F Rosa Rubicondior: Lessons From Nature - How Birds and Berries Gave Us Colour Vision

Sunday 10 November 2013

Lessons From Nature - How Birds and Berries Gave Us Colour Vision

Hips and hawes in an Oxfordshire Hedgerow in Autumn
© 2013 Rosa Rubicondior

We've just spend a lovely afternoon walking over a footpath from Abingdon to the village of Sunningwell, where we had an especially good English Sunday roast dinner with a very drinkable Rioja at the Flowing Well. What made it even more enjoyable, apart from an almost cloud-free blue sky, was the profusion of wild berries amongst the autumn leaves on the hedgerows. The footpath is mostly ancient, so is well stocked with different plant species.

Now, a religious person, and especially a religious person who knew little of evolution and so would almost certainly not believe in it, would probably look at the beauty in the English countryside and marvel at how wonderful his or her god had been in magicking such a pretty planet just for them to marvel at. They might even thank their imaginary friend for being so kind. They would undoubtedly apply their learned confirmation bias and circular reasoning skills to see the countryside as confirmation of their god.

What they would miss entirely is the real wonder in their search for confirmation of their own importance in the scheme of things and their own assumed special relationship with the creator of the Universe rather than for the truth. To appreciate that takes humility and a willingness to accept that it wasn't all made for us. And the berries in the photograph above illustrate this well to anyone who knows a little about them, why they are the colour they are and why they are there at all.

Not that it's particular relevant, but the types of berry you can see here aren't actually berries at all in the botanical sense of the term and they are both fruits of members of the Rosaceae super family. The round fruits are of common hawthorn, Crataegus monogyna and the oval fruits are of the dog rose, Rosa canina. Both these are members of the Angiosperm or flowering plants phylum which has evolved since the evolution of flying insects which created the potential for insect pollination. Until then, most plants had been restricted to living in damp conditions because the male gametes were motile (i.e., they swam to the female gamete using a flagella, much like sperms do). All but the flowering plants still use this method of fertilisation. Insects (and now a few other orders like bats and birds) and wind are the main vectors or pollen dispersal so flowering plants are an example of a niche opened up by one branch of evolving life being exploited by another.

So, the first thing we can tell is that these plants must have evolved after about 320 million years ago when winged insects make their appearance in the fossil record.

The next thing we can see is the colour. Producing colour in fruit isn't simply a chance thing. It involves the production of pigments and that takes resources, so we can be sure that it serves a purpose and it can only serve a purpose by a mechanism which involved detection of colour.

One problem all plants face, and especially the flowering plants freed by evolution from the restriction of needing more or less permanently damp conditions and able to grow in far more diverse habitats, is how to disperse their offspring. The last thing a newly-germinated plant needs is to find itself growing right next to mum, who will have dominated the local environment. Each plant of the same species is a rival for territory with every other one, and not always plants of the same species, and there is no sentimentality involved. It matter not on jot to a plant that it's competing for survival with its offspring or sibling.

It's in an evolving plants 'interest' to evolve dispersal mechanisms which scatter its seeds as widely as possible because that gives a better chance of producing at least one successful offspring during its lifetime (and that is all that is required for a stable population) than by simply dropping the seeds on the ground around it. So, many flowering plants have evolved fruit, which botanically is the apparatus for seed dispersal.

In the case of the hawthorn, the strategy is to get eaten by a bird which digests the red-skinned fruit around the seed but leaves the hard seed unscathed to be dropped some distance away, complete with a little fertiliser to get it started. So, the fruits are small enough to be swallowed whole, so the seed isn't damaged, and attractive enough to be seen. A red haw is a ripe haw and a ripe haw contains sugar and soft pulp, so birds like the common blackbird have evolved to recognise a ready meal sitting there waiting to be eaten.

This dispersal process was possible because birds see in colour, and the colour the species the hawthorn needed to attract recognised red as the signal for ripe fruit. Additionally, red stands out with colour vision but for a species like most mammals which see in shades of grey, red is indistinguishable from green. Were plants also evolving to hide their seeds from mammals with their cushing teeth? Dog rose seeds are dispersed in essentially the same way and at the same time, so they are essentially in competition with other red-fruited autumn species. Without knowing which came first we can only guess which is mimicking the other and evolving in the presence of birds which eat red fruit. It's probably not a coincidence that so many small fruits are red.

Here then we have another mutually beneficial cooperation between totally different species. Plants get their seeds dispersed and birds get fed. This method could not have evolved before there were birds and very many bird species could not have evolved without fruit-bearing flowering plants. Again an example of an opened niche being exploited, then evolution continuing in a new direction that this opened up.

This method is hugely wasteful of course, and hawthorn trees must invest vast amounts of energy into making a huge surplus of seeds to ensure a small number succeed, but that is all it does. That is what the hawthorn tree was created by its genes to do. It has no other purpose other than replicating hawthorn genes into the next generation and hawthorn genes have no ability to make moral judgments about time and resource wasting. Natural selection ensures that whatever strategy produces the most copies will be the winning strategy. Hence the vast abundance of haws; far in excess of what would be needed if they all produced another hawthorn tree. So wasting haws is in the interests of bird genes too.

So, we now know then that hawthorns and dog roses must have evolved after birds, which first appear in the fossil record around 150 million years ago. We also know that whatever is driving this process is neither intelligent nor moral because such waste would be both stupid and immoral when viewed as the product of an intelligent designer.

But why the emphasis on birds as the favoured method of dispersion? Well, apart from the ability of birds to cover much larger distances in the course of a day than mammals normally do, birds see in colour and so can respond to the signal that the fruit is ready for its seeds to be dispersed.

And that leads to the final point: how come, if mammals don't see in colour, humans do?

The reason for that is that of all the mammals, only a few, and almost all of them primates, see the world in other than shades of grey, and the reason for that is that the world of trees in which the primates evolved was one of ripening, high-sugar fruit because birds and flowering plants had been busy creating it for their own mutual benefit for several million years.

Our ancestors found an open niche which they could exploit through the process of evolution by natural selection to either re-activate colour vision which for some reason they had lost after they diverged from the last common ancestor they shared with birds, or it evolved anew. However it happened, birds and flowering plants had created an environment in which it gave our simian ancestors an advantage. To put it bluntly, we parasitised the system of avian plant seed dispersal by treating the fruit and the seeds as a food source. Not all simians have colour vision of course because it serves no purpose in nocturnal species, so the fact that we have colour vision strongly suggests we evolved from a diurnal species.

Of course, evolution did not stop at that point and several mechanisms have evolved since by which plants exploit fruit-eating mammals, hence the abundance of tomatoes close to raw sewerage outlets. Many other plants have evolved other strategies since mammals evolved, like sticking their seeds to mammalian fur for example.

So, turning that round we can predict that the first monkeys with colour vision did not evolve until after birds and flowering plants had jointly evolved fruit. It's not rocket science to predict that it was also after the evolution of trees, of course.

How does this prediction measure up to reality? The evidence is not straightforward with molecular clock estimates at about 85 million years ago, fossil evidence at about 55 million and other estimates at about 65 million years since the first proto-simians evolved. However, these are all well within the timescale predicted by our theory.

So the reason we have colour vision is because birds do and because flowering plants exploited that ability to get their seeds dispersed. Then our ancestors exploited that system to evolve eating fruit. And so we can look at a hedgerow and marvel at the process which brought it about as well as appreciating the aesthetic qualities of autumn colours against a brilliant blue autumn sky in an English country footpath. If it hadn't been for birds eating fruit we probably wouldn't be able to see it in all it's glorious colour.

Pity the poor creationists who only sees spurious confirmation of his own arrogant self-importance and who thus relegates all of nature to a mere utility value whilst missing the entire point.

Thank you for sharing!

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1 comment :

  1. Great read! The colour vision explanation was pretty fascinating. Keep it up!


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