TD Ameritrade announced on Monday that it would acquire Scottrade Financial Services, a rival discount brokerage, for $4 billion, in a bid for scale at a time when small investors are losing their taste for stock trading.

The deal reflects the ongoing evolution of the investment business.

Both companies trace the roots of their business model to a regulatory move in 1975 that banned fixed brokerage commissions, giving rise to a clutch of firms, including Charles Schwab & Company, that offered stock trades with discounted commissions. Discount brokers continued to flourish in the dot-com era, when individual investors, day traders and smaller advisers helped drive a boom in online trading.

But in the current market uncertainty, mom-and-pop investors are more reticent to buy and sell stocks. Index funds and other so-called passive investments have also gained favor, limiting interest in owning individual stocks.

The industry is feeling the pressure. TD Ameritrade, for example, said on Monday that its average client trades per day declined 7 percent in the three months through September.