Hi,

The theory of evolution by natural selection, first formulated in Darwin's book "On the Origin of Species" in 1859, is the process by which organisms change over time as a result of changes in heritable physical or behavioral traits. Changes that allow an organism to better adapt to its environment will help it survive and have more offspring.

Evolution by natural selection is one of the best substantiated theories in the history of science, supported by evidence from a wide variety of scientific disciplines, including paleontology, geology, genetics and developmental biology.

The theory has two main points, said Brian Richmond, curator of human origins at the American Museum of Natural History in New York City. "All life on Earth is connected and related to each other," and this diversity of life is a product of "modifications of populations by natural selection, where some traits were favored in and environment over others".

The theory is sometimes described as "survival if the fittest," but that can be misleading, Pobiner said. Here, "fitness" refers not to an organism's strength or athletic ability, but rather the ability to survive and reproduce.

Hello student!
Darwin's theory of natural selection states that

• Natural selection is the differential survival and reproduction of individuals due to differences in phenotype.
• It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations.
• Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not.
• Natural selection acts on the phenotype, the characteristics of the organism which actually interact with the environment, .
• Over time, this process can result in populations that specialise for particular ecological niches (microevolution) and may eventually result in speciation (the emergence of new species, macroevolution).
• Genotypes can slowly change by random genetic drift, natural selection remains the primary explanation for adaptive evolution.
• In other words, natural selection is a key process in the evolution of a population.

Hope it helps.

Hello
Darwin's Theory

Darwins theory of evolution entails the following fundamental ideas. The first three ideas were already under discussion among earlier and contemporaneous naturalists working on the species problem as Darwin began his research.  Darwins original contributions were the mechanism of natural selection and copious amounts of evidence for evolutionary change from many sources.  He also provided thoughtful explanations of the consequences of evolution for our understanding of the history of life and modern biological diversity.
Species (populations of interbreeding organisms) change over time and space.  The representatives of species living today differ from those that lived in the recent past, and populations in different geographic regions today differ slightly in form or behavior.  These differences extend into the fossil record, which provides ample support for this claim.

All organisms share common ancestors with other organisms.  Over time, populations may divide into different species, which share a common ancestral population.  Far enough back in time, any pair of organisms shares a common ancestor.  For example, humans shared a common ancestor with chimpanzees about eight million years ago, with whales about 60 million years ago, and with kangaroos over 100 million years ago.   Shared ancestry explains the similarities of organisms that are classified together: their similarities reflect the inheritance of traits from a common ancestor.

Evolutionary change is gradual and slow in Darwins view.  This claim was supported by the long episodes of gradual change in organisms in the fossil record and the fact that no naturalist had observed the sudden appearance of a new species in Darwins time.  Since then, biologists and paleontologists have documented a broad spectrum of slow to rapid rates of evolutionary change within lineages.

The primary mechanism of change over time is natural selection, elaborated below.  This mechanism causes changes in the properties (traits) of organisms within lineages from generation to generation.

The Process of Natural Selection:-

Darwins process of natural selection has four components.

Variation.  Organisms (within populations) exhibit individual variation in appearance and behavior.  These variations may involve body size, hair color, facial markings, voice properties, or number of offspring.  On the other hand, some traits show little to no variation among individualsfor example, number of eyes in vertebrates.
Inheritance.  Some traits are consistently passed on from parent to offspring.  Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.
High rate of population growth.  Most populations have more offspring each year than local resources can support leading to a struggle for resources.  Each generation experiences substantial mortality.
Differential survival and reproduction.  Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.

Thank you