Speciation17

Chapter 17 SpeciationKey Concepts17.1 Species Are Reproductively Isolated Lineages on the Tree of Life17.2 Speciation Is a Natural Consequence of Population Subdivision17.3 Speciation May Occur through Geographic Isolation or in Sympatry17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact

Chapter 17 Opening QuestionCan biologists study the process of speciation in the laboratory?

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeSpeciationdivergence of biological lineages and emergence of reproductive isolation between lineages.Most of the species concepts proposed by biologists are different ways of approaching the question What are species?Speciesgroups of organisms that mate with one another.

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeLinnaeus described species based on their appearancethe morphological species concept.Members of species look alike because they share many alleles.He originated the binomial system of nomenclature.

Figure 17.1 Members of the Same Species Look Alikeor Not (Part 1)

Figure 17.1 Members of the Same Species Look Alikeor Not (Part 2)

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeLimitations: members of the same species dont always look alike (e.g., male and female wood ducks).Cryptic speciestwo or more species that are morphologically indistinguishable but do not interbreed.

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed (Part 1)

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed (Part 2)

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeBiological species concept: Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. Ernst MayrReproductive isolation: two groups of organisms can no longer exchange genes.

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeLineage species concept: species as branches on the tree of life.A lineage is an ancestordescendant series of populations followed over time.

Concept 17.1 Species Are Reproductively IsolatedLineages on the Tree of LifeThe various species concepts emphasize different aspects of species or speciation.Reproductive isolation is necessary for lineages to remain distinct over evolutionary time.It is also responsible for morphological differencesmutations that result in morphological changes cannot spread between reproductively isolated species.

Concept 17.2 Speciation Is a Natural Consequenceof Population SubdivisionNot all evolutionary changes result in new species. Speciation requires interruption of gene flow.How can one lineage ever split into two reproductively isolated species?

Concept 17.2 Speciation Is a Natural Consequenceof Population SubdivisionThe DobzhanskyMuller model:A population is subdivided and then the two groups evolve independently.In each lineage, new alleles become fixed at different loci. The new alleles at the two loci are incompatible with one another.Genetic incompatibility between the two isolated populations will develop over time.

Figure 17.3 The DobzhanskyMuller Model

Concept 17.2 Speciation Is a Natural Consequenceof Population SubdivisionIn bats of the genus Rhogeessa, chromosomal rearrangements in different lineages have led to speciation.Hybrids between some types will not be able to produce normal gametes in meiosis.

Figure 17.4 Speciation by Centric Fusion (Part 1)

Figure 17.4 Speciation by Centric Fusion (Part 2)

Concept 17.2 Speciation Is a Natural Consequenceof Population SubdivisionAs species diverge genetically, reproductive isolation increases.Development of reproductive isolation may take millions of years, or may develop in a few generations (e.g., the Rhogeessa bats).

Figure 17.5 Reproductive Isolation Increases with Genetic Divergence

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatryAllopatric speciationwhen populations are separated by a physical or geographic barrier.Barriers can form when continents drift, sea levels rise and fall, glaciers advance and retreat, or climates change.The populations evolve through genetic drift and adaptation to different environments in the two areas.

Figure 17.6 Allopatric Speciation (Part 1)

Figure 17.6 Allopatric Speciation (Part 2)

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatrySome members of a population may cross an existing barrier and establish an isolated population.Finch species in the Galpagos Islands evolved from a single South American species that colonized the islands.The islands are far apart, and have different environmental conditions.

Figure 17.7 Allopatric Speciation among Darwins Finches

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatrySympatric speciationspeciation without physical isolation.May occur with disruptive selectionindividuals with certain genotypes prefer distinct microhabitats where mating takes place.This appears to be taking place with apple maggot flies. One group prefers to lay eggs on hawthorne fruits, the other group lays eggs on apples. They are partially reproductively isolated.

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatrySympatric speciation most commonly occurs by polyploidyduplication of sets of chromosomes within individuals.Autopolyploidychromosome duplication in a single speciesAllopolyploidycombining of chromosomes of two different species

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatryA tetraploid can result if two accidentally unreduced diploid gametes combine.Tetraploid and diploid individuals are reproductively isolated because their hybrid offspring are triploid.But tetraploids can self-fertilize or mate with another tetraploid. Thus polyploidy can result in complete reproductive isolation in two generations.

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatryHybridization between closely related species can disrupt normal meiosis and result in chromosomal doubling.Allopolyploids are often fertile: each chromosome has a partner to pair with in meiosis.

Concept 17.3 Speciation May Occur throughGeographic Isolation or in SympatrySpeciation by polyploidy is common in plants; about 70% of flowering plant species and 95% of fern species.It is more common in plants because many can self-fertilize.It has also occurred in animals (e.g., tree frogs).

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactReproductive isolation may be incomplete when the incipient species come back into contact and hybridization occurs.But hybrids may be less fit, and selection favors non-hybridizing parents.Selection results in reinforcement of isolating mechanisms.

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPrezygotic isolating mechanismsprevent hybridization from occurring. Postzygotic isolating mechanismsreduce fitness of hybrid offspring. Postzygotic mechanisms result in selection against hybridization, which leads to reinforcement of prezygotic mechanisms.

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPrezygotic isolating mechanismsMechanical isolationdifferences in sizes and shapes of reproductive organs. In plants, it may involve pollinators.

Figure 17.8 Mechanical Isolation through Mimicry

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPrezygotic isolating mechanismsTemporal isolationspecies may breed at different times of year or different times of day. Example: closely related leopard frog species.

Figure 17.9 Temporal Isolation of Breeding Seasons (Part 1)

Figure 17.9 Temporal Isolation of Breeding Seasons (Part 2)

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPrezygotic isolating mechanismsBehavioral isolationindividuals reject or fail to recognize mating behaviors of other species.Examples: mating calls of male frogs and coloration of male cichlid fish species.

Figure 17.10 Behavioral Isolation in Mating Calls

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactWhether plant species hybridize may depend on their pollinators.Flower color and shape influences which pollinators are attracted, or alters where pollen is deposited.Two sympatric species of columbines (Aquilegia) have diverged in flower color, structure, and orientation. One is pollinated by hummingbirds, the other by hawkmoths.

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 1)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 2)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 3)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 4)

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPrezygotic isolating mechanismsHabitat isolationwhen two closely related species evolve preferences for living or mating in different habitats.Gametic isolationsperm and eggs of different species will not fuse. Important for aquatic animals that release gametes into the water.

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactPostzygotic isolating mechanismsGenetic differences in diverging lineages may reduce fitness of hybrid offspring:Low hybrid zygote viabilityLow hybrid adult viabilityHybrid infertility

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactReinforcement of prezygotic isolating mechanisms is often detected by comparing sympatric and allopatric populations of potentially hybridizing species.Sympatric populations are expected to evolve more effective prezygotic reproductive barriers than do allopatric populations.Experiments with Phlox flower species are an example.

Figure 17.12 Flower Color and Reproductive Isolation (Part 1)

Figure 17.12 Flower Color and Reproductive Isolation (Part 2)

Concept 17.4 Reproductive Isolation Is ReinforcedWhen Diverging Species Come into ContactIf reproductive isolation is incomplete, hybrid zones may form where population ranges overlap.Hybrid zones may persist for long periods, such as between ranges of European Bombina toad species.

Figure 17.13 A Hybrid Zone

Answer to Opening QuestionSome aspects of speciation can be studied in the laboratory, using organisms with short generation times.Experiments using fruit flies showed that divergence in habitat preference resulted in reproductive isolation in 35 generations.Habitat selection is thought to have had a role in speciation of cichlids in Lake Malawi.

Figure 17.14 Evolution in the Laboratory

*****INTERACTIVE TUTORIAL 17.1 Speciation: Trends*****ANIMATED TUTORIAL 17.1 Founder Events and Allopatric SpeciationAPPLY THE CONCEPT Speciation may occur through geographic isolation*ANIMATED TUTORIAL 17.2 Speciation Mechanisms*****LINK Concept 21.5 Some plants and their pollinators become so tightly adapted to each other that they develop mutually dependent relationships****APPLY THE CONCEPT Reproductive isolation is reinforced when diverging species come into contact****