species formation
Species Formation
1. Introduction: How Do New Species Originate?
Definition of Speciation: Speciation is the evolutionary process by which new species arise from ancestral populations.
Gene Exchange: New species form when populations accumulate enough genetic differences that they no longer exchange genes, even if they come back into contact.
Geographic Influence: The geographic context strongly influences the process of speciation.
Three Major Pathways of Speciation:
Allopatric Speciation: Populations become separated by a physical barrier.
Parapatric Speciation: Populations live in neighboring habitats with limited gene flow.
Sympatric Speciation: Divergence occurs within the same geographic area.
2. Allopatric Speciation
Definition: Allopatric speciation occurs when populations become geographically isolated by barriers such as mountains, rivers, oceans, volcanic islands, etc.
Consequences of Isolation: Without gene flow, isolated populations diverge due to mutation, natural selection, and genetic drift.
A. Speciation by Dispersal
Definition: Dispersal occurs when a small number of individuals move to a new, isolated location.
Founder Populations: If this population becomes isolated from the original, it may diverge into a new species.
Example: Hawaiian Drosophila
Hundreds of Drosophila species occur only in Hawaii.
Many species are restricted to single islands.
Closely related species occur on adjacent islands.
The pattern of speciation follows the geological ages of the islands, supporting the idea that new species formed when small populations colonized new islands and diverged in isolation.
B. Speciation by Vicariance
Definition: Vicariance occurs when a once-continuous population is split into isolated groups due to geological or environmental changes.
Example: Snapping Shrimp
Pairs of snapping shrimp species live on opposite sides of the Isthmus of Panama.
The formation of the isthmus separated ancestral populations.
Each isolated group subsequently diverged into a new species.
3. Parapatric Speciation
Definition: Parapatric speciation occurs when populations occupy neighboring habitats and experience different environmental conditions across a transition zone (ecotone).
Gene Flow: Gene flow is limited because individuals tend to mate with nearby neighbors.
Examples of Ecotones:
Forest ↔ Grassland
Forest ↔ Marshland
Riparian edges
Agricultural field boundaries
Example: Sweet Vernal Grass (Anthoxanthum odoratum)
Some populations grow on heavy-metal-contaminated mine tailings while nearby populations grow in normal soil.
Tailings populations evolved metal tolerance.
These populations also developed different flowering times, which reduces gene flow.
Represents early stages of parapatric divergence.
4. Sympatric Speciation
Definition: Sympatric speciation occurs without geographic separation, with populations diverging while living in the same area.
Drivers of Divergence: Often results from resource specialization or changes in habitat use.
Example: Apple Maggot Fly (Rhagoletis pomonella)
Flies originally used hawthorn fruits.
Some shifted to apples after apple trees were introduced.
Mating occurs on the fruit; larvae develop in the fruit.
Adults return to the same fruit type to reproduce.
Apples and hawthorns fruit at different times → Flies differ in life-cycle timing, leading to reduced gene flow between apple and hawthorn flies, indicating how sympatric divergence can begin through host specialization.
5. Summary of Speciation Pathways
Overview: Speciation is the formation of new evolutionary lineages.
Types of Speciation:
Allopatric Speciation: Results from geographic isolation, categorized into dispersal and vicariance mechanisms.
Parapatric Speciation: Occurs across environmental gradients with limited gene flow.
Sympatric Speciation: Occurs without geographic barriers.
Mechanisms of Divergence: Divergence results from mutation, natural selection, genetic drift, and varying levels of gene flow.
6. Reproductive Isolating Mechanisms
Definition: Reproductive isolating mechanisms prevent gene flow between populations, allowing them to become separate species.
Types of Mechanisms:
Prezygotic Isolating Mechanisms: Prevent mating or fertilization.
Postzygotic Isolating Mechanisms: Act after fertilization and can reduce hybrid viability or fertility.
Prezygotic Isolating Mechanisms
Temporal Isolation
Species breed at different times.
Examples:
Rana aurora breeds earlier in the year than Rana boylii.
Western and Eastern spotted skunks breed in different seasons.
Behavioral Isolation
Differences in courtship signals.
Examples:
Birds have distinct mating songs.
Lacewing species exhibit distinct mating songs.
Mechanical Isolation
Reproductive anatomy does not fit.
Example:
Damselfly males have species-specific claspers.
Ecological Isolation
Species live or breed in different habitats.
Examples:
Blackbird found in woodland, and Ring Ouzel found in moreland.
One garter snake species is aquatic; another is terrestrial.
Postzygotic Isolating Mechanisms
Zygotic Mortality
The egg is fertilized, but the hybrid fails to develop properly.
Example:
Some Ensatina salamander embryos are nonviable.
Hybrid Sterility
Hybrids are sterile.
Example:
Mule (horse × donkey) is sterile.
Ecological Inviability
Hybrids develop normally but suffer low viability due to ecological niche absence or poor fitness choices.
Example:
Hybrids of willow warblers do not follow the correct migration pattern and die in the Sahara Desert.
7. Importance of Reproductive Isolation
Significance: Reproductive isolation maintains species as independent evolutionary units.
Role of Reproductive Isolation: It prevents gene flow and allows divergence to continue even if populations come back into contact.
Combined Mechanisms: Together with geographic processes, reproductive isolation explains how speciation begins and how it becomes complete.