How DropDownList's SelectedIndexChanged() works without PostBack?

You could put the DropDownList into an <asp:UpdatePanel> and set the trigger to the SelectedIndexChanged event of the DropDownList.

Something like this (don't forget the script manager)

<asp:ScriptManager ID="ScriptManager1" runat="server" />

<asp:UpdatePanel ID="UpdatePanel1" runat="server">
   <ContentTemplate>
      <asp:DropDownList ID="drop1" runat="server" OnSelectedIndexChanged="ddlTablo_SelectedIndexChanged" />
   </ContentTemplate>
   <Triggers>
      <asp:AsyncPostbackTrigger ControlID="drop1" EventName="SelectedIndexChanged" />
   </Triggers>
</asp:UpdatePanel>

You can send ajax call, using asp.net UpdatePanel or use jQuery ajax. This wont do postback and your whole page wont get refreshed.

The UpdatePanel is quite straight forward and easy to use. ASP.net ajax will generate the asyn calls for you whereas jQuery ajax will probably need you to render html using javascript.