BIO150-phylogenetic practice-exercise

BIO150-phylogenetic practice-exercise - Reconstructing...

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Reconstructing Evolutionary Relationships S-1 BIO150 2004/05 Practice Exercise: Phylogeny of Terrestrial Vertebrates In this example we will construct a phylogenetic hypothesis of the relationships between seven taxa of terrestrial vertebrates: frogs, turtles, mammals, birds, crocodiles, lizards, and snakes. We will use another taxon (ray-finned fish) – which has some similarities with your terrestrial vertebrates but which is not part of that group – as our out-group to polarize the characters chosen for the analysis. Identifying Characters The first step in every analysis is identification of the characters. This is often the most important and most difficult part of the work (not to mention time and labour consuming). Below is the list of the characters, each of which is accompanied by the corresponding number used in Table 1. Table 1 contains a list of the character states found in each taxon. (You are not expected to learn these characters and their states for the quiz or test.) Amnion (1) : Does the egg have the amnion, an extra-embryonic membrane that evolved to over- come desiccation? Possible states: amnion present, absent. Appendages (2): Early vertebrates evolved fins to swim more efficiently in the water. Limbs allowed movement on land. Although snakes lack external limbs, several groups of snakes have rudimentary appendages, proof of the fact that the ancestors of snakes did have limbs. Possible states: fins, limbs. Body covering (3): Possible states: scales, scutes, hair, feathers. Metabolism (4): Maintenance of a steady internal state (homeostasis) is critical. In vertebrates, body temperature is controlled either internally by metabolic processes (endothermy) or is dependent on the external environment (ectothermy). Possible states: endothermy, ectothermy. Internal nostrils (5): The nostrils of most fishes do not connect with the mouth. The nostril of lungfishes and of all terrestrial vertebrates do. Possible states: internal nostrils present, absent. Temporal fenestrae (6): Number of temporal holes in the skull, which allow for the expansion of jaw muscles (enhancing biting strength). Possible states: none, one, or two. Male genitalia (7): Presence or absence of a hemipenes, or split penis. Snakes and lizards are unusual in that they have a double penis, whereas other vertebrate groups have a single penis. Possible states: hemipenes present, absent. Digestive system (8): Presence or absence of a specialized gizzard, a very muscular portion of the stomach. The muscle action, together with a tough lining of cuticle, and especially grit that is ingested, aid in the grinding of fibrous foods such as seeds. Possible states: gizzard present, absent. Nitrogenous waste (9): Type of nitrogenous waste. Ammonia is converted to either urea or uric acid before excretion.
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BIO150-phylogenetic practice-exercise - Reconstructing...

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