If you look at the flowchart on page 176, a very interesting explanation for evolution over the course of many generations is provided. The key principle emphasized in this schematic is that “descent with modification can build a family tree, or lineage, that we can identify by characters.” (Shubin, 176). This is an easy way to justify how all organisms branched out from one common ancestor, but the one question that the flow chart does not answer is how divergence occurred in the first place? If an original humorless couple is the origin of the entire generation of full bozos, how did the bozos inherit such incredible diversity if the tools for such were not in their original genetic code? Did they intermarry? Obviously that is not possible in the animal kingdom because organisms of different species cannot interbreed.
Consider the theme of evolution and maybe the theme of structure and function. If descent with modification occurs, what causes the modification? Mutations in DNA do occur, which does explain why proteins do not function in the way they are intended, but it doesn’t seem possible that they occur so often that they result in the divergence of species so far over millions of years that jellyfish and humans could possibly be related. Prove me wrong?
Troy Glickstern
The most difficult aspect of evolution to comprehend is the time span in which evolution occurs. Evolution is a painstakingly lengthy and subtle process. Considering that life on Earth began about 3.8 billion years ago, and considering the life spans of modern day organisms, the number of organisms that have lived on Earth since the first single-celled organisms is far too large to comprehend. Therefore we can see how over such a long period of time, the minor mutations will become more widely represented in an organism's population.
ReplyDeleteThe large differences we see in today's animals like jellyfish and humans are really just the sums of many, many small differences. Animals did not simply jump from jellyfish to humans, but rather with random genetic mutations compounding over millions and millions of years. It is difficult to think about how long the process of evolution takes, especially when relying on random and scarce genetic mutations. Troy, they don't occur often at all, especially mutations that will benefit a single organism enough to give it a significant advantage over its competition, leading to the spread of the mutation through reproduction. Those mutations are few and far between, but given enough time (and the history of life on Earth has plenty of time), they make a difference.
http://en.wikipedia.org/wiki/Timeline_of_evolution
http://www.wisegeek.com/contest/how-does-evolution-work.htm
Jeremy Solomon
imabum14@gmail.com
As Jeremy stated, evolution does not occur in the span of few years. It takes millions and millions of years for a new species to emerge, and while these new species arise, millions of others encounter extinction due to various factors. However, what Jeremy did not mention/clarify about evolution is the idea of nature exerting selective pressures on these organisms.
ReplyDeleteA certain species of an organism can grow in population, but its growth is eventually impeded by various selective pressures, such as predation. Once a population encounters an obstacle, those without the selective advantage to overcome it will eventually die out, while those with the advantage will survive and reproduce. Under this system, qualities that are useless or detrimental to a population will not be inherited by its successors, but qualities that are an advantage for the organism's survival will be inherited by their offspring(s). Populations of species will undergo this cycle until their qualities become distinguishable from their ancestors; until they become a new species.
*Note that for species that undergo rapid successions of mutations, such as bacteria, the above occurs at a much faster pace.
One must also take into consideration of the idea of genetic rearrangement. During prophase I of meiosis, chromosomal crossover with non-sister chromatids occur (257). Also, recall the process of how genes are coded for proteins. Throughout the process, genetic rearrangement of the RNA molecule, known as RNA splicing, occurs (334). This (and various other factors, such as mutations) provides for the incredible diversity that this Earth has. Indeed, at first glance humans and jellyfish are no similar than the tortoise and the hare, but under further inspection we find that these differences are an accumulation of these genetic arrangements. Over the years, various species develop certain qualities to survive. Because the selective pressure is different for each individual species, the qualities that are developed are different. As Neil Shubin states, various species "are not completely identical", but "their similarities are profound" (104, Shubin). I hope this answers your question of how jellyfish and humans are related.
If the concept of evolution is still unclear to you, try referring to Chapter 22: Descent with Modification- A Darwinian View of Life. We never read it as a class reading, but it provides a great explanation and examples of why evolution is true.
Works Consulted:
-Campbell Biology
-AP Biology Wikispace
-Your Inner Fish
-Ms. Inselberger's powerpoint on "How to JAE"
(Keigo Tanaka; tanakarus3@hotmail.com)
Although Jeremy and Keigo have covered the time span and genetic basis of evolution, I think that more explanation as to why evolution causes such diversity and development of organisms is needed. Based on genetic mutation to create variation and ultimately speciation over time, it is logical to think that different characteristics might develop in different organisms – however, why would these changes be so drastic from one another, and why would these changes cause an overall trend toward more specific physiological development, like multicellularity or cephalization?
ReplyDeleteOn a small scale, the development of different characteristics makes sense. Different environmental pressures favor certain mutations in natural selection, but this doesn’t explain why such drastic changes exist between different species in the same environment.
The first explanation for this is the variety of different ecosystems in the world. From deserts with virtually no moisture to coral reefs, the Earth provides a vast array of unique environmental pressures to cause different populations to develop different characteristics over time. In the United States and Canada alone there are six different types of ecosystems, and these amount for only a small portion of the total in the world (Campbell 1234).
Furthermore, one development can lead to another, causing a unique branching in the evolutionary tree. As one species evolves over time, its very niche in its environment may change, causing the effective pressures on it to become drastically different. If a carnivorous species develops from an omnivorous one, it suddenly (relatively speaking) has to adapt to catching prey, and any traits it has specifically for foraging vegetation become unnecessary and wasteful.
The overall trend toward higher orders of organization is an interesting pattern. It could make sense that organisms become more and more developed over time, but it might also make intuitive sense for the opposite to be true. According to the ideas of catabolism and entropy, energetic systems naturally degrade to their point of lowest conserved energy (Campbell 143). Wouldn’t this principle indicate that the same would happen for evolution, that organisms would simplify from humans to sponges over evolutionary time?
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ReplyDeleteThe reason this trend is not the case is because of the evolution of more efficient body plans and physiological features. One example of this, on the molecular scale, is the development of the electron transport chain. Before cells had mitochondria that used oxidative phosphorylation to create ATP, they had to rely exclusively on substrate-level phosphorylation. This generated only four ATP molecules per molecule of glucose, whereas oxidative phosphorylation creates about 32 additional ATP molecules (Campbell 176). This new development was much more complex than its predecessors, but it caused those cells that possessed it to generate almost ten times as much energy from the same amount of glucose. The key concept here is that more specific, unique, organized organisms can be more efficient than there simpler counterparts. It is not random chance that causes evolution to favor more developed organisms but rather the benefits complexity can bring, such as increased energy efficiency. This tendency has been coined the Zero-Force Evolutionary Law, stating that “in the absence of selection and constraint, complexity – in the sense of differentiation among parts – will tend to increase” (www.biology.duke.edu/mcshealab).
This is all not to say, however, that natural selection never favors changes that are simply “different.” Although not a direct example of evolution per se, the ability of HIV to constantly change its membrane proteins so as to avoid effective detection and retaliation from the immune system demonstrates that mere changes for the sake of change can be beneficial in a biological context (people.ku.edu/~jbrown/hiv.html). Simple organisms, like unicellular ones and sponges, still do exist and thrive in some areas – the majority of other organisms, however, do feature more advanced development, like arthropods, the most successful phylum on Earth (www.evolution.berkeley.edu/evolibrary/article//arthropods_intro_05).
As the world’s creatures and organisms continue to develop and evolve, they will undoubtedly exhibit a trend toward increased efficiency and improved function. This change can, and quite often does, come from traits that require greater organization to function but also produce disproportionately beneficial results. This leads to the great variety in species we see on the planet today.
- Vincent Fiorentini
(vincent@panatechcomputer.com)