When Robert C. Byrd of West Virginia reached the U.S. Senate in 1959, Democrats supported high levels of defense spending and favored tax cuts to stimulate the economy, even at the risk of deficits. Byrd's party was deeply divided over matters of race and on balance less supportive of civil rights than the Republicans. Democrats also retained an inclination toward freer trade that dated back to the antebellum period. Although party positions were evident on these and other topics, issues that now polarize the parties like abortion and gun control were not on the political agenda.

Five decades later Byrd remains in the Senate and very much a Democrat. Yet in many other respects the identities of the two parties have changed radically. By the latter part of Byrd's tenure, his party was associated with opposition to high levels of defense spending, willingness to raise taxes to balance the budget, and support for civil rights. Support for freer trade had become a Republican cause. Meanwhile, new issues, including abortion and gun control, had arisen and become increasingly partisan.

These shifts occurred over decades during which many politicians came and went. None of the senators with whom Byrd served in 1959 remain in office. Thus many reasonably assume that elite replacement must underlie the changes in party policies that have occurred. Yet this view is largely mistaken. To a great extent, adaptation at the microlevel of individuals has driven change at the macrolevel of parties.