There’s no evolution without mutations. Mutations happen in nature (in people, too) all the time. Usually you cannot really see it with your bare eyes if a mutation has taken place, but sometimes the outcomes of mutations are clearly visible.
A fine example of a visible mutation is the red-leaved downy birch (Betula pubescens f. rubra). Normally B. pubescens individuals have green leaves, but this particular mutated form has red leaves. As we know that this is a local speciality of the Oulu region in Finland, we wanted to find out more, so in the beginning of August, we visited the Botanical Gardens of the University of Oulu. There we met with curator Annu Ruotsalainen, who kindly told us more about the red-leaved downy birch.
The first (and by that time, the only) individual of this red-leaved form was found about 40 years ago in Ylikiiminki, Oulu, Finland. A school girl from a local farm took a branch of a peculiar red-leaved birch tree growing on their yard and showed it to her biology teacher, and the teacher realized that there’s something special about it. The teacher contacted the University of Oulu’s Botanical Department, and eventually specialists at the University confirmed that the red-leaved form is likely a result of a mutation.
This first red-leaved individual didn’t produce any seeds though. As the appearance was so special, the people at the Uni started to test ways to reproduce the tree to get some new ones. They started to clone the tree by taking cuttings and planting them on the ground. Soon they realized that it didn’t work. Then, they carefully took smaller cuttings (actually only parts of the buds) to the laboratory and first placed them in a growing liquid. New trees finally started to grow and succeed! Nowadays there are some nurseries where they grow the red-leaved downy birches for people to buy and decorate their yards with this red peculiarity. All red-leaved downy birches that you come across with have the same ancestor: the tree that was reported from the farm where the girl took the branch to school around 40 years ago.
In addition to historical aspects, Dr. Annu Ruotsalainen also told us some scientific facts and/or assumptions related to the red-leaved downy birch. In the beginning of the summer, when the first leaves appear to a tree representing this particular form, the leaves are almost entirely green, with only a small amount of red pigment (i.e. anthocyanin). After a while, the area with the red pigment starts to widen. Fully-grown leaves are almost entirely, if not entirely, red. Although the color is red, chlorophyll is still present, but the green pigment is covered by the strong red pigment.
So…why did the mutation causing red leaves occur in the first place? Evolution works in a way that there are constantly lots of mutations going on, and the ones that enhance the survival of a species (i.e. phenotypic plasticity) become more common. As this red-leaved form has not been reported anywhere else, this mutation has probably happened only once, that is, in Finland. So, this mutation is not likely to enhance the “quality of life” for this species, but on the other hand, it has not been a step to the wrong direction either. In other words, it seems that this mutation does not have much difference in an evolutionary perspective. It just happened once and by chance (or “by mistake”).
By the way, the offspring of the original mutated birch are able to produce seeds, so in that sense, either, the mutation appears not to have lowered the reproductive success. Also, legends say that the farmer who had the original tree in his yard later saw that new red-leaved birches started to grow, likely from seeds. So maybe the originally reported seedless characteristic of this form might have been a consequence of a bad growing year or some disease? Who knows… Nevertheless, the gardeners who grow the red-leaved downy birches nowadays use the cloning method because it is a 100% accurate method to get the red leaves. With seedlings (i.e. reproduction by seeds), you never know what you get (50% of the offspring have red leaves and 50% “normal”, green leaves; Pihlajaniemi & Siuruainen 2011). It seems that the mutation is heritable, but most likely recessive.
We don’t know if this particular mutation has been studied in a molecular level. There are however some suggestions that the red-leaved forms might be more susceptible towards pathogens. Also, the red pigment, anthocyanin, might be protective for UV radiation. Lots of might bes, so hopefully, at some point, someone digs into the molecular level processes. At least we would be interested to know more. 🙂
As we had our discussion with Dr. Ruotsalainen at the Botanical Gardens, below are some photos taken at the beautiful gardens. It’s a very nice place, so be sure to visit it when you travel to Oulu!
References and other stuff
Pihlajaniemi & Siuruainen (2011) Sorbifolia 42, 16-23.
yle.fi, Naurispellon laidalta suosituksi koristepuuksi – harvinaisen punalehtisen koivun juuret ovat Pohjois-Pohjanmaan mullassa. https://yle.fi/uutiset/3-10237046, 11.6.2018