1. Individuals within a population are not identical. They vary, sometimes only slightly in structure, function and behaviour.
2. Some of this variation is heritable, determined by the organism's genetic make-up, and can be passed from parent to offspring.
3. All living things have a tendency to overproduce. More offspring are produced than are required to keep the population stable.
4. Despite overproduction, populations tend to remain relatively stable. Thus many individuals fail to survive and reproduce.
5. Different individuals leave different numbers od descendants.
More organisms are produced then can possibly survive. This is best illustrated by looking at organisms with a high breeding speed. For instance common brown rats have an average litter of eight young and pregnancy takes only 21 days average. You go figure how long it takes to get one million rats if you start with only two.
(No I can't do maths for crap, that's what I am becoming a biologist for)
There'll be a struggle for existence. At one point the environment will fail in supplying enough food water and space for all organisms produced. Here's were selection comes in. Those individuals best adapted to the environment will logically manage to get the best resources, like food, water, territory, mates, you name it.
Only those best adapted to their environment will survive bad times so they can reproduce for themselves. This how `nature selects' the `fittest', best equipped for surviving.
So the most succesfull indivuals pass on their genes to the next generation. Hereby they spread their genes further trough the population. This happens generation after generation. After a number of generations considerable changes in physiology as a response to the selective environment can be noted.
An extreme case are our common dogs, all dogs belong to the same species, be they a poodle or a golden retriever. In this case man has selected out the dogs with traits they liked, and interbred dogs until they had the desired physical appearance.
Natural selection only works on genetically inheritable characteristics though. Simply because an non genetic adaptation cannot be inherited. A non genetic adaptation can be something an organism learned.
There are for instance chimpansees that have learned to use sticks to catch termites. When a small stick or twig is stuch into a termite hill the termites will start biting the `intruder'. The chimpansee can then just retract the twig and eat the bugs. They can of course teach their young this trick, but the young don't know it by instinct or anything.
Same thing is that human babies don't know language by instinct. The ability to speak has to do with brain, tongue and troath etc. Many genetically inheritable traits have an influence on our ability to speak. But language itself is a learning process.
Natural selection does not make up evolution by itself, but without natural selection evolution would not produce better organisms. In Darwin's Ghost, Steve Jones recounts the classic example of natural selection: As factories sprung up in England, the peppered moth, a gray animal, seemed to turn black. This was natural selection at work. The newly-built factories spewed black smoke into the air. Gray moths, which had been well camouflaged in the past, were now visible to birds while sitting on filthy trees. Many of these moths were killed. Darker moths more often survived to reproduce, and became more and more common. By 1900, in some parts of England, just one peppered moth in fifty was of the gray variety.
This is an example of natural selection that did not progress to speciation, and it is often challenged for this reason. Creationists and others who oppose evolution say things like, "Evolution can make small changes like that, but it can't create a new species." Darwin argues, however, that no "extreme amount of variability is necessary; as man can certainly produce great results by adding up in any given direction mere individual differences, so could Nature, but far more easily, from having incomparably longer time at her disposal." As humans breed dogs through selection of preferred traits, so does nature evolve all species.
As mentioned in the last chapter, this is done in part through the deaths of animals that do not have the traits needed to survive. It is also done through another mechanism: sexual selection. Jones describes evolution as "an examination with two papers." He continues, "The first involves staying alive long enough to have a chance to breed, while the mark in the second depends on the number of progeny." The second is as important as the first.
In some species, such as giraffes, sexual selection takes place through fighting among the males for the rights to the females. (As Jones explains, the length of giraffes' necks is primarily due to their use as clubs; males hit each other with their heads when fighting. This explains why the giraffe's neck is long rather than its legs, which would be just as good for reaching high leaves.) In other species, one gender or the other has a song or plumage that it uses to impress the other. In either case, the result is that animals with certain traits pass on their genes more often than those with other traits, and those genes, and the traits they carry, spread throughout a population.
Natural selection and sexual selection are sometimes in conflict with each other. To quote Jones, "In Uganda in the 1930s, almost every male elephant had tusks, structures evolved (at least in part) as statements of reproductive excellence. Sixty years later, ivory poachers had greatly reduced the number of animals, with those with the largest tusks at greatest risk. Now, a third of adults are tuskless, because the negative effects of ivory on survival outweighed its role in allure." For these elephants, there is a constant balancing act between survival and reproduction.
Natural selection also works against humanity sometimes. Old collections of bacteria, kept in suspended animation, exist going back to 1914. When these bacteria are reanimated, they are susceptible to every antibiotic used today. Contemporary bacteria, however, are not nearly as easy to destroy. Through natural selection, they have evolved resistance to antibiotics. "Twenty years ago," Jones writes, "the drug [penicillin] could kill the bacterium which causes meningitis. In many places—the United States and France included—three quarters can now defy it." At the same time as being a lesson in biology, this is a lesson in caution; if we had not overused antibiotics, this problem would not be nearly so great. Throughout the rest of his chapter, Jones uses examples like this.
Darwin ends his chapter on natural selection by explaining the principle of divergence of character. This section explains several of the toughest parts of the theories of evolution. For instance, why do we get new species at all? Why do perfectly successful species go extinct? The answer to these questions is that, as new varieties of a species evolve through natural selection, they compete vigorously with their parent species for resources. Because the varieties are so similar, they have similar survival needs, and there is simply not enough to go around between the old species or variety and the new one. To quote Darwin, "Hence, all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct."
This tendency of newer species to compete with and destroy older species also explains the "family tree" sort of branching chart that is often drawn to explain evolution. If you look at one of these trees, you'll notice that species generate other species, but that individual species do not appear to survive the entirety of the chart. This type of chart makes sense only if the earlier species die out.
As a result of this constant natural selection, only one in a thousand species that have ever existed are alive today. This is a shocking number when one considers the vast number of species that exist now, but not so shocking when one considers the length of time species have had to evolve and extinguish.
To end our look at this chapter with one more description of natural selection by Darwin, "Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being; and as modern geology has almost banished such views as the excavation of a great valley by a single diluvial wave, so will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure."
< Struggle for Existence | Laws of Variation >
Natural Selection is a mod for Half-Life released on Halloween of 2002 that incorporates elements of real-time strategy into the normal first-person shooter action.
Essentially there are two teams: humans (space marines) and aliens (Kharaa). The human team gets a commander that plays the game from a top-down perspective much like an RTS giving orders that are then carried out by players as well as building structures and keeping track of things. All the other players function a lot like they do in most shooters except they build the buildings the commander places as well as fighting aliens. Aliens, on the other hand, do not have any commander. They get something called hive sight which lets them see each other as well as various buildings and enemies at all times through walls and such (basically sprites marking relative position and color-coded with some text as well).
The overall goal is to destroy the other team and their ability to keep coming back. For humans this means all the aliens and all three of their hives, for aliens all of the humans and the command console inhabited by the commander. In the intervening time there are resources to be gained, area to control, new forms for aliens to evolve into, new weapons and armor for humans to research and purchase, evolutionary traits for aliens to select and so on. In the later stages of the game it's very likely to be fighting cloaking, regenerating, silent aliens shooting acid bombs at you or marines with jetpacks or power armor carrying heavy machine guns and laying waste to all you hold dear. Teamplay is thus an essential component and without it you're toast.
Even an exhaustive writeup would fail to capture all that there is to see and do in this mod. The main site is located at http://www.natural-selection.org
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