Why did Yaakov specify Efrayim and Menasheh, and not children that Yosef might have later?
Seforno (to 48:11): Because Hashem's promise in Luz was directed at them. 1
Why did Yaakov say "v'Atah"?
Ohr ha'Chayim: Now that I see that I did not father other children, I know retroactively that the two sons born to you are the fulfilment of "Mafrecha v'Hirbisicha" (48:4). It was regarding them that Hashem said "Preh u'Reveh" (35:11).
Malbim: Now that I see that I did not father other children, I know that Hashem intended that Bnei Yosef will be two Shevatim. The Bechorah was taken from Reuven (Leah's Bechor) and given to Yosef (Rachel's Bechor).
By what merit did Yosef receive the double portion of the Bechorah?
Maharal #1 (Chidushei Agados Vol. 3, p. 125, to Bava Basra 123a): Yosef supported his father; in contrast to Reuven who had disarranged the bed of his father (35:22).
Maharal #2 (Nesivos Olam, Nesiv ha'Tzeni'us Ch. 1, p. 105): Rachel merited, due to her Tzenius, 1 that her son (Yosef) would be the Bechor. Though Tzenius, one's issue is from an inner, sanctified place, resulting in the sanctity of a Bechor. The trait of Tzenius is thus passed down to the children.
In that she gave over the Simanim to her sister Leah. Also see Chidushei Agados Vol. 3, p. 126, to Bava Basra 123a; refer to 29:25:1.4:1*.
Yaakov puts Efrayim in front of Menasheh, already at this point. Why was Yosef surprised later when Yaakov put his right hand on Efrayim (48:17)?
Ha'amek Davar: Yosef thought that Yaakov put Efrayim first to lower Reuven 1 (that he is below Shimon, just like Efrayim is below Menasheh). (Compare to Gur Aryeh (to 48:20); refer to 48:5:152.1:1.)
If so, he should have said 'Shimon and Reuven will be like Menasheh and Efrayim'! Perhaps he did not want to lower Reuven so overtly (PF).
Yaakov mentions Efrayim before Menasheh, already at this point. Why does the Torah highlight this only later (48:20), and not here?
Gur Aryeh (to 48:20) #1: In our verse, Efrayim is first for a different reason. Had Yaakov said, 'Menasheh and Efrayim shall be like Reuven and Shimon,' Yosef would think that Reuven and Menasheh would each share the status of Bechor. Instead, Yaakov said Efrayim first so that Reuven would parallel Efrayim, to show that neither would be the Bechor. 1
Gur Aryeh (to 48:20) #2: It is less surprising that Efrayim is mentioned first here; after all, he learned Torah with Yaakov (Rashi to 48:1). But when blessing them (in 48:20), we would expect the Bechor (Menasheh) to be first.
Rather, Yosef himself would be Yaakov's Bechor, in that he would have two tribes descend from him. (CS)
QUESTIONS ON RASHI
Rashi writes: "'[... Who were born...] until I came' (Ad Bo'i) - [i.e.] before I came." What is Rashi clarifying?
Gur Aryeh: The term "until" usually implies shortly before; yet we know that Menasheh and Efrayim were born some time before Yaakov's arrival. 1 Rather, as Rashi explains, Yaakov meant that sons born to Yosef any time before his arrival would become tribes -- as opposed to any born after his arrival.
Both of Yosef's sons were born before the onset of the famine (41:50); and Yaakov arrived in the second year of the famine (45:6,11). (CS)
Rashi writes: "'Until I came' - ... I.e. any who were born from when you left me, until I came to you." What is Rashi adding?
Gur Aryeh: The term "until" usually delineates a span of time, with both beginning and ending points. Rashi tells us what these are.
Rashi writes: "'... They are mine' - They are to be counted among the rest of my children, each one to receive his fitting portion in the Land." But each tribe would receive a portion in the Land in accordance with its numbers (see Bamidbar 26:52-56). If so, what was Yosef's advantage in being divided into two Shevatim?
Gur Aryeh #1: Chazal teach that "each and every tribe received within its portion, mountains, lowlands and valleys" (Bava Kama 81b). Now Menasheh and Efrayim would both receive all three types, making it more likely that each individual descendant of Yosef would receive the type of his choice.
Gur Aryeh #2: During the initial years of settling the Land, it was forbidden to marry out of one's Shevet, to ensure that each tribe's inheritance would remain intact (Bamidbar 36:6-9). Efrayim and Menasheh would become distinct tribes in this regard as well.
