Although referred to in this study as "ecotypes", the exact point along the continuum of divergence (from differentiated populations to biological species) remains unknown for S. hystrix, and it is unclear whether additional isolating mechanisms beyond habitat isolation have evolved.
Speciation is often considered a continuum of divergence ranging from continuous variation among individuals within one species to complete discontinuity among groups in genetic, physiological and phenotypic adaptations to specific environments.
Interestingly, the difference between E2F1 + gMCPH1 and E2F1 + mMCPH1 is significant (P <0.05, ANOVA test), suggesting continuum of functional divergence of MCPH1 during primate evolution with a major shift in function between Old World monkeys and apes and a probable further shift during human evolution.
In addition, the gradient change of MCPH1's transcription regulation from macaque to gibbon, and to humans seems to imply a continuum of functional divergence rather than a number of discrete shifts, which calls for further functional tests in extensive primate lineages.
This lack of sharing suggests that the two lineages are at an intermediate stage of divergence along the continuum from complete panmixia to paraphyly and ultimately to reciprocal monophyly [ 106, 107].
This phylogenetic reconstruction demonstrates that the hybridizing color pattern races and incipient species sampled in this study reflect distinct points of divergence along the speciation continuum.
The evolution of species boundaries and the relative impact of selection and gene flow on genomic divergence are best studied in populations and species pairs exhibiting various levels of divergence along the speciation continuum.
New genomic technologies enable the study of high-resolution genomic patterns of divergence across natural speciation continua, where taxa pairs with different levels of reproductive isolation can be used as proxies for different stages of speciation.
Heliconius butterflies offer an exceptional opportunity to explore patterns of divergence at different stages across the speciation continuum.
The degree of divergence among taxa can be quantified along this continuum, typically by estimating for example; levels of reproductive isolation, genetic dissimilarities or the amount of lineage sorting among groups [ 5- 7].
There is a continuum of both sequence and amino acid divergence between definite heteroplasmy (e.g., H. fuscipennis) and definite numts (e.g., H. rugulosus), and due to the high rate of change in mtDNA relative to nuclear DNA, even unusual amino acid changes are not reliable markers- e.g., the coding sequence of H. paradoxicus contains more unique differences than does the numt.
