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Edited by Plasma Prestige: 4/5/2013 11:14:57 PM
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Science Friday, Week 5: Natural Selection

Welcome to week 5 of Science Friday! Last week, we talked about the battery, a marvel of technology which is ubiquitous in the 21st century. This week, let’s talk about the theory of natural selection by evolution. Evolution, being the central tenet of biology, is not a topic I can cover entirely in one week. The theory of evolution by natural selection is perhaps the most poorly understood scientific phenomenon, right next to climate change. Much of the controversy that this has brewed is a direct result of this poor understanding. This week, I hope to clear up some misconceptions. Charles Darwin proposed the theory of evolution by natural selection in the mid-ninteenth century. He made his conclusions about the nature of change in organisms by making keen observations of different animals around the world, especially in how they differed from one another. Darwin came up with two basic facts: one, species are not immutable, that is, they are subject to change over time. Second, the principal mechanism by which species change is [b]natural selection[/b]. But what is natural selection? Natural selection is the differential contribution to future generations by various genetic types. In other words, it is a description of the observation that individuals with certain traits survive better than individuals with different traits. Natural selection acts on the level of the phenotype, the physical characteristic, but the process of evolution is contingent upon the genotype, the alleles that code for the trait. A population is said to be [i]evolving[/i] if individuals survive and reproduce at different rates as a result of differences in genotypes and phenotypes. This is crucially important. This means that [b]only populations evolve[/b], individuals [b]do not evolve[/b]. As a result of natural selection, [i]populations[/i] develop [b]adaptations[/b], which are traits that suit their physical environment better. Adaptations, however, can only develop as incremental changes to the existing gene pool. This is one of the important constraints of evolution to understand. Let’s take a look at a concrete example. Giraffes, at some point in the past, likely did not all have extremely long necks. It is most probable that, in a given population of giraffes, the neck length varied just as human-height may vary. Over the course of many generations, it probably became more advantageous to have longer necks. Longer necks most likely meant easier access to food. As a result, giraffes with longer necks in the population tended to survive better and reproduce more successfully than giraffes with shorter necks. They, in turn, produced offspring more rapidly than giraffes with shorter necks. This type of natural selection is called [b]directional selection[/b]—the selection of one extreme trait over the average trait or other extreme trait (i.e., normal necks and short necks). Three other types of selection include stabilizing, disruptive, and sexual selection. The evolutionary change exhibited by giraffe populations likely took many millennia to manifest itself. Evolution is not necessarily fast, but it is, without a doubt, a fact of nature. We observe it in the way harmful bacteria develop resistances to our antibiotics; we see it in how one species can branch off into many more (stay tuned!); and we see it in the vibrant ecosystems of plant life. Amazingly, natural selection is just one of many mechanisms by which evolution operates. Mutations, genetic drift, gene flow, and nonrandom mating also play smaller, although still pivotal, roles in the change in composition of the tremendous diversity of life we see on Earth. I hope you enjoyed this week of Science Friday. You can, as always, check out my previous postings by clicking on the #sciencefriday tag on this thread. Note, you will have to change the filter settings there to see all threads. If you have any questions or comments about my content or format here, please do not hesitate to comment. Next week, I will most likely continue to talk about evolution, but move on to the discussion of speciation—how common ancestors may branch off into different independent species.

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  • It's funny how all of these "science" threads are composed of basic science that everyone already knows.

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    • Quantum also raises a good point about the role of mutation, which I would like to elaborate upon. Mutation is, by far, the most significant driving force of genetic diversity. Without this genetic diversity, evolution becomes rather arbitrary since there is no potential for certain individuals to survive more successfully than others.

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    • Next week: We'll learn that 1+1=2.

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    • omg its ira flatow

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    • [quote] Charles Darwin proposed the theory of evolution by natural selection in the mid-ninteenth century. [/quote] Ughhh [quote]The idea of evolution of species is found in the writings of men such as ... Lord Monboddo (1714-1799) was perhaps the first to suggest that man is related to chimpanzees. His name was James Burnett. A Scottish judge, he was obviously a scholar and philosopher as well. He is cited by ... Erasmus Darwin (1731-1802) was Charles Darwin's grandfather. He had suggested that "all warm-blooded animals have arisen from one living filament" (Zoönomia). It turns out Darwin's dangerous idea may not have been so original! Jean-Baptiste Lamarck (1744-1829) is perhaps the first well-known name in evolution, being among the first to propose that life in general evolved even from species to species. His name is attached to Lamarckism, the idea that traits developed during a lifetime could be passed to offspring.[/quote] He built upon it, it was already proposed.

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      • I would like to point out that my giraffe example as an instance of directional selection, according to a recent article I have read, is actually not true. The idea that giraffe populations produced long necks as an adaptation for easier accessibility to food has been challenged on the grounds of being supported by weak evidence. Rather, it has been suggested that long necks have actually evolved as a result of sexual selection, which is the acquisition of advantageous traits in a population over time solely for increased success in attracting mates. In fact, most adaptations via sexual selection confer huge costs to the individuals' survival, but persist because of the immense importance of reproduction. Giraffes with longer necks, it has been theorized, were effectively able to combat other males for females, capable of breaking bones. The prevalence of long necks in female giraffes is the biggest issue with this theory at the moment. Usually, sexual selection leads to [b]sexual dimorphism[/b], which is the evolution of drastically different morphologies for the males and females. It has been proposed, consequently, that easier access to food initially prompted the adaptation of long necks but then sexual selection became the main driving force for the evolution of this trait as supported by the observation that male giraffes typically have denser neck necks.

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      • Edited by BC: 4/7/2013 4:09:51 AM
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        Question: what if an animal has no main predator, would it still evolve over time?

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        • Great thread. Although this topic is incomplete without further information on [i]mutation[/i], (hint for your next topic).

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        • Edited by RedBaron86: 4/6/2013 11:30:37 PM
          natural selection made men horny...true fact

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        • As always, thank you for the interesting read. I've always considered evolution a fascinating subject because at first sight it may appear counter intuitive with some apparent order appearing from events that are random, to a major extent. It tells something very profound about the complexity of the universe we live in. What's also interesting is the thought that this whole biodiversity we know, millions of species, all originate from the proteins and strands of DNA. Also, we're going to have an evolution part II?

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        • Edited by kgj: 4/5/2013 11:26:48 PM
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          What most people don't understand about evolution isn't the [I]large[/I] scale, but the fact that evolution can be observed in extremely small timeframes as well. [url=http://www.pbs.org/wgbh/evolution/library/faq/cat06.html]PBS states, "... there are cases in quickly reproducing life forms like bacteria and fruit flies, however, where evolution can be seen happening in a matter of weeks for the bacteria and many months for the flies... "[/url] Also, many people seem to believe that there is an "apex" to evolution. There is not. You are [I]always[/I] evolving- just a lot more slowly some times than others. Humanity is still evolving. Natural selection is still occurring. Another misunderstanding branching from that one is that natural selection is [I]not[/I] occurring due to our "artificial" environment. This is completely false. First off, there is no "artificial" environment. Everything we've created is part of nature- part of our evolutionary arc and set of interactions with the world. Second, regardless of the "artificiality" of the environment, natural selection will still exist. [url=http://www.evotutor.org/Selection/Sl1B.html]An "artificial" environment still fills in the criteria for natural selection.[/url]

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          • Great thread once again. Reminds me of my time in Biology class.

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          • Excellent thread (As usual), and this topic, specifically, is one that I observe many individuals requiring a bit of education on. Misunderstandings about many evolutionary concepts are abundant here.

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            • Relevant.

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