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Intro Bio Week 4

by: thersh

Intro Bio Week 4 BIOL 1010


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Notes from week 4 of class - speciation and extinction
Introduction to Biology
Stephanie Hutchins
Class Notes
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This 3 page Class Notes was uploaded by thersh on Thursday February 18, 2016. The Class Notes belongs to BIOL 1010 at Rensselaer Polytechnic Institute taught by Stephanie Hutchins in Fall 2016. Since its upload, it has received 37 views. For similar materials see Introduction to Biology in Biology at Rensselaer Polytechnic Institute.


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Date Created: 02/18/16
2/18/16 Session 1.6 Speciation and Extinction Biological Species Concept ­ “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups.”    ­  Ernst Mayr ­ It follows that: o Each species has a unique set of alleles that forms the species’ gene pool o Each species’ gene pool is isolated from other gene pools, i.e. alleles do no flow  between species Reproductive Isolation ­ Cornerstone of the BSC ­ Reproductive isolation results from genetic changes causing pre­zygotic or post­zygotic  barriers to reproduction ­ Speciation is the attainment of reproductive isolation ­ How we recognize speciation in nature? Depends on what definition of what a species is. Problems with the BSC ­ Application of BSC to many groups problematic because of hybridization between clearly  delimited species  ­ BSC doesn’t apply & hence of no help in delimiting species in asexual populations (e.g.  rotifers, euglenoids; some green algae, diatoms, self­pollinating plants & Daphnia).   ­ Each individual and its descendents are isolated gene pools,                           so is each  lineage a separate species?  ­ In practice, BSC can’t be applied to delimit species:  breeding experiments are definitive test  of species membership & such tests often impossible with wild populations.  ­ No widely accepted concept of species for prokaryotes  ­  definitions based on DNA ­ rRNA  sequence differences  (> 70% DNA, >98.7% rRNA) Mechanisms of Speciation ­ Geographic barriers – reduce gene flow between separated populations ­ Allopatric Speciation o Speciation in geographically isolated populations o Some type of barrier exists that prevents/reduces gene flow o Effectiveness of barrier varies with species o Barriers in Allopatric Speciation:  Mountain ranges, oceans/bodies of water, land, glacial masses, valleys ­ Parapatric Speciation o Adjacent populations evolve into distinct species o Maintain contact along common border o Often hybridize along contact zone ­ Speciation in progress: o Apple & hawthorn races have evolved reproductive barriers o Adults mate & oviposit on their birth fruit o Apple race emerges 3 weeks earlier o Leads to limited interbreeding o These differences between races mark early stages of sympatric speciation. ­ Sympatric Speciation o “The divergence of a single interbreeding population of one species into two  reproductively isolated populations without any barriers to gene flow.” o Formerly dismissed by biologists especially Ernst Mayr o  But compelling evidence for existence & importance of sympatric speciation since  1960s study of Rhagoletis by Guy Bush o In progress:  Blue indigobird lays eggs in nests of African firefinch & Black­bellied  firefinch  Baby indigobirds learn songs of their respective “foster” finches & have  developed two “races”  Females prefer males who know same finch song as they do & pass  preferences on  Female indigobirds that grow up in African firefinch nests lay eggs in African firefinch nests rather than in Black­bellied firefinch nests. Adaptive Radiation ­ A “burst of speciation and evolutionary divergence ­ Single lineage gives rise to many new species ­ New species fill vacant “adaptive zones” ­ Adaptive zone is a new set of niches or “ways of life” Figure 1 Adaptive Radiation Mass Extinctions ­ Six major mass extinctions have occurred over the past 600 million years. ­  Cretaceous Extinction o Iridium much more common in asteroids & meteors than Earth rocks o Iridium layer at K­T boundary in soil fossil layer marks and separates age of reptiles  & age of mammals o  After several year search geologists found “smoking gun” – huge Chicxulub crater o  120 miles in diameter,  8­10 miles deep o  Controversy over what caused other mass extinctions ­ Recovery from Extinction o Over time species diversity recovers o  3 models:  a. Recovery starts immediately ­ lasts  few million yrs  b. Recovery has time lag ­ then rapid adaptive radiation through + feedback (new species & niches create more & more opportunities Figure 2 Extinction Rate vs. Time for more species)  c. Recovery starts and builds slowly following S­shaped logistic curve  o  Extensive fossil data for marine species favor model b o  Time lag is ~10 MY ­ The Greatest Mass Extinction? o Over the past 300 years, Homo sapiens responsible for 100s of thousands of  extinctions o Extinction rates now 100­1000 times “background” or normal levels of ~1­2 species  per year o If increased rate continues, we are heading towards greatest mass extinction in  Earth’s history


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