Difference between revisions of "Natural selection"
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== Variation and Randomness == | == Variation and Randomness == | ||
| − | Natural selection becomes interesting when there is the possibility of random variation in the duplication of individuals--in other words, when an individual is copied, the copy may be slightly different from the original. | + | Natural selection becomes interesting when there is the possibility of random variation in the duplication of individuals--in other words, when an individual is copied, the copy may be slightly different from the original. It is important to note that, as stated above, the ''mechanism'' for this variation is not explained or even taken into account by natural selection. All that matters is that there is ''some'' process that produces variation. In this case, some of the offspring of an individual will be better (where "better" is defined as "more likely to successfully reproduce"--one can think of it as scoring higher on an evaluation) and some will be worse. The higher-scoring offspring will soon come to dominate the population, and over successive generations the average score will also rise. |
==Role in Life Sciences== | ==Role in Life Sciences== | ||
Revision as of 17:02, September 5, 2007
The fundamental statement of natural selection is that heritable traits which are beneficial will tend to become more numerous in successive generations, while heritable traits which are harmful will tend to become more scarce. The traits are defined to be beneficial if they become more numerous in successive generations. This principle is general, and applies to any system of individuals which reproduce; it is most commonly applied to systems of living organisms. It is sometimes summarized by the slogan, survival of the fittest.
Contents
General Requirements
Natural selection will automatically take place in any system which follows a certain set of rules. These rules are:
1) There must be a set of individuals.
2) These individuals must reproduce somehow.
3) The probability or rate of reproduction must be somehow dependent on a heritable trait of the individual (where a heritable trait is one that is more likely to be present in an individual's offspring than it is in the general population).
In any such system, individuals whose traits allow them to reproduce more effectively will soon come to dominate the population.
Natural selection has been called a tautology because it follows from its definition, and because it has no observable consequences by itself that anyone has ever tested against an alternate theory.
Variation and Randomness
Natural selection becomes interesting when there is the possibility of random variation in the duplication of individuals--in other words, when an individual is copied, the copy may be slightly different from the original. It is important to note that, as stated above, the mechanism for this variation is not explained or even taken into account by natural selection. All that matters is that there is some process that produces variation. In this case, some of the offspring of an individual will be better (where "better" is defined as "more likely to successfully reproduce"--one can think of it as scoring higher on an evaluation) and some will be worse. The higher-scoring offspring will soon come to dominate the population, and over successive generations the average score will also rise.
Role in Life Sciences
Natural selection is a principle that was popularized by Charles Darwin and Alfred Russel Wallace, who noted that organisms which were better adapted to their environment tended to survive longer and reproduce more than less well-adapted organisms. They used the term "natural" to mean that it occurred in nature, as opposed to selection performed by animal breeders or by a deity. The term "survival of the fittest" is often attributed to Darwin but was in fact coined by the philosopher and sociologist Herbert Spencer.
Together with mutation (an altogether distinct phenomenon that should not be confused with natural selection), natural selection forms the basis of evolutionary theory. As stated above, in any system where selection combines with random variation, successive generations will become better adapted to reproduce. Evolution is the theory that this combination of genetic variation (which may be either mutation of the parent or, in most cases, sexual reproduction) and natural selection leads to speciation.
Natural selection has been used to explain many organism traits. For example, deer run fast because slower deer have been eaten by predators, and the faster deer are more likely to pass their traits to the next generation. Selection pressure is seldom so one-sided; for example, the long tail of the peacock leaves it vulnerable to predators. However, since peahens are far less likely to mate with short-tailed peacocks, a long tail is an overall advantage and thus is selected for.
An Example
Here is an example of how an engineer might use an algorithm that is analogous to the combination of mutation and natural selection in the evolution of organisms.
An engineer starts with 10,000 different sets of plans for a hydroelectric dam. His dam must complete two basic tasks: it must hold back a lake with minimal flooding of the surrounding area, and it must maximize power generation. The intrinsic worth of a dam is determined by how well it completes these tasks; a dam which produces 100 kW of electricity while only raising water levels 2 feet is superior to one which produces 10 kW of electricity while raising water levels 5 feet. In our simplified example, let us assume that the design of a dam is based on just a few numerical parameters: height, width, thickness, concrete mixture, hydroelectric turbine size, curvature, etc. After evaluating all the dams, the engineer picks the 100 best and discards the rest. He then copies each of these dams 100 times. When copying, the engineer randomly introduces minor differences: say he rolls a die each time he copies a design parameter; if he rolls a six, he then changes that parameter by 1%, flipping a coin to determine if he will increase or decrease the parameter. Once he has finished copying, he will have 10,000 more dams, most of them very similar to the original 100 but with some small differences. He then evaluates these 10,000 dams again, picks the best 100, and copies them again, introducing random "errors" in the same way. After completing this process several thousand times, the engineer has 10,000 dams, all of which perform excellently. He picks the best of them and thus has an excellent design, without ever doing any real "design" himself. Note that this engineer need not be conscious of the overall goal--he is just applying a simple algorithm over and over. Indeed, the engineer need not be conscious of anything at all--his task could be performed automatically and without sentience or intelligence. Note also that the final dam may be very different from any of the original dams.
Local and Non-Local Improvement
The above section illustrates how natural selection and random variation can combine to create improved individuals. However, it is important to note that there is no long-term planning involved in natural selection. This means that all changes must be locally beneficial in order to survive. Essentially, no improvement can take place if a deterioration must take place first. In order for a change to propagate to future "generations" of individuals, it must not be significantly harmful to any generation. In the language of the above example, let us say that one of the dams would be much improved if the curvature were increased by 10%, but would be worsened considerably if the curvature was 3 to 7% higher. Since curvature can only change by 1% at a time, and a regime of "bad" curvature lies between the current regime and the regime of "good" curvature, the dam will never reach the "good" regime. Thus, in order for a large change to take place over several generations, it must be beneficial (or at least not significantly harmful) at each generation. Again returning to the peacock, it might be an overall advantage to have a very short tail or no tail at all, since predators would be easy to avoid. However, since a slightly shorter tail gives little help in avoiding predators and also is much less attractive to females, short tails are unlikely to come about.
