CHAPTER 24

THE ORIGIN OF SPECIES

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OVERVIEWA. Speciation

•Is defined as the origin of new species (process by which one species splits into two or more species)

•Is at the focal point of evolutionary theory which must explain how new species originate and how populations evolve

B. Microevolution•Consists of adaptations that evolve within a population, confined to one gene pool•Mechanisms—mutation, natural selection, genetic drift, and gene flow

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C. Macroevolution•Refers to evolutionary change above the species level•Deals with the appearance of evolutionary novelties that can be used to define high taxa

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II. Concept 24.1: Biological Species Concept• Emphasizes reproductive isolation• Species is a Latin word meaning “kind” or “appearance”• Today in addition to appearance, we use differences in

physiology, biochemistry, behavior, and DNA sequences to differentiate species

A. Biological Species Concept1. Proposed by Ernst Mayr in 19422. A species is defined as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but who cannot produce viable, fertile offspring with other such populations

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B. Reproductive Isolation1. Defined as the existence of biological factors (barriers)

that impede two species from producing viable, fertile offspring2. Hybrids are the offspring of crosses between different species3. Development of reproductive isolation can possibly lead to the formation of new species4. Reproductive isolation can be classified by whether factors act before (prezygotic) or after fertilization (postzygotic)

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Prezygotic Barriers

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C. Prezygotic Barriers •Five ways prezygotic barriers can block fertilization

between species1. Habitat Isolation

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

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HABITAT ISOLATION

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2. Behavioral IsolationSpecies use unique and elaborate courtship behavior to attract mates

Courtship ritual of blue-footed boobies

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3. Temporal IsolationTwo species that breed during different times of day, different seasons, or different years cannot mix gametes

Eastern spotted skunk(Spilogale putorius)

Western spotted skunk(Spilogale gracilis)

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4. Mechanical IsolationClosely related species may attempt to mate but fail because they are anatomically incompatible and transfer of sperm is not possible

Bradybaena with shells spiraling in opposite directions13

5. Gametic Isolation Gametes of one species may not be able to fertilize eggs of another species

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Postzygotic Barriers

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D. Postzygotic Barriers•Three postzygotic barriers that prevent a hybrid zygote

from developing into a viable, fertile adult1. Reduced Hybrid Viability

Genetic incompatibility between the two species may stop the development of the hybrid at some embryonic stage or produce frail offspring

Ensatina hybrid16

2. Reduced Hybrid FertilityEven though hybrid offspring survives, it may be

sterile

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3. Hybrid BreakdownSome first-generation hybrids are viable and fertile, but when they mate with another species or with either parent species, the offspring of the next generation are sterile or feeble

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E. Limitations of the Biological Species ConceptThe biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

F. Other Definitions of SpeciesOther species concepts emphasize the unity within a species rather than the separateness of different species

1. The morphological species concept defines a species by structural features

• It applies to sexual and asexual species but relies on subjective criteria

•Oldest and still most practical19

2. The ecological species concept views a species in terms of its ecological niche, the set of

environmental resources that a species uses, and its role in a biological community

•Applies to sexual and asexual species3. The phylogenetic species concept defines a species as

the smallest group of individuals that shares a common ancestor and forms one branch on the tree of life

•It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

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II. Concept 24.2: SpeciationA. Two modes of speciation are distinguished by the way gene

flow among populations is initially interrupted:1. Allopatric Speciation

•Occurs when geographic separation of populations restricts gene flow

•Geological process can fragment a population into two or more isolated populations.

•Definition of barrier depends on the ability of a population to disperse

•Separate populations may evolve independently through mutation, natural selection, and genetic drift

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•Evidence of allopatric speciation can be seen in regions with many geographic barriers typically having more species than do regions with few barriers

•Reproductive isolation between populations generally increases as the distance between them increases

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2. Sympatric Speciation•New species arise within the range of the parent population•Occurs when reproductive barriers evolve between

sympatric populations•May occur rapidly

3. Types of Sympatric Speciationa. Polyploidy

•Defined as 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•An allopolyploid is a species with multiple sets of chromosomes derived from different species (more common)•Polyploidy is more common in plants than animals 23

AUTOPOLYPLOID

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AUTOPOLYPLOID

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AUTOPOLYPLOID

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ALLOPOLYPLOID

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ALLOPOLYPLOID

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ALLOPOLYPLOID

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ALLOPOLYPLOID

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b. Habitat Differentiation•Sympatric speciation in animals may result from the appearance of new ecological niches

c. Sexual SelectionB. Adaptive Radiation

•Occurs when many new species arise from a single common ancestor•Typically occurs when a few organisms make their way to new, distant areas or when environmental changes cause extinctions thus opening up niches for survivors•Ex: Darwin’s finches

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Adaptive Radiation

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Allopatric and Sympatric Speciation: 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 34

IV. Concept 24.4: Tempo of Speciation

A. Gradual—proposes that species descended from a common ancestor and gradually diverge more and more in morphology as they acquire unique adapations

-difficult to prove with fossilsB. Punctuated—spurts of rapid change rather than

gradual divergence-based on fossils that suddenly appear and

disappear-proposed by Eldredge and Gould

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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

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