Species Concepts + Mechanisms of Speciation

phenetic species concept (morphospecies)

identify species using morphological similarities (only use when you can’t use other categorization methods)

advantage of morphospecies concept

widely applicable to living or extinct organisms and sexual or asexual organisms

disadvantages of morphospecies concept

• Lack of consistency (dependent on judgement of investigator) 

• Difficult to apply to organisms lacking measurable morphological characters

• Problematic for species with sexual dimorphism 

• Organisms that vary according to environment (phenotypic plasticity) may be misidentified

biological species concept

species are groups of interbreeding natural populations that are reproductively isolated from other groups; must be caused by genetic different, not solely geographic

• species are populations that do not breed with each other

not a species if the organisms do not mate regularly in nature or matings result in infertile offspring (ex. liger)

advantages of biological species concept

objective test of reproductive isolation

disadvantages of biological species concept

• Only relevant for living (no fossils) and sexually reproducing organisms 

• Can be difficult to apply (e.g. could two populations mate if the geographic barrier was removed?)

phylogenetic species concept

species identification is based on monophylyl (aka lineages descended from a common ancestor) → basically species are descended from a common ancestor

• idea is that monophyletic groups possess a number of unique traits (synapomorphies) that are only likely to exist if reproductive isolation occurred

advantages of phylogenetic species concept

• Widely applicable (sexual/asexual and living/extinct) 

• Relatively objective, species are named based on statistically significant differences in the traits

disadvantages of phylogenetic species concept

• To some extent classification is arbitrary 

• Likely to result in a large number of new species and require substantial revision of traditional classification schemes

what are models of speciation?

1. allopatric

2. parapatric

3. sympatric

allopatric speciation

the formation of new species when a population becomes geographically isolated from the parent species and evolves independently over time

• Speciation occurs when populations become geographically isolated and diverge because selection and drift act on them independently

parapatric speciation

• A selective gradient (with different selective pressures) acts on a population 

• new species arises from a continuously distributed population without a geographical barrier, but with varying selective pressures across the population's range

sympatric speciation

• Genetic differentiation happens in a population while they are not geographically isolated

• Error is you think all spaces are the same for everyone → different niches occupy within the space

overview of speciation

You have an interbreeding population (single species) → they get isolated from each other → they diverge from each other → reproductive isolation occurs and they can no longer reproduce with each other

Hawaiian Drosophila (dispersal example)

• Hawaiian islands are diverse (variety of habitats, food sources, and morphological traits) 

• Radiation hypothesized to occur through dispersal, isolation, and subsequent divergence 

• Islands different ages 

• Predictions of dispersal and colonization 

  • Closely related species are found on adjacent islands

  • Branching events correspond to the order of island formation

• an example of prezygotic isolation

vicariance

one habitat that gets split; splitting of a population’s former range into two or more isolated patches (genetic isolation)

• Example: rise of mountain range, formation of a river, continental drift, habitat fragmentation, lava from volcanic eruption

Formation of Panama Isthmus (vicariance example)

• Separated population of marine organisms on Atlantic and Pacific sides

• Based on phenetic (morphospecies) species concept identified six species pairs → in each case one of the species was found on the Atlantic side and the other side on the Pacific side 

• Attempts to mate various pairs were unsuccessful and phylogeny confirmed the species pairs are each other’s closest relative

when can you expect natural selection to influence speciation?

when one population inhabits a novel environment or exploits a novel resource

Rhagoletis flies vs. Hawthorn

• Apple trees intorduced and Rhagoletis parasitizing apple trees mid 1800s

• Flies have food preference for apple or hawthorn and these preferences are heritable and mating is nonrandom because it occurs on the fruit 

• Apples and hawthorns ripen at different times so flies experience different temperatures during pupation → this is what caused divergence of Rhagoletis 

  • Apple and Hawthorn fly populations are considered incipient species (clear divergence, but incomplete isolation due to gene flow)

reinforcement theory

hybrids from diverged populations should have lower fitness

• results in strong selection for assortative mating and finalization of speciation

• predicts secondary contact results in mechanisms to prevent hybridization

Prezygotic isolation

reproductive isolation between populations caused by differences in mate choice or timing of breeding, so that no hybrid zygotes are formed

Postzygotic isolation

reproductive isolation caused by dysfunctional development or sterility in hybrid forms

hybrid zone

a geographic region where differentiated populations interbreed (and hybrids are frequent)