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Biol 121- 2010.03.29- Evolution- Great Changes, Diversity and Divergence (Ch. 27, 34).docx

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BIOL 121
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Biol 121 225 Freeman 556-564, 762-767 Mar 29/31, 10 Evolution: Great Changes; Diversity and Divergence 1) Mechanisms of change -evolution + developmental biology Evo-devo -small changes appear as big changes a. Gene duplications and the Cambrian -did gene duplications trigger the Cambrian explosion? explosion -Hox genes are important in specifying morphological traits and in development -Homeotic genes (order and place) and are responsible for the 3D pattern of structures – was the increase in morphological complexity during the Cambrian b/c of an increase in the # and complexity of homeotic genes? Hypothesis: “new genes, new bodies” -mutation causes new genes to arise -new clusters arise as duplications -increased complexity of Hox complex is related to the increased complexity of organisms Evidence -there are groups that branch off early (like jellyfish) that have fewer Hox genes and simpler body plans -conversely, groups that do not branch off early have more Hox genes and more complex body plans -also genes within a cluster are very similar which is perhaps due to duplication – the whole Hox cluster could have been duplicated twice to lead to vertebrates -zebrafish have 6 clusters of Hox -exception: crustaceans are complex but have only 5 Hox genes -when gene duplication occurred, new Hox genes were greated, b/c genes within the cluster are similar in structure and base sequence -the entire Hox cluster was duplicated, and then duplicated again, in the lineage leading to vertebrates -duplication of Hox genes = important in making elaborate body plans 2) Adaptive radiation -occurs when a single lineage produces many descendant species that live in a wide diversity of habitats and find food in a variety of ways (e.g. Hawaiian honeycreepers) -adaptive radiations produce star phylogenies (on the phylogenetic tree you can see one ancestor branching out into many different monophyletic groups (adaptive radiation leads to rapid speciation and ecological diversification) -the size and shape of beaks of Hawaiian honeycreepers reflect the different feeding strategies -the different seeds and leaf structures of Hawaiian silverswords reflect different moisture levels in the habitat How does adaptive radiation occur? a) Ecological opportunity -availability of new or novel types of resources can lead to adaptive radiation -adaptive radiation often occurs when habitats by unoccupied by competitors -e.g. Anolis lizards of the Carribean -the lineage includes 150 spp, and there is a strong correlation b/t size and shape of each spp and the habitat it occupies -most Anolis species that are twig-dwellers have relatively short legs and tails; those that spend most of their time clinging to broad tree trunks or running on the ground have long legs and tails b) Morphological opportunity -the other trigger for adaptive radiation is morphological innovation – many important diversification events in the history of life started with evolution of a key morphological trait that allowed descendants to live in new areas, exploit new sources of food, or move in new ways 1 Biol 121 225 Freeman 556-564, 762-767 Mar 29/31, 10 -e.g. feathers and wings gave some dinosaurs the ability to fly – the lineage is now called birds and is made of 10K species 3) Mass extinctions -“evolutionary hurricanes” that buffet the tree of life and snap twigs on it -one mass extinction event, about 251 M years ago, nearly uprooted the tree entirely -the end-Permean extinction came close to ending multicellular life on earth, claiming 90% marine spp and 33% terrestrial spp and lasted 5M years -some dinosaur spp survived which can diversity since lots of niches vacant -a mass extinction refers to the rapid extinction of a large number of lineages scattered throughout the tree of life – at least 60% of the spp present are wiped out within 1 million years -mass extinctions result from extraordinary, sudden and temporary changes in the environment -spp do not die out b/c individuals are poorly adapted to normal or gradually changing environmental conditions but rather because of exposure to exceptionally harsh, short-term conditions e.g. flood basalts, volcano eruption, drop in sea level, anoxic ocean, low O2, high CO2, continents moving that create earthquakes + volcanoes -scientists usually consider five large mass extinction events -the end-Cretaceous (K-P) mass extinction as 65 mya and more than 50% marine spp gone, and many terrestrial including most dinosaurs -possible cause: impact hypothesis by meteorite or asteroid -evidence: there is a crater 10 km across and iridium (found in meteors) was found at the site, along with shocked quartz and microtektites which show that a meteorite hit and also dating for the crater is within the expected timeframe
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