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Lecture 4

BIO153 Lecture 4.pdf

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Christoph Richter

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BIO153: Lecture 4 Macroevolution January 14, 2009 Macroevolution = evolution at or above species level “Macroevolution means evolution on the grand scale, and it is mainly studied in the fossil record. It is contrasted with microevolution, the study of evolution over short time periods, such as that of a human lifetime or less. Microevolution therefore refers to changes in gene frequency within a population ... Macroevolutionary events are more likely to take millions, probably tens of millions of years. Macroevolution refers to things like the trends in horse evolution described by Simpson, and occurring over tens of millions of years, or the origin of major groups, or mass extinctions, or the Cambrian explosion described by Conway Morris. Speciation is the traditional dividing line between micro- and macroevolution.” Mark Ridley, 1997 EVOLUTION Oxford University Press, Oxford UK Macroevolution often entails major morphological changes – how do they occur? Completely new features are rare – most “new” structures are modifications of existing features ▯ e.g. mammalian inner ear arose from bones present in the jaw of reptile ancestors ▯ e.g. tetrapod limb: modified functions arise from existing structures (flying, running, burrowing, etc.) Transformation of characters in macroevolution: Changes in: ▯ size ▯ shape ▯ position ▯ association with other parts ▯ differentiation (complexity) 1 Example: serially homologous elements -- repeated elements based on the same developmental plan (e.g. petals, ribs, vertebrae, body segments…) These may evolve in number, position, or differentiation: ▯ may diversify: e.g. reptile teeth are simple and homogeneous, while mammalian teeth are heterogeneous Increasing size & complexity of structures is common (e.g. mammalian lung is more complex than the amphibian lung) But reduction of complexity and loss of structures is common, too! Evolutionary reversal: return to ancestral state of a character ▯ loss of wings in fleas and lice ▯ loss of complex teeth in toothed whales ▯ loss of limbs in snakes Evolutionary reversal is common because of the potentialities of developmental systems (even when the structure is lost, the genes for that structure may be retained) ▯ e.g. since Jurassic, frogs lack teeth in lower jaw ▯ Gastrotheca guentheri has re- evolved true teeth in lower jaw ▯ developmental system for making teeth is still present in frogs; has been genetically “silenced” (more on this in Lecture 5) Thus, simple ▯ ancestral; complex ▯ derived! Developmental plan is very important in creating new features ▯ growth allometry: different structures grow at different rates ▯ e.g. beak of the godwit: beak grows faster than the head; beak is small 2 relative to head in juvenile, but large relative to the head in the adult ▯ what if descendent species had altered development? (e.g. a mutation in the rate of development of the beak?) Heterochrony = changes in timing of development from ancestor ▯ descendent 2 main types: 1. peramorphosis: "elder form" - descendent more “adult-like” than ancestor 2. paedomorphosis: “child form” - descendent more “child-like” than ancestor 1. Peramorphosis ▯ more developed adults in descendent than in ancestor ▯ often mature at a later age & larger size ▯ exaggerated structures: antlers of Irish elk ▯ among deer: diversity in body &
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