2Origin of Species patterns of speciation AnagenesisA single population is transformed enough to be designated a new speciesCladogenesisBranching evolution. A new species arises from a small population that “Buds” from a parent species.Most species probably evolved this way. The branching evolution is the basis of biological diversity
4Origin of Species Species definitions Biological species = a population whose members have the potential to interbreed with one another and produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of another species. (Reproductive Isolation)The biological species concept cannot be applied to asexually reproducing organismsMorphospecies = species defined by their anatomical features
5The biological species concept is based on interfertility rather than physical similarity Eastern MeadowlarkWestern Meadowlark
6Reproductive barriers Any factor that impedes two species from producing fertile hybridsPrevents interbreeding between closely related speciesPrezygotic barriers (impedes mating or hinders ova fertilization)Postzygotic barriers (after fertilization)
7Origin of Species Prezygotic barriers Habitat isolationTemporal isolationBehavioral isolationMechanical isolationGametic isolation
9Temporal IsolationLate-winter mating seasonSummer mating season
10Blue-footed boobies … Courtship ritual as a behavioral barrier between species.
11Mechanical Isolation Gametic Isolation “You can’t fit a square peg in a round hole”Gametic Isolation“gamete recognition”
12Origin of Species Postzygotic barriers Reduced hybrid viability (vigor)Reduced hybrid fertilityHybrid breakdown
13Origin of Species Introgression The transplantation of alleles between species.Alleles occasionally “seep” through all reproductive barriers and pass between gene pools of closely related species. May occur when a fertile hybrid mates successfully with one of the parent species.
14Hercules the ligerLigerFemale Ligers are fertile and can reproduce. If a Female liger mated with a male tiger the offspring would have some lion genes. If that offspring could then mate with a tiger the lion genes would continue on within the Tiger Gene Pool, an example of introgression.
15A summary of reproductive barriers between closely related species
16Origin of Species Modes of speciation Allopatric speciationA population forms a new species while geographically isolated from its parent populationSympatric speciationSpecies formation occurs in geographically overlapping populations
18Origin of Species Allopatric speciation occurs when the initial block to gene flow is a geographical barrier that physically isolates the population.The genetically isolated group can then follow its own evolutionary course as changes in allele frequencies occur and are undiluted by gene flow from other populations
19Has speciation occurred during geographic isolation?
20Allopatric speciation of squirrels in the Grand Canyon White-tailed antelope squirrel (north rim)Harris’s antelope squirrel (south rim)
23Origin of Species Allopatric speciation Adaptive Radiation = emergence of numerous species from a common ancestor introduced into an environment.Example: Darwin’s finches on the Galapagos island chain
26Adaptive Radiation in Cichlids (Specialization in feeding)
27Origin of Species Sympatric speciation Formation of a new species within the geographical range of the parent population (no geographical isolation)Can occur quickly (in one generation) if a genetic change results in a reproductive barrier between mutants and the parent population
28Origin of Species Sympatric speciation IrisPolyploid speciationMany plant species have originated from improper cell division that resulted in polypoidyAutopolyploidy – an organism that has more than two chromosome sets, all derived from the same speciesAllopolyploidy – a polyploid hybrid resulting from contributions by two different species. More common than autopolyploidy. Hybrids are usually sterile but may be very vigorous and propagate asexually.Daylily
29Sympatric speciation by autopolyploidy in plants
30One mechanism for allopolyploid speciation in plants
31Punctuated Equilibrium vs. Gradualism Punctuated equilibrium model suggests evolution occurring in spurts of relatively rapid change instead of gradual divergence