5.15: Theory of Evolution by Natural Selection - Biology

How do new species form?

This is the only illustration in Charles Darwin's 1859 book On the Origin of Species, showing his ideas describing the divergence of species from common ancestors.

Darwin’s Theory of Evolution by Natural Selection

Darwin spent many years thinking about the work of Lamarck, Lyell, and Malthus, what he had seen on his voyage, and artificial selection. What did all this mean? How did it all fit together? It fits together in Darwin’s theory of evolution by natural selection. It’s easy to see how all of these influences helped shape Darwin’s ideas.

For a discussion of the underlying causes of natural selection and evolution see (19:51).

Evolution of Darwin’s Theory

It took Darwin years to form his theory of evolution by natural selection. His reasoning went like this:

  1. Like Lamarck, Darwin assumed that species can change over time. The fossils he found helped convince him of that.
  2. From Lyell, Darwin saw that Earth and its life were very old. Thus, there had been enough time for evolution to produce the great diversity of life Darwin had observed.
  3. From Malthus, Darwin knew that populations could grow faster than their resources. This “overproduction of offspring” led to a “struggle for existence,” in Darwin’s words.
  4. From artificial selection, Darwin knew that some offspring have variations that occur by chance, and that can be inherited. In nature, offspring with certain variations might be more likely to survive the “struggle for existence” and reproduce. If so, they would pass their favorable variations to their offspring.
  5. Darwin coined the term fitness to refer to an organism’s relative ability to survive and produce fertile offspring. Nature selects the variations that are most useful. Therefore, he called this type of selection natural selection.
  6. Darwin knew artificial selection could change domestic species over time. He inferred that natural selection could also change species over time. In fact, he thought that if a species changed enough, it might evolve into a new species.

Wallace’s paper not only confirmed Darwin’s ideas. It also pushed him to finish his book, On the Origin of Species. Published in 1859, this book changed science forever. It clearly spelled out Darwin’s theory of evolution by natural selection and provided convincing arguments and evidence to support it.

Applying Darwin’s Theory

The following example applies Darwin’s theory. It explains how giraffes came to have such long necks (see Figure below).

  • In the past, giraffes had short necks. But there was chance variation in neck length. Some giraffes had necks a little longer than the average.
  • Then, as now, giraffes fed on tree leaves. Perhaps the environment changed, and leaves became scarcer. There would be more giraffes than the trees could support. Thus, there would be a “struggle for existence.”
  • Giraffes with longer necks had an advantage. They could reach leaves other giraffes could not. Therefore, the long-necked giraffes were more likely to survive and reproduce. They had greater fitness.
  • These giraffes passed the long-neck trait to their offspring. Each generation, the population contained more long-necked giraffes. Eventually, all giraffes had long necks.

Giraffes feed on leaves high in trees. Their long necks allow them to reach leaves that other ground animals cannot.

As this example shows, chance variations may help a species survive if the environment changes. Variation among species helps ensure that at least one will be able to survive environmental change.

A summary of Darwin's ideas are presented in the video ‘‘Natural Selection and the Owl Butterfly’’ : (13:29).

KQED: Chasing Beetles, Finding Darwin

It's been over 150 years since Charles Darwin published On the Origin of Species. Yet his ideas remain as central to scientific exploration as ever, and has been called the unifying concept of all biology. Is evolution continuing today? Of course it is.

QUEST follows researchers who are still unlocking the mysteries of evolution, including entomologist David Kavanaugh of the California Academy of Sciences, who predicted that a new beetle species would be found on the Trinity Alps of Northern California. See for more information.

It's rare for a biologist to predict the discovery of a new species. For his prediction, Kavanaugh drew inspiration from Darwin's own 1862 prediction. When Darwin observed an orchid from Madagascar with a foot-long nectar, he predicted that a pollinator would be found with a tongue long enough to reach the nectar inside the orchid's very thin, elongated nectar ‘‘pouch’’, though he had never seen such a bird or insect. Darwin's prediction was based on his finding that all species are related to each other and that some of them evolve together, developing similar adaptations. Darwin's prediction came true in 1903, when a moth was discovered in Madagascar with a long, thin proboscis, which it uncurls to reach the nectar in the orchid's nectar. In the process of feeding from the orchid, the moth serves as its pollinator. The moth was given the scientific name Xanthopan morganii praedicta, in honor of Darwin’s prediction.

As you view Chasing Beetles, Finding Darwin, focus on the following concepts:

  1. the relationship between studying beetles and evolution,
  2. the development of new species,
  3. the relationship between genetic make-up of an organism and evolution,
  4. the role of beneficial mutations,
  5. the role of ‘‘habitat islands’’,
  6. the selection for certain traits among breeders, such as pigeon breeders,
  7. the importance of identifying new species.

For an additional explanation of natural selection, see Darwin, Mice, and Picky Peacocksat


  • Darwin's book On the Origin of Species clearly spells out his theory.
  • Darwin's book also provides evidence and logic to support that evolution occurs and that it occurs by natural selection.

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Use this resource to answer the questions that follow.

  • Charles Darwin & Evolution at
  1. What did Darwin mean by "common descent?"
  2. What did Darwin mean by "gradualism?"
  3. What is meant by "super fecundity?"
  4. What did Darwin say would happen to individuals of the same species in an environment of scarce resources?


  1. Define fitness.
  2. Apply Darwin’s theory of evolution by natural selection to a specific case. For example, explain how Galápagos tortoises could have evolved saddle-shaped shells.
  3. Explain how the writings of Charles Lyell and Thomas Malthus helped Darwin develop his theory of evolution by natural selection.
  4. Discuss the role artificial selection had on Darwin's theory.

Theory of Evolution: Definition, Charles Darwin, Evidence & Examples

In 1831, an inexperienced 22-year-old British naturalist named Charles Darwin jumped on the HMS Beagle and sailed the world on a five-year scientific voyage that earned him a place in science and history.

Known today as the “father of evolution,” Darwin amassed compelling evidence supporting the theory of evolution by natural selection. Earlier scholars, including his grandfather Erasmus Darwin, were mocked for presenting such unorthodox ideas as transmutation of species.

Darwin is credited with being the first scientist to persuasively argue a unifying theory of how species evolve and continue to change.

Darwinism Theory of Evolution (With Criticism) | Biology

In this article we will discuss about the Darwinism theory of evolution with its criticism.

In 1831, Charles Darwin on a voyage on HMS Beagle for five years noted the flora, fauna and geology of the islands of the South Pacific and collected numerous living and fossil specimens. He also sailed to the Galapagos Islands about 600 miles from the west coast of America (Fig. 7a and b).

He observed a number of variations or differences among the organisms that lived on these islands. The common birds of the Galapagos Islands were the finches that were remarkably different from the finches of the mainland. These closely related species of finches had beaks of different shapes and sizes and were adapted for feeding on completely different diets.

In 1838, Darwin read an essay on ‘The Principles of Population’ by Malthus who explained that the rate of reproduction in animal was very rapid and that animal population increases more rapidly than the available food supply. The food supply increases in arithmetic ratio while the population increases in geometric ratio.

Malthus noted that the human population was capable of doubling every 25 years. This increase in population would soon outstrip the food supply, leading to starvation, famine and war, which would ultimately reduce the population.

At the same time, Alfred Wallace, a young English naturalist made similar observations to Darwin. Wallace and Darwin adapted Malthus ideas about how scarce resources could affect populations. Darwin put forth all these ideas in the Journal of Proceedings of Linnean Society in 1859. Darwin also published his observations in a book titled the “The Origin of Species by Natural Selection”. Darwinism is the term coined for the explanation offered by Darwin for the origin of species.

Origin of Species by Natural Selection or Theory of Natural Selection:

The main points of the Theory of Natural Selection are as follows:

a. Over Production or Enormous Fertility:

Living organisms have an innate capacity to produce more individuals to ensure continuity of the race. For example, an oyster may produce over 60-80 million eggs per year. A rabbit produces six young ones in a litter and four litters in a year and the young rabbit becomes reproductively active in six months from birth. A single female salmon produces 28,000,000 eggs in a season.

b. Struggle for Existence:

Organisms multiply in a geometric ratio, while the food supply increases in an arithmetic ratio. This leads to intense competition between organisms to ensure living to obtain maximum amount of food and shelter.

Struggle exists at three levels:

i. Intraspecific struggle is the competition among individuals of the same species or closely related forms. This type of struggle is very severe as the need of the population is the same.

ii. Interspecific struggle is the struggle between organisms of different species living together. Individuals of one species compete with other species for similar requirements.

iii. Struggle with the environment means the various hazards of the nature like extreme heat or cold, excess moisture or drought, storms, earthquakes, volcanoes eruptions, etc. also affect the survival of various organisms.

c. Variations amongst Organisms:

Differences that exist among organisms are called variations. Variations may be harmful, neutral or useful. Variations that are passed on from generation to generation are called heritable variations and these form the raw material for evolution. These variations arise due to changes in the genes or the chromosomes.

d. Survival of the Fittest:

During the struggle for existence, the individuals that exhibit variations beneficial in facing the environment will survive, while those that cannot face the hardship will be eliminated. Those organisms best able to survive and reproduce will leave more offspring than those unsuccessful individuals. This is referred to as survival of the fittest.

According to Darwin, the giraffe exhibited variations in the length of the neck and legs. When the grass on the ground became scarce, giraffes with long necks and legs had an advantage over those with shorter neck and legs, as they could feed on the tall trees. So these forms survived and reproduced and became abundant. Over a period of time, giraffes with short necks starved and became extinct (Fig. 8).

As a result of struggle for existence, variability and inheritance, individuals that are better adapted, survived and became abundant. Slowly over a period of time, this group, which was remarkably different from the original population, becomes established as a new species. This group is also subject to the same forces of change as their ancestors were and this process continues to give rise to new species.

Members of this group may possess variations that may be beneficial to them in another environment. As a result, two or more species may arise from a single ancestral species. Over many generations, unequal reproduction among individuals with different genetic traits changes the overall genetic composition of the population. This is evolution by natural selection. This mechanism can cause a population to change so much, that it becomes a new species. This is known as speciation.

But neither Darwin nor Wallace could explain how the process of evolution occurred how did the inheritable traits, i.e. variations pass on to the next generation? This was because of the fact that during this period, no one knew anything about genetics. During the twentieth century, genetics provided that answer, and was linked to evolution in Neo-Darwinism, also known as Modern Synthesis.

Criticism of Darwinism:

The following points have been raised against the theory of natural selection:

a. Darwin was unable to explain the mechanism of inheritance of characters. Darwin proposed the theory of pangenesis to explain this phenomenon. He said that every cell or organ produces minute hereditary particles called pangenes or gemmules. These were carried through the blood and deposited in the gametes. This theory was not accepted.

b. According to natural selection, only useful organs are favoured by natural selection. The existence of vestigial organs in organisms could not be explained.

c. In some species of deer, the antlers develop beyond the stage of usefulness. These structures are of no functional significance to the animal.

d. Darwin was unable to explain the source of variations in organisms.

Artificial selection is the isolation of natural population and the selective breeding of organisms with characteristic which are useful to humans. In this method, human exert a directional selection pressure that leads to changes in allele and genotype frequencies within the population. This is an evolutionary mechanism which gives rise to new breeds, strains, varieties, races and subspecies.

Darwin studied domestication in plants and animals in detail. He concluded that by artificial selection different varieties of plants and animals could be produced. Cabbage, cauliflower, broccoli, kale, kohlrabi, brussels sprouts could be produced from the common wild mustard (Fig. 9). Similarly, Darwin also raised several types of pigeons from the rock pigeon by artificial selection.

Similarly, the various breeds of fowl are all derived from the jungle fowl, Gallus gallus. Artificial selection has been used by breeders to produce high yielding cows, the Great Dane dog, the Shetland pony, the sleek Arabian horse, etc. The rate of species formation by artificial selection is fast. The analogous process that occurs in nature is natural selection, which proceeds very slowly.

Natural Selection and Darwin's Theory of Evolution

Based on the fossil record, it is clear that species change over time and new species develop while others die off. Before Darwin, there was no explanation of how such changes could take place.

The theory of evolution describes what happens as the characteristics of some individuals of a species become predominant and natural selection describes how this predominance comes about.

Darwin studied natural selection in finches. Even when another mechanism such as mutation changes a population, if the mutation does not confer a natural advantage, it may die out due to natural selection.



It is furthermore believed that life began as a result of spontaneous chemical reactions, which gave rise to a single ancestral cell known as the last universal ancestor. It is believed this hypothetical organism developed either here on Earth or elsewhere through a process commonly called abiogenesis, a strictly naturalistic process that states life can come from non-life. This is completely contradictory to what is already a very well established scientific law of biogenesis.

Common Descent

The theory of evolution purports that the process of biological evolution acting over hundreds of millions of years has given rise to the plethora of organisms on Earth, and therefore evolutionists believe that all lifeforms share a common ancestry.

Darwinists most often point to examples of supposed "homology" as proof of common descent. Because organisms possess a similar cellular makeup and morphological structures, it is argued that there are the result of a shared evolutionary relationship. Creationists instead assert that these traits are merely analogous and derived from being formed by the same creative mind. In reality, common descent is neither observable nor proven, but is nonetheless often touted as being a scientific fact.


In biology, evolution is the change in the inherited traits of a population from generation to generation.

These traits are the expression of genes that are copied and passed on to offspring during reproduction.

Mutations in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms.

New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer.

Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.

Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become more rare.

This occurs because organisms with advantageous traits pass on more copies of these heritable traits to the next generation.

Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment.

In contrast, genetic drift produces random changes in the frequency of traits in a population.

Genetic drift arises from the role chance plays in whether a given individual will survive and reproduce.

One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring.

However, when a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and the selection of novel traits cause the accumulation of differences over generations and the emergence of new species.

The similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.

The theory of evolution by natural selection was proposed roughly simultaneously by both Charles Darwin and Alfred Russel Wallace, and set out in detail in Darwin's 1859 book On the Origin of Species.

In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made.

This powerful explanatory and predictive theory has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.

Watch the video: Η θεωρία της εξέλιξης των ειδών (December 2021).