Concept 24.1: The biological species concept emphasizes reproductive isolation Species is a Latin word meaning “kind” or “appearance” Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms © 2011 Pearson Education, Inc.

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 The biological species concept is based on the potential to interbreed rather than on physical similarity (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

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.

Exploring: Reproductive Barriers Figure 24.3_a Exploring: Reproductive Barriers 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 24.3 Exploring: Reproductive Barriers (k)

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

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

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.

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.

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

Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers © 2011 Pearson Education, Inc.

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

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.

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) Figure 24.3 Exploring: Reproductive Barriers

Figure 24.3j (j) Figure 24.3 Exploring: Reproductive Barriers

Figure 24.3k (k) Figure 24.3 Exploring: Reproductive Barriers

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

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.

Grizzly bear (U. arctos) Figure 24.4 Grizzly bear (U. arctos) Polar bear (U. maritimus) Figure 24.4 Hybridization between two species of bears in the genus Ursus. Hybrid “grolar bear”

Other Definitions of Species 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 © 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.

Allopatric speciation. A population forms a new species while Figure 24.5 Figure 24.5 Two main modes of speciation. (a) Allopatric speciation. A population forms a new species while geographically isolated from its parent population. (b) 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.

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

Reproductive isolation may arise as a result of genetic divergence 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.

(a) Under high predation (b) Under low predation Figure 24.7 (a) Under high predation (b) Under low predation Figure 24.7 Reproductive isolation as a by-product of selection.

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.

A. formosus A. nuttingi Atlantic Ocean Isthmus of Panama Pacific Ocean Figure 24.8 A. formosus A. nuttingi Atlantic Ocean Isthmus of Panama Pacific Ocean Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). A. panamensis A. millsae

Atlantic Ocean Isthmus of Panama Pacific Ocean Figure 24.8b Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).

Figure 24.8c Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). A. formosus

Figure 24.8d Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). A. panamensis

Figure 24.8e Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). A. nuttingi

Figure 24.8f Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus). A. 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.

Degree of reproductive isolation Figure 24.9 2.0 1.5 Degree of reproductive isolation 1.0 0.5 Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders. 0 50 100 150 200 250 300 Geographic distance (km)

Barriers to reproduction are intrinsic; separation itself is not a biological barrier © 2011 Pearson Education, Inc.

EXPERIMENT Initial population of fruit flies (Drosophila Figure 24.10 EXPERIMENT Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch population 1 Starch population 2 Starch Maltose Figure 24.10 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male population 2 Starch Maltose 8 20 12 15 Number of matings in experimental group Number of matings in control group

EXPERIMENT Initial population of fruit flies (Drosophila Figure 24.10a EXPERIMENT Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Some flies raised on maltose medium Figure 24.10 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? Mating experiments after 40 generations

RESULTS Female Female Starch Maltose 22 9 18 15 Male 8 20 12 15 Figure 24.10b RESULTS Female Female Starch population 1 Starch population 2 Starch Maltose Starch population 1 Starch 22 9 18 15 Male Male Maltose Figure 24.10 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? 8 20 population 2 Starch 12 15 Number of matings in experimental group Number of matings in control group

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.

Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not Figure 24.11-4 Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not reduced from 2n to n Normal gamete n = 3 Unreduced gamete with 4 chromosomes Hybrid with 7 chromosomes Figure 24.11 One mechanism for allopolyploid speciation in plants. Normal gamete n = 3 Unreduced gamete with 7 chromosomes 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.

Monochromatic orange light Figure 24.12 EXPERIMENT Monochromatic orange light Normal light P. pundamilia Figure 24.12 Inquiry: Does sexual selection in cichlids result in reproductive isolation? P. nyererei

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 reveal factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids Hybrids are the result of mating between species with incomplete reproductive barriers © 2011 Pearson Education, Inc.

Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone © 2011 Pearson Education, Inc.

B. variegata-specific allele Figure 24.13 EUROPE Fire-bellied toad range Fire-bellied toad, Bombina bombina Hybrid zone Yellow-bellied toad range 0.99 Hybrid zone 0.9 Figure 24.13 A narrow hybrid zone for Bombina toads in Europe. B. variegata-specific allele Frequency of 0.5 Yellow-bellied toad range Fire-bellied toad range Yellow-bellied toad, Bombina variegata 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km)

Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in patches within the same region © 2011 Pearson Education, Inc.

Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: Reinforcement Fusion Stability © 2011 Pearson Education, Inc.

Possible outcomes: Isolated population Hybrid diverges zone Figure 24.14-4 Possible outcomes: Isolated population diverges Hybrid zone Reinforcement OR Fusion OR Figure 24.14 Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Hybrid individual Barrier to gene flow 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 For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations © 2011 Pearson Education, Inc.

Females choosing between these males: Females choosing between Figure 24.15 Females choosing between these males: Females choosing between these males: 28 Sympatric pied male Allopatric pied male 24 Sympatric collared male Allopatric collared male 20 16 Number of females 12 8 Figure 24.15 Reinforcement of barriers to reproduction in closely related species of European flycatchers. 4 (none) Own species Other species Own species Other species Female mate choice Female mate choice

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 For example, researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species This might be causing the fusion of many species © 2011 Pearson Education, Inc.

Pundamilia pundamilia Figure 24.16 Pundamilia nyererei Pundamilia pundamilia Figure 24.16 Fusion: The breakdown of reproductive barriers. 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 © 2011 Pearson Education, Inc.

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 © 2011 Pearson Education, Inc.

The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data © 2011 Pearson Education, Inc.

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 equilibria 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 © 2011 Pearson Education, Inc.

(a) Punctuated pattern Time (b) Gradual pattern Figure 24.17 Figure 24.17 Two models for the tempo of speciation.

Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid For example, the sunflower Helianthus anomalus originated from the hybridization of two other sunflower species © 2011 Pearson Education, Inc.

chromosomes are shown) Figure 24.19 EXPERIMENT H. annuus gamete H. petiolarus gamete F1 experimental hybrid (4 of the 2n = 34 chromosomes are shown) RESULTS H. anomalus Figure 24.19 Inquiry: How does hybridization lead to speciation in sunflowers? Chromosome 1 Experimental hybrid H. anomalus Chromosome 2 Experimental hybrid

chromosomes are shown) Figure 24.19a EXPERIMENT H. annuus gamete H. petiolarus gamete F1 experimental hybrid (4 of the 2n = 34 chromosomes are shown) Figure 24.19 Inquiry: How does hybridization lead to speciation in sunflowers?

H. anomalus Experimental hybrid RESULTS Chromosome 1 H. anomalus Figure 24.19b RESULTS H. anomalus Chromosome 1 Experimental hybrid H. anomalus Figure 24.19 Inquiry: How does hybridization lead to speciation in sunflowers? Chromosome 2 Experimental hybrid

The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years © 2011 Pearson Education, Inc.

Studying the Genetics of Speciation A fundamental question of evolutionary biology persists: How many genes change when a new species forms? Depending on the species in question, speciation might require the change of only a single allele or many alleles For example, in Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene © 2011 Pearson Education, Inc.

In monkey flowers (Mimulus), two loci affect flower color, which influences pollinator preference Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flowers In other species, speciation can be influenced by larger numbers of genes and gene interactions © 2011 Pearson Education, Inc.

M. cardinalis flower-color allele Figure 24.20 (a) Typical Mimulus lewisii (b) M. lewisii with an M. cardinalis flower-color allele Figure 24.20 A locus that influences pollinator choice. (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 © 2011 Pearson Education, Inc.

Tetraploid cell 4n = 12 New species (4n) Figure 24.UN01 Cell division error 2n = 6 Tetraploid cell 4n = 12 2n Figure 24.UN01 In-text figure, p. 495 2n New species (4n) Gametes produced by tetraploids

Allopatric speciation Sympatric speciation Figure 24.UN02 Original population Figure 24.UN02 Summary figure, Concept 24.2 Allopatric speciation Sympatric speciation

Ancestral species: Triticum Wild Wild monococcum Triticum T. tauschii Figure 24.UN03 Ancestral species: Triticum monococcum (2n = 14) Wild Triticum (2n = 14) Wild T. tauschii (2n = 14) Product: Figure 24.UN03 Test Your Understanding, question 10 T. aestivum (bread wheat) (2n = 42)

Figure 24.UN04 Figure 24.UN04 Appendix A: answer to Test Your Understanding, question 10