Speciation and Macroevolution Chapter 20

Learning Objective 1 What is the biological species concept? List two potential problems with the concept

Biological Species Concept one or more populations members interbreed in nature produce fertile offspring do not interbreed with different species

Sterile Hybrid

Problems Biological species concept applies only to sexually reproducing organisms Individuals assigned to different species may occasionally successfully interbreed

KEY CONCEPTS According to the biological species concept, a species consists of individuals that can successfully interbreed with one another but not with individuals from other species

Learning Objective 2 What is the significance of reproductive isolating mechanisms? Distinguish among different prezygotic and postzygotic barriers

Reproductive Isolating Mechanisms Restrict gene flow between species Prezygotic barriers prevent fertilization from taking place Postzygotic barriers prevent gene flow after fertilization has taken place (reproductive isolating mechanisms)

Prezygotic Barriers (1) Temporal isolation two species reproduce at different times of day, season, or year Habitat isolation two closely related species live and breed in different habitats in same geographic area

Temporal Isolation

Figure 20.2: Temporal isolation in wood and leopard frogs. Fig. 20-2a, p. 430

Figure 20.2: Temporal isolation in wood and leopard frogs. Fig. 20-2b, p. 430

Wood frog Leopard frog Mating activity March 1 April 1 May 1 Figure 20.2: Temporal isolation in wood and leopard frogs. March 1 April 1 May 1 Fig. 20-2c, p. 430

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Prezygotic Barriers (2) Behavioral isolation distinctive courtship behaviors prevent mating between species Mechanical isolation incompatible structural differences in reproductive organs of similar species

Behavioral Isolation

Mechanical Isolation

Prezygotic Barriers (3) Gametic isolation gametes from different species are incompatible because of molecular and chemical differences

Postzygotic Barriers (1) Hybrid inviability interspecific embryos die during development Hybrid sterility prevents interspecific hybrids that survive to adulthood from reproducing successfully

Hybrid Sterility

Postzygotic Barriers (2) Hybrid breakdown prevents offspring of hybrids that survive to adulthood and successfully reproduce from reproducing beyond one or a few generations

KEY CONCEPTS The evolution of different species begins with reproductive isolation, in which two populations are no longer able to interbreed successfully

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Learning Objective 3 What is allopatric speciation? Give an example

Allopatric Speciation (1) Evolution of a new species from ancestral population Population becomes geographically isolated from rest of species subsequently diverges

Allopatric Speciation (2) More likely to occur if original isolated population is small makes genetic drift more significant Examples: Death Valley pupfishes Kaibab squirrels Porto Santo rabbits

Allopatric Speciation

KEY CONCEPTS In allopatric speciation, populations diverge into different species due to geographic isolation, or physical separation

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Learning Objective 4 What is sympatric speciation? Give plant and animal examples

Sympatric Speciation Does not require geographic isolation More common in plants than animals

Sympatric Speciation in Plants Usually results from allopolyploidy polyploid individual (>2 sets of chromosomes) is hybrid derived from two species Examples: kew primroses hemp nettles

Allopolyploidy in Plants

Species A Species B 2 n = 6 2 n = 4 P generation n = 3 n = 2 Gametes Figure 20.9: Sympatric speciation by allopolyploidy in plants. When two species (designated the P generation) successfully interbreed, the interspecific hybrid offspring (the F1 generation) are almost always sterile (bottom left). If the chromosomes double, proper synapsis and segregation of the chromosomes can occur and viable gametes may be produced (bottom right). (Unduplicated chromosomes are shown for clarity.) Hybrid AB F1 generation Fig. 20-9a, p. 435

No doubling of chromosome number Doubling of chromosome number Chromosomes either cannot pair or go through erratic meiosis Pairing now possible during meiosis n = 5 Figure 20.9: Sympatric speciation by allopolyploidy in plants. When two species (designated the P generation) successfully interbreed, the interspecific hybrid offspring (the F1 generation) are almost always sterile (bottom left). If the chromosomes double, proper synapsis and segregation of the chromosomes can occur and viable gametes may be produced (bottom right). (Unduplicated chromosomes are shown for clarity.) No gametes or sterile gametes — no sexual reproduction possible Viable gametes — sexual reproduction possible (self-fertilization) Fig. 20-9b, p. 435

No doubling of chromosome number Species A Species B P generation 2 n = 6 2 n = 4 n = 3 n = 2 Gametes Hybrid AB F1 generation No doubling of chromosome number Chromosomes either cannot pair or go through erratic meiosis Doubling of chromosome number 2 n = 10 Figure 20.9: Sympatric speciation by allopolyploidy in plants. When two species (designated the P generation) successfully interbreed, the interspecific hybrid offspring (the F1 generation) are almost always sterile (bottom left). If the chromosomes double, proper synapsis and segregation of the chromosomes can occur and viable gametes may be produced (bottom right). (Unduplicated chromosomes are shown for clarity.) Pairing now possible during meiosis n = 5 Viable gametes — sexual reproduction possible (self-fertilization) No gametes or sterile gametes — no sexual reproduction possible Stepped Art Fig. 20-9b, p. 435

Sympatric Speciation

Sympatric Speciation in Animals Fruit maggot flies

Sympatric Speciation in Animals Cichlids

Figure 20.12: Color variation in Lake Victoria cichlids. Some evidence suggests that changes in male coloration may be the first step in speciation of Lake Victoria cichlids. Later, other traits, including ecological characteristics, diverge. Female cichlids generally have cryptic coloration; their drab colors help them blend into their surroundings. Fig. 20-12a, p. 437

Figure 20.12: Color variation in Lake Victoria cichlids. Some evidence suggests that changes in male coloration may be the first step in speciation of Lake Victoria cichlids. Later, other traits, including ecological characteristics, diverge. Female cichlids generally have cryptic coloration; their drab colors help them blend into their surroundings. Fig. 20-12b, p. 437

Figure 20.12: Color variation in Lake Victoria cichlids. Some evidence suggests that changes in male coloration may be the first step in speciation of Lake Victoria cichlids. Later, other traits, including ecological characteristics, diverge. Female cichlids generally have cryptic coloration; their drab colors help them blend into their surroundings. Fig. 20-12c, p. 437

KEY CONCEPTS In sympatric speciation, populations become reproductively isolated from one another despite living in the same geographic area

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Learning Objective 5 Debate the pace of evolution by representing the views of either punctuated equilibrium or gradualism

Evolution Punctuated equilibrium model Gradualism model evolution proceeds in spurts short periods of active speciation interspersed with long periods of stasis Gradualism model populations slowly diverge from one another by accumulation of adaptive characteristics

Punctuated Equilibrium and Gradualism

Extinction of original species Slow, gradual changes Stasis Stasis Stasis Time Divergence is sudden, with rapid changes Time Divergence is gradual Stasis (little change) Figure 20.13: Punctuated equilibrium and gradualism. In this figure, structural changes in the lizards are represented by changes in skin color. Structural changes Structural changes Fig. 20-13, p. 438

KEY CONCEPTS Speciation may require millions of years but sometimes occurs much more quickly

Learning Objective 6 What is macroevolution?

Macroevolution Large-scale phenotypic changes in populations in taxonomic groups species level and higher new species, genera, families, orders, classes, phyla, kingdoms, or domains

KEY CONCEPTS The evolution of species and higher taxa is known as macroevolution

Learning Objective 7 Discuss novel features of macroevolution, including preadaptations, allometric growth, and paedomorphosis

Macroevolution Includes evolutionary novelties due to changes during development Slight changes in regulatory genes cause major structural changes in organism

Preadaptations Structures originally fulfilled one role changed and adapted for different role Example: feathers

Allometric Growth Varied rates of growth for different parts of body causes overall changes in shape of organism Examples: ocean sunfish male fiddler crab

Allometric Growth Ocean sunfish

Newly hatched ocean sunfish Adult ocean sunfish Tail approx. 1 mm Newly hatched ocean sunfish Adult ocean sunfish Figure 20.14: Allometric growth in the ocean sunfish. The tail end of an ocean sunfish (Mola mola) grows faster than the head end, resulting in the unique shape of the adult. Fig. 20-14a, p. 440

Figure 20.14: Allometric growth in the ocean sunfish. The tail end of an ocean sunfish (Mola mola) grows faster than the head end, resulting in the unique shape of the adult. Fig. 20-14b, p. 440

Figure 20.14: Allometric growth in the ocean sunfish. The tail end of an ocean sunfish (Mola mola) grows faster than the head end, resulting in the unique shape of the adult. Fig. 20-14b, p. 440

Paedomorphosis Juvenile characteristics retained in adult due to changes in timing of development Example: adult axolotl salamanders with external gills and tail fins

Paedomorphosis Salamander

Learning Objective 8 What is the macroevolutionary significance of adaptive radiation and extinction?

Adaptive Radiation (1) Diversification of ancestral species into many new species Adaptive zones new ecological opportunities not exploited by ancestral organism

Adaptive Radiation (2) When many adaptive zones are empty colonizing species diversify and exploit them Example: Hawaiian honeycreepers and silverswords after ancestors colonized Hawaiian Islands

Adaptive Radiation Hawaiian honeycreepers

Rips away bark to find insects Kauai Sips flower nectar Maui parrot bill ‘I‘iwi Oahu Forages among leaves and branches Maui creeper Maui Chisels holes in bark to get insects Extinct Habits unknown Akiapolaau Ula-ai-hawane Extinct Sipped flower nectar Hawaii Figure 20.16: Adaptive radiation in Hawaiian honeycreepers. Compare the various beak shapes and methods of obtaining food. Many honeycreeper species are now extinct or nearing extinction as a result of human activities, including the destruction of habitat and the introduction of predators such as rats, dogs, and pigs. Black mamo Picks food from cracks in the bark Sips flower nectar Akialoa Apapane Feeds on snails and invertebrates Sips flower nectar Poo-uli Crested honeycreeper Fig. 20-16, p. 441

Adaptive Radiation Hawaiian silverswords

Extinction (1) Death of a species When species become extinct adaptive zones they occupied become vacant allows other species to evolve and fill zones

Extinction (2) Background extinction Mass extinction continuous, low-level extinction of species Mass extinction extinction of numerous species, higher taxonomic groups in both terrestrial and marine environments

Mass Extinction

Pterosaurs (flying reptiles, extinct) Ornithischians (dinosaurs, extinct) Crocodilians (alligators, crocodiles) Saurischians (dinosaurs, extinct) Birds Common ancestor of birds and saurischians Figure 20.18: Mass extinction of the archosaurs. (a) Cladogram of the descendants of archosaurs, reptiles that were the ancestors of the dinosaurs. (b) At the end of the Cretaceous period, approximately 65 mya, a mass extinction of many organisms, including the remaining dinosaurs, occurred. (Dinosaurs had already been declining in diversity throughout the latter part of the Cretaceous period.) The only archosauran lines to survive were crocodiles and birds. Archosaur common ancestor Fig. 20-18a, p. 443

Fig. 20-18b, p. 443 Figure 20.18: Mass extinction of the archosaurs. (a) Cladogram of the descendants of archosaurs, reptiles that were the ancestors of the dinosaurs. (b) At the end of the Cretaceous period, approximately 65 mya, a mass extinction of many organisms, including the remaining dinosaurs, occurred. (Dinosaurs had already been declining in diversity throughout the latter part of the Cretaceous period.) The only archosauran lines to survive were crocodiles and birds. Fig. 20-18b, p. 443

Theropods (carnivorous saurischians) Paleozoic Mesozoic Cenozoic Permian Triassic Jurassic Cretaceous Tertiary / Quaternary Theropods (carnivorous saurischians) Birds Sauropods (herbivorous saurischians) Archosaurs Common ancestor Figure 20.18: Mass extinction of the archosaurs. (a) Cladogram of the descendants of archosaurs, reptiles that were the ancestors of the dinosaurs. (b) At the end of the Cretaceous period, approximately 65 mya, a mass extinction of many organisms, including the remaining dinosaurs, occurred. (Dinosaurs had already been declining in diversity throughout the latter part of the Cretaceous period.) The only archosauran lines to survive were crocodiles and birds. Stegosaurs and other ornithischians Pterosaurs Crocodilians Crocodiles Fig. 20-18b, p. 443