Natural Selection: Definition, Darwin's Theory, Examples & Facts

The concept of natural selection was first proposed formally at a biology conference of the Linnean Society. On July 1, 1858, a joint paper on the subject was presented and subsequently published. It included contributions from Charles Darwin and Alfred Russel Wallace.

Both men wrote about the idea that natural selection contributed to earth's evolution through the survival of organisms most suited to their environment. Scientists at the time realized that evolution took place but did not know how species evolved.

After this introduction of natural selection, Darwin elaborated on the subject with his theory of evolution and his book, On the Origin of Species, published in 1859. His work with Darwin's finches and his ideas on survival of the fittest explained the mechanism of natural selection and how it could lead to a proliferation of many different kinds of organisms.

Evolution is the cumulative change in the characteristics of an organism or a population over the next generations. It is sometimes summarized as **descent with modification**. Natural selection is one of the mechanisms that drives evolution.

To be an active characteristic or trait causing natural selection to take place, the trait has to have the following features:

• Heritability. A trait can only influence evolution through natural selection if it is passed on from parents to descendants.
• Functionality. The trait must have a function. Traits must do something for natural selection to take place.
• Advantage. To be selected for passing on to descendants, the trait must confer an advantage on the organism that has it, or make the organism more fit for survival in its environment.
• Origin. The trait must have caused the organisms to evolve because it made the organisms that had it more fit for survival. If the organisms changed due to another mechanism, such as genetic mutation, it was not due to natural selection.

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.

Within a species, a typical population includes individuals with varying traits because they receive half their _genetic code_ from the father and half from the mother. For traits with a genetic basis, this combination of genes from parents results in a wide variety of characteristics in the individuals of the population.

The combination of traits in some individuals gives them an advantage in looking for food, reproducing or withstanding predators or disease. Other individuals receive traits that place them at a disadvantage.

The advantaged individuals will live longer and produce more descendants. Their descendants will mostly receive genes that result in the advantaged traits. Over time, most of the population will evolve with the advantaged traits, and the traits giving a disadvantage will disappear. Natural selection has selected the individuals with positive characteristics.

In 1831, the British navy sent survey vessel the HMS Beagle on a mapping expedition around the world. Charles Darwin came on board as the naturalist assigned to observe local fauna and flora. The expedition took five years and spent a lot of time along the Atlantic and Pacific coasts of South America.

Upon leaving South America for the Pacific crossing to New Zealand, the ship spent five weeks exploring the Galapagos Islands. As he did everywhere, Darwin took extensive notes about the characteristics of the plants and animals he found. Eventually these notes would form the basis for his development of the concept of natural selection and his theory of evolution.

Back in England, Darwin and an ornithologist associate examined Darwin's notes on the finches of the Galapagos Islands. Apparently the islands were home to 13 different species of finches while the nearest South American land mass 600 miles away had only one species. The main difference between the species was the size and shape of the beaks.

Darwin's analysis of his notes led him to draw the following conclusions:

• The finches had different beaks because they lived on different islands in different environments.
• The environment did not cause the differences in beaks because there was no mechanism for such an influence.
• The different beak characteristics must have all been present in the original finch population.
• As the finches from the original population settled on an island, the finches with the beaks best adapted to the local food supply would have an advantage.
• The finches with beaks best suited to the food source on their island would survive in greater numbers than the less adapted finches.
• Eventually, over many generations, the finches on an island would form a distinct species with a distinct beak size and shape because finches with those beaks would be the fittest for their environment.

With these conclusions, Darwin explained the evolution of the finch beaks in the Galapagos Islands by proposing the mechanism of natural selection. He summarized this mechanism as survival of the fittest, where fitness was defined as reproductive success.

For his conclusions, Darwin relied on his notes, his own observations and his interpretation of the writings of Thomas Robert Malthus. Malthus was an English scholar who, in 1798, published his theory that population growth will always outpace the food supply. The corollary is that, in any population, many individuals will die off due to competition for a limited supply of food.

The three observations that allowed Darwin to develop his theory of evolution and natural selection were:

1. The individuals in a population display a variation in traits such as color, behavior, size and shape due to genetic variation.
2. Some of the traits are passed down from parents to descendants and are heritable.
3. The parents in a population overproduce offspring so that some will not survive.

Based on these observations, Darwin proposed that those individuals with traits that made them fitter would be the ones to survive while the least fit would die off. Over time, the population would be dominated by individual with the traits that made them fitter.

Populations of bacteria exhibit very strong natural selection because they can multiply rapidly. They usually multiply until they reach a constraint such as lack of food, space or other resources. At that point, those bacteria best suited to their environment will survive while the rest will die off.

One example of natural selection in bacteria is the development of _antibiotic resistance_. When bacteria cause an infection and the individual is treated with antibiotics, any bacteria that have the antibiotic-resistance trait will survive while all others will die off. The proliferation of antibiotic-resistant bacteria is a major medical problem.

Plants evolve to become suited to their environment through natural selection. Some plants evolve flower colors to attract pollinators of a specific kind and develop special mechanisms to spread their seeds. They have to adapt to more or less sunlight and fight off pests.

Cacti are an example of natural selection in plants. In the desert where they live, there is lots of sunlight, little water and occasionally an animal that would love a juicy bite.

As a result, cacti have developed compact bodies or small, succulent leaves with thick skins to protect against the strong sun and minimize water loss. They can also store water and have sharp spikes to discourage animals. The cacti with these traits were the fittest, and they are still evolving.

Another example is the change in the field mustard plant caused by the drought in Southern California. To survive a drought, plants must grow, flower and distribute their seeds quickly. The Southern California field mustard plants that flowered early became dominant while those flowering later died out.

Animals have more scope for influencing their survival because they can engage in complex behavior patterns. Traits that can determine fitness fall under three main categories. The ability to find enough food through hunting or foraging is a key for survival.

Most animals have predators, and specific traits allow them to avoid being eaten. Finally, the ability to find and attract a mate allows them to pass their positive traits on to offspring.

Typical characteristics that influence natural selection include:

• Movement. The ability to run, swim or fly fast determines whether an animal can hunt successfully or escape predators.
• Camouflage. If an animal can hide successfully, it can evade predators or ambush prey.
• Immunity. Some animals will be more resistant to a disease than others and will survive.
• Strength. Competing for a mate often involves tests of strength with other members of the same species.
• Senses. Animals that can see, smell or hear better may have a better chance of survival.
• Sexual characteristics. Natural selection in animals depends on successful reproduction after attracting a mate.

Animals evolve continuously, first to better adapt to a given environment and then, if the environment changes, to the new environment. Natural selection can cause evolutionary changes in existing populations and can also favor one species over another if two species are competing for the same space and resources.

Natural selection in animals is best seen when the environment changes in some way, and animals with specific characteristics become better suited and soon become dominant.

For example, the peppered moth in London was light-colored with dark spots. During the industrial revolution, buildings became darkened with soot. Birds could easily see the light-colored moths against the dark background, and soon only dark-colored moths were left. Natural selection favored the moths with more and larger dark spots.

In another example, say some insects become resistant to a chemical pesticide very quickly. Even if only a few individuals are resistant, the rest will die off, and the resistant insects will survive. Insects typically produce large numbers of offspring, so the insects with the resistant genes will rapidly take over.

In an example of reproductive preference, female peacocks choose mates based on the size and brightness of their tails. After the effects of natural selection, almost all peacock males today have large, brightly colored tails.

While Darwin is best known for his publications on the theory of evolution, it is natural selection that powers change and adaptation in species. Charles Darwin's 1858 paper, with contributions from Alfred Russel Wallace whose paper was published at the same time, forever changed how people viewed evolution and the natural changes in plants and animals that continuously took place around them.