Are humans the only species where incest causes birth defects?

No - it will happen in every mammalian species. Incest decreases the variety of the gene pool and thus allows (detrimental) recessive mutations to become obvious and cause phenotypes and diseases. If you take two related individuals both heterozygous (and thus phenotypically normal) for a given gene defect and mate them, then 25% of the offspring will be homozygous and thus have the condition. I can’t say (since I am not an expert) for sure whether the same will happen with reptiles, fish and invertebrates but I can’t see any reason why it should not! One of my colleagues will I am sure comment on this latter point.

Incest is relatively bad (excuse the pun) in all species because of the problems raised by David. Many animals have evolved behaviours that help reduce the risk of it happening.

Inbreeding is quite common though and it is likely to increase the rate of speciation by concentrating certain genes within a reproductively isolated population. Fig wasps are an extreme example of organisms that have a large number of species, many of which are closely related, but the barriers that have enabled genetic isolation to occur have been behavioural and relies on brothers mating with their sisters. A small degree of outbreeding reduces problems associated with inbreeding.

Plants have an additional problem, they can sexually reproduce with themselves. To overcome this they will often have their male parts maturing at a different time to their female parts, or they will position the male flowers seperately from the female flowers.

Last edited by Paolo Viscardi (5th Feb 2008 09:41:16)