Source: H. Graetz, History of the Jews, volume II (Philadelphia: Jewish Publication Society of America, 1893) 49-50.
"[The Pharisees'] work commenced with the reorganization of the Synhedrion. The Sadducean members were deprived of their seats, the penal code which they had added to the Jewish penal laws was set aside, and the old traditionary methods again made valid. The people had nothing to complain of in this change, for they hated the severity of the 'eye for eye' punishment of the Sadducees. On the other hand, certain days of rejoicing, disregarded by the Sadducees, were proclaimed as half-holidays by the Pharisees. Witnesses in the law courts were no longer to be questioned merely upon the place where and the time when they had seen the crime committed, but they were expected to give the most detailed and minute evidence connected with it, so that the judge might be better able to pronounce a correct judgment and to detect the contradictory statements of witnesses. This was particularly designed as a protection against the charges of informers, who were numerous enough in an age when the conquerors and the conquered were constantly changing parts. A salutary measure also was enforced to lessen the number of divorce cases, which the literal interpretation of the divorce laws, as administered by the Sadducees, had failed in doing. The High Court, as reorganized by the Pharisees, ordered the husband to give his repudiated wife a certain sum of money, by which she could support herself, and, as there was but little current coin amongst a people whose wealth consisted principally in the fruits of the soil or in cattle, the husband would often pause before allowing a momentary fit of passion or excitement to influence his actions."
Graetz doesn't cite sources for any of this, so I'm taking a risk in assuming that what he says is true. But he's describing the time under Salome Alexandra in Judea, when she put the Pharisees in charge. This occurred in the first century B.C.E.
Graetz acts as if the Sadducees supplemented the Torah with another law, while the Pharisees put Israel back on track by returning her to her ancestral traditions. But his description shows that the Sadducees relied on a literal interpretation of the Torah alone, whereas the Pharisees had oral traditions that made the law more reasonable. When the Sadducees read "eye for an eye," they took that to mean "an eye for an eye" (ouch!). The Pharisees, by contrast, made it a matter of monetary compensation.
I don't understand what Graetz is talking about in his discussion on witnesses and informers. What's one have to do with the other? And why should we assume that the old Torah system allowed someone to be executed without much evidence? Two witnesses were needed, and they had to give the same testimony about where and when something happened. If one was lying, he was put to death. That sounds like a fairly reasonable way to get at the truth. But maybe Graetz is just saying that the Pharisaic system focused more on intricate details, which allowed contradictions in testimony to be better identified.
On divorce, the Pharisees allowed it. But they added a more humane dimension to it, by requiring the husband to give his wife money when he divorced her. They were not consciously undermining the Torah, but they had this other set of laws that they also deemed to be divinely authoritative. So they were somehow able to adjust the rules, making them more humane or practical.