frequency. Genetic drift is also an important factor in evolutionary change. 

5. Chromosomal aberrations: Chromosomal aberrations are structural and morphological changes in chromosomes that occur as a result of rearrangement. 

It alters the arrangement of the genes and results in variation.  

These changes occur either in the number of genes (deletion and duplication) or in the position of genes (inversion). 

Chromosomal aberrations may occur due to –  

1. Deletion: Involves the loss of genes from the chromosome and may be terminal or intercalary. 

2. Duplication:  Involves the doubling or repetition of genes in a chromosome. It may be cyclic. There is no loss or gain of the chromosome’s gene component here. 

3. Translocation: It involves the transfer of a part of chromosome or a set of genes to a non-homologous chromosome. It may be simple or reciprocal type. 

2. Natural selection: 

Darwin believed that natural selection is the driving force behind evolution. He suggested that genetic variations arise within the population. The ‘fittest’ will have a selective advantage and will be more likely to produce offspring than the rest; as the ‘fit’ continues to thrive in terms of survival and reproductivity, new species will emerge. 

Natural selection is the process through which better-adapted organisms grow and reproduce more offspring in a population. It creates evolutionary changes by favoring differential reproduction of genes which alters the gene frequency from one generation to the next generation. 

Selection against harmful mutations results in a mutation balance in which the allele frequency of harmful recessives remains constant generation after generation. Natural selection favors those genes or traits that ensure a higher rate of adaptive efficiency between a population and its environment. 

Examples of Natural selection: 

One of the good examples of natural selection is industrial melanism. 

Before industrialization, the grey white-winged moths (Biston betularia) outnumbered the black-winged moths (Biston carbonaria) in Britain. These moths are nocturnal and rest on tree trunks during the day. White-winged moths can blend in well with lichen-covered trees, allowing them to avoid predatory birds. On the other hand, the black-winged moths resting on lichen-covered tree trunks were easy prey for the predatory birds and so their number was reduced. 

During the industrial revolution, numerous industries emerged in Great Britain. The industries emitted black sooty smoke that killed the lichens and turned the tree black due to pollution. This change benefitted the black-winged moth as it blended well with the black tree trunks, and their number increased while the white-winged moth became prey for predatory birds resulting in a decline in their population.  

The establishment of phenotypic traits in changing environmental conditions occurs due to natural selection. 

3. Isolation of species: 

The separation of the population of a certain species into smaller units in order to prevent interbreeding between them is termed isolation. 

Any barrier that restricts gene flow or exchange of genes across isolated populations is called an isolating mechanism. Numerous isolating mechanisms are operated in nature which results in divergence and speciation.  

There are two types of isolating mechanisms, namely, geographical isolation and reproductive isolation. 

1. Geographical isolation: It is also known as physical isolation. It occurs when geographical barriers such as rivers, oceans, mountains, glaciers, etc., divide an original population into two or more groups. These barriers prevent interbreeding between the separated groups.  

The separated groups are exposed to different environmental conditions, and they acquire new traits through mutations. The separated populations develop different gene pools, and they do not interbreed. Thus, new species are created by geographical isolation. E.g., Darwin’s Finches. 

2. Reproductive isolation: It occurs because of changes in genetic material, gene pool, and structure of genital organs. It prevents interbreeding between populations. 

Types of Isolating Mechanisms: 

1. Pre-mating or pre-zygotic isolating mechanism: It prevents fertilization and zygote formation.  

1. Habitat isolation: Members of a population who live in the same geographical area but occupy different habitats so that potential mates do not meet. 

2. Seasonal or temporal isolation: Members of a population occupy the same geographical area but mature sexually at various times of the year or in different years. 

3. Ethological isolation: Due to different mating behavior, the members of the population do not mate.  

4. Mechanical isolation: Members of two populations have anatomical differences in reproductive organs. 

2. Post-mating or Post-zygotic barriers: 

1. Gamete mortality – Gametes have a short life span. If due to any reason, the two gametes do not unite in the given period, it results in gamete mortality. 

2. Zygote mortality – in this case, the egg is fertilized, but the zygote dies due to some other reasons. 

3. Hybrid sterility – Hybrids mature but become sterile due to improper gametogenesis (meiosis). E.g., Mule is an intergeneric hybrid that is sterile. 

Summary

• Neo-Darwinism is the modified version of Charles Darwin’s theory of evolution by natural selection. It is the integration of Darwinism, Mendelian genetics, and mathematical population genetics.

• According to neo-Darwinism, variations, mutations, natural selection, genetic drift, and isolation of species are the factors responsible for the formation of new species.

• Genetic variations are the changes occurring in genes and gene frequencies. These variations are caused due to various aspects of mutation, genetic drift, gene flow recombination, and migration.

• Natural selection is the process through which better-adapted organisms grow and reproduce more offspring in a population. It creates evolutionary changes by favoring differential reproduction of genes which alters the gene frequency from one generation to the next generation.

• The separation of the population of a certain species into smaller units in order to prevent interbreeding between them is termed as isolation.

• There are two types of isolating mechanisms, namely, geographical isolation and reproductive isolation that leads to speciation.