Biology 1B Evolution Lecture 11 March 19 2010 Insights from the Fossil Record and Evo Devo Extinction Important points on extinction rates Background rate of extinctions per million species per year About 1 extinction per million species per year so if there are roughly 10 million species on the planet that would imply that about 10 species go extinct per year Environmental pressures such as changing temperatures or volcanism commonly cause extinction of species but extinction also occurs in a constant environment Broadly a typical species lasts 2 10 million years Figure 25 14 page 521 8th edition Huge extinction the result of a comet impact more below Cretaceous Paleogene Tertiary Extinction K T Boundary Most importantly this marked the demise of the dinosaurs save for modern dinosaurs which are birds With the end of the dinosaurs came the adaptive radiation of the mammals Luis and Walter Alvarez of UC Berkeley worked to find what caused the K T boundary extinction of the dinosaurs Walter a paleonotologist noticed a layer of dark silt between fossils of organisms between the Cretaceous large bodied animals and Paleogene smaller bodied animals He asked his father a physicist what might be studied in the isotopes in the dark silt layer Moritz Lecture 11 Notes 1 They found a huge spike in the amount of iridium which suggested a comet impact Iridium spike indicating comet impact Despite the scrutiny and disbelief of the scientific community the crater the comet would have left in the Yucatan was eventually found The massive impact of the comet ejected sediments into the air halting photosynthesis and changing the raw material of the planet It took about 10 000 years to restore the Earth s normal ecological processes Are we in a sixth mass extinction Very difficult to know for sure as we don t even know how many species there are on the planet we ve only described 1 6 million of perhaps 10 15 million total species Amphibians are closely tied to environmental conditions and 30 of known species are now endangered Better evidence of a mass extinction might come from the loss of megafauna smilodons mastodons that might be the result of humans hunting climate change or both North American and Australian megafauna went extinct roughly after humans arrived in North America Africa still has many megafauna suggesting that they evolved with the humans and human ancestors there We also see extinctions of large birds such as rails and moas soon after the recent colonization of Pacific Islands by humans Moritz Lecture 11 Notes 2 Major Transitions in Phenotypes Vertebrate Fish Point of transition from exclusively aquatic organisms to terrestrial vertebrates is important Amphibian Tetrapods Figure 34 20 page 711 8th edition There is a huge jump between these groups particularly in phenotypes which cannot be constructed from modern fauna this is where the fossil record comes into play The transition from ancestors of lungfish to ancestors of tetrapods and key insights are coming from fossils Acanthostega has a moving neck lungs limbs but no wrists and a weak rib cage that would have made breathing on land very difficult so it probably lived in a swamp Tiktaalik however has the evolved traits of Acanthostega plus wrists and a stronger rib cage see image below The extinct forms are not necessarily direct ancestors but they do allow us to see how and when certain traits adaptations evolved Evolution of Novel Structures and Exaptation The hammer and anvil bones of the middle ear of mammals that act as transmitters from the outer ear into the inner ear There is a complete set of fossil transitions that takes us from the original function of the bones articulation points of the jaw in cynodonts and synapsids Moritz Lecture 11 Notes 3 Figure 34 31 page 721 8th edition see also Figure 25 6 page 513 8th edition Co opting structures from one function to another is known as exaptation don t need a new structure to arise can simply modify an existing one Feathers are another example their original purpose may not have been for flight but rather for attracting mates or thermoregulation new studies have found dinosaurs with feathers that were colored but not for flight suggesting their use in display Evolutionary Novelties Development and Deep Homology Re programming of developmental pathways can cause changes in timing location of expression of key regulatory genes Within sets of genes there are often duplicates allowing one copy to be kept and others to be used for new functions Figure 25 22 page 527 8th edition Moritz Lecture 11 Notes 4 In insects the Ubx gene suppresses leg development to 6 legs but it can have a different effect in other species Deep Homology Very different structures that have a common site of developmental genes These structures have very different anatomy yet they have similar patterns of Hox gene expression Origin of Novelty the Eye Eyes of humans and squid are analogous i e independently evolved between vertebrates and mollusks the diagram below shows multiple types of eyes Homologous genes code for these types of eyes yet there is a range of structures and consequent sight capability within the two groups Moritz Lecture 11 Notes 5 Moritz Lecture 11 Notes 6
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