Chapter 24 The Origin of Species.

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 © 2011 Pearson Education, Inc.

Video: Galápagos Tortoise
© 2011 Pearson Education, Inc.

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 changes in allele frequency in a population over time Macroevolution refers to broad patterns of evolutionary change above the species level © 2011 Pearson Education, Inc.

Animation: Macroevolution Right-click slide / select “Play”

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.

(a) Similarity between different species
Figure 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

(a) Similarity between different species
Figure 24.2a Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (a) Similarity between different species

(b) Diversity within a species
Figure 24.2b 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.

Prezygotic barriers (a) (c) (e) (f) (g) (d) (b) Figure 24.3_b 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 - prevent mating or fertilization between species
Temporal isolation - two species breed at different times Habitat isolation – live in different habitats  don’t meet Behavioral isolation - there is little or no sexual attraction between species, due to specific behaviors Mechanical isolation - female and male sex organs or gametes are not compatible; morphological differences Gametic isolation – gametes die or fail to unite Figure 14.3A Figure 14.3B Figure 14.3C

Postzygotic Barriers - Operate after hybrid zygotes are formed
Hybrid inviability – hybrids don’t develop; Genes of the different parent species may interact and impair the hybrid’s development Hybrid sterility - hybrid offspring between two species are sterile and therefore cannot mate (example: mule) Hybrid breakdown – first generation is fertile, 2nd generation sterile or feeble Figure 14.3D

Video: Albatross Courtship Ritual

Video: Giraffe Courtship Ritual

Video: Blue-footed Boobies Courtship Ritual

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.

MECHANISMS OF SPECIATION (the “HOW”)
Geographic isolation can lead to speciation In allopatric speciation a population is geographically divided, and new species often evolve A. harrisi A. leucurus Figure 14.4

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.

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

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. Geographic distance (km)

Barriers to reproduction are intrinsic; separation itself is not a biological barrier

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 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 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 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

Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
Figure 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 Figure One mechanism for allopolyploid speciation in plants.

Figure 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 One mechanism for allopolyploid speciation in plants.

Figure 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 One mechanism for allopolyploid speciation in plants. Normal gamete n = 3 Unreduced gamete with 7 chromosomes

Figure 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 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

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 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 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.

Gene flow Population Barrier to gene flow Figure 24.14-1
Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Barrier to gene flow

Isolated population diverges Gene flow Population Barrier to gene flow
Figure Isolated population diverges Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Barrier to gene flow

Isolated population Hybrid diverges zone Gene flow Population Hybrid
Figure Isolated population diverges Hybrid zone Figure Formation of a hybrid zone and possible outcomes for hybrids over time. Gene flow Population Hybrid individual Barrier to gene flow

Possible outcomes: Isolated population Hybrid diverges zone
Figure Possible outcomes: Isolated population diverges Hybrid zone Reinforcement OR Fusion OR Figure 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 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 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 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 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 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 Inquiry: How does hybridization lead to speciation in sunflowers? Chromosome 2 Experimental hybrid

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.

The Origin of Species Chapter 24 Jung Choi Michael Dini
Questions prepared by Jung Choi Georgia Institute of Technology Michael Dini Texas Tech University

Biological Species Two animals are considered members of different species if they _____. are members of different populations cannot mate and produce viable, fertile offspring are geographically isolated live in different habitats look different Answer: b

Biological Species Cynotilapia afra, introduced at West Thumbi Island in Lake Malawi in the 1960s, has split into two genetically distinct populations, located at the north and south ends of the island. How can scientists determine whether these populations are now different species, according to the biological species concept? See whether the two populations are morphologically different from each other: coloring, bone structure, and so on. Determine whether captured individuals from the two different populations will mate and produce offspring in a laboratory fish tank. Determine whether individuals from one population will interbreed with individuals from the other population when introduced into each other’s native habitats. Answer: c 77

Phylogenetic Species According to the phylogenetic species concept, what is a species? a clone of genetically identical organisms a population that is physically able to mate even if there are no offspring or the offspring are infertile a population that interbreeds and produces fertile offspring a set of organisms with a unique genetic history a population with a distinct fossil record Answer: d

Mechanism of Reproductive Isolation
If the two populations of C. afra at West Thumbi Island have different striping patterns, what might be a likely mechanism contributing to speciation? polyploidy female mating preferences differing habitat preferences temporal isolation mechanical isolation Answer: b 79

Species Concepts Which species concept is least useful for classifying prokaryotes? biological species concept phylogenetic species concept ecological species concept morphological species concept Answer: a 80

Adaptive Radiation Cichlids in Lake Victoria are thought to have diversified from a relatively few species to 600 hundred species in about 12,000 years (the last time the lake dried up). What is the best way to test this hypothesis? Examine the fossil record in lake sediments. Look for morphological characteristics shared among Lake Victoria species that are distinct from the species present in other lakes. Look for genetic characteristics shared among Lake Victoria species that are distinct from species present in other lakes. This is not a testable hypothesis. Answer: c A phylogenetic analysis should provide the most conclusive answer: The Lake Victoria species should form one or a few clades distinct from the clades in other African great lakes. 81

Mechanisms of Adaptation
In vertebrate radiation, exemplified by African cichlids, modification of jaws occurs during adaptation to different food sources. Such rapid modifications could result from which of the following? changing the expression of genes that control the growth of different jaw bones during development evolving new genes that generate new jaw bones acquiring genes by hybridizing with other species that exploit the same food source individuals exercising their jaw muscles in different ways, causing the jaws to grow accordingly Answer: a 82

Allopatric Speciation
Which population characteristic would make the population more likely to undergo allopatric speciation? having a large number of individuals being highly mobile existing on the edge of the home range having a relatively homogeneous home range having a relatively homogeneous gene pool Answer: c 83

Speciation and Physical Environment
In what type of environment is a scientist most likely to find examples of ongoing speciation? areas that have a large range of altitudes, such as mountains and deep valleys areas such as large plains that facilitate migration of animals extreme physical environments that are marginal for supporting life areas that are already species-rich areas that have undergone recent environmental disturbance or change Answer: e 84

Reproductive Isolation Mechanisms
Sea urchins are broadcast spawners: They release their gametes into the water, without courtship. Which of these reproductive isolation mechanisms might operate for sea urchin species? behavioral isolation or mechanical isolation gametic isolation or temporal isolation habitat isolation Answers: b 85

Human Brain Evolution A comparison of the human and chimpanzee genomes shows that they are 99% identical, with very few differences in protein coding genes. What mechanisms may be responsible for evolution of the human brain? One or a few novel genes organized new brain structures. The timing of the expression of brain development genes altered. Brain development genes acquired mutations that made them more active in promoting brain growth. Genes that had other functions acquired new functions in brain development (exaptation). Answer: b The most likely explanation is b, but the other options cannot be ruled out at this point. 86

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

Chapter 24 The Origin of Species.

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