From these verses, we find that David descends from Ram. One opinion in Sotah (11b) says that David comes from Miryam [and Kalev, Ram's brother - Divrei ha'Yamim I, 29]!
Sechel Tov (Shemos 2:10): It is not clear that David comes from Miryam and Kalev 1 .
Bartenura (on Rashi Shemos 1:18), Maharsha (11b): A female descendant of Miryam and Kalev married Ram or his paternal seed; David comes from both of them.
I heard that Ram married Miryam 2 , and died without children. Kalev did Yibum, and fathered Aminadav, therefore he is called on Ram's name. (PF)
The Gemara connotes, but did not explicitly say, that David comes from Kalev. Perhaps David comes from a son of Miryam from another husband. Refer to 4:19:1:3*. (PF)
Miryam was almost 50 years older than Kalev (Yehoshua 14:7). Why did this awesome Tzadekes from the greatest family not marry in her youth?! If she was married beforehand, this is not difficult. However, perhaps no one wanted to marry her because she was very ill; Kalev's face turned black amidst fasting (Sotah 12a), i.e. for her to recover (Maharsha 12a). (PF)
