Evolution - Darwin's Theory of Evolution, its Causes and its Mechanism

What Is Evolution?

Evolution is the prerequisite process by which species will change with time through variations in their genetic makeup. This is one of the basic concepts in biology explaining diverse life on earth. Before Darwin, there existed ideas on evolution but they needed to be clearly explained. Early ideas advanced by Lamarck and contemporaries held that an organism could change in its lifetime and transmit such changes to its offspring.

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The English naturalist Charles Darwin provided another scientific explanation of the emergence of life through his theory of evolution by natural selection. The work gave a scientific explanation of how species evolve, influencing countless biological disciplines and giving insight into the interrelationship of all life forms.

Darwin's Theory Of Evolution

It has been nurtured with the basic concepts of:

• Natural Selection: Better-fitted organisms for the environment have a greater tendency to survive and reproduce.

• Descent with Modification: Species change over generations to give way to new species.

• Survival of the Fittest: Variations which were advantageous in terms of fighting ability vis-à-vis outside pressures would be passed on to the offspring for future survival, like vying for food sources.

Darwin's trip on the HMS Beagle was quite imperative for the development of his theory. It was in the Galapagos Islands, with its different species of finches with beaks of different shapes adapted to different food sources, that he had made the bulk of his most important observations. Coupled with keen research and experimentation, through these findings among others, it is that Darwin came up with his revolutionary theory expounded in "On the Origin of Species" in 1859.

Causes Of Evolution

Evolution is a result of several factors that introduce and propagate genetic variations in populations.

Genetic Variation

The process of evolution requires genetic variation and this variation has multiple sources:

• Mutations:

Changes to the DNA molecule that arise spontaneously or because of some environmental influence and result in new, potentially novel traits.

• Gene Flow:

Movement of genes between populations because of migration. For instance, genetically determined variations in the shape of the beak in Galapagos finches and variations in the colouration pattern in peppered moths are such examples.

• Environmental Factors:

The environment is not less important in the course of generating species at the hands of selective pressures.The role of environmental changes like temperature, resource availability, or interactions with other species determines which traits become conclusively beneficial or otherwise.

Examples: Climate change may either result in adaptation of the polar bears in the colder environments or the destruction can make habitat forcing for quick adaptation or extinction.

Mechanisms Of Evolution

Several mechanisms result in evolutionary force. Each of these, in some way or another increases the genetic diversity and adaptability of species.

Natural Selection

This is the fundamental mechanism of evolution wherein differential survival and, as a consequence, reproduction takes place for different types. Types of natural selection include -

Stabilising Selection:

A type of selection which favors average traits hence reducing variations.

Directional Selection:

When one extreme is favoured over the other, a shift in traits occurs

Disruptive Selection:

Extreme traits on both ends are selected for thus increasing variability.

Genetic Drift

• Genetic drift involves random changes in allele frequencies in a population:

• Significant in small populations where random events may change genetics drastically.

• Examples: bottleneck effect—a drastic reduction in the size of a population. Founder effect—a new population being started by a few individuals.

Gene Flow

• Gene flow is the exchange of genes between populations:

• Gene flow can result in the addition of new genes to a population leading to a decrease in differences among populations.

• Examples: Individuals of birds migrating between isolated populations or pollen moving between plant populations.

Mutation

• Mutations are random changes of DNA:

• They represent the ultimate source of genetic variation, providing new traits on which natural selection can then act.

Types:

• Point mutations: Change in a single nucleotide

• Chromosomal mutations: Change in the number or structure of chromosomes.

Evidence For Darwin's Theory

The theory of evolution is overwhelmingly supported by a huge amount of evidence from many very different fields of science.

Fossil Record

• Fossils offer a chronological record of the life of the past:

• Fossils show changes in species over time and transitional forms that link major groups.

• Archaeopteryx, a transitional fossil between dinosaurs and birds, and the many hominid fossils tracing human evolution.

Comparative Anatomy

Comparative anatomy examines the anatomy of diverse species and looks for evidence of both similarities and differences:

Homologous Structures:

Structures shared in different species because they have evolved from a common ancestor. A good example is the forelimbs of mammals.

Analogous Structures:

Structures that serve a similar function but of different origins, like the wings of birds and insects.

Molecular Biology

Molecular biology includes genetic material to reveal evolutionary relationships:

DNA Sequencing:

This carries genetic similarities and dissimilarities among species, thus proving common ancestry.

Examples:

Genetic closeness of human beings with chimpanzees, and conserved sequences of crucial genes in all species.

Biogeography

Biogeography is the study of the geographic distribution of species :

Distribution Across the Globe

The geographic distribution of species reflects evolution; for example, unique species on islands from common ancestors.

Examples

Variations of finches on the Galapagos Islands and the distribution of marsupials in Australia.

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