Evolution & Natural Selection

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Darwin Pond Simulation  
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Types of natural selection - selection and change in allele frequency






Evolution and Natural Selection  
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 History of ideas of Life on Earth

17th Century

Most people believed in Creationism, which considered that all life was created just as it is now. This was not based on any evidence, but was instead a belief.

18th Century

Naturalists began systematic classification systems (especially Linnaeus 1707-1778) and noticed that groups of living things had similar characteristics and appeared to be related. So their classifications looked a bit like a family tree.


European naturalists travelled more widely and discovered more fossils, which clearly showed that living things had changed over time, so were not always the same. Extinctions were also observed (e.g. dodo), so species were not fixed.

19th Century

Lamark (1809) proposed a theory that living things changed by inheriting acquired characteristics. e.g. giraffes stretched their necks to reach food, and their offspring inherited stretched necks. This is now known to be wrong, since many experiments (and experience) have shown that acquired characteristics are not inherited, but nevertheless Lamark's theory was the first to admit that species changed, and to try to explain it.


Charles Darwin (1859) published "On the origin of species by means of natural selection, or the preservation of favoured races in the struggle for life", which has been recognised as one of the most important books ever written. A very similar theory was also proposed by Alfred Wallace, and Darwin and Wallace agreed to publish at the same time.


Darwin's Theory of Evolution by Natural Selection   [back to top]

Darwin's theory was based on four observations:

Darwin concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection.

Darwin explained the giraffe's long neck as follows. In a population of horse-like animals there would be random genetic variation in neck length. In an environment where there were trees and bushes, the longer-necked animals were better adapted and so competed well compared to their shorter-necked relatives. These animals lived longer, through more breeding seasons, and so had more offspring. So in the next generation there were more long-neck genes than short-neck genes in the population. If this continued over very many generations, then in time the average neck length would increase. [Today it is thought more likely that the selection was for long legs to run away from predators faster, and if you have long legs you need a long neck to be able to drink. But the process of selection is just the same.]

Darwin wasn't the first to suggest evolution of species, but he was the first to suggest a plausible mechanism for the evolution - natural selection, and to provide a wealth of evidence for it.

Darwin used the analogy of selective breeding (or artificial selection) to explain natural selection. In selective breeding, desirable characteristics are chosen by humans, and only those individuals with the best characteristics are used for breeding. In this way species can be changed over a long period of time. All domesticated species of animal and plant have been selectively bred like this, often for thousands of years, so that most of the animals and plants we are most familiar with are not really natural and are nothing like their wild relatives (if any exist). The analogy between artificial and natural selection is a very good one, but there is one important different - Humans have a goal in mind, nature does not.

Types of Natural Selection - selection and change in allele frequency 
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There are three kinds of Natural Selection.

1.      Directional Selection  [back to top]

This occurs whenever the environment changes in a particular way. There is therefore selective pressure for species to change in response to the environmental change e.g.

Populations do not have to decide to adapt, or mutate, after an environmental change.  The mutation, or combination of alleles giving resistance, have to already be there by chance, otherwise the population may become extinct.

"Environment" includes biotic as well as abiotic, so organisms evolve in response to each other. e.g. if predators run faster there is selective pressure for prey to run faster, or if one tree species grows taller, there is selective pressure for other to grow tall. Most environments do change (e.g. due to migration of new species, or natural catastrophes, or climate change, or to sea level change, or continental drift, etc.), so directional selection is common.


2.      Stabilising (or Normalising) Selection.  [back to top]

This occurs when the environment doesn't change. Natural selection doesn't have to cause change, and if an environment doesn't change there is no pressure for a well-adapted species to change. Fossils suggest that many species remain unchanged for long periods of geological time. One of the most stable environments on Earth is the deep ocean e.g.

Anther example of stabilising can be seen in the birth weight of humans.  The heaviest and lightest babies have the highest mortality and are less likely to survive to reproduce and pass on their alleles.


3.      Disruptive (or Diverging) Selection.  [back to top]

This occurs where an environment change may produce selection pressures that favour two extremes of a characteristic e.g.


Speciation   [back to top]

A species is defined as a group of interbreeding populations that are reproductively isolated from other groups. Reproductively isolated can mean that sexual reproduction between different species is impossible for physical, ecological, behavioural, temporal or developmental reasons. For example horses and donkeys can apparently interbreed, but the offspring (mule) doesn't develop properly and is infertile. This definition does not apply to asexually reproducing species, and in some cases it is difficult distinguish between a strain and a species.

New species usually develop due to:


Geographical Isolation (Allopatric Speciation)   [back to top]

1.       Start with an interbreeding population of one species.


2.       The population becomes divided by a physical barrier such as water, mountains, desert, or just a large distance. This can happen when some of the population migrates or is dispersed, or when the geography changes catastrophically (e.g. earthquakes, volcanoes, floods) or gradually (erosion, continental drift).

3.       If the two environments (abiotic or biotic) are different (and they almost certainly will be), then the two populations will experience different selection pressures and will evolve separately. Even if the environments are similar, the populations may change by random genetic drift, especially if the population is small.

4.       Even if the barrier is removed and the two populations meet again, they are now so different that they can no longer interbreed. They are therefore reproductively isolated and are two distinct species. They may both be different from the original species, if it still exists elsewhere.

It is meaningless to say that one species is absolutely better than another species, only that it is better adapted to that particular environment. A species may be well-adapted to its environment, but if the environment changes, then the species must adapt or die. In either case the original species will become extinct. Since all environments change eventually, it is the fate of all species to become extinct (including our own).


Reproductive Isolation (Sympatric Speciation)  [back to top]

Reproductive Isolation is a type of genetic isolation.  Here the formation of a new species can take place in the same geographical area, e.g. mutations may result in reproductive incompatibility.  A new gene producing, say, a hormone, may lead an animal to be rejected from the mainstream group, but breeding may be possible within its own groups of variants.  When this mechanism results in the production of a new species it is known as sympatric speciation.




Last updated 20/06/2004

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