Shadow Wraith

I've currently finished an article on evolution which I plan on posting on the net. I need someone to review it for any mistakes I might have made or any suggestions for change to make it better. I don't have it uploaded to a viewable webpage yet, so I hope you don't mind me posting it here.
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Evolution

In the 19th century, Charles Darwin hit upon one of the most important discoveries ever made. He found out that species were not fixed, but they rather changed over time, bringing new species into existence over the course of Earth's history, and that all species developed from common ancestors. While the idea of the mutability of species was not totally original (people like Jean-Baptiste Lamarck asserted that the fixity of species was an illusion long before Darwin did, as did groups such as saltationists, transmutationists, and the like), Darwin was the first to develop it into the scientific theory we recognize today. What was especially remarkable about his theory was that not only did he propose a descriptive process of an observation of nature, he also proposed a plausible causal mechanism: natural selection. In my personal opinion, Darwin is one of the greatest scientific geniuses to ever live, his exemplary use of the empirical method being something to live up to. We will now go into a basic description of the Theory of Evolution (more detailed info can be found in the links page).


Evolutionary Mechanisms

There are several mechanisms that bring about changes in populations of organisms and the eventual rise of new species. Such mechanisms include genetic drift, recombination, and gene flow. The most familiar, though, is natural selection. This process favors traits that allow populations within a species to survive in its particular environment--kind of like a naturally occurring form of the selective breeding humans use to create new breeds/species of animals (dogs, pigeons, fruit flies, etc.). Traits that are advantageous to the survival of populations withing a particular species are favored by natural selection, and thus the individuals in the population tend to live longer and therefore leave the most offspring. These offspring inherit the genes of their parent(s), thus resulting in further spread of the selected traits throughout the population. Characteristics that are not advantageous to the survival of a population are not favored by selection. As such, poorly adapted organisms tend to not live long and leave less offspring and eventually that population in a particular environment becomes either extinct or a rarity.

A great example of natural selection lies within our own species: the connection between sickle cell anemia and malaria. Malaria is a highly lethal disease caused by the protozoan Plasmodium falciparum. It is transmitted through mosquito bites and is common in Central America, northern South America, and especially sub-Saharan Africa. Sickle cell anemia is a hemoglobin deficiency carried by a mutant allele of the hemoglobin gene. The normal hemoglobin gene is designated HbA while the mutant gene is called HbS. The genes are carried in pairs, and as such can be arranged into any of three configurations--HbA/HbA, HbA/HbS, and HbS/HbS. People with two copies of the HbS gene tend to not live very long, usually dying before they are old enough to have children. However, people with only one copy suffer the effects of the anemia much less and tend to live a lot longer. It turns out that people with normal hemoglobin (two copies of the HbA allele) are highly supsceptible to malaria. However, the mutant HbS allele confers immunity to malaria, even if an individual has only one copy of it. So in areas where malaria is common, people that have two copies of either the HbA OR the HbS gene tend to not survive long. A human having normal blood where malaria is rampant does not have survival advantage over a person with double sickle cell alleles! Since people with one copy of the HbS gene live longer than people with two copies, and since said mutant gene gives immunity to malaria, they live longer and as a result produce the most offspring. In areas where malaria is rampant, natural selection favors the HbA/HbS configuration of hemoglobin genes as it gives its possessors the best overall survival/reproductive advantage.


Evolution's Flow: The Origin of Life, the Fossil Record, Extinction, and the Future of Life

The first chemical signs of life date back some 3.8 billion years, and the oldest known fossils--cyanobacteria--date back 3.5 billion years. But how did life get here? The answer lies in a similar yet separate field called "abiogenesis" research--the study of the prebiotic chemical evolution of the Earth and how it led to life's origin. Currently, no one knows for sure exactly how life came to be, but a few ideas exist. Among those are Sidney Fox's "protenoid microsphere" experiments and the "RNA World" theory--two of the more widely accepted abiogenesis theories. While we are far from knowing exactly how life originated, we do know that it is possible for life to come from non-life. Viruses are a great example of a transitional form between life and non-life (though the early proto-life was obviously not what we would identify as viral). Whatever the cause may be for life's origins, we know that any theory describing it must conform to what we know about the environment and conditions of early Earth and with chemistry/biochemistry. Note that abiogenesis should NOT be lumped into the same category as spontaneous generation of complex organisms from non-living matter (flies from garbage, bacteria from soup, etc.) which was disproved by Louis Pasteur. Prebiotic chemical evolution is just like biological evolution--it is a gradual stepwise process. Just as a "walking" catfish doesn't suddenly give birth to a salamander, the first proto-cells or even the first nucleic acids did not just spotaneously assemble from simple compounds.

Though we are just beginning to understand the origins of life, its progress from the first microbes to the millions of species alive today is a much better understood subject. The fossil record, though spotty in many places, testifies to the development of life over the last few billion years. I will focus here on the basic phylogeny of life on Earth, specifically the origins of major groups & taxons, animal phyla, and the development of humans from their ancestors. After the origins of life some 3.5-3.8 billion years ago, organisms remained unicellular for a long time. The first unicellular organisms were prokaryotes, which were followed by the devolopment of the first eukaryotes--most likely through the symbiosis of two prokaryotic organisms--some 1.8 to 2.7 billion years ago. Practically all non-bacterial organisms are eukaryotes. One of the earliest major developments in eukaryote evolution was their development of mitochondria (found in almost all eukaryotes) and chloroplasts (found almost exclusively in plants and certain protists like Euglena), which came about through the symbiosis of purple bacteria and cyanobacteria, respectively, with the early eukaryotes. At least 600 million years ago, the first multicellular organisms--animal phyla like Placozoa, Porifera, and Coelenterata, as well as simple plants--came into being (based on newer molecular evidence, biologist think that multicellular life could have been around as early as 1 billion years ago). Complex multicellularity of this sort obviously originated through colonies of unicellular organisms. Modern colonial animals include the Portugese man-o-war, and there exist colonial protists, called choanoflagellates, which are prime candidates for the ancestor of multicellular animals (the aforementioned Placozoans are the simplest known animals, and may also be representatives of the first multicellular animals, but little is known about this phylum). Multicellularity in plants probably originated in a similar fashion through colonial protists that possessed chlorplasts. Once we get to the Cambrian period over 500 million years ago, we see an "explosive" proliferation of many new types of organisms. This "Cambrian explosion," as it is known*, saw the rise of the first arthropods (trilobites, etc.), mollusks (ammonites, etc.), echinoderms (urchins, etc.) and also the first chordates (an animal phylum containing the subphylum Vertebrata).

The first chordates, which were descended from echinoderms, were probably primitive organisms related to modern urochordates (tunicates, etc.). Later on, fish-like organisms resembling modern lancelets diverged from them, probably developing from the free-swimming larval forms of the urochordates via a process called "neoteny." Cathaymyrus diadexus and Pikaia gracilens, the two oldest known chordate species (both of which did happen to resemble lancelets), and the later-ocurring fishlike conodonts are examples of these early chordates. Their ancestors were probably hagfish-like, and those were in turn followed by the first true fishes (which were the first fully vertebrate animals), which arose in the Ordovician. Plants spread onto land in the Silurian period. Arthropods became the first land-dwelling animals shortly thereafter.

In the later parts of the Devonian, amphibians--the first tetrapods--developed from the lobe-finned crossopterygian fishes. For example, the crossopterygian fish Eusthenopteron is directly linked to the early fish-like amphibians Acanthostega and Icthyostega. Reptiles diverged from amphibians in the Carboniferous period. For example, Seymouria shows features of both amphibians and reptiles, so it is a great example of the amphibian/reptile transition. Mammals diverged from a group of reptiles called synapsids during the Triassic period; specifically, from a group of synapsids called therapsids (like the cynodonts). Birds diverged from reptiles in the Jurassic period (Archaeopteryx lithographica is the most famous reptile/bird transitional). Which reptile group has been debated, though. Two major ideas exist. One group of scientists holds that birds came from thecodonts--small reptiles that gave rise to dinosaurs and crocodilians. The other much larger group of researchers postulates that birds evolved from small bipedal theropod dinosaurs (e.g., Coelophysis). The latter has the most evidence in their favor.

The first primates arose in the Eocene era, with the first hominids diverging from the apes about 7 million years ago (late Miocene era). Our own genus, Homo, arose 2.5 million years ago from the ape-human transitional genus Australopithecus, which includes such species as A. afarensis (e.g., the famous "Lucy") and A. africanus. Our own species H. sapiens, originated within the last 150,000 years, having diverged from either H. erectus or H. heidelbergensis. H. neanderthalensis,--which came about some 100,000 years earlier than us and was the last hominid to face extinction (they died out 30,000 years ago)--is a "sister species" to H. sapiens, as they most likely have the same ancestor.

Despite being spotty in many places,--the odds of a fossil forming, avoiding destruction by geological processes, and later being found by humans are very slim--the fossil record speaks loudly that evolution occured.

During the course of the Earth's history, the biosphere has suffered several mass extinctions. Two of the most familiar are the Permian-Triassic extinction, the largest in Earth's history, and the Cretaceous-Tertiary (K-T) extinction, which is famous for killing off the dinosaurs. While several theories exist as to the cause of the Permian-Triassic extinction (read here), the causes of the K-T extinction are somewhat more conclusive. Evidence suggests a large meteor measuring some 10 km in diameter slammed into the Yucutan peninsula (which was underwater at the time). There are large amounts of iridium in the K-T boundary, which is a rare metal on Earth but is known to be common in some meteors. There are also large amounts of shocked quartz and other things that are formed by impacts. Also, there is what appears to be a large crater buried underground in the Yucatan peninsula and is estimated to be the same age as the K-T event. While this theory is the most widely accepted, many paleontologists suggest that a meteor impact was probably not the sole perpetrator in the K-T extinction. For example, a meteor impact cannot explain the "sudden" (in geological terms) decrease in sea levels that happened 65 million years ago. Such a decrease in sea levels would result in land bridges between certain land masses, which would in turn result in biological invasions of foreign animals into the territories of other animals. The results of biological invasions are quite often disatrous, as can be witnessed whenever humans introduce foreign species to a new ecosystem (zebra mussels in America's waterways, snakes in Guam, wild pigs in Hawaii, and so on). Thus, certain land animals may have been "softened up" by biological invasions and then finished off by the meteor impact. While the meteor theory is the best overall for explaining the K-T extinction, it is not held unanimously. For example, some paleontologist suggest that volcanism might be the perpetrator in the K-T event.

Mass extinctions can exterminate wildly variable percentages and types of organisms when they occur. For example, the Permian-Triassic was the largest of all time, killing off the majority of life on Earth--some 90-95% of marine species, 70% of all vertebrate families, most plants, and 33% of all insect orders. Mass extinctions are influential to evolution as they open new ecological niches to the surviving species. This gives evolution a lot to work with and produces many new species over time from the survivors of the extinctions. For example, the K-T extinction killed off some 90% of all species, including the dinosaurs. This allowed birds and mammals--orders of animals that were still relatively young at the time and lost in head-to-head competition with the dinosaurs--to achieve dominance.

What will the future of evolution be like? No one knows for sure, as no one can really predict what will happen. Only imaginary scenarios have been constructed (such as in the recently aired TV program "The Future is Wild"). One of the most widely asked questions about this topic is if we humans are still evolving. Most biologists say that we aren't. We are currently an evolutionarily stagnant species, meaning we are changing very little and are actually becoming more and more similar to each other instead of becoming more varied (which would eventually lead to new species of humans). This is caused by our lack of geological isolation from each other, our extremely advanced rate of geographic mobility, and our resulting interbreeding with each other, which ultimately results in a lack of truely distinct popultations. Our control of natural selective process with the advent of modern medicine, agriculture, and tecnhology is another major factor to take into consideration. This isn't a bad thing at all, and is actually extremely benificial in many ways, but it does keep humans from having another speciation event. For example, humans do not have to wait countless generations to evolve coats of fur to survive cold weather. We can simply make thick clothing or, more recently, use environmental control to make our homes warm. Our resourcefulness has given us many wonderful things, with probably the most important achievements in recent times being genetic engineering and space travel. We may be able to artificially control our evolution by manipulating our genes. Many biologists see this as a very good possibility in the near future. We may become a spacefaring species within the next 100 years as well. This excursion into space could, in time, open new ecological niches for us, thus giving evolution the chance to change us. However, there is currently no sign of any speciation events among humans anytime soon.

* It is important to note that the term "Cambrian explosion" is a bit of a misnomer. In actuality, many of the main phyla of animals had already originated by the the Cambrian. However, the Cambrian saw the rise of the majority of hard-bodied organisms--arthropods, mollusks, and so forth--which fossilize much better than their soft-bodied predecessors, thus leading to an apparent "explosion" of life during the period
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Well, there you have it. Any comments. Thanks.

~Shadow

Dr.GH

I just skimmed it. What is your traget audience? Are you writing for kids, or adults.. creationists or fence-sitters?

There are some technical issues as well, but first identify your readership.

Shadow Wraith

My target audience? Mainly a general audience. It isn't meant as a critique of creationism, nor is it targeted towards younger readers. I guess just for mainly teenage and up laypersons who want a general description of evolution.

Peez

I agree that one should know the purpose of the article, and the level of knowledge assumed in the target group, before detailed commentary is warranted. However, a few quick comments:Although you do subsequently make it clear that Darwin did not originate the idea that species are not fixed, your second line states that he did. I would avoid that. I would also suggest defining exactly what evolution is, as early as is convenient.Your description of natural selection is making an unfortunately common error: natural selection is about traits that favour individual reproductive success, generally regardless of the success of the population or the species.Note that the genus and species should be in italics. Sickle cell anemia is not a hemoglobin "deficiency," rather it is caused by having plenty of "abnormal" hemoglobin. Are people with one HbS gene completely immune, or merely more resistant? References would be helpful. This is a good example of how natural selection favours what works in a given environment (rather than what is "better" in some absolute sense), but it is a rather complex example of general natural selection.This may be interesting, but perhaps the title of the article should be changed if this material is included. Note that a virus is a very poor example of something transitional between life and non-life in this context.The evolution of eukaryotic cells did include some other processes.The portugese man-o-war is a colony of multicellular organisms.I would recommend that you be careful about naming living species as "prime candidates for the ancestor of" any other living species. Perhaps it would be better to state explicitly that these living Placozoans might be relatively unchanged from the first multicellular animals (though I note that they are not radially symmetrical, and the earliest animals are thought to have been likely radially symmetrical).It is worth highlighting the fact that these early chordates were pretty much worms.Or, at least, shared a relatively recent common ancestor.Italics for genus and species.Many authorities consider neanderthals to have been a subspecies of H. sapiens.I would avoid the colloqial use of the term "theory" here, since you have already started using it in the scientific sense earlier.??? I would disagree, do you have a reference?How does this relate to evolution?This might retard speciation, but not necessarily evolution.Yes, but that simply means that selective forces have changed, not disappeared.It is also important to note that this "explosion" took many millions of years.

This has just been a quick run through and a few quick comments. Overall it is interesting, but is not as focused as it could be. I would recommend breaking it into sections, e.g. abiogenesis, evolution, the theory of evolution, and descent with modification. Subsections could be used, where appropriate, and references would be helpful. However, don't let my criticisms stop you from posting a useful article!

Peez

P.S. I noted a few typos as well.

Shadow Wraith

Thanks for the suggestions, peez. I have already corrected the spelling errors, and I am going to italicize the genus & species names when I add the html coding. As soon as I've made some more changes, I'll send the revised version to your personal messages here at the board. Thanks again.

~Shadow

Secular Pinoy

What, a discussion on the fossil record yet no reference to punctuated equilibrium? That must be fixed.