The Origin of Species

Macroevolution Macroevolution: the origin of new taxonomic groups Speciation: the origin of new species Anagenesis (phyletic evolution): accumulation of heritable changes Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity)

What is a species? Biological species concept a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring genetic exchange is possible and that is genetically isolated from other populations Proposed in 1942 by Ernst Mayer

Prezygotic Reproductive Isolation Biological factors impede 2 different species from producing viable offspring Prezygotic barriers: impede mating between species or hinder the fertilization of the ova Habitat (snakes; water/terrestrial) Behavioral (fireflies; mate signaling) Temporal (salmon; seasonal mating) Mechanical (flowers; pollination anatomy) Gametic (frogs; egg coat receptors)

Postzygotic Reproductive Isolation Postzygotic barriers: fertilization occurs, but the hybrid zygote does not develop into a viable, fertile adult Reduced hybrid viability (frogs; zygotes fail to develop or reach sexual maturity) Reduced hybrid fertility (mule; horse x donkey; cannot backbreed) Hybrid breakdown (cotton; 2nd generation hybrids are sterile)

Reproductive Isolation

Modes of speciation Allopatric: Sympatric: populations segregated by a geographical barrier can result in adaptive radiation (island species) Sympatric: reproductively isolated subpopulation in the midst of its parent population (change in genome) polyploidy in plants cichlid fishes

Causes of Sympatric Speciation Polyploidy Autopolyploid Individual doubles chromosome number (becomes tetraploid) Allopolyploid Cells have a different chromosome number due to successful hybridization Habitat Differentiation Different food sources Sexual Selection Females select for different characteristics in males

Adaptive Radiation Evolution of many diversely adapted species from a common ancestor Introduced to new environments Ex: Species on the Hawaiian Islands

Tempo of Speciation Gradual change vs. divergence in rapid bursts Punctuated Equilibrium Niles Eldredge and Stephen Jay Gould (1972) Helped explain the non-gradual appearance of species in the fossil record

Evo-Devo Evolution has affected developmental biology Heterochrony Rate of developmental events differs between organisms Allometric growth (proportioning that gives body its distinct form) Paedomorphosis Organism retains characteristics of larval stage in adult body

Heterochrony Paedomorphism

No Goal in Sight Evolution is not goal-oriented Branches could end up in a “trend” or in a “dead-end” Individual species undergo natural selection “species selection”