Chapter 24

Speciation: The Origin of Species

Introduction to Speciation

Speciation is the process by which one species splits into two or more species.
It is a source of tremendous diversity of life and helps explain the unity of life.

Microevolution vs. Macroevolution

Speciation forms a conceptual bridge between microevolution and macroevolution.
Microevolution: Changes in allele frequency in a population over time.
Macroevolution: Broad patterns of evolutionary change above the species level (e.g., the rise and fall of dinosaurs).

The Biological Species Concept

Species (Latin): "kind" or "appearance."
Morphologically distinct species are discrete groups, confirmed by comparisons of physiology, biochemistry, and DNA sequences.
The genetic code is the most reliable determination of species.
Biological Species Concept: A species is a group of populations whose members:
- Have the potential to interbreed in nature.
- Produce viable (able to survive) and fertile (able to reproduce) offspring.
- Do not produce viable, fertile offspring with members of other such groups.
Gene flow between populations holds a species together genetically.
Two different species do not produce viable and fertile offspring with each other.

Reproductive Isolation

Reproductive isolation: Biological barriers that impede members of two species from interbreeding and producing viable, fertile offspring.
These barriers limit the formation of hybrids (offspring from interspecific mating).
Reproductive isolation is classified by whether factors act before (prezygotic) or after (postzygotic) fertilization.
A zygote is a fertilized egg.

Prezygotic Barriers

Block fertilization from occurring by:
- Impeding different species from attempting to mate.
- Preventing the successful completion of mating.
- Hindering fertilization if mating is successful.

Types of Prezygotic Barriers

Habitat Isolation
- Two species occupy different habitats within the same area and rarely encounter each other.
- Example: Apple maggot flies vs. blueberry maggot flies - they feed and lay eggs on different fruits.
Temporal Isolation
- Species breed at different times of day, in different seasons, or different years and cannot mix gametes.
- Example: Western spotted skunks (mate in summer) vs. eastern spotted skunks (mate in winter).
Behavioral Isolation
- Courtship rituals and other behaviors unique to a species are effective barriers to mating.
- Example: Blue-footed boobies mate only after a unique courtship display.
Mechanical Isolation
- Mating is attempted, but morphological differences prevent its successful completion.
- Example: Genital openings of snails in the genus Bradybaena do not align if their shells spiral in opposite directions.
Gametic Isolation
- Sperm of one species may not be able to fertilize eggs of another species.
- Common in aquatic animals.
- Example: Surface proteins on sperm and eggs of different sea urchin species bind poorly preventing fusion and zygote formation.

Postzygotic Barriers

Prevent hybrid zygotes from developing into viable, fertile adults through:
- Reduced hybrid viability.
- Reduced hybrid fertility.
- Hybrid breakdown.

Types of Postzygotic Barriers

Reduced Hybrid Viability
- Genes of different parent species may interact in ways that impair the hybrid’s development or survival in its environment.
- Example: Hybrid offspring of different subspecies of salamanders of the genus Ensatina do not usually complete development (dies in utero or stillbirth).
Reduced Hybrid Fertility
- Meiosis may fail to produce normal gametes, resulting in sterility if the parent species have chromosomes of different number or structure.
- Example: Mule (hybrid offspring of a male donkey and a female horse) is robust, but sterile.
Hybrid Breakdown
- First-generation hybrids are viable and fertile, but offspring in the next generation are feeble or sterile.
- Common in plants.
- Example: Hybrids between certain strains of cultivated rice are vigorous and fertile, but members of the next generation are small and sterile.

Limitations of the Biological Species Concept

Emphasizes the absence of gene flow, but gene flow occurs between many morphologically and ecologically distinct species.
Example: Grizzly bears and polar bears can occasionally mate to produce "grolar bears."

Other Definitions of Species

The biological species concept emphasizes the separateness of different species due to reproductive barriers.
Several other definitions emphasize the unity within a species.

Alternative Species Concepts

Morphological Species Concept
- Distinguishes a species by its structural features.
- Applies to sexual and asexual species and does not require information on the extent of gene flow.
- Disadvantage: Relies on subjective criteria.
Ecological Species Concept
- Defines a species by its ecological niche (the sum of its interactions with the nonliving and living parts of the environment).
- Applies to sexual and asexual species and emphasizes the role of disruptive selection.

Modes of Speciation

Speciation can occur in two main ways:
- Allopatric speciation: Populations are geographically isolated.
- Sympatric speciation: Populations are not geographically isolated.

Allopatric Speciation (“Other Country”)

Gene flow is interrupted when a population is divided into geographically isolated subpopulations.
Example: Water in a lake subsides and forms one lake which separates the populations into two separate ponds.
Allopatric speciation can occur without geographic change when individuals colonize a remote area.
Example: The flightless cormorant of the Galápagos likely originated from a flying species on the mainland.
The gene pools of isolated populations may diverge through mutation, natural selection, and genetic drift.
Reproductive isolation may arise as a by-product of genetic divergence.
Example: Isolated populations of mosquitofish have become reproductively isolated as a result of selection under different levels of predation.
There is evidence for allopatric speciation in nature.
Example: Sister species of snapping shrimp (Alpheus) diverged 9 to 3 million years ago as populations were isolated by the Isthmus of Panama.
Isolated or highly subdivided regions usually have more species than those with fewer barriers.
Example: The Hawaiian Islands have many unique plants and animals.
Reproductive isolation between populations generally increases with geographic distance.
Physical separation due to geographic isolation prevents interbreeding, but is not a biological barrier to reproduction.
Biological barriers are intrinsic to the organisms themselves.

Sympatric Speciation (“Same Country”)

Speciation occurs in populations that live in the same geographic area.
Sympatric speciation is less common than allopatric speciation.
Occurs if gene flow is reduced by factors such as:
- Polyploidy
- Sexual selection
- Habitat differentiation

Polyploidy

Accidents during cell division can cause polyploidy (the presence of extra sets of chromosomes).
This process can form a new species within a single generation without geographic separation.
Polyploidy is common in plants but rare in animals.
${* There are two types of polyploids: autopolyploids and allopolyploids.}
Autopolyploids: Have more than two sets of chromosomes, all derived from a single species.
In plants, mitotic errors can result in the production of a tetraploid (4n) $cell from a diploid$ (2n) cell.
Fertile offspring (4n) can be produced through self-fertilization or mating among tetraploids.
Mating between tetraploids and diploids produces triploid (3n)$$ offspring with reduced fertility.
Allopolyploids: Have 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 can reproduce asexually.
Allopolyploids are formed if the chromosome number doubles in subsequent generations.
Allopolyploids can successfully interbreed with each other, but not with either parent species.
The diploid number of the new allopolyploid species equals the sum of the diploid number of both parents.
Many important agricultural crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids.
New polyploid agricultural species are produced using chemicals to induce errors in cell division and other genetic modified organism strategies.

Sexual Selection

Sympatric speciation can be driven by sexual selection.
Example: Speciation of cichlids in Lake Victoria was likely driven by female mate choice based on male breeding coloration.
Remember sympatric speciation is in the same area – based on color in same area of Lake Victoria.
Allopatric speciation must be separated geographically.

Habitat Differentiation

Sympatric speciation can also result from the exploitation of new habitats or resources.
Example: Apple maggot flies evolved in North America after switching hosts from hawthorn to apple.
Maggot flies mate on their host plant, resulting in habitat isolation between groups using different hosts.
Apple-feeding flies develop faster than hawthorn-feeding flies, resulting in temporal isolation.
Alleles that benefit flies using one host plant harm those using the other, causing post-zygotic isolation.

Review of Allopatric and Sympatric Speciation

Allopatric Speciation:
- Geographic isolation restricts gene flow between populations.
- Intrinsic barriers to reproduction arise due to genetic change driven by processes including divergent selection and genetic drift.
- Reproductive barriers prevent interbreeding even if contact is restored between populations.
Sympatric Speciation:
- A reproductive barrier isolates a subset of a population without geographic separation from the parent species.
- Sympatric speciation can result from polyploidy, sexual selection, or natural selection resulting from a switch in food source or habitat.

Hybrid Zones

A hybrid zone is a region in which members of different species mate and produce hybrid offspring.
Hybrids are the result of mating between species with incomplete reproductive barriers.

Patterns Within Hybrid Zones

Some hybrid zones form as narrow bands where habitats of two or more closely related species meet.
Example: Two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone.
Hybrids often have reduced survival and reproduction compared with parent species.
Outside the hybrid zone, gene flow may be impeded by obstacles such as natural selection in the different parental habitats.
Hybrid zones are typically located wherever habitats of interbreeding species meet.
This often occurs as isolated patches scattered across the landscape, rather than a continuous band.
Changing environmental conditions can drive the production of new hybrid zones.
Example: The range of the southern flying squirrel expanded northward into the range of the northern flying squirrel in response to a series of warm winters.
Alleles can be transferred from one parent species to the other through breeding between parents and hybrids.
The transfer of novel alleles may help parent species cope with changing environments.

Outcomes for Hybrid Zones Over Time

If hybrids do not become reproductively isolated from their parent species, then three alternate outcomes are possible:
- Reinforcement
- Fusion
- Stability

1. Reinforcement: Strengthening Reproductive Barriers

If hybrids are less fit than the parent species, then strong selection for prezygotic barriers should reduce hybrid production.
This process is called reinforcement because it reinforces reproductive barriers.
Reinforcement should be stronger for sympatric than allopatric populations.
Hybrids are LESS FIT.

2. Fusion: Weakening Reproductive Barriers

There can be substantial gene flow between species if hybrids are as fit as their parents.
Reproductive barriers can weaken, and the two parent species may fuse into a single species.
Example: Pollution in Lake Victoria has reduced the ability of female cichlids to visually distinguish between males of their own and different species.

3. 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.
Example: Members of both parent species of Bombina routinely migrate into the narrow hybrid zone, resulting in ongoing hybridization.
The 2 parents and the hybrid population populations are stable at about the same population levels.

Speciation Rates

Many questions remain concerning how long it takes for new species to form and how many genes change when one species splits into two.
The rate of speciation can be studied by observing broad patterns in the fossil record.
Morphological and molecular data can also be used to assess the time interval between speciation events in particular groups.

Patterns in the Fossil Record

The fossil record includes many episodes where new species appear suddenly, persist unchanged through several strata, and then disappear.
Punctuated equilibria describes these periods of apparent stasis punctuated by sudden change.
Rather than a punctuated pattern, other species appear to have changed gradually over time.

Quantifying Speciation Rates

The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid.
Example: The sunflower Helianthus anomalus was formed by hybridization between two other sunflower species followed by rapid speciation.
In a study of 84 groups of plants and animals, the interval between speciation events ranged from 4,000 years (cichlids) to 40 million years (beetles).
The average time between speciation events was 6.5 million years.

From Speciation to Macroevolution

Differences accumulate with successive speciation events; eventually new groups of organisms form that differ greatly from their ancestors.
Other groups shrink in size as species are lost to extinction.
Macroevolution is the cumulative effect of many speciation and extinction events.