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Natural Selection

Natural selection is the process through which species adapt to their environments. It is the engine that drives evolution.

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ENCYCLOPEDIC ENTRY 179

Natural Selection

Natural selection is the process through which species adapt to their environments. It is the engine that drives evolution.

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On the Origin of Species

English naturalist Charles Darwin wrote the definitive book outlining his idea of natural selection, On the Origin of Species. The book chronicled his studies in South America and Pacific islands. Published in 1859, the book became a best seller.

PHOTOGRAPH BY IAN FORSYTH VIA GETTY IMAGES

English naturalist Charles Darwin developed the idea of natural selection after a five-year voyage to study plants, animals, and fossils in South America and on islands in the Pacific. In 1859, he brought the idea of natural selection to the attention of the world in his best-selling book, On the Origin of Species.

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others. Individuals with adaptive traits—traits that give them some advantage—are more likely to survive and reproduce. These individuals then pass the adaptive traits on to their offspring. Over time, these advantageous traits become more common in the population. Through this process of natural selection, favorable traits are transmitted through generations.

Natural selection can lead to speciation, where one species gives rise to a new and distinctly different species. It is one of the processes that drives evolution and helps to explain the diversity of life on Earth.

Darwin chose the name natural selection to contrast with “artificial selection,” or selective breeding that is controlled by humans. He pointed to the pastime of pigeon breeding, a popular hobby in his day, as an example of artificial selection. By choosing which pigeons mated with others, hobbyists created distinct pigeon breeds, with fancy feathers or acrobatic flight, that were different from wild pigeons.

Darwin and other scientists of his day argued that a process much like artificial selection happened in nature, without any human intervention. He argued that natural selection explained how a wide variety of life forms developed over time from a single common ancestor.

Darwin did not know that genes existed, but he could see that many traits are heritable—passed from parents to offspring.

Mutations are changes in the structure of the molecules that make up genes, called DNA. The mutation of genes is an important source of genetic variation within a population. Mutations can be random (for example, when replicating cells make an error while copying DNA), or happen as a result of exposure to something in the environment, like harmful chemicals or radiation.

Mutations can be harmful, neutral, or sometimes helpful, resulting in a new, advantageous trait. When mutations occur in germ cells (eggs and sperm), they can be passed on to offspring.

If the environment changes rapidly, some species may not be able to adapt fast enough through natural selection. Through studying the fossil record, we know that many of the organisms that once lived on Earth are now extinct. Dinosaurs are one example. An invasive species, a disease organism, a catastrophic environmental change, or a highly successful predator can all contribute to the extinction of species.

Today, human actions such as overhunting and the destruction of habitats are the main cause of extinctions. Extinctions seem to be occurring at a much faster rate today than they did in the past, as shown in the fossil record.

adapt Verb

to adjust to new surroundings or a new situation.

DNA Noun

(deoxyribonucleic acid) molecule in every living organism that contains specific genetic information on that organism.

evolution Noun

change in heritable traits of a population over time.

extinction Noun

process of complete disappearance of a species from Earth.

gene Noun

part of DNA that is the basic unit of heredity.

genetic Adjective

having to do with genes, inherited characteristics or heredity.

invasive species Noun

type of plant or animal that is not indigenous to a particular area and causes economic or environmental harm.

molecule Noun

smallest physical unit of a substance, consisting of two or more atoms linked together.

mutation Noun

sudden variation in one or more characteristics caused by a change in a gene or chromosome.

natural selection Noun

process by which organisms that are better -adapted to their environments produce more offspring to transmit their genetic characteristics.

predator Noun

animal that hunts other animals for food.

speciation Noun

process by which one or more populations of a species become genetically different enough to form a new species.

species Noun

group of similar organisms that can reproduce with each other.

Source : education.nationalgeographic.org

Evolution through natural selection

In this free course, Evolution through natural selection, we describe the theory of evolution by natural selection as proposed by Charles Darwin in his book, first published in 1859, On the Origin ...

2 Darwin and natural selection

While Darwin knew nothing about the mechanism of inheritance, he was very aware of many other aspects of living organisms. Among these, three are particularly emphasized in his theory:

The species that inhabit the Earth today are not the same species that existed in the past, although they do resemble them. This aspect of evolution was very apparent to Darwin from the fossil record.

Each species possesses a number of characters that adapt individuals within that species to their way of life and their particular environment. Much of is devoted to detailed descriptions of the adaptations of individual species, for example the various beak shapes of finches on the Galapagos Islands.

Selective breeding of domestic species can produce characters in a diversity of forms. For example, dog breeders have produced numerous breeds that differ in characters such as ear length, stature and behaviour: different breeds have different forms of a character.

Darwin's theory of natural selection can be expressed as four propositions. These propositions are so important to an understanding of evolution through natural selection that you should try to remember them, although not necessarily word-for-word.

Darwin's four propositions

Within a given species, more individuals are produced by reproduction than can survive within the constraints (e.g. food supply) imposed by the species' environment.

Consequently, there is a struggle for existence, because of the disparity between the number of individuals produced by reproduction and the number that can survive.

Individuals within a species show variation; no two individuals are exactly alike (not even those we call 'identical' twins). Those with advantageous characters have a greater probability of survival, and therefore of reproducing, in the struggle for existence.

Individuals produce offspring that tend to resemble their parents (the principle of inheritance). Provided that the advantageous characters that promote survival are inherited by offspring, individuals possessing those characters will become more common in the population over successive generations because they are more likely than individuals not possessing those characters to survive and produce offspring in the next generation.

The essence of Darwin's theory is that natural selection will occur if three conditions are met. These conditions, highlighted in bold above, are a struggle for existence, variation and inheritance. These are said to be the conditions for natural selection to occur. To say that the three conditions are means that, unless all three conditions are met, natural selection will not occur. Thus, it will not occur if reproduction does not produce more progeny than can survive, it will not occur if a character does not show variation, and it will not occur if variation does not have a heritable basis. To say that the three conditions are means that, if all three conditions are met, natural selection will inevitably occur and this lead to change in the characters of a population from one generation to the next.

Darwin was concerned with evolution, i.e. change over time, and he proposed a process, natural selection, that could bring about such change. Evolution through natural selection is our main focus here. However, it is important to bear in mind that natural selection is also a process that can change, i.e. promote stability. In other words, natural selection can occur evolution. Furthermore, there are factors other than natural selection that affect evolution (some of which are considered in Section 3). The three conditions listed above are necessary and sufficient for natural selection to occur, rather than for evolution to occur. Nevertheless, the vast majority of biologists accept that .

Let us look a little more closely at the three necessary and sufficient conditions and consider how likely it is that they will be met. The first, a struggle for existence, is probably almost always met, because living organisms produce more progeny than are required to replace their parents when they die. The second condition, variation, is often but not always met. Some characters show virtually no variation between members of a species, whilst other characters show considerable variation. The third condition, inheritance, is only sometimes met; not all variation has a heritable basis. For example, toads vary in size. The two factors which make the largest contribution to variation in the body size of toads are variation in age (toads continue to grow throughout their lives) and variation in their environment (e.g. a good food supply). These are both external causes (i.e. body size is not a result of particular characters possessed by the toad). So body size in toads is not primarily an inherited character.

This last point brings us to an important aspect of natural selection, which was much discussed when Darwin first proposed his theory. This debate concerns the possible . As well as growing, individual organisms may develop particular skills or physical characters during the course of their lives as a result of differences in the way they live. Consider the human practices of ear-piercing, circumcision and decorative body scars. These characters, which are acquired deliberately during the course of an individual's life, are not inherited by that individual's offspring even though the practice may have been carried out for hundreds of generations. Likewise, a plant that has grown particularly large in a patch of good ground, or a toad that has grown very big because it lives in a garden full of food, will not pass their large size on to their progeny. So, inheritance of characters does not occur.

Source : www.open.edu

What is natural selection?

Discover what natural selection theory is, how adaptations work, the story of Darwin's finches and whether we are still evolving.

WHAT ON EARTH?

What is natural selection?

By Emily Osterloff 465

Natural selection is a mechanism of evolution. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success. This process causes species to change and diverge over time.

Natural selection is one of the ways to account for the millions of species that have lived on Earth.

Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) are jointly credited with coming up with the theory of evolution by natural selection, having co-published on it in 1858. Darwin has generally overshadowed Wallace since the publication of On the Origin of Species in 1859, however.

The Museum's Library holds the world's largest concentration of Darwin works, with 478 editions of On the Origin of Species in 38 languages. You can get your own copy of this famous work, based on an original edition, from the Museum's shop.

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In Darwin and Wallace's time, most believed that organisms were too complex to have natural origins and must have been designed by a transcendent God. Natural selection, however, states that even the most complex organisms occur by totally natural processes.

Prof Adrian Lister, a researcher at the Museum says, 'It's not that biologists don't understand that organisms are complex and functional, and it does seem almost miraculous that they exist. We realise that, but we think we've found another way of explaining it.'

Wallace (L) and Darwin (R) came up with very similar theories on evolution. Darwin has generally overshadowed Wallace's contributions, however.

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How does natural selection work?

In natural selection, genetic mutations that are beneficial to an individual's survival are passed on through reproduction. This results in a new generation of organisms that are more likely to survive to reproduce.

For example, evolving long necks has enabled giraffes to feed on leaves that others can't reach, giving them a competitive advantage. Thanks to a better food source, those with longer necks were able to survive to reproduce and so pass on the characteristic to the succeeding generation. Those with shorter necks and access to less food would be less likely to survive to pass on their genes.

The evolution of a long neck is an adaptation that helps giraffes survive in their environment © FluffyCreature via Flickr (CC BY-NC 2.0)

Adrian explains, 'If you took 1,000 giraffes and measured their necks, they're all going to be slightly different from one another. Those differences are at least in part determined by their genes.

'The ones with longer necks may leave proportionally more offspring, because they have fed better and have maybe been better in competing for mates because they are stronger. Then, if you were to measure the necks of the next generation, they're also going to vary, but the average will have shifted slightly towards the longer ones. The process carries on generation after generation.'

What is an adaptation?

An adaptation is a physical or behavioural characteristic that helps an organism to survive in its environment.

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But not all characteristics of an animal are adaptations.

Adaptations for one purpose can be co-opted for another. For instance, feathers were an adaptation for thermoregulation - their use for flight only came later. This means that feathers are an exaptation for flight, rather than an adaptation.

Adaptations can also become outdated, such as the tough exterior of the calabash fruit (). This gourd is generally thought to have evolved to avoid being eaten by Gomphotheres, a family of elephant-like animals. But these animals went extinct around 10,000 years ago, so the fruit's adaptation no longer has a survival benefit.

The large, spherical calabash fruit has an extremely tough exterior. But this adaptation is now outdated  © Wendy Cutler via Flickr (CC BY-NC 2.0)

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Selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift or gene flow.

What does 'survival of the fittest' mean?

In terms of evolution, an animal that is 'fit' is one that is adapted to its environment. This concept is at the core of natural selection, although the term 'survival of the fittest' has often been misunderstood and may be best avoided.

There is also a degree of randomness to evolution, so the best-adapted animal won't always be the one to survive.

Source : www.nhm.ac.uk