# Natural selection

Favorable traits pass on to more successful offspring than ones with less favorable traits, making the traits more common in the population. The Galapagos islands experience a drought and there were less soft, small seeds for the birds to eat. There was increase however with larger and hard seeds. The birds with a bigger beak ate the seeds more easily and passed on those traits. Several philosophers of the classical era, including Empedocles and his intellectual successor, the Roman poet Lucretius, expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by Aristotle in Book II of *Physics*. He posited natural teleology in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof. Nevertheless, he accepted in his biology that new types of animals, monstrosities (τερας), can occur in very rare instances (*Generation of Animals*, Book IV). As quoted in Darwin's 1872 edition of *The Origin of Species*, Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived

> So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, and whatsoever things were not thus constituted, perished, and still perish.
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— Aristotle, *Physics*, Book II, Chapter 8

## Examples

There are now quite a number of examples of natural selection in natural populations.

### Antibiotic resistance

Resistance to antibiotics is increased by the survival of individuals which are immune to the effects of the antibiotic. Their offspring inherit the resistance, creating a new population of resistant bacteria.
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.

### Camouflage

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.

Mimicry is 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.

## Sexual selection

Illustration from *The Descent of Man and selection in relation to sex* by Charles Darwin showing the Tufted Coquette *Lophornis ornatus*, female on left, ornamented male on right.
**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.

Ernst Mayr said:

"Since Darwin’s days it has become clear that this kind of selection includes a far wider realm of phenomena, and instead of sexual selection it is better referred to as *selection for reproductive success*... genuine selection, not elimination, is involved, unlike survival selection. Considering how many new kinds of selection for reproductive success are discovered year after year, I am beginning to wonder whether it is not even more important than survival selection, at least in certain higher organisms".