Evolution for Beginners

[Smartmarzipan]

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?

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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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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What is Natural Selection?

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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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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What is Speciation?

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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Please feel free to add more!

[Fidel_Castronaut]

Further Evidence of Speciation:

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[Smartmarzipan]

Different Types of Speciation  You are not allowed to view links. Register or Login

Sympatric Speciation

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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Allopatric Speciation

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

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.

Sympatric Speciation  You are not allowed to view links. Register or Login

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
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[Smartmarzipan]

Evolution of the Eye

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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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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[Colanth]

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?

[Solitary]

         

The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding.



Should we consider trilobite A as a separate species from trilobite D, and if so, where should we divide the lineage into separate species?
Solitary

[SGOS]

Geeze, Smartzie.  You're really serious about this!

[Smartmarzipan]

Geeze, Smartzie.  You're really serious about this!

I don't fuck around when it comes to SCIENCE!! *Maniacal laughter* *cue lightning bolt*

[Smartmarzipan]

What is a Ring Species?  You are not allowed to view links. Register or Login

Some critics of the theory of evolution argue that it doesn't convincingly explain the origin of new species. They say that members of one species couldn't become so different from other individuals through natural variation that they would become two separate non-interbreeding species.

One of the most powerful counters to that argument is the rare but fascinating phenomenon known as "ring species." This occurs when a single species becomes geographically distributed in a circular pattern over a large area. Immediately adjacent or neighboring populations of the species vary slightly but can interbreed. But at the extremes of the distribution -- the opposite ends of the pattern that link to form a circle -- natural variation has produced so much difference between the populations that they function as though they were two separate, non-interbreeding species.

In concept, this can be likened to a spiral-shaped parking garage. A driver notices only a gentle rise as he ascends the spiral, but after making one complete circle, he finds himself an entire floor above where he started.

A well-studied example of a ring species is the salamander Ensatina escholtzii of the Pacific Coast region of the United States. In Southern California, naturalists have found what look like two distinct species scrabbling across the ground. One is marked with strong, dark blotches in a cryptic pattern that camouflages it well. The other is more uniform and brighter, with bright yellow eyes, apparently in mimicry of the deadly poisonous western newt. These two populations coexist in some areas but do not interbreed -- and evidently cannot do so.

Moving up the state, the two populations are divided geographically, with the dark, cryptic form occupying the inland mountains and the conspicuous mimic living along the coast. Still farther to the north, in northern California and Oregon, the two populations merge, and only one form is found. In this area, it is clear that what looked like two separate species in the south are in fact a single species with several interbreeding subspecies, joined together in one continuous ring.

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The evolutionary story that scientists have deciphered begins in the north, where the single form is found. This is probably the ancestral population. As it expanded south, the population became split by the San Joaquin Valley in central California, forming two different groups. In the Sierra Nevada the salamanders evolved their cryptic coloration. Along the coast they gradually became brighter and brighter.

The division was not absolute: some members of the sub-populations still find each other and interbreed to produce hybrids. The hybrids look healthy and vigorous, but they are neither well-camouflaged nor good mimics, so they are vulnerable to predators. They also seem to have difficulty finding mates, so the hybrids do not reproduce successfully. These two factors keep the two forms from merging, even though they can interbreed.

By the time the salamanders reached the southernmost part of California, the separation had caused the two groups to evolve enough differences that they had become reproductively isolated. In some areas the two populations coexist, closing the "ring," but do not interbreed. They are as distinct as though they were two separate species. Yet the entire complex of populations belongs to a single taxonomic species, Ensatina escholtzii.

Ring species, says biologist David Wake, who has studied Ensatina for more than 20 years, are a beautiful example of species formation in action. "All of the intermediate steps, normally missing, have been preserved, and that is what makes it so fascinating."

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[Smartmarzipan]

The Ages of the Earth

Geologic Time Scale  You are not allowed to view links. Register or Login

The geologic time scale (GTS) is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth's history. The table of geologic time spans presented here agrees with the dates and nomenclature set forth by the International Commission on Stratigraphy standard color codes of the International Commission on Stratigraphy.

Evidence from radiometric dating indicates that the Earth is about 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating major geological or paleontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous�Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by the absolute age.

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What Epoch Do We Live In?

The Holocene  You are not allowed to view links. Register or Login

The Holocene is a geological epoch which began at the end of the Pleistocene (at 11,700 calendar years BP) and continues to the present. The Holocene is part of the Quaternary period. It has been identified with the current warm period, known as MIS 1 and based on that past evidence, can be considered an interglacial in the current ice age.

he Holocene also encompasses within it the growth and impacts of the human species world-wide, including all its written history and overall significant transition toward urban living in the present. Human impacts of the modern era on the Earth and its ecosystems may be considered of global significance for future evolution of living species, including approximately synchronous lithospheric evidence, or more recently atmospheric evidence of human impacts. Given these, a new term Anthropocene, is specifically proposed and used informally only for the very latest part of modern history and of significant human impact since the epoch of the Neolithic Revolution (around 12,000 years BP).

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[Smartmarzipan]

I'm having a lot of fun doing this, so I'm just going to keep going. <!-- s:) -->:)<!-- s:) --> Everyone is welcome to add whatever pieces of information they want! The more, the better.

Introduction to Human Evolution  You are not allowed to view links. Register or Login

Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called �pygmy chimpanzees�) and gorillas -- share a common ancestor that lived between 8 and 6 million years ago.

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Human evolution is the evolutionary process leading up to the appearance of modern humans. While it began with the last common ancestor of all life, the topic usually covers only the evolutionary history of primates, in particular the genus Homo, and the emergence of Homo sapiens as a distinct species of hominids (or "great apes"). The study of human evolution involves many scientific disciplines, including physical anthropology, primatology, archaeology, linguistics, evolutionary psychology, embryology and genetics.

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Evidence

The closest living relatives of humans are bonobos and chimpanzees (both genus Pan) and gorillas (genus Gorilla). With the sequencing of both the human and chimpanzee genome, current estimates of the similarity between their DNA sequences range between 95% and 99%. By using the technique called the molecular clock which estimates the time required for the number of divergent mutations to accumulate between two lineages, the approximate date for the split between lineages can be calculated. The gibbons (family Hylobatidae) and orangutans ( genus Pongo) were the first groups to split from the line leading to the humans, then gorillas followed by the chimpanzees and bonobos. The splitting date between human and chimpanzee lineages is placed around 4-8 million years ago during the late Miocene epoch.

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There is little fossil evidence for the divergence of the gorilla, chimpanzee and hominin lineages. The earliest fossils that have been proposed as members of the hominin lineage are Sahelanthropus tchadensis dating from 7 million years ago, Orrorin tugenensis dating from 5.7 million years ago and Ardipithecus kadabba dating to 5.6 million years ago. Each of these have been argued to be a bipedal ancestor of later hominins but, in each case, the claims have been contested. It is also possible that one or more of these species are ancestors of another branch of African apes, or that they represent a shared ancestor between hominins and other apes.

The question of the relationship between these early fossil species and the hominin lineage is still to be resolved. From these early species, the australopithecines arose around 4 million years ago and diverged into robust (also called Paranthropus) and gracile branches, one of which (possibly A. garhi) probably went on to become ancestors of the genus Homo. The australopithecine species that is best represented in the fossil record is Australopithecus afarensis with more than one hundred fossil individuals represented, found from Northern Ethiopia (such as the famous "Lucy"), to Kenya, and South Africa. Fossils of robust australopithecines such as A. robustus (or alternatively Paranthropus robustus) and A./P. boisei are particularly abundant in South Africa at sites such as Kromdraai and Swartkrans, and around Lake Turkana in Kenya.

The earliest members of the genus Homo are Homo habilis which evolved around 2.3 million years ago. Homo habilis is the first species for which we have positive evidence of the use of stone tools. They developed the oldowan lithic technology, named after the Olduvai gorge in which the first specimens were found. Some scientists consider Homo rudolfensis, a larger bodied group of fossils with similar morphology to the original H. habilis fossils, to be a separate species while others consider them to be part of H. habilis - simply representing species internal variation, or perhaps even sexual dimorphism. The brains of these early hominins were about the same size as that of a chimpanzee, and their main adaptation was bipedalism as an adaptation to terrestrial living.

During the next million years, a process of encephalization began and, with the arrival of Homo erectus in the fossil record, cranial capacity had doubled. Homo erectus were the first of the hominina to leave Africa, and this species spread through Africa, Asia, and Europe between 1.3 to 1.8 million years ago. One population of H. erectus, also sometimes classified as a separate species Homo ergaster, stayed in Africa and evolved into Homo sapiens. It is believed that these species were the first to use fire and complex tools.

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For almost a century after Darwin, most views on human origins posited a linear progression (orthogenesis) from an ape to modern humans. This is often illustrated as a march of hominids (see picture at right), usually starting with chimpanzees, which become more human-like as they progress across the page (usually left to right). This view echoed the religious ideas of the time, namely the Great Chain of Being (a ladder of forms, with humans as the top of the physical forms, just below the angels and God). Even great evolutionary scientists, such as Thomas Huxley, were of the opinion that humans were a very special type of animal, somehow the pinnacle of biological evolution.

This anthropocentric view began to diminish as paleoanthropologists found it difficult to find fossils that fit within this progressive lineage. During the 1950s and 1960s, fossil finds of primates and humans led to the conclusion that humans did not evolve from an extant ape species, but both apes and humans evolved independently from a common ancestor (which would be classified as an ape). Indeed, independently divergent evolution became a central view throughout evolutionary theory, and scientists began to appreciate the many speciation events that led to humans. No longer was it assumed, as Theodosius Dobzhansky and Ernst Mayr had proposed, that only one species of hominin could exist at once. Molecular genetics confirmed the view of cladogenesis and multiple extinction events, rather than anagenetic human origins.

 
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[MrsSassyPants]

Geez this has been incredibly interesting to read.  And only SLIGHTLY over my head.  Thanks yall!!!!

[Icarus]

Exploring the origins of life:

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[josephpalazzo]

A good video in You are not allowed to view links. Register or Login from Szostak, the Medicine Nobel Prize winner.

[AllPurposeAtheist]

I gotta tell you.. very few, if ANY of the religious nitwits I know would bother reading much past: see evolution sticky note..
God would certainly render them blind and they automatically qualify for the government dole..
Sorry to be such a stick in the mud. :)