Darw r i w n n a nd n t h t e e G ala l pa p gos Biolo l gy g L c e t c u t re r e 16 1 : 6 Ma M cr c o r ev e olu l ti t on

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1 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 Natural selection leads to great variation of traits within a species Could these differences within one species become so great that another species could emerge? This concept is called speciation This is at the boundary between microevolution and macroevolution Traditional view Animals that look similar are the same species Massive problems with this view Looks aren t everything Looks aren t everything Eastern Meadowlark Western Meadowlark Worker Soldier 1

2 Currently the most accepted definition is the Biological species concept 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 How much of a reproductive barrier is necessary? Ring species All these salamanders breed with their immediate neighbors except these guys Asexual Reproduction - how to classify organisms that don t mate? Morphological body shape, size Ecological how they live Paleontology Are two similar fossils the same species? How similar are the two fossils? Are their differences due to gene flow? Are their differences due to normal variation within a species? 2

3 Can t observe reproductive barriers How to determine whether they would mate? Would they mate with modern organisms? What about chronospecies? Reproductive barrier = time Where do we draw the speciation line? 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 Short term: barrier can be broken and populations reconnected Possibilities: Reproductive Isolation 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 Long term: genomes diverge, no longer compatible Possibilities: Reproductive Isolation Populations can no longer reproduce Hybrids are less viable 3

4 Geographic A physical barrier forms between populations Habitat Mate and live in different environments Water Land Temporal Mate at different times of day or year Behavioral Mates attracted by different behaviors Spring Fall Mechanical Structural differences prevent mating Post-Mating Barriers Gametic incompatibility Egg and sperm do not recognize each other 4

5 Post-Mating Barriers Reduced hybrid viability Once a hybrid forms, it does not survive Post-Mating Barriers Reduced hybrid viability Hybrid is strong but cannot reproduce Liger 1. Rana pipiens 2. Rana blairi 3. Rana sphenocephala 4. Rana berlandieri Allopatric Speciation Geographic separation restricts gene flow Sympatric Speciation 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

6 Rapid speciation from one common ancestor Fills ecological niches Minimizes competition Also called divergent evolution Many examples Adaptive Radiation Hawaiian islands isolated, diverse Silversword Alliance Single ancestor on one island diverged into more than 50 species Adaptive Radiation Adaptive Radiation How Fast Is Evolution? Galapagos finches Gradualism Ground and Cactus Finches Tree Finches Vegetarian Tree Finch Warbler Finches Proposed by Darwin fuliginosa magnirostris conirostris Camarhynchus parvulus Camarhynchus pauper Cactospiza pallida Certhidea fusca Slow, gradual changes over long periods of time Punctuated Equilibrium fortis scandens difficilis Camarhynchus psittacula Cactospiza heliobates Platyspiza crassirostris Certhidea olivacea 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