Evolution

Evolution is a theory in Biology that describes the change in the characteristics of a species of plants, animals, and other living things on Earth generation by generation and this depends upon the process of natural selection. At the time of reproduction, the genes are passed from parents to offsprings. The characteristics are expressions of these genes. Different characteristics exist in the given population of species either due to genetic variation or due to mutation. Over a period of time, these characteristics tend to become more common or rare in a given population of species.

HISTORY

The scientific theory of Evolution through the method of natural selection was introduced by ‘Charles Darwin’ and ‘Alfred Russel Wallace’. This came into existence in the 19th century. The introduced theory is commonly known as Darwin’s Theory of Evolution as well. Darwin even wrote a book on the topic of Evolution that was named ‘On the Origin of Species’. According to Darwin’s observation in the theory of natural selection, the offsprings are often produced more in number but the survival rate is low. 

There are three things observed with regard to this theory:

1. In all the species, the characteristics vary with regard to their physiology, morphology, and behavior.

2. The rate of survival and reproduction depends upon the characteristic that has passed on.

3. Characteristics can pass on from generation to generation.

So we can say that with each passing generation the offspring tend to survive by getting the particular trait from the parents and transform in their features and characteristics with respect to certain environmental conditions that enable them to adapt to a particular habitat. This is how evolution takes place generation by generation.

Whatever the form of creatures that are present today on Earth, have the Last Universal Common Ancestor (LUCA) that lived nearly around 3.5 to 4 billion years ago. This can be assessed by studying fossil records that include Biogenic Graphite to fossilized organisms underground or some evolution data encrypted in each passing generation that can give us an approximate characteristic detail of any living being in the history. 

The study of the existing pattern of biodiversity has been classified under 3 categories:

• Speciation( this is the formation of new species with time )

• Anagenesis( the changes that occur within a particular species with respect to each passing generation )

• Extinction( loss of any particular species with time like Dinosaurs ) 

The Greek philosophers like Empedocles and Anaximander of the presocratic times also believed that there is a possibility a particular type of organism can descend from another variety. These thoughts were carried forward even to Roman times. During medieval times people followed the concepts of Aristotle who was considered as the father of western logic, however, much to the contradiction of the classical data the people believed that whatever was present in nature has something to do with divinity in-universe. Hence, this understanding was brought under Christian learning. However, the examples that were set by Aristotle that how new types of living things could come into existence was still considered.

Then came the Pre-Darwinian times when the modern approach of understanding rejected the ideologies put forward by Aristotle. They only accepted the explanation of natural phenomena with regard to physical laws. The term ‘Species’ was given by John Ray to some fixed natural types. The biological classification was then introduced by Carl Linnaeus in the year 1735. Later, Erasmus Darwin put presented the idea that all warm-blooded animals evolved from a single microorganism.

The first theory of Evolution came into existence in the 19th century( 1744-1829 ) when Jean Baptiste Lamarck proposed the theory of ‘Transmutation of species’.

ROLE OF GENES IN EVOLUTION

The process of Evolution takes place at the Genomic level. Genes are the basic unit of inheritance. Genes are composed of DNA. Any changes in the sequence of DNA will affect the composition and also affect the way Genes are expressed. Our ancestral history and the evolutionary data are encrypted in the DNA. When two different species have similar DNA and Genes, then we can predict that those species might have shared similar Ancestors. For example, humans are said to be evolved from apes. With time, the DNA undergoes changes. While some changes are beneficial for a living being and help them to adapt to the environment and habitat better, other changes might tend to become harmful especially if any defect is involved as this defect can be carried further into the passing generations. The rest of the Genes remain as it is throughout the Evolution.

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HEREDITY

Heredity depends upon two main factors which are Genotype and Phenotype.

There are certain traits that are controlled by the Genes. This is called as Genotype. For example, some facial features and eye color of the person are inherited from one of the parents. To be precise, if one of the parents has the blue color of the eye and it is inherited to the offspring then it is called the ‘blue eye trait’.

The set of characteristics of an individual with regard to the interaction between their genotype and the environment are called Phenotype. It is due to this factor, many features of the individual’s phenotype are not inherited. Phenotype is observable by nature. There is a strong combination of Genes with the environment that presents itself on the appearance and behavior of the organism. Various examples can be accounted for that, ranging from the size of the bird’s beak, stripes on the animals back, and hair color, etc. 

GENE FLOW

Gene flow is also called as migration. This is nothing but carrying the genetic material by an individual from one population to another. This migration takes place by several factors like pollens flying from one place to another, mating between humans of different races and ethnicity will yield to offsprings carrying traits of both parents. For example, an individual of India shifts to the USA, and mates with an individual of the American race will yield to children who have characteristics of both parents. Gene transfer between two species will also lead to the formation of Hybrid organisms. 

Gene flow between different species take place in two ways;

1. Vertical gene transfer: The gene flow between two populations of the same species is done through migration and carried ahead through reproduction, thus Vertical gene transfer from parents to offsprings take place.

2. Horizontal gene transfer: This is the gene transfer between two different species. The genes can transfer from lower organisms to higher organisms or the gene transfer takes place from Endosymbiont to the host. 

The genetic variation of the population depends upon the Gene flow. Gene flow can be restricted within specific species by putting barriers between the population separating them thus maintaining the unique identity of the particular population.

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NATURAL SELECTION

The theory of Natural Selection proposed by Darwin has four major components:

• Inheritance: few traits are indefinitely passed on from parent to offsprings whereas few traits have the influence of the environment and Habitat.

• Variation: Even within a population each individual exhibits different variations like color, body size, features, etc though they belong to the same species.

• High population growth rate: Few species have more offsprings each year than the number of resources present for them. This leads to a struggle for resources between the population.

• Differential survival and reproduction: Those organisms that have the trait which can survive the struggle for resources will lead to reproduction and thus will contribute more offsprings to the future generation.