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TableĀ 2 Full key speciation concepts and short codes

From: A community-informed list of key speciation concepts for undergraduate education

Short code Full key concept statements
SC1 a A species is a population or group of populations that experiences evolutionary processes independently from other populations. Biologists use species concepts to draw boundaries between species, and develop tests of hypothesized species boundaries. No species concept is appropriate for all biological contexts and biologists often rely on more than one species concept to confidently determine species boundaries
SC2 The criterion for determining species boundaries under the biological species concept is reproductive isolation. A species is composed of populations whose members are capable of mating and producing viable and fertile offspring, or would do so if they came into contact. Members from different species could not mate freely or could not produce viable and fertile offspring
SC3b The criterion for determining species boundaries under the phylogenetic species concept is a unique combination of shared, derived character states. A species is composed of the smallest monophyletic group that shares a unique combination of derived characters that is different from all other populations
SC4 The criterion for determining species boundaries under the phenetic species concept is degree of phenotypic similarity, including morphological, physiological, or behavioral similarity. A species is composed of populations with shared phenotypes
SM1 a Ultimately, speciation results from lack of gene flow between populations. When barriers to gene flow exist, populations will begin to diverge genetically because they independently experience mutation, selection, and genetic drift
SM2 Lack of gene flow commonly occurs when populations are geographically isolated. This can happen when a large population is subdivided into two or more isolated populations because of the development of geographical barriers OR when one or more small populations are geographically separated from the main large population
SM3 Lack of gene flow can also result from chromosomal mutations. One example of this is mutations resulting in a change in number of chromosome sets. A daughter population that has four copies of chromosomes (tetraploid) generally cannot reproduce with a diploid parent population, creating an immediate barrier to gene flow. Another example is gene inversions, which are mutations that invert a section DNA within a chromosome. This change prevents recombination and thus acts as a barrier to gene flow
SM4 Genetic divergence can result from natural selection based on the environment, including differences in resources, habitat, and interactions with other species. This is called ecological speciation and it can occur when populations are geographically isolated and when they are not geographically isolated
SM5 Sexual selection can also contribute to genetic divergence. This can occur when changes in mate preference and secondary sexual traits within a population lead to assortative mating, such that individuals prefer to mate with other individuals of the same population
SM6 Genetic divergence between populations tends to increase as a result of genetic drift. The impact of drift on genetic divergence will be greater in smaller populations and populations that experience less gene flow
RB1 Reproductive barriers are biological features of organisms that prevent species from interbreeding or from producing viable and fertile offspring
RB2 Prezygotic reproductive barriers prevent gametes from meeting to form zygotes
RB2.1 Behavioral isolation is when differences in behavior prevent individuals of one species from mating with individuals of another species
RB2.2 Ecological isolation is when differences in habitat and resource use prevent individuals of one species from mating with individuals of another species. Examples of ecological isolation include differences in timing and location of breeding
RB2.3 Mechanical isolation is when differences in reproductive structures prevent the successful mating of different species
RB2.4 Gametic isolation is when transferred gametes of one species are unable to fertilize eggs of individuals of another species
RB3 Postzygotic reproductive barriers reduce the fitness of hybrids
RB3.1 Intrinsic postzygotic isolation is when hybrids exhibit biological problems that prevent them from producing offspring or reduce the viability or fertility of their offspring. These problems are independent of the environment. One example is incompatible interactions between genes inherited from parent populations
RB3.2 Extrinsic postzygotic isolation is when hybrids have lower fitness as a result of interactions between their biological features and the environment. One example is hybrids not surviving as well because they are not adapted to forage in the available habitats
OH1 Secondary contact occurs when populations that have been geographically separated come back into contact with each other. Hybridization occurs when individuals from these formerly separated populations mate and produce offspring. Possible evolutionary outcomes of hybridization are hybrid zones and hybrid swarms, hybrid speciation, and reinforcement
OH2 Hybrid zones are geographical areas in which individuals from distinct populations or species mate and produce hybrid offspring. Hybridization can lead to introgression, in which alleles from one population are incorporated into the gene pool of another population
OH3 If populations of hybrids survive beyond the first hybrid generation and continue to interbreed with parental populations, then these hybrid populations are called hybrid swarms. Hybrid swarms are often highly variable phenotypically and genetically due to gene flow between parental populations. A hybrid swarm can lead to the loss of distinguishable species, which is known as species collapse. This process can be exacerbated by human influences on habitats, including the introduction of invasive species and climate change
OH4 Hybrid speciation may occur if hybrid offspring have higher fitness in a habitat that is different from the habitat(s) of the two parent populations, and they are reproductively isolated from both parent populations
OH5 Reinforcement is natural selection that favors individuals who preferentially mate with individuals from the same population. This mating preference increases reproductive success when hybrid individuals have lower fitness than individuals from the parental populations. Reinforcement reduces the frequency of hybrids
  1. Italics indicate overarching concepts
  2. SC, speciation concept; SM, speciation mechanism; RB, reproductive barriers; OH, outcome of hybridization
  3. aSC1 and SM1 are both versions of the statements from Survey 2. All other statements are from Survey 3
  4. bSC3 was generated based on feedback from experts. The final version was not evaluated by experts