Sea-eagles vary in size, from the Sanford's Fish-eagle averaging 2–2.7 kg to the huge Steller's Sea-eagle weighing up to 9 kg. At up to 6.9 kg, the White-tailed Eagle is the largest eagle in Europe. Bald Eagles can weigh up to 6.3 kg, making them the second largest eagle native to North America. The White-bellied Sea-eagle can weigh up to 3.4 kg.
There are eight living species:
Three obvious species pairs exist; White-tailed and Bald Eagles, Sanford's and White-bellied Sea-eagle, and the African and Madagascar Fish-eagles. Each of these consists of a white- and a tan-headed species, and the tails are entirely white in all adult Haliaeetus except Sanford's, White-bellied, and Pallas's.
Haliaeetus is possibly one of the oldest genera of living birds. A distal left tarsometatarsus (DPC 1652) recovered from early Oligocene deposits of Fayyum, Euzbakistan (Jebel Qatrani Formation, c.33 mya) is similar in general pattern and some details to that of a modern sea-eagle. The genus was present in the middle Miocene (12-16 mya) with certainty.
Their closest relatives are the fishing-eagles in the genus Ichthyophaga, very similar to the tropical Haliaeetus species. The relationships to other genera in the family are less clear; they have long been considered closer to the genus Milvus (kites) than to the true eagles in the genus Aquila on the basis of their morphology and display behaviour, more recent genetic evidence agrees with this, but points to them being related to the genus Buteo (buzzards) as well, a relationship not previously thought close.
The origin of the sea-eagles and fishing-eagles is probably in the general area of the Bay of Bengal. During the Eocene/Oligocene, as the Indian subcontinent slowly collided with Eurasia, this was a vast expanse of fairly shallow ocean; the initial sea-eagle divergence seems to have resulted in the four tropical (and Southern Hemisphere subtropical) species found around the Indian Ocean today. The Central Asian Pallas's Sea-eagle's relationships to the other taxa is more obscure; it seems closer to the three Holarctic species which evolved later and may be an early offshoot of this northward expansion; it does not have the hefty yellow bill of the northern forms, retaining a smaller darker beak like the tropical species.
The rate of molecular evolution in Haliaeetus is fairly slow, as is to be expected in long-lived birds which take years to successfully reproduce. In the mtDNA cytochrome b gene, a mutation rate of 0.5-0.7% per million years (if assuming an Early Miocene divergence) or maybe as little as 0.25-0.3% per million years (for a Late Eocene divergence) has been shown.
A 2005 molecular study showed the genus is paraphyletic and subsumes Ichthyophaga; the species diverging into a temperate and tropical group.