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Now, this topic might be controversial to some, but be that as it may, it currently provides the best explanation to how all life on Earth are the way they are. Note that we use the word "currently", as the theory of evolution through natural selection is ever changing with every new evidence discovered: fossils, taxonomy, genetics.
It is quite poetic that one of the most controversial theories in science also goes through the process of evolution. Maybe we'll cover how the idea changed since Darwin, but for now, we must first understand how it works. |
"Natural selection, the blind, unconscious, automatic process which Darwin discovered, and which we now know is the explanation for the existence and apparently purposeful form of all life, has no purpose in mind. It has no mind and no mind’s eye. It does not plan for the future. It has no vision, no foresight, no sight at all. If it can be said to play the role of watchmaker in nature, it is the blind watchmaker."
Richard Dawkins, 'The Blind Watchmaker' (1986)
Natural selection is a central concept of evolution. It was an idea of Charles Darwin and Alfred Russel Wallace, and is sometimes called the survival of the fittest. Darwin chose the name as an analogy with artificial selection (selective breeding).
Natural selection is the process where organisms with favourable traits are more likely to reproduce. In doing so, they pass on these traits to the next generation. Over time this process allows organisms to adapt to their environment. This is because the frequency of genes for favourable traits increases in the population.
Members of a species are not all alike, partly because of differences in heredity (genetics). This is true even with children of the same parents. Some of these differences might make one organism better at surviving and reproducing than others in a particular habitat. When this organism reproduces, its children get the genes, which gave it the advantage. Some adaptations are extremely long-lasting, useful in many habitats. The evolution of wings in birds is an example. Others are good only as long as the environment stays the same. If the environment changes enough, then another organism might do better.
The Process
Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. The premise works thusly:
Examples
A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to fight bacterial diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some have mutations that make them slightly less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. The elimination of individuals which have no resistance is an example of natural selection.
Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge. This leads to what is known as an evolutionary arms race, or co-evolution, in which bacteria continue to develop strains that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. A similar situation occurs with pesticide resistance in plants and insects, and with malarial resistance to quinine.
One famous case study is the study of peppered moth evolution, and there are many other examples. Most of these day-flying moths were light in colour, but just a few of the moths were dark. At first, the light coloured moths survived better because they were camouflaged against the light colour of the nearby trees. This made it hard for birds to see them.
When factories were built, the pollution made all the trees look black. Now the light coloured moths were obvious against the dark bark. The dark coloured moths had the advantage after the environment changed. The genes controlling dark colour spread through the population of moths. After the second world war, controls against pollution worked to make the environment cleaner. Then the lighter moths once again had the advantage, and are now much more common.
Another example: Some harmless insects mimic other insects which are dangerous, or which taste foul. Mimicry evolves because the better mimics survive better. They live to produce more offspring than the less good mimics. The genes of the better mimics become more common in the species. Over time, mimic species get closer to their models. This is the process of evolution by natural selection.
Natural selection is the process where organisms with favourable traits are more likely to reproduce. In doing so, they pass on these traits to the next generation. Over time this process allows organisms to adapt to their environment. This is because the frequency of genes for favourable traits increases in the population.
Members of a species are not all alike, partly because of differences in heredity (genetics). This is true even with children of the same parents. Some of these differences might make one organism better at surviving and reproducing than others in a particular habitat. When this organism reproduces, its children get the genes, which gave it the advantage. Some adaptations are extremely long-lasting, useful in many habitats. The evolution of wings in birds is an example. Others are good only as long as the environment stays the same. If the environment changes enough, then another organism might do better.
The Process
Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. The premise works thusly:
- All living things have such fertility that their population size could increase rapidly forever.
- Actually, the size of populations does not increase to this extent. Mostly, numbers remain about the same.
- Food and other resources are limited. So, there is competition for food and resources.
- No two individuals are alike. Therefore, they do not have the same chance to live and reproduce.
- Much of this variation is inherited. The parents pass the traits to the children through their genes.
- The next generation comes from those that survive and reproduce. The elimination is caused by the relative fit between the individuals and the environment they live in. After many generations, the population has more helpful genetic differences, and fewer harmful ones. Natural selection is really a process of elimination.
Examples
A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to fight bacterial diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some have mutations that make them slightly less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. The elimination of individuals which have no resistance is an example of natural selection.
Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge. This leads to what is known as an evolutionary arms race, or co-evolution, in which bacteria continue to develop strains that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. A similar situation occurs with pesticide resistance in plants and insects, and with malarial resistance to quinine.
One famous case study is the study of peppered moth evolution, and there are many other examples. Most of these day-flying moths were light in colour, but just a few of the moths were dark. At first, the light coloured moths survived better because they were camouflaged against the light colour of the nearby trees. This made it hard for birds to see them.
When factories were built, the pollution made all the trees look black. Now the light coloured moths were obvious against the dark bark. The dark coloured moths had the advantage after the environment changed. The genes controlling dark colour spread through the population of moths. After the second world war, controls against pollution worked to make the environment cleaner. Then the lighter moths once again had the advantage, and are now much more common.
Another example: Some harmless insects mimic other insects which are dangerous, or which taste foul. Mimicry evolves because the better mimics survive better. They live to produce more offspring than the less good mimics. The genes of the better mimics become more common in the species. Over time, mimic species get closer to their models. This is the process of evolution by natural selection.
Sexual Selection
Sexual selection is a special kind of natural selection. It is a theory of Charles Darwin that certain evolutionary traits can be explained by competition within a species. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex". It is usually males who fight each other. Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.) and sometimes referred to as 'weapons'. Traits selected by mate choice are called 'ornaments'.
Females often prefer to mate with males with external ornaments—exaggerated features of morphology. Genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism—features that also benefit females. This idea is known as the 'good genes' hypothesis. Sexual selection is still being researched and discussed today.
Sexual selection is a special kind of natural selection. It is a theory of Charles Darwin that certain evolutionary traits can be explained by competition within a species. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex". It is usually males who fight each other. Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.) and sometimes referred to as 'weapons'. Traits selected by mate choice are called 'ornaments'.
Females often prefer to mate with males with external ornaments—exaggerated features of morphology. Genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism—features that also benefit females. This idea is known as the 'good genes' hypothesis. Sexual selection is still being researched and discussed today.
A male peafowl presenting its tail feathers
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Female deers prefer a strong mate
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For humans, it's success and wealth
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Artificial Selection
Charles Darwin lived in a world where animal husbandry and domesticated crops were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.
Nikolai Vavilov showed that rye, originally a weed, came to be a crop plant by unintentional selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like the wheat, rye was able to become a crop plant in harsh areas, such as hills and mountains.
There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record".
Some have thought that artificial selection could not produce new species. It now seems that it can.
New species have been created by domesticated animal husbandry, but the details are not known or not clear. For example, domestic sheep were created by hybridisation, and no longer produce viable offspring with Ovis orientalis, one species from which they are descended. Domestic cattle, on the other hand, can be considered the same species as several varieties of wild ox, gaur, yak, etc., as they readily produce fertile offspring with them.
The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, Drosophila melanogaster, using a maze with three different choices of habitat such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.
After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.
Diane Dodd was also able to show how reproductive isolation can develop from mating preferences in Drosophila pseudoobscura fruit flies after only eight generations using different food types, starch and maltose.
Ponder this
How many generations does it take to create a new species?
Are there limits to what can be manipulated through artificial selection? Can we give new abilities to a species, or just physically change them?
Discuss
Try to draw a Tree of Life, and see how living things (animals, plants, fungi, humans) you can think of that are related to each other. Discuss their traits, research on their genetic similarities.
Further readings
"On the Origin of Species – Chapter 4: Natural Selection", the specific chapter on natural selection from Darwin's seminal work.
"Principles of Geology", by Charles Lyell, who used geological proof to determine that the Earth was far older than what had been thought.
"An Essay on the Principle of Population", Thomas Malthus' work on pressures against population growth. An important concept in Darwin's work.
