Have you ever tried to understand the naming and relationships of the numerous Citrus varieties available? It's not simple (some would say it's chaotic!) because of the usual problems with common & varietal names that can differ greatly round the world and can be assigned for all sorts of non-botanical reasons, and even with binomials it can be confusing because species in the genus hybridise so readily and some hybrids are incorrectly named as individual species. For example globally, sweet orange is the most commonly grown Citrus and is frequently referred to as C sinensis, but it's actually a complicated hybrid that was naturally formed from other progenitor species and introgressions eons ago, and further hybridisation has continued through to the present day with domestication and breeding. The binomial is not officially accepted.
A very thorough study published in Nature (GA Wu et al, (2018), 554, 311-316) has addressed the genomics and evolution of ten commonly known citrus types using genomic, phylogenetic and biogeographic analyses of 58 accessions representing diverse Citrus germplasms. Their data suggest the genus originated in south east Asia and began to diversify through the late Miocene (11-5 million years ago), with variously admixed genomes leading to the major commercial types we're familiar with. They found evidence of an extensive network of relatedness between mandarins and sweet oranges, indicating the role domestication has played with these two major crops. The picture summarises their findings.
At the top, the three pivotal progenitor species are shown - citron (C medica), mandarin (C reticulata) and pummelo (C maxima, with 2 haplotypes P1 and P2); the other two species playing lesser roles are nagami kumquat (Fortunella margarita) and C micrantha. For those not familiar with biology, the small black circles with an arrow or cross indicate males and females respectively involved in sexual reproduction. The blue lines with arrows show the direction of simple crosses between male and female parents, forming for example calamondin and Mexican lime. Lemons are derived from a male citron and female sour orange, which itself was formed from a mandarin pummelo cross. Type 2 mandarins are formed by a cross of pummelos and pure (type 1) mandarins, and these have variously crossed again with pummelos to produce sweet oranges. Somatic mutations (sports) of a common genome then underlie many modern sweet orange cultivars. As an example of the genetic content in mandarin pummelo crosses, type-2 and 3 mandarins have 1-10 and 12-38% pummelo genes respectively. The authors suggested that with the initial cross of mandarin and pummelo to produce type 2 mandarins, mandarin genes were successively diluted by repeated mandarin backcrosses. Pummelo parentage has played a major role in fruit size and acidity. For example regarding size, mandarins with an average fruit diameter of 4-7cm had pummelo genetic content of 5-20%, sweet oranges of 7-8cm dia had 45-50%, and grapefruit of 9-12cm had 60-65%. There was a similar effect with acidity, eg sour orange being more acid than type-1 mandarins. The study confirmed the appropriateness of Fortunella margarita now belonging in the Citrus clade (as C japonica); C micrantha is also now classed as a synonym of C hystrix.