Charles Darwin's hypothesis includes the ideas of descent with modification due to natural selection and explains how organisms, over time, have become adapted to their environment and how life shows not only unity, but a great diversity.
After traveling on the H.M.S. Beagle from England in 1831 and taking part in a five-year journey, Darwin--(in the role of the ship's naturalist) collected specimens of various plants and animals from South America. He noted that organisms, while similar to others from Europe, were quite different as well. When he reached the Galapagos Islands, he found unique species that resembled live species in South America (the closest land mass) but were unique in their adaptations and found nowhere else on Earth.
Upon his return to England, he pondered all he had seen and in 1859 published his work On The Origin of Species. Another scientist named Alfred Wallace also came up with similar ideas around the same time but Darwin's ideas were much more supported by data and he is credited with the Theory of Evolution by Natural Selection.
"Descent with modification" refers to the idea that all organisms share a common ancestor in the past. This shows unity among living things. The descendants of this ancestor accumulate many diverse modifications, also known as adaptations. These allow them to survive in their unique environment.
Natural selection explains these patterns of evolution, or change over time. Darwin's theory had several key ideas.
The first is variation- no two individuals are exactly alike and vary in their inherited traits. The second is overpopulation or overproduction of offspring, and this refers to the idea that living things produce more offspring than can possibly be supported by the environment. Some will live and reproduce and others will perish. There are limitations to population growth including predators, lack of food, diseases, space, etc.
The next idea is struggle for existence-- a competition for resources will occur and those individuals with traits that give them a higher chance of surviving in relation to others will do so and will probably leave more offspring. This is called differential reproductive success. Those individuals who survive are deemed "fit" if they live as well as reproduce. This idea is called survival of the fittest.
Natural selection will occur--nature acts as the selecting agent where the best adapted individuals survive and reproduce causing favorable traits to accumulate in future generations.
Darwin's theory showed that natural selection increases the number of best adapted individuals to their environment. If that environment should change in the future, natural selection may result in new adaptations being selected for and perhaps a new species will arise with so many accumulated differences from the ancestral species.
I have included a link which shows the mechanism of natural selection using beetles in the diagrams. It may be a nice way to visualize these ideas.
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