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.