Chapter 24 The Origin of Species

Overview: That “Mystery of Mysteries” In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth

Animation: Macroevolution Speciation, the origin of new species, is at the focal point of evolutionary theory Evolutionary theory must explain how new species originate and how populations evolve Microevolution consists of adaptations that evolve within a population, confined to one gene pool Macroevolution refers to evolutionary change above the species level Animation: Macroevolution

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 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

(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

Figure 24.3 Does gene flow occur between widely separated 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 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

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

Figure 24.4 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 24.4 Reproductive barriers (k) Water-dwelling Thamnophis Terrestrial Thamnophis Eastern spotted skunk(Spilogale putorius) Western spotted skunk (Spilogale gracilis) Courtship ritual of blue-footed boobies Bradybaena with shell spiraling in opposite directions g) Sea urchins h) Ensatina hybrid Donkey Horse Mule (sterile hybrid) Hybrid cultivated rice plants with stunted offspring (center)

Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Mechanical isolation: Morphological differences can prevent successful mating Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown

Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile 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 Limitations: The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

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

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

Speciation can occur in two ways: Concept 24.2: Speciation can take place with or without geographic separation Speciation can occur in two ways: Allopatric speciation Sympatric speciation

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

Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift

Fig. 24-6 A. harrisi A. leucurus Regions with many geographic barriers typically have more species than do regions with fewer barriers Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

Millions of years ago (mya) Fig. 24-7 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

Reproductive isolation between populations generally increases as the distance between them increases Barriers to reproduction are intrinsic; separation itself is not a biological barrier

Degree of reproductive isolation Fig. 24-8 2.0 1.5 Degree of reproductive isolation 1.0 0.5 Figure 24.8 Variation in reproductive isolation with distance between populations of dusky salamanders 50 100 150 200 250 300 Geographic distance (km)

EXPERIMENT RESULTS Initial population Some flies raised on Fig. 24-9 EXPERIMENT Initial population Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch Starch Starch Maltose population 1 population 2 Figure 24.9 Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male Maltose 8 20 12 15 Starch population 2 Mating frequencies in experimental group Mating frequencies in control group

Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in geographically overlapping populations

Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division An autopolyploid is an individual with more than two chromosome sets, derived from one species

2n = 6 4n = 12 2n 4n Failure of cell division after chromosome Fig. 24-10-3 2n = 6 4n = 12 2n 4n 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

An allopolyploid is a species with multiple sets of chromosomes derived from different species 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 Normal gamete n = 3 Species A 2n = 6

Polyploidy is much more common in plants than in animals Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids Ancestral species: Fig. 24 AA BB DD Wild T. tauschii (2n = 14) Triticum monococcum (2n = 14) Wild Triticum (2n = 14) AA BB DD Product: T. aestivum (bread wheat) (2n = 42)

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

Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Monochromatic orange light Fig. 24-12 EXPERIMENT Monochromatic orange light Normal light P. pundamilia Figure 24.12 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

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

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 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. 24-13 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 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

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

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. 24-15 Sympatric male pied flycatcher Allopatric male pied flycatcher 28 Pied flycatchers 24 Collared flycatchers 20 16 Figure 24.15 Reinforcement of barriers to reproduction in closely related species of European flycatchers Number of females 12 8 4 (none) Females mating with males from: Own species Other species Own species Other species Sympatric males 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

Pundamilia pundamilia Fig. 24-16 Pundamilia nyererei Pundamilia pundamilia Figure 24.16 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 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 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

(a) Punctuated pattern Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern Figure 24.17 Two models for the tempo of speciation

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

Figure 24.18 Rapid speciation in a sunflower hybrid zone (a) The wild sunflower Helianthus anomalus H. anomalus Chromosome 1 Experimental hybrid H. anomalus Chromosome 2 Figure 24.18 Rapid speciation in a sunflower hybrid zone Experimental hybrid H. anomalus 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 (b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids

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

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