Big Idea #1 Part C Evolution Continues in a Changing Environment 1. Speciation and Extinction (Rates/adaptive radiation) 2. Role of Reproductive Isolation 3. Populations continue to evolve

The Biological Species Concept The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations Gene flow between populations holds the phenotype of a population together © 2011 Pearson Education, Inc.

Figure 24.2 (a) Similarity between different species (b) Diversity within a species

Reproductive Isolation Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring Hybrids are the offspring of crosses between different species Reproductive isolation can be classified by whether factors act before or after fertilization © 2011 Pearson Education, Inc.

Figure 24.3_a Prezygotic 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 FERTILIZATION VIABLE, FERTILE OFFSPRING Postzygotic barriers (a)(c)(e) (d) (b) (g) (k) (h) (i) (j) (l) (f)

Figure 24.3_b Prezygotic barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Individuals of different species MATING ATTEMPT FERTILIZATION (a) (c) (e) (f) (b) (g) (d)

Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown FERTILIZATION VIABLE, FERTILE OFFSPRING Postzygotic barriers (k) (h) (i) (j) (l) Figure 24.3_c

Prezygotic barriers block fertilization from occurring by: Impeding different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful © 2011 Pearson Education, Inc.

Figure 24.3a

Figure 24.3b

Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers © 2011 Pearson Education, Inc.

Figure 24.3c

Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes © 2011 Pearson Education, Inc.

Figure 24.3e (e)

Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers © 2011 Pearson Education, Inc. Video: Blue-footed Boobies Courtship Ritual Video: Giraffe Courtship Ritual Video: Albatross Courtship Ritual

Figure 24.3f (f)

Mechanical isolation: Morphological differences can prevent successful mating © 2011 Pearson Education, Inc.

Figure 24.3g (g)

Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species © 2011 Pearson Education, Inc.

Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown © 2011 Pearson Education, Inc.

Figure 24.3h (h)

Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development © 2011 Pearson Education, Inc.

Figure 24.3i (i)

Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile © 2011 Pearson Education, Inc.

Figure 24.3l (l)

Hybrid breakdown: Some first- generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile © 2011 Pearson Education, Inc.

Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) The biological species concept emphasizes absence of gene flow However, gene flow can occur between distinct species For example, grizzly bears and polar bears can mate to produce “grolar bears ” © 2011 Pearson Education, Inc.

Figure 24.4 Grizzly bear (U. arctos) Polar bear (U. maritimus) Hybrid “grolar bear”

Other Definitions of Species Other species concepts emphasize the unity within a species rather than the separateness of different species morphological species concept The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

Concept 24.2: Speciation can take place with or without geographic separation Speciation can occur in two ways: Allopatric speciation Sympatric speciation © 2011 Pearson Education, Inc.

Figure 24.5 (a) (b) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. Sympatric speciation. A subset of a population forms a new species without geographic separation.

Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland © 2011 Pearson Education, Inc.

The Process of Allopatric Speciation The definition of barrier depends on the ability of a population to disperse For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen © 2011 Pearson Education, Inc.

Figure 24.6 A. harrisii A. leucurus

Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a result of genetic divergence For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds © 2011 Pearson Education, Inc.

Figure 24.7 (a) Under high predation (b) Under low predation

Evidence of Allopatric Speciation 15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters © 2011 Pearson Education, Inc.

Figure 24.8 A. formosus Atlantic Ocean A. nuttingi Isthmus of Panama Pacific Ocean A. panamensisA. millsae

Regions with many geographic barriers typically have more species than do regions with fewer barriers Reproductive isolation between populations generally increases as the distance between them increases For example, reproductive isolation increases between dusky salamanders that live further apart © 2011 Pearson Education, Inc.

Figure 24.9 Degree of reproductive isolation Geographic distance (km) Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Figure EXPERIMENT RESULTS Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Mating experiments after 40 generations Some flies raised on maltose medium Female Starch Maltose Male Maltose Starch Number of matings in experimental group Female Starch population 1 Male Starch population 2 Number of matings in control group Starch population 2 Starch population 1

Figure 24.10b RESULTS Female Starch Maltose Male Maltose Starch Number of matings in experimental group Female Starch population 1 Male Starch population 2 Number of matings in control group Starch population 2 Starch population 1

Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations © 2011 Pearson Education, Inc.

Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division Polyploidy is much more common in plants than in animals An autopolyploid is an individual with more than two chromosome sets, derived from one species © 2011 Pearson Education, Inc.

An allopolyploid is a species with multiple sets of chromosomes derived from different species © 2011 Pearson Education, Inc.

Figure Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes

Figure Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes

Figure Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3

Figure Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Meiotic error; chromosome number not reduced from 2n to n Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Unreduced gamete with 7 chromosomes Normal gamete n = 3 New species: viable fertile hybrid (allopolyploid) 2n = 10

Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids © 2011 Pearson Education, Inc.

Habitat Differentiation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees © 2011 Pearson Education, Inc.

Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria © 2011 Pearson Education, Inc.

Figure Normal light Monochromatic orange light P. pundamilia P. nyererei EXPERIMENT

Allopatric and Sympatric Speciation: A Review In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented © 2011 Pearson Education, Inc.

In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection © 2011 Pearson Education, Inc.

Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids

Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region

Fig EUROPE Fire-bellied toad range Hybrid zone Yellow-bellied toad range Yellow-bellied toad, Bombina variegata Fire-bellied toad, Bombina bombina Allele frequency (log scale) Distance from hybrid zone center (km)

Fig a Yellow-bellied toad, Bombina variegata

Fig b Fire-bellied toad, Bombina bombina

Fig c Fire-bellied toad range Yellow-bellied toad range Hybrid zone Allele frequency (log scale) Distance from hybrid zone center (km)

Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: Strengthening of reproductive barriers Weakening of reproductive barriers Continued formation of hybrid individuals

Fig Gene flow Population (five individuals are shown) Barrier to gene flow

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid

Fig Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid Possible outcomes: Reinforcement OR Fusion Stability

Reinforcement: Strengthening Reproductive Barriers The reinforcement of barriers occurs when hybrids are less fit than the parent species Over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

Fig Sympatric male pied flycatcher Allopatric male pied flycatcher Pied flycatchers Collared flycatchers Number of females (none) Females mating with males from: Own species Other species Sympatric males Own species Other species Allopatric males

Fig a Sympatric male pied flycatcher Allopatric male pied flycatcher

Fig b Pied flycatchers Collared flycatchers (none) Number of females Females mating with males from: Own species Other species Sympatric males Own species Other species Allopatric males

Fusion: Weakening Reproductive Barriers If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species

Fig Pundamilia nyerereiPundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water

Stability: Continued Formation of Hybrid Individuals Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers

Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species

The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

Patterns in the Fossil Record The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence

Fig (a) Punctuated pattern (b) Gradual pattern Time

Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years

Fig (a) The wild sunflower Helianthus anomalus H. anomalus (b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids Chromosome 1 Chromosome 2 Chromosome 3 Experimental hybrid Key Region diagnostic for parent species H. petiolaris Region diagnostic for parent species H. annuus Region lacking information on parental origin

Fig a (a) The wild sunflower Helianthus anomalus

Fig b (b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids Region lacking information on parental origin Region diagnostic for parent species H. petiolaris Region diagnostic for parent species H. annuus Key Experimental hybrid Chromosome 3 Chromosome 2 Chromosome 1 H. anomalus

Studying the Genetics of Speciation The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation Depending on the species in question, speciation might require the change of only a single allele or many alleles

Fig

Fig (a) Typical Mimulus lewisii(b) M. lewisii with an M. cardinalis flower-color allele (c) Typical Mimulus cardinalis(d) M. cardinalis with an M. lewisii flower-color allele

From Speciation to Macroevolution Macroevolution is the cumulative effect of many speciation and extinction events

Fig. 24-UN1 Original population Allopatric speciation Sympatric speciation

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

Fig. 24-UN3

You should now be able to: 1. Define and discuss the limitations of the four species concepts 2. Describe and provide examples of prezygotic and postzygotic reproductive barriers 3. Distinguish between and provide examples of allopatric and sympatric speciation 4. Explain how polyploidy can cause reproductive isolation 5. Define the term hybrid zone and describe three outcomes for hybrid zones over time

Concept 25.4: The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations The history of life on Earth has seen the rise and fall of many groups of organisms Video: Lava Flow Video: Lava Flow Video: Volcanic Eruption Video: Volcanic Eruption

Continental Drift At three points in time, the land masses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago Earth’s continents move slowly over the underlying hot mantle through the process of continental drift Oceanic and continental plates can collide, separate, or slide past each other Interactions between plates cause the formation of mountains and islands, and earthquakes

Fig (a) Cutaway view of Earth (b) Major continental plates Inner core Outer core Crust Mantle Pacific Plate Nazca Plate Juan de Fuca Plate Cocos Plate Caribbean Plate Arabian Plate African Plate Scotia Plate North American Plate South American Plate Antarctic Plate Australian Plate Philippine Plate Indian Plate Eurasian Plate

Fig a (a) Cutaway view of Earth Inner core Outer core Crust Mantle

Fig b (b) Major continental plates Pacific Plate Nazca Plate Juan de Fuca Plate Cocos Plate Caribbean Plate Arabian Plate African Plate Scotia Plate North American Plate South American Plate Antarctic Plate Australian Plate Philippine Plate Indian Plate Eurasian Plate

Consequences of Continental Drift Formation of the supercontinent Pangaea about 250 million years ago had many effects A reduction in shallow water habitat A colder and drier climate inland Changes in climate as continents moved toward and away from the poles Changes in ocean circulation patterns leading to global cooling

Fig South America Pangaea Millions of years ago Mesozoic 251 Paleozoic Gondwana Laurasia Eurasia India Africa Antarctica Australia North America Madagascar Cenozoic Present

Fig a South America Millions of years ago 65.5 Eurasia India Africa Antarctica Australia North America Madagascar Cenozoic Present

Fig b Pangaea Millions of years ago 135 Mesozoic 251 Paleozoic Gondwana Laurasia

The break-up of Pangaea lead to allopatric speciation The current distribution of fossils reflects the movement of continental drift For example, the similarity of fossils in parts of South America and Africa is consistent with the idea that these continents were formerly attached

Mass Extinctions The fossil record shows that most species that have ever lived are now extinct At times, the rate of extinction has increased dramatically and caused a mass extinction

The “Big Five” Mass Extinction Events In each of the five mass extinction events, more than 50% of Earth’s species became extinct

Fig Total extinction rate (families per million years): Time (millions of years ago) Number of families: Cenozoic Mesozoic Paleozoic E OS D C P Tr J Era Period 5 C P N

The Permian extinction defines the boundary between the Paleozoic and Mesozoic eras This mass extinction occurred in less than 5 million years and caused the extinction of about 96% of marine animal species This event might have been caused by volcanism, which lead to global warming, and a decrease in oceanic oxygen

The Cretaceous mass extinction 65.5 million years ago separates the Mesozoic from the Cenozoic Organisms that went extinct include about half of all marine species and many terrestrial plants and animals, including most dinosaurs

Fig NORTH AMERICA Chicxulub crater Yucatán Peninsula

The presence of iridium in sedimentary rocks suggests a meteorite impact about 65 million years ago The Chicxulub crater off the coast of Mexico is evidence of a meteorite that dates to the same time

Is a Sixth Mass Extinction Under Way? Scientists estimate that the current rate of extinction is 100 to 1,000 times the typical background rate Data suggest that a sixth human-caused mass extinction is likely to occur unless dramatic action is taken

Consequences of Mass Extinctions Mass extinction can alter ecological communities and the niches available to organisms It can take from 5 to 100 million years for diversity to recover following a mass extinction Mass extinction can pave the way for adaptive radiations

Fig Predator genera (percentage of marine genera) Time (millions of years ago) Cenozoic Mesozoic Paleozoic E O S DCP Tr J Era Period C P N

Adaptive Radiations Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities

Worldwide Adaptive Radiations Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods

Fig Millions of years ago Monotremes (5 species) ANCESTRAL CYNODONT 0 Marsupials (324 species) Eutherians (placental mammals; 5,010 species) Ancestral mammal

Regional Adaptive Radiations Adaptive radiations can occur when organisms colonize new environments with little competition The Hawaiian Islands are one of the world’s great showcases of adaptive radiation

Fig Close North American relative, the tarweed Carlquistia muirii Argyroxiphium sandwicense Dubautia linearis Dubautia scabra Dubautia waialealae Dubautia laxa HAWAII 0.4 million years OAHU 3.7 million years KAUAI 5.1 million years 1.3 million years MOLOKAI MAUI LANAI

Fig a HAWAII 0.4 million years OAHU 3.7 million years KAUAI 5.1 million years 1.3 million years MOLOKAI MAUI LANAI

Fig b Close North American relative, the tarweed Carlquistia muirii

Fig c Dubautia waialealae

Fig d Dubautia laxa

Fig e Dubautia scabra

Fig f Argyroxiphium sandwicense

Fig g Dubautia linearis