Speciation
Key Concepts
• Species Are Reproductively Isolated Lineages on the Tree of Life
• Speciation Is a Natural Consequence of
Population Subdivision
• Speciation May Occur through Geographic
Isolation or in Sympatry
• Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Speciation —divergence 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?”
Species —groups of organisms that mate with one another.

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Linnaeus described species based on their appearance —the morphological species concept .
Members of species look alike because they share many alleles.
He originated the binomial system of nomenclature.

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Limitations: members of the same species don’t always look alike (e.g., male and female wood ducks).
Cryptic species — two or more species that are morphologically indistinguishable but do not interbreed.

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

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Biological species concept : “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” Ernst Mayr
Reproductive isolation : two groups of organisms can no longer exchange genes.

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Lineage species concept : species as branches on the tree of life.
A lineage is an ancestor –descendant series of populations followed over time.

Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
The 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 differences —mutations that result in morphological changes cannot spread between reproductively isolated species.

Concept 2 Speciation Is a Natural Consequence of Population Subdivision
Not 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 2 Speciation Is a Natural Consequence of Population Subdivision
The Dobzhansky –Muller 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 Dobzhansky
–Muller Model

Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision
In 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.

Concept 2 Speciation Is a Natural Consequence of Population Subdivision
As 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).

Concept 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Allopatric speciation —when 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 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Some members of a population may cross an existing barrier and establish an isolated population.
Finch species in the Gal ápagos 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 Darwin’s Finches

Concept 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Sympatric speciation —speciation without physical isolation.
May occur with disruptive selection —individuals 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 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Sympatric speciation most commonly occurs by polyploidy —duplication of sets of chromosomes within individuals.
Autopolyploidy —chromosome duplication in a single species
Allopolyploidy —combining of chromosomes of two different species

Concept 3 Speciation May Occur through
Geographic Isolation or in Sympatry
A 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 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Hybridization 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 3 Speciation May Occur through
Geographic Isolation or in Sympatry
Speciation 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).

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Reproductive 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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms —prevent hybridization from occurring.
Postzygotic isolating mechanisms —reduce fitness of hybrid offspring.
Postzygotic mechanisms result in selection against hybridization, which leads to reinforcement of prezygotic mechanisms.

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Mechanical isolation —differences in sizes and shapes of reproductive organs.
In plants, it may involve pollinators.

Figure 17.8 Mechanical Isolation through Mimicry

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Temporal isolation —species 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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Behavioral isolation —individuals 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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Whether 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)

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Habitat isolation —when two closely related species evolve preferences for living or mating in different habitats.
Gametic isolation —sperm and eggs of different species will not fuse. Important for aquatic animals that release gametes into the water.

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Postzygotic isolating mechanisms
Genetic differences in diverging lineages may reduce fitness of hybrid offspring:
• Low hybrid zygote viability
• Low hybrid adult viability
• Hybrid infertility

Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Reinforcement 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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
If 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

Chapter 17 Opening Question

Answer to Opening Question
Some 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