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Adaptations & Comparative Anatomy For a sticker: 1. Speciation occurs when a population diverges because a ______ limits ____ ____. 2. Name the four prerequisites or conditions for natural selection. (Hint: they start with V, I, O, and L.) Homework Recap Variation + Inheritance + Overreproduction + Limited Resources = Natural Selection, individuals bearing traits providing reproductive advantage have more babies. Natural selection produces cumulative effects over time. Evolution = allele frequency changes over time. Sufficient cumulative effects can yield speciation. Populations diverge (speciate) when gene flow is limited by a barrier. Three kinds of speciation. How rapidly speciation occurs depends on amount of selective pressure, and generation length. Objectives Learn three methods scientists use in determining a species’ ancestry Differentiate between different kinds of anatomical patterns that evolution produces Be able to explain how you can use anatomy to determine descent Determining Descent Scientists have many approaches to studying evolution. The rest of the unit, we will focus on three of those approaches. We will study how scientists use anatomy, molecular evidence, and the fossil record. We will skip biogeography, biochemistry, behavior, and active experimentation, unfortunately. Determining Descent These are all different methods of determining descent. Organisms have common descent if they come from a common ancestor. The most recent ancestor that they all have in common is called the last common ancestor. Common Descent The more recent the last common ancestor, the more closely related and the more similar the organisms are. Today, we will focus on anatomy: morphology and embryology. Adaptations An adaptation is a trait that developed/spread because it conferred a survival advantage. What adaptations have you learned about in previous lessons? Adaptations An adaptation is a trait that developed/spread because it conferred a survival advantage. What adaptations have you learned about in previous lessons? Adapatations can include features like camouflage, limb shape, tooth size, bone density, scales, etc. Adaptations and Structures QuickTime™ and a decompressor are needed to see this picture. An adaptation that arises in a species will be passed down to its descendents, perhaps with modification, unless the descendents inhabit an environment that makes the adaptation disadvantageous. We can track common ancestry by tracking who inherited the same adaptations. Adaptations & Structures Homologous structures are anatomical features that are similar between two organisms because they came from the same ancestral form. They don’t necessarily have the same function anymore. “Homo” root, remember what that means? A great example is the shape of the forelimb in animals. Homology Same structure, diverse functions: 1 bone, then 2, then a wrist, then radiating digits. The common ancestor for all these organisms had the same limb pattern, which was modified in descendents depending upon the pressures of their environments. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Homology vs Analogy This is different from an analogous structure. Analogous structures: Perform the same function but evolved separately. Remember what an analogy is? For instance, the wing of a butterfly, bat, and bird. All structures enable flight, none of these organisms inherited the structure from one of the others. Consequently, they have similar physics but obviously different underlying anatomy. • Analogous structures are useful to ecological study, but not to studying descent. Vestigiality Also very useful are a type of homologous structure called a vestigial structure. When a structure becomes disadvantageous to an organism, it’s rarely instantly lost. Instead, its significance gradually diminishes, just like its significance originally gradually appeared. The structure may never go away entirely. If it gets small and insignificant enough that it no longer affects the organism, selection can’t act on it anymore so that last tiny shard of it may remain in the species for a very long time. Vestigiality A homologous structure that has lost part of all of its original function is called a vestigial structure. A “vestige” is a “remnant.” A vestigial structure is a leftover remnant of a structure that was whole and important in an ancestor. Examples… Vestigiality Examples The human appendix. Our primate and other vertebrate ancestors had a caecum at the end of their intestines, which held a colony of specialized bacteria that increased the efficiency of cellulose digestion. Over the history of human evolution, it shrank and now is quite useless at its former job. All it does is get infected 7% of people get acute appendicitis in their lifetimes. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Vestigiality Examples Flightless birds haven’t yet lost their wings. The wings are vestigial. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Vestigiality Examples Whales have tiny vestigial hind legs, remnants of their terrestrial ancestry. Some snakes also have vestigial legs from their lizard ancestors. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Vestigiality Examples Human erector pilli (the muscles that make your hair stand on end), the human coccyx, and human wisdom teeth are more examples from our species. Our primate ancestors had larger jaws than us that could accommodate more teeth. But our jaw size has been shrinking faster than our wisdom teeth, so we still have them. Given another few million years, maybe we’ll be rid of the nuisance! Determining Descent So one way that scientists track descent by examining physical characteristics is by seeing which organisms: Have the same characteristics • All mammals have hair, so it’s logical to conclude that their last common ancestor also had hair. Have homologous characteristics • Amphibians, reptiles, birds, and mammals all have homologous limb structures, so it’s logical to assume that their last common ancestor had that limb structure. Have vestigial structures, and which organisms have the functional versions of those structures Determining Descent Another comparison of physical characteristics is made by examining the embryos of organisms. Genes don’t just code for how an adult is built, they also code for how an embryo is built. An organism’s instructions for how to build an embryo are inherited by its descendents just like any other genes. Therefore, the more structures two species’ embryos share, the more closely related the species are. Determining Descent What’s more, because evolution is much more likely to proceed by modifying existing structures than by making whole new ones, embryos will often have the same structure that an ancestor did, but the structure will change or disappear by the time of birth. For instance… For instance, birds have limbs with three fused digits. Their reptile ancestors had five digits. It would require a significant genetic change for a bird to only make three digits, but it’s a relatively minor change to make five digits and then, while the bird is still in the egg, bind them together while they’re still cartilage. And so, bird embryos have five digits, indication of their reptilian ancestry. QuickTime™ and a decompressor are needed to see this picture. Determining Descent Dr. Kenneth Miller on some lines of descent that were in the news when the movie was made (2006). Ch. 9-13 Recap Definitions: adaptation, homologous structure, analogous structure, vestigial structure Scientists can determine common ancestry by comparing anatomy. Identical structures Homologous structures • Adult homologous structures • Adult vestigial structures • Embryo homologous structures Coming Up Learning how to make and read a phylogenetic tree (a map or “family tree” of ancestry) More sources of evolutionary evidence: molecular (DNA & proteins), fossils Putting it all together with the example of human evolution After MCAS, we may come back and address biogeography and applications of evolutionary principles (poisons, antibiotics, crime scenes, agriculture…)