EVOLUTION
UNENDING DIVERSITY
Evolution proves that all of life’s diversity is a result of modifications of preceding life forms. That evolution has occurred is a fact. The mechanisms that cause this diversification are the theory part of evolution. As with any scientific theory, much evidence exists to support the species diversification mechanisms. But these mechanisms, including natural selection, are still being researched, with the outcome being a greater body of knowledge supporting the workings of evolution. An important point to remember is that all professional scientific organizations support the teaching of evolution. (http://www.talkorigins.org/faqs/evolution-fact.html; http://stephenjaygould.org/library/gould_fact-and-theory.html; http://actionbioscience.org/evolution/lenski.html)
As some of you know, the teaching of evolution in our public high schools has become a controversial issue. This opposition mostly comes from those Christians who interpret the Bible literally. Yet most Christian and Jewish faiths find evolution to be compatible with their beliefs. They do not combine life’s creation with how species, including ‘man,’ originated. (For example, over 7,000 Christian clergy have signed a letter saying, “…the theory of evolution is a foundational scientific truth… .To reject this truth or to treat it as ‘one theory among others’ is to deliberately embrace scientific ignorance… .”) (http://www.uwosh.edu/colleges/cols/religion_science_collaboration.htm) Although evolution does not require a Creator, it does not preclude one either. Creationism’s Trojan Horse: The Wedge of Intelligent Design and Evolution vs. Creationism: An Introduction are good summaries of the latest efforts to include creationism in the science classroom. A letter to all members of the National Academy of Sciences summaries the seriousness of this threat can be found at web site: http://www.nationalacademies.org/evolution .
Following is a brief explanation that hopefully will entice you to seek a fuller understanding of the evidence proving the occurrence of evolution, as well as an understanding of the various ideas regarding the mechanisms behind this occurrence. Evolutionary understanding did not stop with Darwin but instead incorporates a rich 150 years of research that continues today!
The basic driving forces for an individual organism are:
1. Securing energy for itself. For the most part, although there are exceptions, energy (food) is obtained directly or indirectly from the sun. Organisms containing chlorophyll, such as the green plants we see around us, obtain energy directly from the sun through the process of photosynthesis. Other organisms obtain energy indirectly by (a)consuming the sun-derived energy organisms, or (b) consuming organisms that fed upon sun-derived energy organisms.
2. Keeping from being converted (eaten) to energy by being consumed by an energy seeking organism; and,
3. Most importantly, successfully reproducing before being converted to energy. (Granted, some of us elect not to have children out of choice. But our evolved mind allows for this choice, just as it does in helping us identify and solve moral issues.)
Charles Darwin’s primary mechanism, or causal agent, behind evolution is Natural Selection. His and co-discover Alfred Russel Wallace Natural Selection explanation, a fairly simple concept concerning Earth’s complex diverse life, is one of the greatest scientific ideas developed by our evolved consciousness. Natural Selection recognizes that each species (the total number of like interbreeding individuals) produces a surplus of young that must compete for the limited amount of available energy. Only those individuals within the “surplus” that meet the three basic driving forces can be deemed successful; only they are able to continue to perpetuate themselves through their genes.
Darwin used the idea of the domestication of animals by humans to buttress the idea of Natural Selection. Varieties of sheep, cattle, dogs, chickens, cats, horses, etc. resulted from the purposeful breeding of individuals of each respective species. Dog varieties as different as dachshund and Great Danes were developed from the originally domesticated wolf, both for utilitarian purposes (i.e., hunting) and companionship.
It was Darwin’s insight to postulate that this human role of selecting which individuals would breed with each other, was assumed by the mechanism of Natural Selection in Nature. Those individuals that both successfully obtained food and kept from becoming food, produced offspring with similar successful traits. Success leading to a collie resulted from the continually breeding of longhaired dogs (humans ensured that the other two basic forces of food and safety were met); success leading to a tiger resulted from successful cat like organisms interbreeding and passing on genes allowing their offspring to meet all three of the basic forces through strength, speed, camouflage, stealth and mating.
Another mechanism driving evolution that was proposed by Charles Darwin is Sexual Selection. Here a female of the species selects a male with which to breed based on some physical aspect of the male, such as large antlers, or brightly colored bird feathers or fish fins or a behavioral trait, such as songs or courtship rituals. Each spring we are fortunate at our house to be able to observe male turkeys with their brightly colored blue heads and red throats, strutting to impress the females, their tail feathers in a large fan shape and wings lowered towards the ground. Some biologists postulate that these traits advertise the fitness of that individual male. The peacock is frequently used to illustrate this idea, as the large tail feathers of a male peacock are seen as a detriment to his survival abilities.
Since Darwin’s time, much additional understanding in paleontology, geology, field biology, genetics and in the last thirty years molecular biology has given us greater understanding of the workings of evolution. More recently embryology or Evo Devo (evolutionary developmental biology) has provided even more knowledge of this process. (An excellent explanation of how form and pattern develop in the embryo to produce the extraordinary biodiversity that now surrounds us, is found in Sean Carroll’s book, Endless Forms Most Beautiful.) That Charles Darwin was so insightful in beginning our current understanding of the evolutionary process is remarkable given the limited knowledge constraints under which he was conducting his research and formulating his conclusions. For example genes were unknown to Darwin and he did not develop a complete explanation for speciation. Thus, Charles Darwin is not the be all and end all in the understanding of evolution and any criticism of evolution based exclusively on what Charles Darwin said is not only unwarranted but also unfair.
In addition to Darwin’s Natural and Sexual Selection, other species selection influences include: (1) chance (an acorn falling in a stream has less of a chance of growing than one falling on the ground, and one never knows when an asteroid is going to hit and eliminate much of life as it has done in the past), (2) genetic drift (number of genes in a population due to chance), (3) genetic and physical constraints (you cannot develop a needed trait if genes do not exist to begin its development, nor can one develop a needed trait that defies physics), and, (4) the role of self-organization (spontaneous emergence of pattern, structure and order) in complex systems. The understanding of these and other species diversification mechanisms, the limiting factors under which they operate, as well as the affects of parasitism, symbiosis, and ‘cooperation,’ are helping unlock the mysteries of how evolution works.
Understanding the role of genes in evolution began, even though the work was rediscovered at a later time, when Gregor Mendel’s experimentations showed that an individual organism’s physical characteristics are inherited separately from each parent, after having been rearranged in meiosis, the cell division process used to create sex cells. The rules of dominant and recessive characteristics (encoded in genes) began a process that has seen other geneticists, field researchers, and molecular biologists confirm the concept of adaptive radiation (diversity) through such mechanisms as natural selection.
It is the individual variety of the offspring, caused by recombination of genes and modified genes (mutations) that Natural Selection acts upon. Those individuals best able to secure energy and not be converted to energy pass on their successful genes to future generations.
Genes that express themselves do so as the behavioral (e.g., migration) and physical (e.g., long legs, eyesight) traits that allow an organism to occupy a niche in its environment. Organisms both shape and are shaped by their environment. Over periods of time environments change, as do the organisms that occupy them.
Speciation
The process of how species arise still has a number of unanswered questions. But this does not mean that much is not already known about the mechanisms that cause speciation to occur. One known mechanism for causing new species to occur is called allopatric speciation. Allopatric speciation occurs when a geographic barrier, such as the rise of a mountain range, a change in climate, or the change in a river’s course, causes a population of a species to be isolated from the other populations of that species over a long period of time. The divergent population’s gene makeup becomes different through interbreeding and adapting to its ecological niche, to the point that even if it again comes in contact with the other populations from which it became separated, it cannot, or does not, interbreed with these former populations. The divergent population assumes the status of a species.
The Smithsonian Institution’s Natural History Museum’s excellent new mammal exhibit gives one such example; that of polar bears evolving, or branching off in the ‘tree’ metaphor, 70,000 years ago from brown bears. In this case the physical barrier was a large glacier. Some time between 250,000 and 100,000 years ago an Alaskan glacier separated off a portion of the bears. Through time, against the snowy background, the lighter colored, of this isolated bear population, had greater success in capturing seals. These lighter colored bears were not only more successful in obtaining food, but this success in obtaining food allowed them to pass on the genes for hair color to their offspring. This isolating glacial barrier also resulted in other changes, such as wider webbed paws for swimming, better insulating hair, and rough groves in their paws for better traction on the snow. All of these changes allowed polar bears to utilize their environmental niche more efficiently.
A second way speciation occurs is known as sympatric speciation. Here a geographic barrier does not occur. Instead, this type of speciation seems to be most prevalent when certain individuals of a population begin to secure their energy needs from a different source than other members of the population. For example, some individuals of a particular species of insect that feeds exclusively on one type of plant, may begin to feed on another type of nearby plant. These individuals breed with other individuals also feeding on the second type of plant. When enough genetic differences arise between the insect population still feeding exclusively on the first host plant and the population feeding on the new host plant, a new species comes into being.
Another way animal speciation may take place is through hybridization. Already well documented in plant speciation, Penn State researchers led by Dietmar Schwarz, are investigating the colonization of the introduced Asian honeysuckle bush by a hybridized fruit fly. The hybridized fruit flies are the offspring of two native flies, the blueberry maggot and the snowberry maggot. These fruit flies are very specific to feeding only on one host plant. But when the newly introduced honeysuckle bush became available 250 years ago as an additional energy source in the northeast United States, the evolutionary process may have quickly (within 250 years!) produced a new species (Lonicera fly) capable of utilizing this new source of energy.
It will be interesting to see if future research reveals hybridization to be a major casual factor of animal diversification. A thought that comes to mind is have hybridized plants, as new host food sources, been instrumental in the formation of newly hybridized insect species?
All cells are basically one of two forms: they are either prokaryote (cells without a nucleus), or eukaryote (cells with a nucleus). All organisms that are visible to our naked eye (animals, plants, and fungi) consist of eukaryotic cells, although our red blood cells are prokaryotic cells. Also, a few types of microorganisms, such as the protists, are eukaryotic cells. All the rest of life, bacteria and bacteria-like archae, are prokaryotes.
For the first portion of the 3.8 billion years of the history of life on earth, prokaryotic organisms were alone. It was not until about 2 billion years ago that eukaryotic organisms joined them . Later still, only about 600 million years ago, multi-celled life, all of which are eukaryotic organisms, appeared.
Eukaryotic evolution conforms to the conventional idea of evolution being a branching bush. It is thought that the first eukaryotic cell arose from either a pool of gene exchanging bacteria, or a bacteria and archebacteria fusing together. The resulting organism is the last common ancestor from which all eukaryotic celled organisms, including us, arose. Each major eukaryotic branch (animal, plant, fungi) in turn has a “common ancestor.” Eukaryotic celled organisms, except possibly the protists, evolve mostly by vertical gene transfer; that is, two organisms of the same species through sex recombine the genes. (Richard Dawkins’ latest book, The Ancestor’s Tale, provides an excellent explanation in tracing the ‘twig’ of the bush on which we reside, back to the beginnings of life’s evolution.) (http://tolweb.org/tree/phylogeny.html)
For prokaryotes the explanation is not quite as straight forward and the “branching tree” metaphor is replaced by a circular ring of life. This is because bacteria and archabacteria, as well as the organisms that were Earth’s first life forms that differentiated into bacteria and archabacteria, evolve mostly by horizontal gene transfer, as well as by asexual cell division. All can exchange genes; there is not a species-specific limitation. (http://www.college.ucla.edu/treeoflife.htm)
Dr. Carl Woese also rejects the single ancestor idea and proposes that the three major branches of life, (bacteria, archaeabacteria, and the eukaryotes) are actually three separate trunks that arose separately from proto cells that freely shared their DNA.
Nobody said this is easy! As you can see much about evolution is still being worked out, but not the fact of evolution itself.
Our “DIVERSITY OF ONE” bumper sticker, included free with each T-shirt purchase, sums up what has occurred evolutionary wise, at least with eukaryotic organisms. (See preceding section Types Of Cells.) Thus, the multi-celled organisms most familiar to us-- plants, animals and fungi-- resulted from the diversification that began with the first eukaryotic organism. (This first eukaryotic organism is also the “Last Common Ancestor” for the other seven to nine extant eukaryotic kingdoms.)
The “DIVERSITY OF ONE” actually occurs at several hierarchical levels in the modern classification of eukaryotic organisms. For example, chimpanzees, orangutans, gorillas, and humans are all twigs on the same ape branch off of another branch within the animal kingdom. All four evolved from the same common ancestor, which had in turn, evolved from another common ancestor, and so forth, until you arrive back to the “Last Common Ancestor” for eukaryotes.
The bird drawing, a large ground finch (Geospiza magnirostris), on the bumper sticker is a reproduction of one of Charles Darwin’s drawings of the finches he observed on his Galapagos Island visit. Darwin’s finches are one of the icons of evolution, even though Darwin himself, at first, did not realize their significance in demonstrating the workings of evolution by natural selection. Although somewhat technical, Peter and Rosemary Grant’s paper, Genetics and the origin of bird species, which is accessible online at the Proceedings of the National Academy of Sciences web site (see below), explains that a minimum of 30 individuals of one South America mainland finch species diversified into the thirteen different finch species.
This founding population of finches serves as a textbook example of how one species evolves into other species as they adapted to take advantage of the different available sources of food for energy. For example, some finches evolved thin beaks to capture insects and others thick beaks to feast upon buds, fruits, and seeds. One finch even evolved the behavioral skill to use a tool (stick) to extract grubs from trees. (A great read on this is The Beak Of The Finch: A Story Of Evolution In Our Time by Jonathan Weiner.)
Just as fascinating as the adaptive radiation concept (diversification) of the founding finch population is the “Last Common Ancestor” of all birds! In the give and take that underpins science there are two main schools of thought—some think that birds evolved from dinosaurs, while others propose a non-dinosaur reptile origin. Most evolutionary biologists now feel the evidence points to a dinosaur origin. The May, 2005 issue of Natural History has an excellent flow diagram (cladogram) of the dinosaur/bird “family tree.” (http://www.ucmp.Berkeley.edu/diapsids/avians.html; http://www.abc.net.au/science/slab/dinobird/story.htm)
An excellent website on the subject of “Common Descent” is: http://www.talkorigins.org/faqs/comdesc/ but you should read Dr. Woese’s paper which can be accessed at the National Academy of Science (see below); for evolution in general: http://evolution.berkeley.edu/evosite/evohome.html, http://www.talkorigins.org/origins/faqs-evolution.html, http://www.actionbioscience.org/evolution/index.html, http://www.pbs.org/wgbh/evolution/ and http://www.millerandlevine.com/km/evol/ ; fossil evidence: http://www.agiweb.org/news/evolution.pdf and the genetic code: http://psyche.uthct.edu/shaun/SBlack/geneticd.html and http://www.evolvingcode.net/uni_code.php.
To stay current with what’s new in evolution think, you can consult ScienceDaily: http://www.sciencedaily.com and the National Academy of Science (NAS) Proceedings at http://www.pnas.org/ (just type the word ‘evolution’ in the search box). Another good NAS publication is Teaching About Evolution And The Nature Of Science at http://www.nap.edu/readingroom/books/evolution98/front.html.
An excellent group that promotes the teaching of evolution that you may wish to join is the National Center for Science Education. Additionally, they have much information regarding the teaching of evolution in school controversy. They are located at the website: http://www.natcenscied.org .
Recommended books include various books by Stephen Jay Gould, Richard Dawkins, Niles Eldredge, Mark Ridley, Helena Cronin, Matt Ridley, Ernst Mayr, Sean Carroll, and Carl Zimmer. The November, 2004 issue of National Geographic contains a good introductory article on evolution. Perhaps the best book to begin with is E.O. Wilson’s, The Diversity Of Life. There are also good bibliographical books about Charles Darwin by Janet Brown, and by Adrian Desmond and James Moore. Author Peter Raby’s biography of Alfred Russel Wallace is also a good read.
Lastly, if you have not already done so, read Charles Darwin’s Origin Of Species. It is available on the Internet at several sites.
Thanks,
DIVESITY OF ONE, LLC
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