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Download Biology 102 Lecture 16: Macroevolution 16: Macroevolution
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11/28/2015 Biology 102 Lecture 16: Macroevolution Darwin and the Galapagos • Darwin recognized that the Galapagos were a place of genesis for new species • Many plants and animals exist there that exist nowhere else Speciation What is a Species? Natural selection leads to great variation of traits within a species • Traditional view • Animals that look similar are the same species • Could these differences within one species become so great that another species could emerge? • Massive problems with this view… • This concept is called speciation • This is at the boundary between microevolution and macroevolution • What is a Species? • Looks aren’t everything… What is a Species? • Looks aren’t everything… Worker Eastern Meadowlark Soldier Western Meadowlark 1 11/28/2015 Problems With the Biological Species Concept What is a Species? • Currently the most accepted definition is the • Individuals that can produce viable, fertile offspring are of the same species • Unable to reproduce with other species • Reproductive isolation leads to genetic divergence • There are some problems with this too… “Biological species concept” • How much of a reproductive barrier is necessary? Problems With the Biological Species Concept • Ring species • All these salamanders Problems With the Biological Species Concept • breed with their immediate neighbors Asexual Reproduction - how to classify organisms that don’t mate? • Morphological – body shape, size • Ecological – how they live except these guys… Problems With the Biological Species Concept • Paleontology – Are two similar fossils the same species? • How similar are the two fossils? • Are their differences due to gene flow? Problems With the Biological Species Concept • Are their differences due to normal variation within a species? 2 11/28/2015 Problems With the Biological Species Concept Problems With the Biological Species Concept • Can’t observe reproductive barriers • What about chronospecies? chronospecies? • How to determine whether they would mate? • Reproductive barrier = time • Would they mate with modern organisms? • Where do we draw the speciation line? Problems With the Biological Species Concept • Phylogenetic - Look for DNA sequence similarities Reproductive Isolation • How do two populations become two species? • A reproductive barrier must form that prevents the exchange of genes • Two populations become isolated and evolve separately Reproductive Isolation Reproductive Isolation • Short term: barrier can be broken and populations reconnected • Long term: genomes diverge, no longer compatible • Possibilities: • Possibilities: • Population fusion - gene flow could erase differences; populations freely interbreed • One population could go extinct • Founding of stable hybrid zone • Creation of a new species by hybridization • Populations can no longer reproduce • Hybrids are less viable 3 11/28/2015 PrePre-Mating Barriers PrePre-Mating Barriers • Geographic • Habitat • A physical barrier forms between populations • Mate and live in different environments Water PrePre-Mating Barriers Land PrePre-Mating Barriers • Temporal • Behavioral • Mate at different times of day or year • Mates attracted by different behaviors Spring Fall PrePre-Mating Barriers PostPost -Mating Barriers • Mechanical • Gametic incompatibility • Structural differences prevent mating • Egg and sperm do not recognize each other 4 11/28/2015 PostPost -Mating Barriers PostPost -Mating Barriers • Reduced hybrid viability • Reduced hybrid viability • Once a hybrid forms, it does not survive • Hybrid is strong but cannot reproduce 1 2 3 Liger 4 1. Rana pipiens 2. Rana blairi 3. Rana sphenocephala 4. Rana berlandieri Allopatric Speciation • Geographic separation Sympatric Speciation • restricts gene flow Reproductive barriers evolve within a population Allopatric Speciation Sympatric Speciation Part of a fly population that lives only on hawthorne trees moves to an apple tree Part of a mainland population reaches to an isolated island “Habitat isolation” The isolated populations begin to diverge due to genetic drift and natural selection Divergence may eventually become sufficient to cause reproductive isolation The flies living on the apple tree do not encounter the flies living on the hawthorne tree, so the populations diverge 5 11/28/2015 Adaptive Radiation Adaptive Radiation • Rapid speciation from one common ancestor • Hawaiian islands – isolated, diverse • Fills ecological niches • “Silversword Alliance” • Minimizes competition • Single ancestor on • Also called “divergent evolution” one island diverged • Many examples… into more than 50 species Adaptive Radiation • How Fast Is Evolution? Galapagos finches • Ground and Cactus Finches Geospiza fuliginosa Geospiza magnirostris Vegetarian Tree Finch Tree Finches Camarhynchus parvulus Geospiza conirostris Camarhynchus pauper Cactospiza pallida Warbler Finches Geospiza scandens Geospiza difficilis Camarhynchus psittacula Cactospiza heliobates • Proposed by Darwin • Slow, gradual changes over long periods of time Certhidea fusca • Geospiza fortis Gradualism Platyspiza crassirostris Certhidea olivacea Punctuated Equilibrium • Proposed by Stephen Jay Gould • Rapid evolution over short periods, followed by short periods of no change How Fast Is Evolution? • Which is correct? • Depends on organism and environment • Larger animals show gradual change • Smaller animals more punctuated 6
