Chapter 24: The Origin of Species

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) Animation: Macroevolution

What is a species? Biological species concept (Mayr) 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)

Reproductive Isolation (isolation of gene pools), I 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) Video: Albatross Courtship Ritual Video: Giraffe Courtship Ritual Video: Blue-footed Boobies Courtship Ritual

Fig. 24-4a Prezygotic barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Individuals of different species Mating attempt (a) (c) (e) (f) (d) Figure 24.4 Reproductive barriers (b)

Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Fig. 24-4i Prezygotic barriers Postzygotic barriers Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Viable, fertile offspring Fertilization (g) (h) (i) (l) (j) Figure 24.4 Reproductive barriers (k)

Reproductive Isolation, II 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)

Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria

The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

Modes of speciation (based on how gene flow is interrupted) Allopatric: 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

(a) Allopatric speciation (b) Sympatric speciation Fig. 24-5 Fig 24.5 Two main modes of speciation (a) Allopatric speciation (b) Sympatric speciation

Isolated population diverges Fig. 24-14-4 Isolated population diverges Possible outcomes: Hybrid zone Reinforcement OR Fusion Gene flow Hybrid OR Figure 24.14 Formation of a hybrid zone and possible outcomes for hybrids over time Barrier to gene flow Population (five individuals are shown) Stability

Punctuated equilibria Tempo of speciation: gradual vs. divergence in rapid bursts; Niles Eldredge and Stephen Jay Gould (1972); helped explain the non-gradual appearance of species in the fossil record

You should now be able to: Define and discuss the limitations of the four species concepts Describe and provide examples of prezygotic and postzygotic reproductive barriers Distinguish between and provide examples of allopatric and sympatric speciation Explain how polyploidy can cause reproductive isolation Define the term hybrid zone and describe three outcomes for hybrid zones over time