The Alvarez - magnetic reversals in the stratigraphic...

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The Alvarez's from Berkeley proposed that the K/T extinctions were caused by impact of a large asteroid. Some compelling evidence supports the notion: Excess of iridium (iridium anomaly or iridium "spike") at the K/T boundary (see fig. 23.6, pg. 649). This element is rare in earth's crust, but not uncommon in meteorites. The presence of "shocked" quartz (likely to be formed at asteroid impact, less likely to be formed by normal Earthly geological processes) is also in excess at the K/T. Evidence for these diagnostic markers of impact have been sought at the other "big five" mass extinctions and only one has any blip of excess iridium (no where near the spike at the K/T). There are some problems with the impact explanation: why was it so selective? and: where is the impact crater? Well, sure, you know, ah, it, ah, it landed in the ocean! Or maybe it landed near a subduction zone and the evidence has been conveniently tucked under some continental plate. Also, there is evidence from
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Unformatted text preview: magnetic reversals in the stratigraphic record that the K/T transition is varies in time from place to place. Every couple of years someone publishes a paper indicating that they found the impact crater; the most recent focus is somewhere near the Yucatan peninsula or western Caribbean. Keep an eye on Nature and Science. The issue of impact extinction puts all that we learned about population genetics and adaptation in a very different perspective. So what if one allele is more fit than another, or the rate of evolution depends on the amount of additive genetic variation in the population, if an asteroid is going to blow us away tomorrow, then microevolution really is decoupled from macroevolution . But what about those lineages that sail through the K/T boundary unaffected? Maybe they were adapted, pre-adapted or just exapted for the impact and there is a coupling....
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