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Life in the fast lane: how quickly can a new species evolve?

The world is changing fast, and evolution is not staying behind. The curious case of a new species of flower that evolved in the north isles of Scotland shows that evolution can create new species in a matter of years.

The environment around us is changing rapidly as a human population of seven billion consumes resources more quickly than the planet can replenish them, and technology helps us reach the most remote corners of the planet. There are probably no places on Earth that are not affected in one way or another by human activities.

The effects of climate change, land-use change, urbanization, and global trade and travel on ecological systems are well documented. In the past decades we have accumulated a number of examples that human-driven changes cause evolutionary change.

With peppered moths turning black in response to environmental pollution and then back to peppered when lichens came back after the Clean Air Acts, warfarin resistance rats, antibiotic-resistant bacteria, and plants that can survive weed-killers, we have plenty of examples of rapid evolutionary change.

But one area where evolution may have a hard time keeping up the pace is the origin of new species. The evolution of a new species requires more than just a change in wing colour or a mutation in the metabolism of toxic compounds. For a new species to evolve, you also need to ensure that the new organism is reproductively incompatible with other similar organisms. In other words, you need reproductive isolation.

Without reproductive isolation, emerging species run the risk of merging back with their ancestors, resulting in incomplete speciation. But evolving reproductive isolation is often a gradual process that requires many changes (mutations) which accumulate over a long period of time. We tend to think of the origin of species as something that takes thousands or even millions of years. Speciation is not usually a fast phenomenon. Unless you are talking about a polyploid.

What are polyploids?

One of the most dramatic mutations that an organism can suffer is the duplication of its entire genome, a phenomenon known as polyploidy. Polyploid organisms have twice (or more) the number of chromosomes of their ancestors. For example, humans usually have 23 pairs of chromosomes, and a polyploid human would have 46 pairs. But such a drastic change in the genome of an organism is often deadly, and in fact there are no humans with that many chromosomes. The exceptions to this rule include plants and other organisms, such as some fish and amphibians, which are much more tolerant to carrying twice the usual number of chromosomes.

Flowering plants also seem to be particularly robust to genome doubling. In fact, all flowering plants have had one or more episodes of genome duplication in their evolutionary history. But what does polyploidy have to do with speciation?

Magic traits

Polyploidy can be a mechanism of “instantaneous” speciation. This is because plants that survive the doubling of their genome are not only different in morphology, but are also reproductively isolated from their immediate ancestors. The reason for their reproductive isolation is simple: organisms with different numbers of chromosomes usually produce sterile offspring. An example of this is horses and donkeys, which produce offspring (mules) that are sexually sterile, and therefore an evolutionary dead-end.

Traits that cause the evolution of both different characteristics and reproductive isolation have been called by theoretical biologists “magic traits”. Polyploidy, or genome doubling, is a good candidate for such a magic trait.

One of the youngest species on the planet?

The Shetland archipelago, off the north coast of Scotland, is home to one of the few known examples of a new type of plant that has evolved through genome duplication in the last 200 years. The Shetland monkeyflower evolved from the common monkeyflower (Mimulus guttatus), a species native to North America, but which invaded the British Isles in the 1800s. Because its ancestor has only been around Shetland for about 200 years, we know that the new plant must be younger than that.

A Shetland monkeyflower (Mimulus guttatus), next to a 50 pence piece to demonstrate size. Copyright of Mario Vallejo-Marin and used with permission.

The Shetland monkeyflower evolved on a roadside ditch among other wildflowers. It is superficially similar to its ancestor: It has bright yellow flowers, with tiny red spots in the centre. Under UV light, both of them display a “bull’s eye” pattern, which although invisible to us, can be detected by bees and other pollinators.

But what really makes the Shetland monkeyflower special is its number of chromosomes. It has twice the number of chromosomes as its ancestor (14 vs. 28 pairs), and exemplifies the evolution of a new type of plant through genome doubling.

The doubling of its genome also produces some subtle but important differences in the way the new plant looks. The Shetland monkeyflower has larger leaves, larger flowers, and takes longer to flower than its ancestor. Having twice the amount of DNA in their cells, seems to have some immediate repercussions.

The discovery of this new plant is so recent, that it has not yet been formally named. Naming a new species requires careful descriptions and comparisons with other related species. But the lack of name also reflects a common prejudice among botanists. Polyploid organisms often remain officially nameless because a doubling of the genome is sometimes not deemed sufficient to recognise a different species despite the fact that it is likely to be reproductively isolated.

Variation in the number of chromosomes characterizes some older evolutionary lineages (sometimes called polyploid complexes), and members of these lineages often share the same scientific name (e.g. the Scottish bluebell, Campanula rotundifolia). This raises an interesting question. When should we name a new species? Is reproductive isolation and genome doubling enough? If not what else is required?

Regardless of whether we give the Shetland monkeyflower a new scientific name or not, the evolution of this new plant in a wind-swept roadside ditch of a remote island represents a beautiful example of rapid evolution in action. In a rapidly changing world, the origin of new species via polyploidy is a reminder that evolution, even speciation, is not necessarily a slow phenomenon.

Featured image credit: A Shetland monkeyflower (Mimulus guttatus). Copyright of Mario Vallejo-Marin and used with permission.

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