Natural Selection
by Saul McLeod 
published 2007
Only the best adapted to their environment will survive long enough to reproduce, and so pass on their genes to the next generation. A successful organism is one that is well adapted to its environment, so it can pass on it characteristics to its offspring.
The term natural selection refers to the process whereby characteristics (such as strong legs) are maintained and continued in the animal according to their survival value in a particular environment, according to their adaptation value.
The animals who survive the process of competition and go on to breed will tend to have characteristics that are better suited to the environment than those who do not. Thus, there is natural selection or "survival of the fittest" (the term fitness refers to the animal’s ability to survive and produce offspring). But where did these ideas about natural selection originate?
Charles Darwin is the one who is credited with coming up with the idea of natural selection, although there were others too, notably the American, Alfred Wallace.
Darwin noted four things from his observations:
1. All organisms produce an abundance of offspring, many more than is necessary
2. There is a fairly constant population size over time for any particular organism
3. Within a species there is a wide range of features, due to different alleles
4. Some variations are passed on and inherited by the offspring
From these Darwin decided that all organisms struggle for survival and so have to produce many offspring to ensure that some do survive.
He also concluded that for a species to survive, the best (or ‘fittest’) of them must survive long enough to reproduce and pass on their genes.
These assumptions where developed from naturalistic observations throughout the Voyage of the Beagle. During the voyage Darwin visited the Cape Verde Archipelago, the Falkland Islands, the South American coast, the Galapagos Islands, New Zealand and Australia, meeting primitive peoples, seeing natural wonders, and above all, collecting considerable quantities of specimens.
We have mentioned various characteristics of animals that might improve their fitness, but how do these characteristics arise in the first place? How did the first ever giraffe to have an extra long neck come about? It is only by passing on genes that inherited characteristics can be given to a new organism. So where did the gene for the giraffe’s extra long neck (or any other animal’s particular characteristics) come from?
To answer that, we need to go back to the discussion of reproduction. Firstly, recall that the combination of chromosomes at fertilization, half from each parent, yields a completely novel set of chromosomes. This means that this unique individual can inherit a combination of characteristics that give it an advantage over either of its parents. Secondly, occasionally in the process of inheritance a mutation appears by chance in a gene carried by one (or both) of the parents. As a result of this, the offspring is different from either of its parents in a way that could not be predicted. Sometimes this mutation is advantageous to the animal (e.g. a giraffe’s long neck) and sometimes it is not (e.g. if the animal is born without lungs and lives on land). If the mutation is advantageous then the animal will survive and pass on the mutated characteristic to its offspring, if it is disadvantageous it will not survive to pass the characteristic on. Selective pressure refers to any property of environment which tends to favor one form of species over another.
What does all this have to do with Psychology? The genes we have today, that, through interactions with the environment, make us behave in the way that we do, have evolved via the process of natural selection. The nervous system is the physical basis for the processes that make us behave in the way that we do. This system has been shaped by natural selection. Our brains are made up of thousands of millions of neurons and the processes that make us what we are (e.g. consciousness, memory, learning, emotion, intelligence, thought, language) take place somewhere in these neurons in the brain: it is through these neurons that genes exert their influence on behavior.
This should give you some idea of the influence and importance of the biological approach.
Evolution PDF Downloads
Evolutionary Psychology BPS Article
Evolution Downloads
Evolution and Natural Selection
Evolution Websites 
The Institute of Human Origins
Cambridge University: Leverhulme Centre for Human Evolutionary Studies
University of California, Santa Barbara: Department of Anthropology
Washington State University: Overview of Human Origins
Watch Videos about Charles Darwin
How to cite this article:
McLeod, S. A. (2007). . Retrieved from
