Campbell Biology Twelfth Edition: The Origin of Species

Overview of Speciation and Evolutionary Change

Definition of Speciation: This is the process by which one species splits into two or more species. It represents the focal point of evolutionary theory because it explains both the diversity of life and the underlying unity of life.
Conceptual Bridge: Speciation connects two fundamental scales of evolution: - Microevolution: Refers to changes in allele frequencies within a single population over time. - Macroevolution: Refers to broad patterns of evolutionary change above the species level, such as the emergence of entirely new groups of organisms through multiple speciation events.
Operational Taxonomic Units (OTUs): These are pragmatic, sequence-based clusters (typically requiring a $97\%$ similarity threshold) used primarily to classify microorganisms when exact species identification is not feasible. While species are rigid biological or phylogenetic ranks defined by interbreeding or genetic identity, OTUs serve as flexible placeholders in taxonomic research.
Taxonomic Hierarchy Example: The Red fox (Vulpes vulpes) follows the hierarchy: Domain Eukarya, Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Carnivora, Family Canidae, Genus Vulpes, Species Vulpes vulpes.

The Biological Species Concept

Definition: According to this concept, a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but do not produce viable, fertile offspring with members of other such groups.
Primary Mechanism: The genetic cohesiveness of a species is maintained by gene flow, which is the transfer of alleles between populations.
Etymology: The word species is Latin for "kind" or "appearance."
Reproductive Isolation: The existence of biological factors (barriers) that impede members of two different species from interbreeding and producing viable, fertile offspring. These barriers limit the formation of hybrids, which are offspring resulting from interspecific mating.

Prezygotic Barriers

Prezygotic barriers block fertilization from occurring by impeding mating attempts, preventing completion of mating, or hindering fertilization.

Habitat Isolation: Two species occupy different habitats within the same area and rarely, if ever, encounter each other. - Example: Apple maggot flies (Rhagoletis pomonella) feed and lay eggs on apples, while their close relatives, blueberry maggot flies, use blueberries, keeping them isolated.
Temporal Isolation: Species breed at different times of the day, different seasons, or different years. - Example: The western spotted skunk (Spilogale gracilis) mates in late summer, whereas the eastern spotted skunk (Spilogale putorius) mates in late winter.
Behavioral Isolation: Courtship rituals and other behaviors unique to a species act as effective mating barriers. - Example: Blue-footed boobies mate only after a specific high-step courtship display that advertises their bright blue feet. Other examples include Albatross and Giraffe courtship rituals.
Mechanical Isolation: Mating is attempted, but morphological differences (anatomical incompatibility) prevent its successful completion. - Example: In snails of the genus Bradybaena, if the shells spiral in opposite directions, the genital openings do not align.
Gametic Isolation: The sperm of one species is unable to fertilize the eggs of another species. This may be due to the inability of the sperm to survive in the reproductive tract or biochemical incompatibilities. - Example: Surface proteins on the sperm and eggs of different sea urchin species bind poorly to each other, preventing zygote formation.

Postzygotic Barriers

Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult after fertilization has occurred.

Reduced Hybrid Viability: The genes of different parent species may interact in ways that impair the hybrid's development or survival. - Example: Hybrid offspring of salamander subspecies in the genus Ensatina usually fail to complete development, and those that do are very frail.
Reduced Hybrid Fertility: Even if hybrids are vigorous, they may be sterile if the chromosome numbers or structures of the parent species differ, preventing normal meiosis. - Example: A mule is the robust but sterile offspring of a male donkey and a female horse.
Hybrid Breakdown: The first-generation ( $F_1$ ) hybrids are viable and fertile, but when they mate with each other or with either parent species, the offspring of the next generation ( $F_2$ ) are feeble or sterile. - Example: Certain strains of cultivated rice produce vigorous hybrids, but the subsequent generation consists of small, sterile plants.

Limitations and Alternatives to the Biological Species Concept

Limitations: - It cannot be applied to fossils, as mating behavior cannot be observed. - It does not apply to asexual organisms, including all prokaryotes. - It is complicated by species that are morphologically and ecologically distinct yet still exchange genes, such as grizzly bears and polar bears mating to produce "grolar bears."
Morphological Species Concept: Defines species by structural features. It applies to both sexual and asexual organisms and does not require gene flow data, but it relies on subjective criteria.
Ecological Species Concept: Defines species based on its ecological niche (the sum of its interactions with living and nonliving parts of the environment). This emphasizes the role of disruptive natural selection and applies to sexual and asexual species.
Diversity of Definitions: More than $20$ other species definitions exist; choice depends on the specific research question.

Allopatric ("Other Country") Speciation

Process: Gene flow is interrupted when a population is divided into geographically isolated subpopulations. This can occur through geological changes (e.g., a lake subsiding into two smaller lakes) or colonization of remote areas.
Example (Natural): The flightless cormorant of the Galápagos likely evolved from a flying ancestor on the mainland that colonized the islands.
Example (Experimental): Snapping shrimp (Alpheus) diverged into sister species as they were isolated by the formation of the Isthmus of Panama between $9$ and $3$ million years ago. Deep-water species diverged first.
Mechanism of Divergence: Isolated gene pools diverge through mutation, natural selection (such as different predation pressures), and genetic drift. - Mosquitofish Example: Populations in ponds with predators evolved body shapes for rapid bursts of speed ( $70$ to $110\,\text{m/s}^2$ escape acceleration), while those without predators evolved shapes for long, steady swimming. This divergence led to reproductive isolation.
Geographic Distance: Reproductive isolation typically increases as the geographic distance between populations increases.

Sympatric ("Same Country") Speciation

Process: Speciation occurs in populations that live in the same geographic area. It is less common than allopatric speciation and requires a reduction in gene flow via specific mechanisms.
Polyploidy: Accidental extra sets of chromosomes during cell division, common in plants but rare in animals. - Autopolyploid: An individual with more than two chromosome sets derived from a single species. A tetraploid ( $4n$ ) can result from a failure of cell division in a diploid ( $2n$ ) cell. Tetraploids can produce fertile $4n$ offspring by self-fertilizing or mating with other tetraploids, but mating with diploids produces triploids ( $3n$ ) with reduced fertility. - Allopolyploid: An individual with multiple sets of chromosomes derived from different species. These hybrids are usually sterile because chromosomes cannot pair during meiosis, but they can become fertile if the chromosome number doubles in subsequent generations. The new species' diploid number equals the sum of the parents' diploid numbers. - Example: Two new species of Tragopogon (goatsbeard) evolved from three diploid parent species ( $2n=12$ ) to form polyploids ( $2n=24$ ). - Agriculture: Crops like oats, cotton, potatoes, tobacco, and bread wheat (Triticum aestivum, $2n=42$ ) are polyploids.
Sexual Selection: Female mate choice based on male appearance can drive sympatric speciation. - Example: Cichlids in Lake Victoria diversified because females selected males based on specific breeding coloration.
Habitat Differentiation: Occurs when a subpopulation exploits a habitat or resource not used by the parent population. - Example: North American apple maggot flies switched from host hawthorn trees to apple trees. This caused habitat isolation, temporal isolation (apples ripen faster), and postzygotic isolation due to host-specific alleles.

Hybrid Zones and Outcomes

Hybrid Zone: A region where members of different species meet, mate, and produce offspring of mixed ancestry.
Patterns: Some are narrow bands (e.g., Bombina toads in Europe), while others are scattered patches. Hybrids often have lower fitness than parents.
Outcomes over Time: - Reinforcement: If hybrids are less fit, natural selection strengthens prezygotic barriers to reduce hybrid production. This is stronger in sympatric than allopatric populations. (e.g., Pied and Collared flycatchers look different in sympatry but similar in allopatry). - Fusion: If reproductive barriers are weak and hybrids are fit, gene flow increases until the two species fuse into one. (e.g., Pollution in Lake Victoria making water turbid, preventing cichlid females from distinguishing mates). - Stability: Continued production of hybrid individuals because gene flow from outside the zone overwhelms selection against hybrids. (e.g., Bombina toads migrating into the hybrid zone).

Time Course and Genetics of Speciation

Punctuated Equilibria: Periods of apparent stasis (no change) in the fossil record punctuated by sudden, rapid change. This contrasts with a gradual model where species change slowly over long periods.
Speciation Rates: The interval between speciation events varies wildly. - Cichlids: Can be as short as $4,000$ years. - Beetles: Can be as long as $40$ million years. - Average: Approximately $6.5$ million years.
Genetics of Speciation: Speciation can be driven by a single gene or many genes. - Single Gene: The direction of shell spiral in Euhadra snails prevents mating between opposite-spiraling individuals. - Few Genes: Flower color in monkey flowers (Mimulus) is controlled by at least two loci, determines whether bees or hummingbirds pollinate them, and leads to isolation.
Macroevolutionary Patterns: Macroevolution is the cumulative effect of many speciation and extinction events over vast periods of time, resulting in groups that differ significantly from their ancestors.