Speciation

Species

Population(s) that can interbreed with members of the same group in nature and produce viable, fertile offspring

Macroevolution

Evolutionary change at or above the species level

• Origin of new traits, such as feathers in dinosaurs

• Divergence of new clades

• Catastrophic events (e.g., mass extinctions)

Microevolution

Evolutionary change at or below the species level, characterised by changes in the allele frequencies in a population over generations

Zygote

A fertilized egg from the union of reproductive cells

Hybrids

Offspring from the mating of two different species

Biological Species Concept

Populations that can interbreed or have the potential to interbreed with members of the same group in nature, and produce viable, fertile offsprings

Reproductive Barrier

The existence of biological factors that impede members of two species from producing viable, fertile offspring

Prezygotic

A reproductive barrier that impedes mating between species or hinders fertilization if interspecific mating is attempted

Habitat Isolation

Two species that live in different habitats within the same location encounter each other rarely or not at all

Temporal Isolation

Two species that breed at different times, leading to rare or no encounters

• Migratory species arrive for mating at different time

• Nocturnal species vs diurnal species

Behavioural Isolation

Species use unique courtship rituals and other behaviours to attract mates

Mechanical Isolation

Anatomical differences prevent successful mating

Gametic Isolation

Sperms of one species cannot fertilize the eggs of another species

Types of Prezygotic Barriers

• Habitat Isolation

• Temporal Isolation

• Behavioural Isolation

• Mechanical Isolation

• Gametic Isolation

Types of Postzygotic Barriers

• Reduced Hybrid Viability

• Reduced Hybrid Fertility

• Hybrid Breakdown

Postzygotic Barrier

A reproductive barrier that prevents hybrid zygotes produced by two different species from developing into viable, fertile adults

Reduced Hybrid Viability

Genes from parents of different species may interact to impair the hybrid’s development or survival

• Most hybrid animals do not complete embryonic development

  • The observable hybrids are typically frail and would most likely be eliminated by natural selection

Reduced Hybrid Fertility

Hybrids may be sterile and could not reproduce further, even if the hybrids can survive

Hybrid Breakdown

Reproductive failure that appears after the F2 generation of crosses between different species

• Possibly occurs due to the accumulation of deleterious genetic traits (ex. alleles, hybridization across strains of rice)

Limitation of BSC

• Cannot be applied to asexually reproducing organisms (e.g., prokaryotic bacteria)

• Cannot be examined in most fossil species

• Strong emphasis on the absence of gene flow between species

  • BSC implies that different species would not mate and produce surviving offsprings

  • Surviving hybrids do exist in nature

Morphological Species Concept

Defines species in terms of measurable anatomical features

• Observable and measurable features of a given species are distinct from other species

• Applicable to both extant and extinct organisms

• Applicable to both sexually and asexually reproducing organisms

Ecological Species Concept

Defines species by ecological niches, which encompass how organisms of the same species interact with biological and abiotic aspects of their environment

• Individual of the same species share similar preferences/tolerances to habitat conditions

• Species are grouped/separated by partitioning of ecological niches

• Applicable to both sexually and asexually reproducing organisms

Phylogenetic Species Concept

Defines a species as the smallest group of individuals on a phylogeny

• This definition is based on evolutionary history and shared ancestry

• A species can be one of the tips/terminal taxa on a phylogeny

• Applicable to both sexually and asexually reproducing organisms

• Delineating phylogenetic species is difficult:

  • Phylogenies are updated frequently

  • Tips can be expanded into a monophyletic group

Speciation

An evolutionary process in which one species splits into two or more species

• Darwin envisioned speciation as branching events descending from common ancestors

Allopatric Speciation

The formation of new species in populations that are geographically isolated from one another, and gene flow is interrupted

• Supporting evidence: number of species is higher in regions with many geographic barriers

• Whether a geographic construct is geographic barrier depends on how the population migrates

• Most speciation is thought to be allopatric, involving two geographically separated populations

Dispersal

The movement of individuals away from the rest of the population

• This movement sometimes expands the geographic range of a population or species

Vicariance

The range of a species is split by a change in the environment, creating two subpopulations

• Subpopulations may be subjected to different selective agents and pressures

Types of Allopatric Speciation

• Dispersal

• Vicariance

Sympatric Speciation

The formation of new species in populations that live in the same geographic area

• A geographic barrier is absent, but some individuals stop interbreeding with others in the population (i.e., stopping gene flow on a local level)

• New species are established only when reproductive isolation is fully established

Types of Sympatric Speciation

• Habitat Differentiation

• Sexual Selection

Habitat Differentiation

Subpopulations of the same species adapt to different environments, leading to reproductive isolation

• Reproductive barriers can be either prezygotic or postzygotic

Sexual Selection

Individuals with certain inherited characteristics are more likely than others to obtain mates

• This selection for traits leads to increased mating

• Can lead to sexual dimorphism

• Traits can be deleterious to survival of the individuals

• Sexual selection can be seen as a form of natural selection for mating success

• Sexual selection can drive sympatric speciation

Speciation and Natural Selection

Speciation can occur with and without natural selection

• Accumulation of genotypic difference leads to speciation

• Natural selection acts on traits of the new species

• New species deemed unfit would be removed

Hybrid Zones

A geographic region in which members of different species meet and mate, producing at least some offspring of mixed ancestry

• Hybrids are often less fit, but can sometimes lead to speciation

• A hybrid zone can be partial overlap in the ranges of two species

• Three possible outcomes: reinforcement, fusion, and stability

Hybrid Zones Reinforcement

Reproductive barriers strengthen due to hybrid offspring being less fit than the parents, slow being removed by natural selection

Hybrid Zones Fusion

Reproductive barriers weaken due to hybrid offspring being as fit as parents, allowing gene flow to be maintained between hybrids and parent populations

Stability

Production of hybrid individuals remains stable