Interbreeding of Water frog species

Of the more commonly found frog species within the British Isles, there are two groups:

1. Brown frogs, including the Common frog and Agile frog.
2. Green frogs (or Water frogs), which include the Pool frog, Marsh frog and Edible frog.

These three species of Water frog are capable of interbreeding. The mechanism of this interbreeding is complex, and this page attempts to explain what can happen, and why.

Pool frogs and Marsh frogs are two distinct and 'true' species. The Edible frog is a hybrid (technically a klepton or synklepton, which is a fertile hybrid) of these two species. The results of interbreeding between water frog species is explained in the diagram below.

Edible frogs have different biological traits, depending on the combination of parent species present. The most common system in the UK and Western Europe is known as the 'L-E system' (Pool-Edible), which comprises Pool frogs and Edible frogs.

Table 1 Interbreeding Results (L-E System)	Male Pool frog	Male Edible frog	Male Marsh frog [not present in L-E system]
Female Pool frog	Pool frogs m:f = 1:1	Edible frogs (Females only) [rarely occurs]	Edible frogs m:f = 1:1 [rarely occurs]
Female Edible frog	Edible frogs m:f = 1:1	Marsh frogs (Females only) [rarely survives]	Marsh frogs m:f = 1:1
Female Marsh frog [not present in L-E system]	Edible frogs m:f = 1:1	Marsh frogs (Females only)	Marsh frogs m:f = 1:1

For a detailed description of the possible genome combinations, and an explaination of the above results, see: Water Frog Genetics

Population ratios

The ratio of frog population numbers by species cannot be determined by genetic analysis alone. Other factors, including mate preference, fecundity and habitat suitability all play an important part in determining the population ratios.

General rules of thumb can be applied: If the habitat is a large pool or lake with little vegetation, then Marsh frogs are likely to dominate the numbers. If the pool is small with dense vegetation, then the Pool frog will be most numerous. As is so often the case, the Edible frog is somewhere between the two parent species in terms of habitat selection.