In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Figure 24.1 How did this flightless bird come to live on the isolated Galápagos Islands?

(a) Similarity between different species Fig. 24-2 (a) Similarity between different species Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity (b) Diversity within a species

Gene Flow Between Populations EXPERIMENT Example of a gene tree for population pair A-B Allele Population Gene flow event 1 B Allele 1 is more closely related to alleles 2, 3, and 4 than to alleles 5, 6, and 7. Inference: Gene flow occurred. 2 A 3 A 4 A 5 B Alleles 5, 6, and 7 are more closely related to one another than to alleles in population A. Inference: No gene flow occurred. 6 B 7 B RESULTS Pair of populations with detected gene flow Estimated minimum number of gene flow events to account for genetic patterns Distance between populations (km) Figure 24.3 Does gene flow occur between widely separated populations? A-B 5 340 K-L 3 720 A-C 2–3 1,390 B-C 2 1,190 F-G 2 760 G-I 2 1,110 C-E 1–2 1,310

Reproductive Barriers Between Species Prezygotic Barriers Postzygotic Barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Individuals of different species Mating attempt Viable, fertile offspring Fertilization (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) Figure 24.4 Reproductive barriers (k)

Water-dwelling Thamnophis Habitat Isolation Figure 24.4 Reproductive barriers Water-dwelling Thamnophis

Terrestrial Thamnophis Habitat Isolation Figure 24.4 Reproductive barriers Terrestrial Thamnophis

Courtship ritual of blue-footed boobies Behavioral Isolation Occurs Without Appropriate Mating Rituals Figure 24.4 Reproductive barriers Courtship ritual of blue-footed boobies

Mechanical Isolation: Figure 24.4 Reproductive barriers Mechanical Isolation: Bradybaena with shells spiraling in opposite directions

Mule: sterile hybrid offspring between horse and donkey PostZygotic Reproductive Barrier Figure 24.4 Reproductive barriers Mule: sterile hybrid offspring between horse and donkey

Speciation (a) Allopatric speciation (b) Sympatric speciation Fig 24.5 Two main modes of speciation (a) Allopatric speciation (b) Sympatric speciation

Allopatric Speciation A. harrisi A. leucurus Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

Continental Drift Caused Allopatric Speciation Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 80 60 40 20 1 2 3 Figure 24.7 Allopatric speciation in frogs Millions of years ago (mya) 1 2 3 India Madagascar 88 mya 65 mya 56 mya

Sympatric Speciation via Polyploidy is Common in Plants Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile. Fig 24.10 Sympatric speciation by autopolyploidy in plants

Sympatric Speciation - Polyploidy --> Allopolyploid Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Meiotic error Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10 Figure 24.11 One mechanism for allopolyploid speciation in plants Normal gamete n = 3 Species A 2n = 6

Hybrid Zones EUROPE Fire-bellied toad range Hybrid zone Fire-bellied toad, Bombina bombina Yellow-bellied toad range Yellow-bellied toad, Bombina variegata 0.99 0.9 Fig 24.13 A narrow hybrid zone for B. variegata and B. bombina in Europe Allele frequency (log scale) 0.5 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km)

Hybrid Zones Over Time Possible outcomes: Reinforcement Fusion into Isolated population diverges Hybrid zone Reinforcement of gene flow barrier. OR Fusion into one species. 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: Continued formation of hybrid individuals.

Patterns in Speciation Punctuated Equilibrium pattern Change / Time Gradualism pattern Figure 24.17 Two models for the tempo of speciation

Speciation Review Allopatric speciation Sympatric speciation Original population Fig. 24-UN1 Allopatric speciation Sympatric speciation

Speciation Ancestral species: AA BB DD Triticum monococcum (2n = 14) Wild Triticum (2n = 14) Wild T. tauschii (2n = 14) Product: Fig. 24-UN2 AA BB DD T. aestivum (bread wheat) (2n = 42)

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.