body size is not a result of particular characters possessed by the toad). 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. The third condition, inheritance, is only sometimes met not all variation has a heritable basis. Some characters show virtually no variation between members of a species, whilst other characters show considerable variation. The second condition, variation, is often but not always 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. 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. Nevertheless, the vast majority of biologists accept that natural selection is the most important process by which evolution is brought about. The three conditions listed above are necessary and sufficient for natural selection to occur, rather than for evolution to occur. Furthermore, there are factors other than natural selection that affect evolution (some of which are considered in Section 3). In other words, natural selection can occur without evolution. However, it is important to bear in mind that natural selection is also a process that can prevent change, i.e. Evolution through natural selection is our main focus here. change over time, and he proposed a process, natural selection, that could bring about such change. To say that the three conditions are sufficient means that, if all three conditions are met, natural selection will inevitably occur and this can lead to change in the characters of a population from one generation to the next.ĭarwin was concerned with evolution, i.e. 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 necessary means that, unless all three conditions are met, natural selection will not occur. These are said to be the necessary and sufficient conditions for natural selection to occur. These conditions, highlighted in bold above, are a struggle for existence, variation and inheritance. The essence of Darwin's theory is that natural selection will occur if three conditions are met. 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. Individuals produce offspring that tend to resemble their parents (the principle of inheritance). Those with advantageous characters have a greater probability of survival, and therefore of reproducing, in the struggle for existence.
Individuals within a species show variation no two individuals are exactly alike (not even those we call 'identical' twins).
food supply) imposed by the species' environment.Ĭonsequently, there is a struggle for existence, because of the disparity between the number of individuals produced by reproduction and the number that can survive. Within a given species, more individuals are produced by reproduction than can survive within the constraints (e.g. 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. 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.ĭarwin's theory of natural selection can be expressed as four propositions. Selective breeding of domestic species can produce characters in a diversity of forms. Much of The Origin of Species is devoted to detailed descriptions of the adaptations of individual species, for example the various beak shapes of finches on the Galapagos Islands. This aspect of evolution was very apparent to Darwin from the fossil record.Įach species possesses a number of characters that adapt individuals within that species to their way of life and their particular environment. The species that inhabit the Earth today are not the same species that existed in the past, although they do resemble them. Among these, three are particularly emphasized in his theory: While Darwin knew nothing about the mechanism of inheritance, he was very aware of many other aspects of living organisms.
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