The exceptions of biology


Credit: Stanford University

I like stating in my lectures that biology is a science of exceptions.  This in no way invalidates the rules; rather, they give us a new appreciation for them.  I have to confess that I love these exceptions.  In a previous post I wrote about organisms that were at the same time heterotrophs (they have to eat other organisms for nourishment) and autotrophs (capable of using chemical or radiant energy to make their nutrients). A recent example of such organisms are photosynthesizing insects!

Another favorite example of mine are the marine cone snails that feed on fish. These snails are slow, as any self-respecting snail would and yet they are able to actively hunt fish, and not slow fish either…

Other examples that come to mind:

**Egg-laying mammals
**Poisonous birds
**Differences in the genetic code of various types of organisms (yep, the rumors of an absolute universal genetic code have been greatly exaggerated)
**Jellyfish with complex eyes and no apparent brain to send light signals to
**Entities that are unicellular or multicellular depending on the environment
**Fungi that trap and eat worms
**In humans, XXs are girls and XYs are boys, yet there exist individuals XXY, and just X. How do we classify them? Moreover, there are cases of XY individuals who lack a certain type of sex hormone receptors. These individuals look and act biologically female…
**Viruses that infect viruses
**Plants that use neurotransmitter-like molecules
**Eusocial mammals
**The use of quantum phenomena in photosynthesis
**Social spiders
**Venomous mammals
**Heat-producing plants
**One of the 20 amino acids present in proteins is not an amino acid at all

And many more…

Can you think of others?

Most interestingly, some possible biological exceptions may point out to alternate form of life, fundamentally different from our own. Some propose that right now there may be a “shadow biosphere”, a concept originally proposed by Dr. Carol Cleland; in her own words: “The idea is straightforward. On Earth we may be co-inhabiting with microbial lifeforms that have a completely different biochemistry from the one shared by life as we currently know it.” (cited from

keepcalm biologist




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  1. I believe P.C.W. Davies and Charles H. Lineweaver were first to suggest the idea of a shadow biosphere, although Carol and Shelley D. Copley did coin the term and do much to elaborate the concept.
    Also, I’ve always been a huge fan of the rampant exceptions to ecological rules (such as Cope’s Rule, Foster’s Rule, etc.). Historically, the black swan was a key biological exception. Carnivorous plants and walking are also particularly strange.


  2. I suppose this is something I could google, but because you’re a scientist, I might as well ask: how does someone with just an x (and no y or other x, I presume) come about? Is it a “spontaneous” conception sort of thing? Or am I just reading too much into it? At any rate, this post sounds fascinating….I had never heard of photosynthesizing insects or poisonous birds.


    1. The short answer is that as with all the other chromosomes, one of the sex chrom in you came from your mother and the other one from your father. When there is a defect in the egg or spermcells, say that one has two sex chroms or none, the embryo will have an abnormal # of sex chroms. I hope this helps. If you want to look into it google “meiosis non Disjunction. Thanks for your comment!


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