Science Section > Science General Discussion

Evolution for Beginners

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I think we should have an Evolution sticky post to refer to and add to whenever we get an inquiring newbie or even an obstinate creationist. We can cover the basics at first, and then delve into the more nuanced details of the theory.

Anyway, here goes.

What is Evolution?

--- Quote ---In biology, the process of evolution is the change in a population's genetic structure over successive generations. Specifically, it is the change in allele frequency over time. The many sub-processes of evolution account for the diversity of life, such as genetic inheritance, which accounts for the continuity of traits, mutation, which accounts for novel traits, and natural selection, which accounts for the environmental filtering of traits.
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What's an allele?

Processes of Evolution[/u]

Well, what causes these changes in a population's genetic structure over time, anyway? [url=]Mutation plays a key role.

--- Quote ---Initially variation is introduced by mutation, a process which can create new alleles. Mutations are commonly seen as something bad, often associated with cancer, but can also be good, or have no effect at all. Tissue cells divide by mitosis, and cells are dividing constantly in large multi-cellular organisms, like humans. Mutations, therefore, occur in humans every second. These may make the daughter cell more, or less, efficient. They may damage the mechanisms which control mitosis, making the cell divide uncontrollably, forming a tumor, or they may affect a gene which is never expressed in the cell, having no effect. Mutations in mitosis are never passed on, but mutations in meiosis are. Meiosis is the type of cell division which occurs in gametogenesis, the process by which gametes (sex cells) are produced. The same processes of mutation take place in both types of cell division.
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How do cells divide?Process of Gametogenesis

What is Natural Selection?

--- Quote ---Natural selection is described as "survival of the fittest", or a "struggle for survival". There is, of course, no conscious struggle, it is simply that those good at surviving survive, while those who aren't, don't. Genes, therefore, that make an individual good at surviving, also survive. Those which aren't so good don't survive.
Genes are selected for, or against, when there is a selection pressure, or collection of them. A commonly used example of a selection pressure is that of predator and prey running speed. This is also an example of different species "co-evolving" (changes to one species phenotype exert a selection pressure on another species). A population of big cats living in the African grasslands run at approximately the same speed as their preferred prey. By the mechanisms of variation the running speed of the prey increases slightly. The slowest big cats can no longer catch enough food, and they all die out. The average running speed of the population of big cats increases. By the mechanisms of variation the running speed of the big cats increases slightly - the new phenotype might be slightly longer legs or more streamlined body - the individuals who carry the new genes are better at catching prey, live longer and reproduce more. The new gene spreads through the population. By the mechanisms of variation the running speed of the big cats may decrease, the individuals carrying the new gene are less well adapted to hunting and starve, removing the deleterious gene from the gene pool.
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What is a Phenotype?Darwin's Finches

What is Sexual Selection?

--- Quote ---In addition to simple survival, organisms must reproduce for their genes to be passed on to later generations. There are many genes which control the processes involved in sexual reproduction, and this includes the choosing of partners. In sexual selection genes are selected for because genes for a particular phenotype and genes for finding that phenotype desirable in a mate are passed on to offspring such that those genes spread throughout the population.
Sexual selection can be, but does not have to be, exclusive of natural selection. In English slang the word "fit" is often used to describe an attractive body, because attractive bodies are fit and healthy. When humans were hunters and gatherers, people who were fit and healthy were better at providing food for their families, so that their offspring, who carried their genes, did not starve but grew up to pass on their genes.

Sexual selection can occur against the pressures of natural selection, however. Or, at least, against many of the obvious pressures of natural selection. Peacocks and male Birds of Paradise have large bright feathers, which provide no advantage to flying, and might actually impede flight, thus attracting predators. These spectacular feathers have evolved because they attract potential mates.
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Asexual Reproduction

What is Speciation?

--- Quote ---Speciation is the evolutionary process by which new biological species arise.

There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric, parapatric, and sympatric. Speciation may also be induced artificially, through animal husbandry, agriculture, or laboratory experiments.
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Observable speciation
Speciation 2

Please feel free to add more!

Further Evidence of Speciation:


2nd link

Different Types of Speciation  Source

Sympatric Speciation

--- Quote ---Sympatric speciation occurs when populations of a species that share the same habitat become reproductively isolated from each other. This speciation phenomenon most commonly occurs through polyploidy, in which an offspring or group of offspring will be produced with twice the normal number of chromosomes. Where a normal individual has two copies of each chromosome (diploidy), these offspring may have four copies (tetraploidy). A tetraploid individual cannot mate with a diploid individual, creating reproductive isolation.

Sympatric speciation is rare. It occurs more often among plants than animals, since it is so much easier for plants to self-fertilize than it is for animals. A tetraploidy plant can fertilize itself and create offspring. For a tetraploidy animal to reproduce, it must find another animal of the same species but of opposite sex that has also randomly undergone polyploidy.
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What's a polyploid?

Allopatric Speciation

--- Quote ---Allopatric speciation, the most common form of speciation, occurs when populations of a species become geographically isolated. When populations become separated, gene flow between them ceases. Over time, the populations may become genetically different in response to the natural selection imposed by their different environments.
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Parapatric Speciation

--- Quote ---Parapatric speciation is extremely rare. It occurs when populations are separated not by a geographical barrier, such as a body of water, but by an extreme change in habitat. While populations in these areas may interbreed, they often develop distinct characteristics and lifestyles. Reproductive isolation in these cases is not geographic but rather temporal or behavioral. For example, plants that live on boundaries between very distinct climates may flower at different times in response to their different environments, making them unable to interbreed.
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Sympatric Speciation  Source

--- Quote ---Sympatric speciation refers to the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.
Often-cited examples of sympatric speciation are found in insects that become dependent on different host plants in the same area.[10][11] However, the existence of sympatric speciation as a mechanism of speciation is still hotly contested. Scientists have argued that the evidences of sympatric speciation are in fact examples of micro-allopatric, or heteropatric speciation.[/quote
What's Heteropatric speciation?

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Evolution of the Eye

--- Quote ---When evolution skeptics want to attack Darwin's theory, they often point to the human eye. How could something so complex, they argue, have developed through random mutations and natural selection, even over millions of years?

If evolution occurs through gradations, the critics say, how could it have created the separate parts of the eye -- the lens, the retina, the pupil, and so forth -- since none of these structures by themselves would make vision possible? In other words, what good is five percent of an eye?

Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain. Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like structure, a light-sensitive pigmented spot on the skin, could have gone through changes and complexities to form the human eye, with its many parts and astounding abilities.

Through natural selection, different types of eyes have emerged in evolutionary history -- and the human eye isn't even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an "intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design.

Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through.
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--- Quote ---Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.

Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.

In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch.
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Eye Evolution

We're still going to get "but getting an eye doesn't make a fish give birth to a cat".  A thread that explains it all simply enough to a creationist is going to be about as large as one of Dawkins' books.  (It actually takes a few of his books to  fully explain evolution.)

One thing they should be made aware of is that "species" is an arbitrary division invented by man - it's not like the dividing line between land and ocean.  Are the donkey and the horse different species?  The lion and tiger?  Simple question, complex answer.

And would anyone want to post a couple of examples of ring species?


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