All the animals on the Earth are different. Some have two legs while some have four. Some have two eyes, and others have over ten! Sometimes it's very hard to compare one animal to another. For example, try comparing a human to a jellyfish and you'll find yourself stumped! It seems as if the two have nothing in common! Shubin, however, believes "all animals are the same...like a cake recipe passed down from generation to generation- with enhancements to the cake in each-the recipe that builds our bodies has been passed down, and modified, for eons. We may not look much like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them" (115). In a nutshell, Shubin says that humans are a small sliver in the "recipe of animals". We are simply modified versions of animals that weren't as environmentally fit as us. But what exactly modifies us? What makes changes to the "recipe" we are part of? How could our "recipe" possibly have originated from a recipe that called for a jellyfish? The answer is Evolution. Explain the concept of Evolution. Then use the terms natural selection, ecological fitness, variation, selective advantage, and adaption to describe how our "recipe" was modified to cook-up the animals we are today.
Mikey Ling
(mikeyling@ymail.com)
Evolution is when a species changes in some way over many generations because of some (most likely environmental) pressure. This process causes the organisms best fit for this new pressure to survive which reproduce. This process keeps occurring until an adaptation occurs in which a majority of the organisms can survive. This occurred among the first ancestors of animals to a whole bunch of different environments as animals began to spread out. This caused the main functioning portion of the animal (like nervous and circulatory system) to change minimally considering these new environments were actually like the organisms now-a-day in the way that they are extremely similar despite their obvious differences.
ReplyDeleteNatural selection allows the organism that is most fit for the environment (in the case of evolution, from the same species) survive and produce offspring. This means that the organisms that survive and don't survive are almost identical, except for one characteristic. After generations, eventually these small differences add up causing new species to form and old ones to die, causing variation from the same origin. Variation is the key to evolution because if every organism from a single species was identical then none have an advantage over the other which would cause either all to live or die. Selective advantage allows some species to survive while others die off due to being less suited for the environment which causes the animals with the edge to live on and become even more prepared to face the environmental factor. Ecological fitness increases as the species evolves to survive the ecosystem it inhabits better. This causes a rise in population which allows the species to live on and possibly further evolve. Adaptations are the result of evolution where a species has changed from its original to have unique features to face some outside pressure. This adaptation can cause a split in a species as, "a trait may have been adaptive at some point in an organism's evolutionary history, but a change in habitats caused what used to be an adaptation to become unnecessary or even a hindrance" (http://en.wikipedia.org/wiki/Adaptation#Genetic_change) therefore either changing or removing this former adaptation while another population of this species keeps this adaptation.
Jackie James
(jackie.james@comcast.net)
Evolution is the core theme of biology; it links species to a common ancestor. Jackie did a pretty good job explaining evolution, but I will recap some points she made. First, to answer your question on how modification/variation occurs, variation occurs within a population through sexual reproduction and/or mutation. In zygote formation, half of the genes come from the father’s sperm, while the other half of the genes come from the mother’s ovum. When they fuse, their genes combine creating variation among generations via sexual reproduction. Furthermore, variation can occur at an earlier stage in meiosis as the cells forming four gametes from a single diploid cell (in spermatogenesis all four normally survive, while in oogenesis three haploid cells become polar bodies and one haploid gamete matures into an ovum), genetic material between homologous chromosomes can be exchanged. Obviously mutations on the genetic level can also create variation in the population. Sometimes mutations are inherited from parents or during sexual reproduction there are complications that lead to mismatch in DNA. In addition, mutations can occur during the life time because of environmental factors such as DNA damage from UV radiation and chemical carcinogens (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T9P-429XTXN-1&_user=10&_coverDate=12%2F31%2F2000&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0d25ed6de2f31f7a0914e7e4f67a95d1&searchtype=a).
ReplyDeleteSo as variations exist in the population, some drastic environmental or biology change (Jackie) as Jackie mentioned puts natural selection at work. As Jackie said, “natural selection allows the organism that is most fit for the environment (in the case of evolution, from the same species) survive and produce offspring” (Jackie). Those organisms that have the ability to survive, reproduce, and pass along the advantageous trait to their offspring will survive and will be considered to have the selective advantages over those organisms without the advantageous trait that was developed through variation of the population. After a long period of time, the advantageous trait appears in the entire population with variation. Then, over an immense period of time, there are a lot of successive events of natural selection for numerous advantageous traits (in contrast to Jackie hint at it was only one trait that changed). This leads to a population that is so different from the genes of the original population, that theoretically, the new species could not breed with the original (which is most likely extinct). Speciation is the term biologists use for the formation of this population of new species.
Now to address the changes we see in the animal “cake recipes,” we can look at morphological and molecular phylogeny trees. All animals, according to the theory of evolution, are derived from the same common ancestor. From this common ancestor emerged animals that lacked true tissues (sponges) and animals that had true tissues (eumetazoa). Sponges may be very different from humans, but they are just a variation in the recipe. Sponges may be sessile, lack true tissues, and have a spongocoel to draw in water and food (food for which the amoebocytes digest), sponges also have been highly researched to find connections between human embryonic stem cells and sponge embryonic stem cells because we do share a common ancestor. In addition, it has been found that “nearly 95% nearly 95 percent of all genes associated with human disease can be found [in] sponges” (http://www.sciencealert.com.au/news/20092907-19478.html).
Following the path of the animals with true tissues, another divergence occurred between animals that have bilateral symmetry and those that don’t. In class, the animals that do not have bilateral symmetry that diverge at this point were from class cnideria such as sea anemones and jellyfish. For sea anemones and jellyfish it was selectively advantageous to have radial symmetry. Their nerve net can sense different stimuli in all directions to capture prey. However for most other animals, bilateral symmetry became a common trait. These animals include roundworms, mollusks, arthropods, and echinoderms. Then, diverging from an ancestral deuterostome, notochords evolved. All of these animals have diverged from the same ancestral animal, however from slight variation and speciation evolved new animals and variations in the common recipe.
ReplyDeleteLastly, I think the best example of the process of evolution and natural selection comes from the example at the beginning of the year (from what I can remember, there may be a slight variation in the story). This analogy works perfectly for thinking while writing FRQ’s about evolution. Within the classroom existed many variations. One of which was the difference in hair color: some of the students had black hair, others green hair. Then an ENORMOUS MONSTER came into the classroom and ate all of the black haired students because they were easier to spot. The green haired students had the selective advantage and overtime after many generations of reproduction, the entire population had green hair. Then another ENORMOUS MONSTER came into the classroom and this time, singled out those students with white skin as opposed to the students with orange skin. Once again, time passed, and orange skin along with green hair appeared throughout the population. Then, another ENORMOUS MONSTER came into the classroom. The short students had the selective advantage because they could hide more easily, so all of the tall students were eaten. Time passed again, and over time and many years of reproduction, no longer could we call these students students. A new species was created via speciation: umpa lumpas.
Also, don’t trust anyone that gives you a cake recipe. You’ll probably find that some variation from the original cake recipe exists!
(Bobby Muttilainen, rmuttilainen@gmail.com)
One key feature of evolution than neither Jackie nor Bobby touched on was the set of mechanisms in different organisms to resist or encourage genetic variation. In different circumstances, variation, either by means of DNA mutation or by sexual reproduction, may be a desirable characteristic or a negative one.
ReplyDeleteSexual reproduction combines different sets of genes to create a new organism that may thrive and reproduce more than its genetically distinct counterparts, encouraging natural selection to cause the population to move toward evolution. Crossing over in meiosis, an integral part of sexual reproduction, for example, introduces genetic variation that may help a species, on an evolutionary time scale on the whole, survive and reproduce more effectively than it would without any introduced change (http://www.accessexcellence.org/RC/AB/BC/Genetic_Recombination.php).
In contrast, certain circumstances favor an actual lack of sexual reproduction that would introduce variation. In addition to the costs of sexual reproduction, asexual reproduction, such as fragmentation, can be the more favorable option. In some groves of aspen trees, for example, the organisms reproduce by asexual reproduction because their environment is stable, and the current set of genes functions well in its niche (Campbell 812).
On the molecular scale, the "wobble" effect can help prevent mutations from effecting phenotypic variations. Because the third nucleotide in a codon can sometimes not affect the amino acid coded for, the resulting protein and subsequent phenotype would be the same in this silent mutation (Campbell 339). This is an advantage in many instances, particularly on a short-term scale because, although it does not allow greatly for evolutionary variation, it does reduce the chance of a potentially fatal or detrimental change in phenotype.
- Vincent Fiorentini
(vincent@panatechcomputer.com)