NEW YORK (AP) -- Tractor Supply Co. is joining the benchmark Standard & Poor's 500 index, replacing Life Technologies Corp.
The life sciences company is being acquired by Thermo Fisher Scientific Inc.
Shares of Tractor Supply, an operator of retail and ranch stores, rose $2.07, or 2.8 percent, to $76.38 in after-hours trading Thursday.
When a stock is added to a major index, such as the S&P 500, it tends to rise. That's because investment funds that track the index must buy the shares.
S&P Dow Jones Indices, which manages the S&P 500, announced the change on Thursday. It's effective after the close of trading next Thursday.