Evolusjon! – Evolution!

Does the function make the organ?

By Maria Capa

You may have heard the famous sentence attributed to Jean Baptiste Lamarck ‘The function makes the organ’. It captures the fact that organisms can provide their offspring with some characteristics acquired during lifetime resulting in the species’ gradual adaptation to the environment. This could explain why giraffes have long necks and some mantis look so much alike the orchid they live on.

Considering Lamarck merely for this contribution to Science is oversimplifying the great role this man had over the scientific community and society since he published Philosophie Zoologique (1809), the first account of a cohesive evolution theory breaking through creationist and static paradigms of that time.

However, the observations related herein seem to be contrary examples to Lamarck’s assertion.

If there is no function… why maintaining the organs?

Fig. 1. Megalomma sp. within its tube, with two distal compound radiolar eyes sticking out of the radiolar crown (Photo: Alexander Semenov)
Fig. 1. Megalomma sp. within its tube, with two distal compound radiolar eyes sticking out of the radiolar crown (Photo: Alexander Semenov)
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Fig. 2 Groups of radiolar eyespots in Notaulax sp. (Photo: Alexander Semenov)

 

Something that has called my attention for years is the great diversity of type and arrangement of eyes found in the feather-duster worms (sabellids). These marine creatures are tube-dwelling worms. They spend most (if not all) of their lives within their opaque tubes made of mucus and sand and only expose their often colourful radiolar crown out of them to capture the suspended particles from the water column they feed on (Fig. 1). It makes sense that some species bear eyespots on this structure because, even though they are not able to build proper images, they can distinguish light and dark, and that allows them to withdraw rapidly into their protective tubes when a hungry fish passes by (Figs 1, 2).

But these amazing animals can also bear compound eyes (similar to those present in many insects) on their radiolar crown (Fig. 3). Furthermore, they can have cerebral eyes (near the brain), paired segmental eyes along their body (Fig. 4), and also eyespots in the back ends (Fig. 5). And if that was not enough, the type of photoreceptors and pigments present in these eyes show also variations even in a single individual!

Fig. 3. Scanning electron micrograph of a compound eye in Megalomma phillisae (Photo: Maria Capa)
Fig. 3. Scanning electron micrograph of a compound eye in Megalomma phillisae (Photo: Maria Capa)

For sure all these eyes may have some sort of utility, avoiding predators in case they have to leave their tubes and build a new one, for example. But it is sticking that so much energy is devoted in developing and maintaining such structures in organisms that barley live their tubes and often live below 100 m with little or none light to see… And still they are one of the organisms exhibiting the most number and diversity of eyes!

Evolutionary pathways are limited by genetic and developmental constrains. But on the other hand, genetic variation is the major responsible for the phenetic plasticity found in organisms (like the diversity of eyes found in feather-duster worms). The presence, arrangement and type of eyes in sabellids have an evolutionary potential to respond to environmental conditions, but unless we discover the function of some of these eyes, could they also be vagaries of nature?

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Fig. 4. Segmental eyes in Sabella spallanzanii (Photo: Eunice Wong)
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Fig. 5. Pygidial eyes in Bispira porifera (Photo: Maria Capa)

 



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