Origins of Biodiversity: Allopatric and Sympatric Speciation

Introduction to Speciation

• Reproductive Context: The primary mechanism maintaining separate species is reproduction. If organisms do not interbreed to produce viable, fertile offspring, they remain separate lineages.
• Review of Reproductive Barriers:
      * Pre-zygotic Barriers: Mechanisms that prevent the egg and sperm from fusion (e.g., behavioral or temporal isolation).
      * Post-zygotic Barriers: Mechanisms that act after fertilization occurs. This includes the prevention of a viable offspring from developing or the production of an offspring that is infertile (e.g., a hybrid that cannot reproduce).
• The Speciation Event: Speciation is the process by which new species arise from existing ones.

Mechanisms of Population Divergence

• Primary Evolutionary Drivers: Speciation begins when populations of the same species stop interbreeding and mating with one another. Once interbreeding ceases, divergence begins.
• Local Adaptation: This is the primary reason for divergence during speciation. If populations exist in different parts of a habitat or environment, they will adapt differently to their local conditions over time (ranging from several years to centuries).
• Gene Flow:
      * Gene flow (the random movement of alleles between gene pools) acts to unite populations and keep them as a single species.
      * Barriers to gene flow divide populations, setting them on a path toward divergence because they are no longer exchanging alleles.
• Mutations: The accumulation of unique mutations in isolated populations can lead to enough differentiation for the populations to be classified as separate species.
• Genetic Drift: Random sampling events can significantly narrow the diversity of a gene pool.
      * This may result in the fixation of certain alleles and the loss of others in each population.
      * Populations lose access to alleles present in the original group, potentially leading to the emergence of different species.

Allopatric Speciation

• Definition: Reproductive isolation occurring because populations are separated by a geographic barrier. This is the most common form of speciation.
• Mechanisms of Geographic Separation:
      * Dispersal: A few individuals from a population move to a new area and found a new population. If they are unable to return or travel back, they become isolated.
          * Plant Example: Pollen released and carried by the wind into a new area where it establishes a distant population.
          * Newt/Organism Example: A few individuals cross a major river. Due to currents or river size, they cannot cross back, leading to local adaptation over decades or hundreds of years.
      * Environmental/Geologic Barriers: Physical changes in the landscape.
          * Antarctica: Geographically isolated by the circumpolar current. This current is so massive and strong that it acts as a physical barrier, preventing movement between the Antarctic side and landmasses like South America.
      * Island Isolation:
          * Channel Island Foxes: Mainland foxes were taken to the Channel Islands off the coast of California (likely by humans in boats). Isolated by the water barrier and lacking gene flow with the mainland, they diverged into a separate species.
          * Galapagos Finches: These species derived from a single population of mainland South American finches. A small number moved to the islands and stayed isolated by the large body of water. This resulted in approximately $50$ to $60$ different finch species.
• Adaptive Radiation: A process where a population moves into an area with many open, uninhabited niches. Through local adaptation, the population diverges into many new species. The Galapagos finches are a primary example of this.

Case Study: Nancy Dalton and the Isthmus of Panama

• Geologic Context: Approximately $3 imes 10^6\, ext{years}$ ago, a land bridge (the Isthmus of Panama) formed between North and South America due to tectonic movement and volcanic activity. This area was previously open water.
• Impact: The land bridge created a barrier for marine populations, splitting previously united habitats into the Pacific Ocean and the Caribbean/Atlantic Ocean.
• The Snapping Shrimp Hypothesis: Researcher Nancy Dalton (Smithsonian) tested whether the Isthmus of Panama caused the speciation of various snapping shrimp.
• Experimental Scenarios:
      * Scenario 1: Pacific and Caribbean populations became reproductively isolated recently (roughly $3 imes 10^6\, ext{years}$ ago), corresponding with the bridge formation.
      * Scenario 2: Populations have been isolated for a long time (approximately $50 imes 10^6\, ext{years}$ ago), meaning isolation preceded the land bridge.
• Phylogenetic Evidence: Researchers mapped the phylogeny of shrimp labeled "P" (Pacific) and "C" (Caribbean) using morphological data, behavior, and DNA sequencing.
      * Finding: The tree showed a pattern of sister taxa where every Pacific species had an immediate, closely related Caribbean counterpart (couplets).
      * Conclusion: The presence of these closely related P-C pairs confirms that the speciation event occurred recently, caused by the formation of the isthmus roughly $3 imes 10^6\, ext{years}$ ago.

Sympatric Speciation

• Definition: A speciation event occurring within the same habitat without a physical or geographic barrier.
• Commonality: Less common than allopatric speciation, but frequent in plants.
• Mechanisms:
      * Polyploidy: A plant reproductive process involving the doubling of chromosomes, which can immediately create a new species.
      * Habitat Differentiation: The most common sympatric mechanism in animals/mobile organisms. Populations in the same area begin using the habitat differently, eventually leading to genetic and reproductive isolation.
• Example: Apple Maggot Flies:
      * Native to North America, these flies originally laid eggs on Hawthorn fruits.
      * In the mid-$1800s$, apple trees were introduced to North America.
      * Some flies began laying eggs on apples instead of Hawthorn berries. Over time, flies hatched on apples tended to lay their own eggs on apples (specialization).
      * Despite living in the same orchards, the "Hawthorn-fly" population and the "Apple-fly" population diverged into separate species due to their different habitat use.
• Example: Cichlid Fishes:
      * Found in the African Great Lakes (e.g., Lake Victoria), over $1000$ species arose via adaptive radiation in a single habitat.
      * Different populations specialized in different niches: surface insects, detritus, etc. Their specialization led to reproductive isolation within the same massive lake.

Questions & Discussion

• Q: Why would some mice in a controlled experiment with perfect resources suddenly stop mating?
      * A: In highly controlled, non-natural environments, mating might stop due to difficulty finding mates or shifts toward more efficient asexual reproduction (e.g., parthenogenesis/axolotls).
• Q: What would cause a species to leave an area if not for a natural disaster?
      * A: Competition is a major driver. If competition for a food resource is too high, populations may migrate to areas with fewer competitors. Other drivers include changes in climate and resource availability.
• Q: If an allopatric barrier (like a mountain or river) disappears after speciation has already occurred, what happens?
      * A: Even if the environment is mixed back together, the two groups are likely already reproductively isolated. They are adapted to their local environments, meant that even if they encounter each other, they are unlikely to produce viable offspring or successfully interbreed.
• Q: Which type of speciation involves a geographic barrier?
      * A: Only allopatric speciation involves a geographic barrier. Sympatric speciation occurs in the same location.
• Q: What type of isolation is occurring in a scenario where one fly population lives on cattle and another on antelope in the same area?
      * A: This is sympatric speciation driven by habitat isolation, as the organisms are separated by their choice of micro-habitat (the host animal) despite being in the same general geographic area.
• Q: If a population's trait shifts toward one extreme over time, what type of selection is it?
      * A: Directional selection.