2 Origin 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
4 Origin 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
5 The biological species concept is based on interfertility rather than physical similarity Eastern MeadowlarkWestern Meadowlark
6 Reproductive 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)
7 Origin of Species Prezygotic barriers Habitat isolationTemporal isolationBehavioral isolationMechanical isolationGametic isolation
9 Temporal IsolationLate-winter mating seasonSummer mating season
10 Blue-footed boobies … Courtship ritual as a behavioral barrier between species.
11 Mechanical Isolation Gametic Isolation “You can’t fit a square peg in a round hole”Gametic Isolation“gamete recognition”
12 Origin of Species Postzygotic barriers Reduced hybrid viability (vigor)Reduced hybrid fertilityHybrid breakdown
13 Origin 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.
14 Hercules 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.
15 A summary of reproductive barriers between closely related species
16 Origin 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
18 Origin 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
19 Has speciation occurred during geographic isolation?
20 Allopatric speciation of squirrels in the Grand Canyon White-tailed antelope squirrel (north rim)Harris’s antelope squirrel (south rim)
23 Origin 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
25 A model for adaptive radiation on island chains
26 Adaptive Radiation in Cichlids (Specialization in feeding)
27 Origin 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
28 Origin 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
29 Sympatric speciation by autopolyploidy in plants
30 One mechanism for allopolyploid speciation in plants
31 Punctuated Equilibrium vs. Gradualism Punctuated equilibrium model suggests evolution occurring in spurts of relatively rapid change instead of gradual divergence