Origin of Species I - Medium

• A species represents the smallest independently evolving unit, though there is constant disagreement on how to define it.
• Definitions for extinct species are often based on whether they hybridised and changes in morphological features.
• Definitions for extant species consider behaviour, geography, and dispersal tendencies in addition to morphology.

Specific Species Concepts:

• Phylogenetic Species Concept (PSC): Defines a species as the smallest possible group descending from a common ancestor, recognizable by unique, derived traits. It is useful for systematics, focusing on phylogenetic history.
• Biological Species Concept (BSC): Defines species as groups of potentially interbreeding populations that are reproductively isolated from other groups. It works well for sexually reproducing animals and is the most common definition.
• General Lineage Species Concept (GLSC): Defines species as metapopulations of organisms that evolve independently from other metapopulations. A metapopulation is a group of spatially separated populations of the same species that interact at some level, with significant gene flow indicating a single gene pool.

Underlying Concepts

• Species follow independent evolutionary trajectories.
• Reproductive isolation is a key factor in distinguishing species.
• Gene flow (or lack thereof) plays a significant role in maintaining or separating species.
• Geographic barriers can lead to the physical separation of populations (allopatry).
• Reproductive barriers, both pre- and postmating, prevent interbreeding.
• Phylogenetic history and unique derived traits are central to the PSC.
• The BSC emphasises the potential for interbreeding and reproductive isolation.
• The GLSC focuses on the independent evolutionary trajectory of metapopulations.
• Speciation involves barriers to reproduction.

Processes and Mechanisms in Speciation

• Geographic barriers prevent gene flow and physically separate populations (allopatry).
• Reproductive barriers are features of organisms that prevent interbreeding, even in sympatry (when populations are in the same geographic area).
• Speciation involves barriers to reproduction.
• Premating barriers occur before mating and include differences in the timing of reproduction (allochrony) and pollinator isolation.
• Postmating barriers occur after mating but before the formation of a viable offspring.
• Prezygotic postmating barriers include physical damage during hybridisation and gametic incompatibility.
• Postzygotic barriers result in hybrids with low fitness due to hybrid inviability, hybrid sterility, ecological inviability, and behavioural sterility.
• Genetic incompatibilities between gametes can prevent interbreeding.
• Genetic incompatibilities that reduce hybrid fitness can be important barriers to gene flow, arising from Bateson-Dobzhansky-Muller (B-D-M) incompatibilities, which are genetic incompatibilities of hybrids resulting from epistatic interactions at two or more loci.
• Allopatric speciation can occur through vicariance and dispersal.
• Vicariance: the physical splitting of a habitat, leading to two isolated populations.
• Dispersal: a population moves to a new habitat, colonises it, and founds a new, isolated population.
• Reinforcement is a process where selection favours prezygotic isolation mechanisms, preventing the formation of hybrids with reduced fitness when previously allopatric populations come back into sympatry.

Types of Reproductive Isolation

• Reproductive Barriers are features of organisms that prevent interbreeding, and are effective even when populations are in the same geographic area (sympatry).
• Premating Barriers (Occur before mating)
  • Timing of reproduction (Allochrony): Populations reproduce at different times.
  • Pollinator isolation: Different pollinators are attracted to different populations.
• Postmating Barriers (Occur after mating)
  • Prezygotic (before the formation of a zygote):
  • Physical damage: Hybridisation attempts can physically harm the female reproductive tract.
  • Gametic incompatibility: Sperm or pollen fails to fertilise the eggs of another species.
  • Postzygotic (after the formation of a hybrid zygote): Hybrids are produced but have low fitness.
  • Hybrid inviability: Hybrids do not survive.
  • Hybrid sterility: Hybrids survive but cannot reproduce.
  • Ecological inviability: Hybrids are not well-suited to the environment.
  • Behavioural sterility: Hybrids have incorrect mating behaviours.

Genetic Incompatibilities

• Genetic incompatibilities between gametes can prevent interbreeding.
• Genetic incompatibilities that reduce hybrid fitness (Bateson-Dobzhansky-Muller incompatibility) can act as barriers to gene flow.
• Genetic incompatibilities can act as barriers to gene flow, either between gametes or by reducing hybrid fitness.
• Bateson-Dobzhansky-Muller (B-D-M) incompatibility is a specific type of genetic incompatibility in hybrids, arising from epistatic interactions at two or more loci, resulting in hybrid inviability.
  • Monkeyflowers: studies uncovered two hybrid instability genes, Hms1 and Hms2, whose specific combinations from different species (guttatus and nasutus) lead to inviable offspring.
  • Swordfish: hybrid inviability is observed in Xiphophorus malinche and Xiphophorus birchmanni.

Mechanisms of Geographic Isolation and Allopatric Speciation

• Geographic barriers are extrinsic properties of the landscape that prevent gene flow, leading to allopatry.
• Allopatric speciation occurs when populations are geographically separated over time, reducing gene flow.
• Vicariance is one mechanism of allopatric speciation, involving the physical splitting of a habitat, leading to two isolated populations where genetic drift and selection act independently.
• Dispersal is another mechanism of allopatric speciation, involving a population moving to a new habitat, colonising it, and founding a new isolated population, where genetic drift and selection act.
• Islands provide opportunities for allopatric speciation following dispersal.
• Darwin's finches are an example of allopatric speciation by dispersal from Ecuador to the Galápagos Islands.