Speciation

Introduction to Speciation

• Speciation: the process in which biological lineages diverge due to reproductive isolation, resulting in new species
  • AKA macroevolution

Defining a Species:

• Species Concepts: proposals for how to define a species
• Morphological species concept:
  • Defines a species based on observable morphological characteristics
  • Ex: body shape, size, color, and other structural features
  • Carolus Linnaeus
  • Two organisms are a species if they look alike
  • Problems:
  • Members of the same species may not look alike: sexual dimorphism
  • Members of different species may look similar: cryptic species look alike but do not interbreed
• Biological species concept:
  • Defines a species as groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups
  • Ernst Mayr
  • Two species are the same species if they can mate with one another
  • Reproductive isolation: a situation in which two groups of organisms are capable of exchanging genes
  • Problems:
  • The definition does not apply to asexual organisms, like prokaryotes
  • The definition cannot be applied to extinct species, as fossils do not tell us about sexual isolation of animals
  • Many species hybridize, like mules
• Lineage species concept:
  • Defines a species as two groups that share a branch on the tree of life
  • Relies on molecular similarity and phylogeny
  • Advantage: accommodates asexual organisms and hybridization
  • Disadvantage: may classify organisms as two different species based on unimportant genetic differences

The Role of Reproductive Isolation:

• Reproductive isolation is the most important factor in speciation

• Speciation requires interruption in gene flow between two groups

• Ways gene flow can be disrupted:

  • Behavioral - assortative mating
  • Geographical - habitat selection
  • Genetic
  • Physiological barriers

• Dobzhansky-Muller Method: simple model that explains how a single lineage can split into two reproductively isolated species

  • Explains role of mutation on speciation
  • Steps:
  1. Something causes two groups to become reproductively isolated
  2. Over evolutionary time, a different mutation may occur at a different loci in each group
  3. If these mutations convey an advantage, they will become fixed in the population over time
  4. The mutant allele from one group may then be incompatible with the mutant allele in the other group, preventing the groups from interbreeding
  • Applies to major chromosomal rearrangements and point mutations

The Relationship Between Genetic Divergence and Reproductive Isolation:

• Reproductive isolation tends to increase as species diverge genetically
• Two groups of organisms are more likely to be classified as separate species if their DNA is similar
• Can develop quickly over a course of a single generation or can develop over the course of millions of years
• Genetically distant pairs = high reproductive isolation
• Genetically similar pairs = low reproductive isolation
• Yet, it is still possible to have high levels of reproductive isolation even without much genetic divergence

Incomplete Hybridization and Hybrid Zones:

• Hybrid zones: regions where populations of two closely related species overlap, such that they can hybridize
• Existence of hybrid zones indicates that reproductive isolation between two populations is incomplete
• Usually very narrow, bc strong selective pressure against hybrids
  • Natural selection favors mechanisms that prevent species from creating a hybrid in the first place

Allopatric vs. Sympatric Speciation:

• Speciation requires the isolation of gene flow (migration)
• 2 ways that can happen: allopatric or sympatric

Allopatric Speciation:

• Allopatric speciation: occurs when a population is divided by a physical or geographic barrier, forming two reproductively isolated populations that evolve independently
• Major form of speciation in most organisms (especially animals)
• Examples: climate change, glacial advance and retreat, a change in sea levels, continental drift
• Allopatric speciation can also result from founder events: cases in which individuals from one population colonize another region
  • Drosophila flies: Hawaii is home to more than 800 species of Drosophila flies, due to around 45 founder events
  • Galapagos finches: Finches on each island developed specific characteristics as a result of different environmental conditions from island to island
• Examples of adaptive radiations, which occur when a single species undergoes multiple speciation events so that each newly formed species is able to occupy a specific niche in the environment

Sympatric Speciation:

• Sympatric speciation: occurs when species arise from a population that remains connected
• More common in plants than animals
• 3 major causes of sympatric speciation: disruptive selection, assortative mating, and polyploidy
• Disruptive selection: favors two extreme phenotypes and can result in divergence between two species
• Assortative mating: assortative mating (nonrandom mating) could also result in disruptive selection
• Polyploidy: occurs when a mistake during cell division results in an individual with cells that have more than one copy of the genome (viable = can live and reproduce, but genetically isolated = cannot reproduce with normal diploid individuals)
  • In plants, they are able to self fertilize, create offspring and thus propagate a new species

Reproductive Isolation:

• Incipient species: refer to species that are in the process of diverging into separate species, but that can currently interbreed
• When they come back to reproduce (hybridization) two possibilities:
  • Reinforcement: hybrids may be less reproductively fit, hybrid offspring may be weak or sterile
  • Natural selection will favor parents who mate with their own species over parents who hybridize
  • Known as post-zygotic reproductive isolation
  • Merger: sometimes, hybrids are no less fit than offspring of non-hybridizing parents
  • Populations may interbreed to a point that the formerly separate species become a single species again
• Reproductive barriers:
  • Prezygotic isolating mechanisms: prevent hybridization in the first place
  • Postzygotic isolating mechanisms: operate after zygote development and reduce the fitness of the hybrid offspring, cause selection against hybridization, leading to reinforcement of prezygotic isolating mechanisms

Prezygotic Isolating Mechanisms:

• Prezygotic isolating mechanisms: reproductive barriers that prevent mating or fertilization between two individuals
  • Mechanical isolation: some animals are simply not physically compatible with other animals due to anatomical differences
  • Temporal isolation: some organisms are isolated such that they never have the opportunity to reproduce with one another due to timing of reproduction
  • Behavioral isolation: some animals have mating rituals that must occur before mating, and pairing of mates does not occur unless the correct mating rituals or signals are performed
  • Habitat isolation: some animals are isolated by space such that they never have the opportunity to come in contact with one another
  • Gametic isolation: some animals produce sperm and egg cells that will simply not fuse when they come into contact

Postzygotic Isolating Mechanisms:

• Postzygotic isolating mechanisms: reproductive barriers that prevent interbreeding after the development of the zygote and reduce fitness of hybrid offspring
  • Low hybrid zygote viability: Sometimes, an offspring is created but is not viable and die soon after fertilization
  • Low hybrid adult viability: when the offspring mate with one another or with either parent species, offspring in the next generation become feeble or sterile
  • Hybrid infertility: form offspring that can live but not reproduce

Natural Selection and Reproductive Isolation:

• Natural selection favors prezygotic rather than postzygotic
  • A lot more efficient to prevent reproduction from occuring in the first place through prezygotic
• Populations living together (sympatry) tend to evolve more effective prezygotic reproductive barriers than populations living apart (allopatry)