Origin of Species I - Easy

• A species represents the smallest independently evolving unit, though there is constant disagreement on its definition
• Definitions for extinct species are often based on whether they hybridized and changes in morphological features
• Definitions for extant species consider morphology, behavior, geography, and dispersal tendencies

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
• The PSC 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 such groups
• The BSC 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 of Speciation

• Speciation involves barriers to reproduction
• Geographic barriers prevent gene flow and physically separate populations (allopatry)
• Reproductive barriers are features of organisms that prevent interbreeding, even in sympatry

Reproductive Barriers

• 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
• This can arise 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

• 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
• Islands can provide opportunities for allopatric speciation following dispersal
• 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
• This often occurs through reproductive character displacement, where traits involved in mate recognition diverge more in the zone of overlap

Types of Reproductive Isolation

• Reproductive barriers are features of organisms that prevent interbreeding
• They are effective even when populations are in the same geographic area (sympatry)

Premating Barriers

• Timing of reproduction (Allochrony): Populations reproduce at different times
• Pollinator isolation: Different pollinators are attracted to different populations (e.g., monkeyflowers pollinated by bees vs. hummingbirds)

Postmating Barriers

• 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

Genetic Incompatibilities

• Genetic incompatibilities result in
• 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 between gametes can prevent interbreeding
• Genetic incompatibilities that reduce hybrid fitness (Bateson-Dobzhansky-Muller incompatibility) can act as barriers to gene flow

Hybrid Incompatibilities and Genetic Differences

• Genetic incompatibilities can act as barriers to gene flow
• These incompatibilities can occur between gametes, preventing interbreeding
• They can also reduce hybrid fitness, acting as important barriers
• It arises from epistatic interactions at two or more loci
• It can result in hybrid inviability (hybrids do not survive)

Examples of B-D-M incompatibility

• Monkeyflowers: studies uncovered two hybrid instability genes (Hms1 and Hms2), whose specific combinations from different species (guttatus and nasutus) lead to inviable offspring
• Swordfish (Xiphophorus malinche and Xiphophorus birchmanni): hybrid inviability is observed

Mechanisms of Geographic Isolation

• Geographic barriers are extrinsic properties of the landscape that prevent gene flow
• These barriers lead to allopatry, where populations are physically separated
• Allopatric speciation occurs when populations are geographically separated over time, reducing gene flow
• Vicariance involves the physical splitting of a habitat
• This chance separation leads to two isolated populations, and genetic drift and selection act independently on these isolated populations
• Dispersal involves a population moving to a new habitat, colonising it, and founding a new isolated population
• Genetic drift and selection act on the new, isolated population
• 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