2.2. On the biology of speciation: Nongeographical barriers to gene flow
Reproductive isolating barriers can restrict gene flow between species by preventing successful interbreeding among closely related species due to intrinsic or extrinsic factors (Coyne & Orr, 2004). These reproductive isolating barriers can act either early in an organism’s lifecycle (prezygotic, before zygote formation), or can be late-acting (postzygotic, after zygote formation) (Dobzhansky, 1937). Studies examining reproductive isolating barriers found that prezygotic isolating barriers typically exhibit a stronger influence than postzygotic barriers in restricting gene flow between closely related species (Ramsey, Bradshaw, & Schemske, 2003; Dopman, Robbins, & Seaman, 2009; Sanchez-Guillen, Cordoba-Aguilar, Cordero-Rivera, & Wellenreuther, 2014), and this is likely due to their early action in the organism’s lifecycle. However, examining how these reproductive isolating barriers emerge and restrict gene flow remains poorly understood. In this section, we review the most prevalent reproductive isolating barriers that occur between species of Daphnia , and the implications for advancing the field of speciation research.