Editing Solidago (section)

===Evolutionary relationships within ''Solidago''===
Chromosome counts and advances in molecular systematics have enabled greater understanding of evolutionary relationships within ''Solidago''. At the time a taxonomy of ''Solidago'' was published,<ref name="Phytologia-1994" /> related taxa causing contention, such as ''Chrysoma'', ''Euthamia'', ''Oligoneuron'', and ''Petradoria'', were excluded from this genus. The number of ''Solidago'' species has remained relatively stable, around 120, with approximately 80 in North America.<ref name="Semple-2016" /><ref name="Phytologia-1994" /> Due to monophyletic support for the New World taxa<ref name="Beck-2004" /><ref name="Hood-2003" /> and taxonomic difficulties with Old World taxa, the taxonomy provided in the 1990s<ref name="Phytologia-1994" /> only includes North American taxa and thus treats ''Solidago'' as non-monophyletic. Existing molecular-based phylogenies provide monophyletic support for ''Solidago''<ref name="Schilling-2008" /><ref name="Beck-2004" /><ref name="Semple-2016" /><ref name="Zhang-1996" /> given its inclusion of ''Oligoneuron''.

Chromosome counts have proven to be a valuable character in ''Solidago'' taxonomy and in elucidating the cytogeographic history of the genus. Similar chromosome counts may indicate close evolutionary relationships, while different chromosome numbers may suggest distant relationships through reproductive isolation. Chromosome counts have been studied extensively in North America;<ref>{{Cite journal|last1=Cook|first1=Rachel E.|last2=Semple|first2=John C.|date=2008-11-13|title=Cytogeography of ''Solidago'' subsect. ''Glomeruliflorae'' (Asteraceae: Astereae)|journal=Botany|volume=86|issue=12|pages=1488–1496|doi=10.1139/B08-087|issn=1916-2790}}</ref><ref>{{Cite journal|last1=Semple|first1=John|last2=Watanabe|first2=Kuniaki|date=2013-03-02|title=A Review of Chromosome Numbers in Asteraceae with Hypotheses on Chromosomal Base Number Evolution|url=https://www.researchgate.net/publication/48460015}}</ref> all ''Solidago'' species have a base chromosome number of x=9, but the following ploidy levels have been observed: 2x, 3x, 4x, 6x, 8x, 10x, 12x, and 14x.

Though negligible differences in karyotype among ''Solidago'' and related genera were found,<ref name="Kapoor-1966" /> ''Solidago'' taxa with multiple cytotypes are more common than those with one.<ref name="Semple-2016" /> Although chromosome count is a useful metric for differentiating among ''Solidago'' taxa, it may be problematic due to the frequent variation in ploidy levels. Cytogeographic patterns in the ''Solidago gigantea'' complex, with tetraploids occurring in eastern North America and hexaploids in Oregon and Washington, were observed.<ref>{{Cite journal|last1=Semple|first1=John C.|last2=Ringius|first2=Gordon S.|last3=Leeder|first3=Colleen|last4=Morton|first4=Gary|date=1984-07-01|title=Chromosome Numbers of Goldenrods, ''Euthamia'' and ''Solidago'' (Compositae: Astereae). II. Additional Counts with Comments on Cytogeography|journal=Brittonia|language=en|volume=36|issue=3|pages=280–292|doi=10.2307/2806528|issn=0007-196X|jstor=2806528|bibcode=1984Britt..36..280S |s2cid=186241866}}</ref> Cytogeographic patterns are also observed in the ''[[Solidago canadensis]]'' complex: hexaploids within ''S. canadensis'' have been observed east of the Great Plains and are treated as ''[[Solidago altissima]]'', and diploids and tetraploids occurring in the Great Plains are treated as ''Solidago gilvocanescens''. The taxonomic status of ''Solidago ptarmicoides'' created an extensive debate due to frequency hybridization of ''S. ptarmicoides'' with members of the ''Ptarmicoidei'' section of ''Solidago''.<ref name=fna/> It was asserted that ''S. ptarmicoides'' should be united with ''Solidago'' rather than the genus ''Aster'' due to external morphological features such as similar pappus length as well as the same chromosome base (x=9). Information about chromosome number is still a crucial part of current understanding and phylogenies of ''Solidago''.<ref name="Semple-2016" />