Evolution is descent with modification. I'm specifically talking about biological or organic evolution here. It's necessary to make this distinction because the word itself precedes its biological usage. Anyone who adheres to the traditional sense of the word might be misled into thinking that individual organisms transform themselves from one form to another! We can avoid this by making a second bold declaration: Populations evolve, not individuals. Even exceptions to this broad rule can be subsumed when start to see everything as a population; an individual is, after all, a population of cells. Let's skirt around the edges of this issue of levels of evolution (which just begs for a write-up of its own) and stick to the task at hand.
Evolution happens to populations of living things that reproduce. (If there was a population of critters that didn't reproduce, they'd eventually die off. Even immortal critters have to knock off eventually. So, conveniently enough, any living thing that happens to still be walking around is probably reproductive. Even those noding E2.)
It isn't enough to simply reproduce for there to be biological evolution, however. There must also be some system of genetic inheritance. At least some of the generative information contained in the parent must get passed onto the offspring.
Reproduction and genetic inheritance: it so happens that having these two ingredients is enough for biological evolution. How evolution applies strictly to populations may become clearer now: because some individuals may leave more offspring than others (by dumb luck or otherwise -- genetic drift and natural selection) causes the membership of the population to change between generations.
We've had to assume that there are genetic differences between families. One exception to the bare minimum for evolution I gave above is when everyone is genetically identical. The changing makeup of a population that is evolution is strictly genetic change. Pierced noses and dyed hair don't count.
I feel comfortable about making this assumption because genetic variation is virtually guaranteed. Mutations are always being introduced in the genetic code because of mistakes made during replication. Mutation is the fuel of the engine that is evolution.
Where does this leave us? There are two more things that I want to mention about evolution: speciation and evolution's place in society.
Did you notice that the fundamental processes that I mentioned above don't account for the proliferation of species? Heck. Neither did the Origin of Species. Speciation is a special case of evolution.
This write-up is dedicated to the late Stephen Jay Gould, who would have done a much better job of it.
There is a fundamental and common mistake being made here that cmmike has touched upon; and it is also worth elaborating on. As a biological term, evolution refers to heritable change. This can be caused by selection as pi has described, but also by mutation and sampling error (drift). A good analogy for the latter process comes in the form of black and white beans in an urn. A handful of beans will not necessarily reflect the colour proportions within the urn.
Ever since the development of evolution as a principal biological discipline, other disciplines have drawn analogies from evolutionary processes including history, sociology and the computational sciences. It is an important caveat that transmission and reproduction within these systems will have very different constraints than a genetic (flesh and blood) system. Hence, these analogies are often inaccurate and misleading, succeeding only in generating excitement, misplaced legitimacy and confusion.
The development of cumulative changes in the characteristics of populations of organisms from each generation to the next. Evolution occurs by mutations in the structure of the genetic material, and the passing on of these changes from ancestor to descendant. Evolution is a fact; it is the way in which it occurs that is a theory.
Evolution is as much a fact as the earth turning on its axis and going around the sun. At one time this was called the Copernican theory; but, when evidence for a theory becomes so overwhelming that no informed person can doubt it, it is customary for scientists to call it a fact. That all present life descended from earlier forms, over vast stretches of geologic time, is as firmly established as Copernican cosmology. Biologists differ only with respect to theories about how the process operates. - Martin Gardner
Biological evolution can take place in many ways. Many of the other writeups here describe the process in terms of passing along those traits that allow an organism to survive for longer. In theory, organisms that avoid being eaten will have more opportunities to reproduce. That is how we end up with insects that look just like leaves and zebras whose markings make it difficult to tell them apart when they stampede.
As quincyfree described earlier, a species can also evolve as a result of genetic drift and mutation. A salient example of this is the lack of genetic variability between harbor seals. This species was hunted almost to extinction; recently, the number of harbor seals has increased a great deal, but because the current population was derived from a very small population, the genetic difference between individual harbor seals is very small. Genetic drift and mutation are the main causes of evolution in species that reproduce asexually.
One additional force that drives species evolution (at least of sexually reproducing organisms) is sexual selection. Sexual selection works independently of what has so far been referred to as "natural selection," and favors those organisms that have traits that allow them to reproduce more often. Basically, sexual selection acts to preserve the heritable traits that make a particular organism sexy. The classic example of this is the peacock's tail. If you have ever been around a peacock, you know that the big fancy tail is little more than a convenient handle. However, peahens find that same elaborate peacock tail practically irresistable. It is possible that sexual selection is responsible for the entire population of people that are cute, but stupid.
So, in the end, there are basically three traits any individual organism can have that increase the chances that its genes will make it into the next generation. It can be smart (natural selection), it can be lucky (mutation, genetic drift), or it can be attractive (sexual selection).
Good luck transmitting those genes!
What amounts for the similarity among many species?
Creationist- The divine plan of creation (purpose unknown) produced the basic "kinds" of organisms.
Lamarckian- Decent from a common ancestor.
Darwin- Descent from a common ancestor.
Modern Biology- Descent from a common ancestor.
What accounts for the origin of variations among members of a species?
Creationist- Although they can be environmentally caused, they are part of the divine plan of creation.
Lamarckian- Environmentally caused.
Darwin- At times: Unknown causation. At times: environmental changes cause new variations, although the variations may be in any direction.
Modern Biology- Heritable differences are caused by random changes (mutation) in the genetic material. Noninheritable differences are caused by the environment.
What accounts for the presence of particular organs and structures through time?
Creationist- They were initially designed so by the creator. Many present creationists believe that organ defects, diseases, etc., are caused by the fall of humans from divine grace and/or intervention by the devil.
Lamarckian- Use enhances the development of adaptive variations, and disuse eliminates nonadaptive ones.
Darwin- Natural Selection perpetuates only adaptive traits and eliminates nonadaptive traits. At times: use and disuse.
Modern Biology- Primarily natural selection but other forces may be involved. (such as mutation)
What accounts for the variation among species?
Creationist- The seperate creation of each species. Many present creationists believe that the original "kinds" of organisms were perfect, and variations leading to species have been degenerative.
Lamarckian- Each species has responded to different environmental needs by developing new organs or discarding old ones.
Darwin- At times: Selective differences among species account for their changed inheritance. At times: Differences in the use and disuse of particular organs has changed inheritance.
Modern Biology- Changes occur in the genetic material of each species through the process of mutation and the various forces that change gene frequencies.
What accounts for the resemblance of organisms to their parents?
Creationist- Mechanisms unknown, but acquired characters are inherited as part of the divine plan.
Lamarckian- Those characters acquired through use and disuse are inherited through a pangenesislike process.
Darwin- At times: unknown At times: pangenesis
Modern Biology- Transmission of genetic material through the germ plasm.
- From Strickenberger's "Evolution, 3rd Edition"
'Evolution' is an old word, and a plain English word meaning development, unrolling, or increasing complexity. The Webster definitions hint at some of this as much as Webster ever reveals any meaning mutter.
Darwin's theory of evolution is so familiar now, and has been applied so widely, that it seems natural to think of evolution as Darwinian evolution. But really all it means is 'change' in some expanding way. So the solar system has evolved. The English language has evolved. These do not imply that any natural selection, or any analogous process such as artificial selection, has taken place.
The evolution of organisms is an old idea; it might well go back to the Greeks, though I can't think of specific instances: it would be a characteristically Epicurean idea, for one. In modern times Erasmus Darwin and Lamarck put forward theories of evolution. A lot of people before (Charles) Darwin believed that species evolved from other species. They just didn't know how.
(An added confusion is that before Charles Darwin the word was also used for an entirely different theory, Webster's 6.(b), which no-one believes in any more.)
The radically new theory, discovered independently first by Patrick Matthew, then by Charles Darwin, then by Alfred Russel Wallace, was a theory of how evolution occurred. They came up with the idea of natural selection. Their idea is now known to be true, and Lamarck's idea of how evolution occurs is known to be false.
And to round off, Darwinism is this combination: that evolution occurs by means of natural selection; what we may also call "Darwinian evolution".
Gorgonzola has pointed out that in modern scientific discussion the word Darwinism is sometimes used more narrowly, with the added sense of gradualism. This is particularly the case with the Stephen Jay Gould camp who attack more orthodox Neo-Darwinism. But Darwin himself was not committed to gradualism, which was a term in nineteenth-century geology opposed to catastrophism. He phrased his arguments in gradualist terms to emphasize how slight the change needed to explain evolution was.
Evolution is the brainchild of a man named Charles Darwin. It was originally conceived by him on the ship HMS Beagle from 1831 - 1836.
While in the island chain Galapagos Archipelago Mr. Darwin observed interesting discrepancies between finch species. Some finches had curved beaks for probing flowers, while other had long straight beaks, which appeared to have been designed for drilling holes in wood for insect food.
Previously any scientist observing this discrepancy would have assumed that God had simply designed the finches differently, so they would not have to share the same food source. Darwin postulated something different. He suggested that perhaps once in a while a finch would be born with a slightly different beak than its parents. This beak may have been better designed for acquiring food than that of the other finches in the area. Now let's say during the lifetime of this finch the food supply became scarce for some reason or another. While the other finches may starve, the finch with the special beak would have an alternate source of food. This finch would survive and reproduce.
Now although genetics had not really been introduced to the world at that time, it had been observed that offspring often inherited traits from their parents. Mr. Darwin hypothesized that some of the offspring would be likely to share this (no longer quite) unique feature. Should the period of low food supply continue into their lifetime, or should another one occur, the offspring with the trait would be more likely to survive and produce another generation. This time however the offspring produced are more likely to have the specialized beak, especially since chances are higher that both parents will have the desirable trait. The chance of this happening increases over time, so eventually an entire new race of finches will appear, all with the specialized beak. What happened to the original species? The food shortage probably did not kill them all off, and once the problem is resolved it is more than likely their population climbed back to it's previous level. Perhaps they migrated to another island, one more abundant in the resources they require for life.
While this theory is sound, it doesn't account for finches turning into a completely different species. Mr. Darwin theorized that perhaps, over many, many generations, changes like this would "pile up". One resource shortage might cause a change in the race's predatory weapons, allowing them to hunt different types of prey. But this new prey might be harder to digest, leading some time later to the development of different digestive and eating organs. Perhaps this prey is fast so new legs need to be developed. And enhanced intellect certainly has no considerable disadvantages. So these changes keep on piling up, until one day, you have something so wildly different from the original species that it can be classified as an entirely different creature.
This, the theory of macroevolution, is the most widely accepted explanation for the diversity of life on Earth. However debate continues to this day about its accuracy, and recent discoveries in numerous fields cast serious doubt on macroevolution's future as a serious theory, therefore this author shall certainly not attempt to reach any conclusion in the bounds of this document. The goal of this document was merely to overview the theory of evolution from an objective point of view, and that being accomplished, is now concluded.
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