Chapter 24

Speciation: The Origin of New Species
Fundamental Concepts
What is Speciation?
• Speciation: The process by which one species splits into new species, producing tremendous diversity of life
• Species origin: New species originate from existing species through genetic divergence and reproductive isolation
• Forms a conceptual bridge between microevolution (changes in allele frequencies within a population) and macroevolution (broader patterns of evolutionary change above the species level)
The Biological Species Concept
• "A species is a group of populations whose members have the potential to interbreed in nature, produce viable fertile offspring, and do not produce viable fertile offspring with members of other such groups"
• This is the primary definition used in this chapter and essential for understanding speciation
• Gene flow: Movement of genes between populations that holds species together genetically
• Physical similarity is not always a reliable indicator of genetic relatedness or species membership
Evolution is Non-Linear
• Evolution branches in many different directions
• Environmental factors and selective pressures constantly change, creating a "moving goal post"
• Processes that work well in an environment are reinforced; those that don't work are eliminated
• As external factors change, selective pressures acting on species also change
Genetic Divergence and Reproductive Isolation
• When populations have gene flow, genes are shared and passed along
• Reproductive barrier: A biological barrier preventing gene flow between populations
• When reproductive barriers prevent interbreeding, populations can begin to diverge genetically
• Reproductive isolation and absence of interbreeding are the first steps toward speciation

Reproductive Barriers
Overview
• Reproductive isolation: Results when biological barriers impede members of two species from interbreeding and producing viable fertile offspring
• Barriers limit formation of hybrids: offspring resulting from interspecific mating (mating between two different species)
• Barriers classified as pre-zygotic (before zygote formation) or post-zygotic (after zygote formation)
• The distinction depends on whether a zygote is actually formed
Pre-Zygotic Barriers
Block fertilization by preventing species from attempting to mate, preventing successful completion of mating, or preventing fertilization if mating does occur.
Habitat Isolation
• Two species in different habitats within the same geographic area encounter each other rarely or never
• Example: Apple maggot flies isolated from blueberry maggot flies because they feed and lay eggs on different fruits
Temporal Isolation
• Species breed at different times: different times of day, different seasons, or different years
• Organisms cannot reproduce if they are not breeding simultaneously
• Example: Western and eastern spotted skunks have habitat overlap but breed at different times of year
Behavioral Isolation
• Species have unique courtship rituals or mating behaviors that effectively prevent mating with other species
• Example: Blue-footed booby has complex, unique mating displays that are not recognized by other similar species
• Requires correct and complete performance of mating behavior for female choice and successful reproduction
Mechanical Isolation
• If mating is attempted, morphological differences prevent successful completion
• Example: Snails in genus Bradybania have genital openings that do not align at the shell because shell spirals in opposite directions
• Physical incompatibility prevents reproduction even when organisms meet
Gametic Isolation
• Sperm of one species cannot fertilize eggs of another species
• Surface proteins on sperm and eggs from different species bind poorly to each other
• Example: Sea urchin species have surface proteins on sperm and eggs that prevent binding and zygote formation
• Prevents fertilization even if mating occurs
Post-Zygotic Barriers
Prevent a hybrid zygote from developing into a viable fertile adult through one of three mechanisms:
Reduced Hybrid Viability
• Genes from different parent species interact in ways that impair hybrid development or survival
• Example: Salamander hybrids do not fully complete development or metamorphosis if they hybridize
• Hybrids cannot complete their life cycle in the environment
Reduced Hybrid Fertility
• Meiosis fails to produce normal gametes, resulting in sterility
• Parent species have chromosomes of different numbers or structures
• Example: Mule (offspring of male donkey and female horse) is robust and healthy but sterile and cannot reproduce
• Successful reproduction between species produces offspring that cannot reproduce themselves
Hybrid Breakdown
• First generation hybrids are fertile but offspring in next generation are feeble or sterile
• Example: Hybrid fruits or crops—if you plant seeds from a hybrid fruit, resulting plant lacks the vigor of the hybrid parent
• Common in cultivated rice, corn, and other agricultural crops
• Hybrid strain has strength and disease resistance, but these traits do not pass to subsequent generations

Alternative Species Concepts
Limitations of the Biological Species Concept
• Cannot be applied to fossils: No way to observe mating behavior of extinct organisms or assess reproductive compatibility
• Cannot be applied to asexual organisms: No sexual reproduction or genetic mixing; includes all prokaryotes
• Emphasizes separateness and reproductive barriers rather than unity within species
• Gene flow occurs between morphologically and ecologically distinct species in some cases
Example of Gene Flow Between Distinct Species
• Grizzly bears and polar bears: Very distinct species that occasionally mate and produce "growler bears"
• Likely increasing with climate change as polar bear habitat warms and overlaps with brown bear range
• Raises question: Could this hybridization avenue lead to a new species?
Morphological Species Concept
• Distinguishes species by structural features
• Applicable to sexual and asexual species
• Can be used for extinct species through examination of fossil evidence and physical characteristics
• Based on morphological or physical appearances
Ecological Species Concept
• Defines species by its ecological niche: the sum of interactions with living and nonliving parts of the environment
• Applicable to sexual and asexual species
• Emphasizes the role of disruptive selection in maintaining species

Allopatric Speciation
Definition and Mechanism
• Allopatric: Roughly translates to "other country"
• Speciation occurs when populations are geographically isolated, preventing gene flow
• Gene flow is interrupted when populations are divided into geographically isolated subpopulations
• Can occur without geographic change when individuals colonize remote areas (island biogeography)
How Geographic Isolation Works
• Water bodies that become disconnected may develop unique species of fish over time
• Flying species or aquatic species commonly become stranded on islands
• Can occur with land animals on islands, but most common in flying species
• Geographic barriers effect: Depends on organisms' ability to move around
• Canyon creates barrier for small rodents but not for birds or coyotes
Genetic Mechanisms of Divergence
• Genes of isolated populations may diverge through mutation, natural selection, or genetic drift
• Reproductive isolation: May arise as a byproduct of genetic divergence
• Isolated populations accumulate genetic differences over time
• Example: Mosquito fish isolated populations became reproductively isolated as a result of selection under different predation levels
• With predators: body shape enables rapid bursts of speed
• Without predators: body shape favored for long, steady swimming
Evidence for Allopatric Speciation
Laboratory Evidence
• Reproductive barriers develop between isolated laboratory populations subjected to different environmental conditions
• Model organisms: Drosophila (fruit flies) commonly used because they have short generational cycles, easy to raise, very small, quick generational turnaround
• Can observe many generations quickly and push populations in different directions
• Example experiment: Feed fruit fly populations different diets (starch vs. maltose medium) over 40 generations
• Mates adapt to same diet
• After 40 generations, different populations can digest different food sources
• Can change outcomes by changing selective pressures
• Ethanol experiments: Isolated populations develop different capacities to digest alcohol
• Different selective pressures cause natural selection to drive genetic change
Natural Examples
• Sister species of snapping shrimp (genus Alpheus): Diverged 3-9 million years ago as populations were isolated by isthmus of Panama
• Different species now exist on either side of the isthmus
• Hawaiian islands: Have unique plants found only on individual islands
• Island chains: Great places to observe speciation—Galápagos and Hawaiian islands show isolation from mainland and between islands
• General pattern: Highly subdivided regions usually have more species than those with fewer barriers
• Geographic distance: Reproductive isolation between populations generally increases with geographic distance
Important Note on Physical vs. Biological Barriers
• Physical separation due to geographic isolation prevents interbreeding but is not a biological barrier to reproduction
• If geographically isolated populations are brought back together, it's possible they could still be physically capable of reproducing (unless other barriers have evolved)

Sympatric Speciation
Definition and Occurrence
• Sympatric: "Same country"—speciation occurs in populations living in the same geographic area
• Much less common than allopatric speciation
• Gene flow reduced by factors such as polyploidy, different sexual selection pressures, or habitat differentiation
Polyploidy
Overview
• Polyploidy: Accidents during cell division cause presence of extra chromosomes
• Can form new species within a single generation
• Primarily seen in plants, rarely in animals (not the typical mutation model seen in X-Men)
Auto-polyploidy
• More than two sets of chromosomes all derived from a single species
• Comes from mitotic errors
• Can result in production of tetraploid ($4n$) cell from diploid ($2n$) cell
• Fertile offspring: $4n$ can be produced through self-fertilization or mating among tetraploids
• Triploid mating: Mating between tetraploid and diploid produces triploid offspring ($3n$) with reduced fertility due to chromosome misalignments in gametes
Allo-polyploidy
• More than two sets of chromosomes derived from different species
• Chromosomes from different species do not pair during meiosis, resulting in hybrid sterility
• Sterile hybrids: Cannot reproduce sexually
• Asexual reproduction: Serial hybrids can reproduce asexually
• Chromosome doubling: Allo-polyploids form if chromosome number doubles in subsequent generations
• Diploid cells from species A and B combine to form sterile hybrid with $N = 5$ (3 + 2)
• Mitotic or meiotic errors in hybrid plant cell double chromosome numbers
• Results in viable fertile allopolyploid with $2N = 10$ (sum of diploid numbers of both parents)
Allopolyploid Characteristics
• Successfully interbreed with each other but not with either parent species
• Diploid number of new allopolyploid species equals sum of diploid numbers in both parents
• At least five new species have originated by polyploidy since 1850
• Example: Two allopolyploid species have evolved from diploid parent species from genus Tragopogon
Agricultural Importance
• Many important crops are polyploids: oats, cotton, potatoes, tobacco, wheat
• New polyploid agricultural species produced using chemicals to induce errors in cell division
• Used to exaggerate desired characteristics: fruit production, disease resistance, etc.
Sexual Selection
• Different mate preferences or sexual selections drive reproductive isolation
• Female choice and male competition cause sexual selection and sympatric speciation
• Example: Cichlids in Lake Victoria—sexual selection based on color
• When fish put under monochromatic light, females could not distinguish between different species with distinct coloration
• Sexual selection changed based on visual perception ability
• This weakened reproductive barriers
Habitat/Resource Exploitation
• Speciation results from exploitation of new habitats or resources
• Example: Apple maggot flies in North America
• Evolved after switching hosts from hawthorns to apples (closely related plants)
• Maggot flies mate on their host plant, creating habitat isolation between groups using different hosts
• Temporal isolation: Apple-feeding flies develop faster than hawthorn-feeding flies
• Alleles: Those benefiting flies using one host plant differ from those using the other, causing post-zygotic isolation
• Multiple mechanisms contribute to true speciation and habitat differentiation

Hybrid Zones
Definition and Formation
• Hybrid zone: Region where members of different species mate and produce hybrid offspring
• Hybrids result from mating between species with incomplete reproductive barriers
• Form along narrow bands where habitats overlap
• Example: Fire-belly toads (genus Bombina) interbreed along narrow hybrid zone where two species overlap
Three Possible Outcomes
Reinforcement
• Definition: Strong selection for pre-zygotic barriers, driving species further apart
• Occurs when hybrids are less fit than parent species
• Strong selection against hybrid production
• Reinforcement stronger for sympatric than allopatric populations
Fusion
• Species come back together into single species
• Occurs when hybrids are as fit as (or more fit than) parent species
• Weak reproductive barriers: Can be overcome by substantial gene flow
• Reproduction barriers weaken and two parent species merge
• Example: Lake Victoria cichlids—pollution has made it difficult for females to differentiate males by color
• Weakened reproductive barriers lead to fusion occurring
• Turbid water variant may become single fused species
Stability
• Separate lineages maintained alongside hybrid lineage in middle
• Extensive gene flow from outside hybrid zone can overwhelm selection for increased reproductive isolation within zone
• Example: Fire-belly toads maintain separate species with stable hybrid population in middle

Speciation Rates and Patterns in the Fossil Record
Open Questions
• How long does it take for new species to form? No single static answer exists
• Rate depends on intrinsic factors specific to each species and individual organisms
• Change likely depends greatly on the species involved and unique circumstances
Speciation Rates from Fossil Record
• Can observe broader patterns in fossil record
• Relies heavily on morphological understanding of species
• Molecular data also used to determine time intervals between speciation events in particular groups
Fossil Record Patterns
Punctuated Equilibrium
• Definition: Describes periods of apparent stasis (no change) punctuated by sudden change
• Sudden events that produce rapid change in speciation
• Sometimes dramatic loss of genetic diversity
• Example: Major extinction events bottleneck genetics followed by rapid speciation
• "The model that we use to describe this is called punctuated equilibrium. So punctuated equilibrium describes these periods of apparent stasis punctuated by sudden change"
Gradual Change Model
• Some species show gradual change over time rather than punctuated pattern
• Gentle, slow separation from common ancestor into two separate lineages over long period
• Slow and steady change observed in some lineages
Both Patterns Observed
• Fossil record shows evidence for both punctuated equilibrium and gradual change
• Both models happening simultaneously in different lineages
• No one-size-fits-all answer to speciation timescale
• Depends on unique circumstances surrounding species, population, and sometimes subpopulation

Key Biological Processes
Gene Flow and Speciation
• Gene flow shares genes between populations
• Reproductive barriers interrupt gene flow
• Without gene flow, populations diverge genetically
• Absence of gene flow allows accumulation of genetic differences
Natural Selection and Speciation
• Different selective pressures in isolated populations drive genetic divergence
• Selective pressures change with environmental conditions
• Directional selection can accumulate different alleles in isolated populations
• Disruptive selection emphasized in ecological species concept
Genetic Drift
• Random genetic changes in isolated populations
• One of three mechanisms (along with mutation and natural selection) driving divergence
• More pronounced in small populations
Meiosis and Reproductive Barriers
• Zygote formation: Result of sexual reproduction fertilization between male and female sex cells from genetically different organisms
• Gamete production via meiosis can fail in hybrids
• Chromosome alignment problems in hybrids with different chromosome numbers
• Mitotic errors can cause polyploidy

Summary and Context
Darwin's Work
• Very interested in process of speciation
• Process by which one species splits into new species
• Speciation explains tremendous diversity AND unity of life—everything shares common ancestor but diverse mechanisms produce different life forms
Chapter Scope
• Focuses on defining species and speciation
• Examines genetic mechanisms enabling speciation
• Discusses processes through which new species originate from existing species
• Explores role of adaptation and environmental influence