Chapter 16: Mechanism of Speciation

Introduction

• Species: the smallest evolutionary independent unit that is isolated by lack of gene flow
• Evolutionary forces: mutations, selection, gene flow (migrations), drift

Species Concepts

Morphospecies Concept (MSC)

• Morphospecies Concept: 2 populations are treated as different species if they are discontinuously separate in morphology, meaning that the border between the species is marked by the absence of some combination of characteristics (discrete/continuously variable traits)

• Advantages of MSC: based only on the pattern (no taxon-specific processes), likely to work for a variety of organisms.

• Disadvantages of MSC: Without additional criteria, the MSC cannot handle different morphologies during the life cycle of the same species or polymorphism and cryptic species

  • Cryptic species: species that are indistinguishable morphologically, but different in other traits

Biological Species Concept (BSC)

• Proposed by Earnst Mayr in 1942

• Biological Species Concept: “groups of interbreeding populations that are reproductively isolated from other such groups.”

• Criterion for identifying evolutionary independence → Reproductive Isolation

• Strength of BSC: Confirms lack of gene flow (biologically intuitive)

• BSC Species: potential to interbreed and produce fertile offspring determines species membership

• BSC limitations: Knowledge about reproductive isolation in nature is hard to achieve for the majority of organisms, how do we deal with fossils, assumes sexual process when most taxa are asexual and difficult to apply to plants because many divergent populations can hybridize

Phylogenetic Species Concept (PSC)

• Phylogenetic Species Concept: the fundamental grouping criteria is monophyly
• Monophyletic Group: a taxon that contains ALL descendants of a common ancestor originating in a single event.
• PSC Species: the smallest monophyletic group of common ancestry
• To be applied to separate phylogenetic species, synapomorphy must have occurred
• PSC Advantages: can be applied to sexual and asexual organisms, can be applied to fossils, clear evolutionary perspective, testable
• PSC Disadvantages: Time, money, careful analysis, will species change when phylogenetic hypothesizes change, how well do we know phylogeny, species, and genetics are not always correct

3 Stages of Speciation

Introduction

1. Genetic Isolation

   1. speciation begins when gene flow is disrupted and the population becomes genetically isolated

2. Divergence in Traits

   1. Mating system
   2. habitat use

3. Produce Reproductive Isolation

   1. Secondary contact: two populations that have diverged in isolation from a common ancestor are reunited geographically

Step 1: Physical Isolation as a barrier to Gene Flow

• Geographical speciation is called allopatric because the diverging populations are found in separate (more or less discontinuous) areas
• When populations are isolated geographically, evolutionary processes (such as mutation, selection, etc) may lead to speciation, given sufficient time
• Allopatric model: speciation occurs when populations become geographically isolated and diverge because speciation acts on them independently
• There are two types of allopatric speciation: Dispersal and Vicariance

Dispersal

• Dispersal of individuals across geographical, environmental, or ecological barriers
• Dispersal and Colonization of Hawaiian Flies
  • ~1000 species in 2 genera with high diversity in habitats, food sources, elaborate traits in combat as well as courtship displays
  • Dispersal and colonization hypothesis: speciation events begin when small populations colonize new geographical areas
  • It predicts that closely related species should be found in adjacent islands and that some sequences of branching events should correspond to the order of the island formation
  • DeSalle and Giddings (1986) and Bonacum et al. (2005) tested the mitochondrial DNA of 5 closely related species which are consistent with predictions.

Vicariance

• Vicariance: the formation of barriers due to environmental changes
• Knowlton et al (1943) Snapping Shrimp Isthmus of Panama Study
  • The Isthmus of Panama closure occurred about 3 million years ago
  • Compare Marine populations on the Atlantic and Pacific sides
  • Snapping Shrimp hunt by paralyzing prey with shock waves of imploding gas bubbles
  • Morphological Species → 7 pairs of closely related species pair with one member of each pair found on each side
  • Sister species on the Pacific side and Caribbean side
    • Sister Species: the species diverged from the same ancestral node on a phylogenetic tree
  • Put males and females from opposite sides together
  • They snapped aggressively instead of courting
  • Almost none of the pairs that formed during the courtship experiment produced fertile offspring
  • In conclusion, the Pacific and Caribbean Shrimp populations are separate species, based on the 3 species concepts

Role of Mutation

• Mutation can create polyploidy and other chromosome changes as a barrier to gene flow
• Sympatric Speciation: the formation of 2 or more descendants species from a single ancestral species all occupying the same geographical location
  • Possibilities → Spontaneous speciation with polyploidy
  • Polyploidy is common in angiosperms (50-70%) and ferns/allies (95%), frequent in bryophytes and algae, and rare in fungi and gymnosperms

Step 2: Divergence

• Mechanism of Divergence: genetic drift, natural selection, sexual selection

Drift

• Drift can produce rapid genetic divergence in small isolated populations
  • Nonrandom sample of the ancestral population
  • drift causes random loss or fixation of alleles
  • divergence from the ancestral population
• Bottleneck Effect: A large-scale but short-term reduction in population size followed by an increase in population size

Natural Selection

• Divergence due to natural selection on food or habitat
• Apple introduced to North America by European immigrants, the Native hawthorn maggot fly quickly became an orchid pest.
  • Some of hawthorn flies ate their normal food while some switched over to the apples (apple flies).
  • Genetic markers indicate divergence between the apple flies and hawthorn flies
  • Only 6% of observed matings are between apple flies and hawthorn flies
  • So we’re on the way to speciation but not quite
  • host fidelity

Sexual Selection

• Sexual selection: divergence due to differences on how females choose mates, how males attract females, and how male compete for females.

Step 3: Secondary Contact

• After genetic isolation and divergence, the recently diverge populations may come back in contact and have the opportunity to interbreed - hybridization
• Hybridization is not uncommon in plants and birds
• Fate of hybrids
  • Hybrids interbreed with parental populations, erase the divergence
  • hybridization creates a new species
  • hybrids have reduced fitness

Reinforcement

• Dobzhansky (1940) “Occurance of hybridization between races and species constitutes a challenge to which they may respond by developing or strengthening isolation mechanisms that would make hybridization difficult or impossible
• Hybrids have reduced fitness
• Reinforcement: Natural selection that results in assortative mating in recently diverged populations in secondary contact; it finalizes the speciation process by producing complete reproductive isolation
  • Assortative mating: occurs when individuals tend to mate with other individuals with the same genotype or phenotype
• Reinforcement prevents hybridization when second contact occurs
• There are two types of reproductive isolation
  • Prezygotic
  • Postzygotic

Prezygotic Isolation

• Prezygotic Isolation: reproductive isolation between population caused by difference in mate choice or timing of breeding, so no hybrid zygotes were formed
  • Habitat isolation: populations of concern occur in different habitats in the same geographical region
  • Seasonal/Temporal: Mating or flowering times occur at different time zones
  • Sexual/ethological: Mutual attraction between the seces of different species is weak or absent
  • Mechanical: physical attributes of animal genetalia are specific to each species or flower parts prevent transfer of pollen
  • Isolation by different Pollinators: In flowering plants, related species may be specialized to attract different insect as pollinators
  • Gametic: Incompatibility of male and female gametes preventing the formation of hybrid zygote
  • ex: human fertility issues

Postzygotic Isolation

• Postzygotic isolation: reproductive isolation between populations caused by dysfunctional development, sterility, or decreased fitness in hybrid forms
  • Hybrid inviability: hybrid zygotes have lower viability or are inviable
  • Hybrid Sterility: The F1 hybrids of 1 sex or both sexes fail to produce functional gametes
  • Hybrid Breakdown: The F2 backcross hybrids have lower viability or fertility

When the Hybirds turn out well → Hybridization

• When hybrids survive and reproduce well…
  • the formation of new species contact and gene flow
• Hybrid Zone: a geographical region where interbreeding between divergent population occurs and hybrid offspring are frequent