For several years, I’ve noticed that in many different locations, dandelions located on a southern slope are the first to bloom in the springtime. It happens because the slope receives more direct rays from the sun, causing the soil to heat up faster. This presumably signals the plants that it’s time to grow and reproduce. It’s related to the reason why our noses, if they are sloped, get sunburnt before the rest of our faces. For centuries, people have been aware of the southern slope advantage, given that it’s been used as an optimal location for vineyards.
It’s not the kind of thing I could have noticed in my youth, given that municipalities would apply weed-killer to get rid of dandelions. Nowadays, to the dismay of herbicide manufacturers, many cities have bylaws prohibiting the use of such compounds, so dandelions abound. There are also less people who are obsessive about upkeeping weedless lawns, so in May especially, dandelions abound, and many children’s favorite color is in full display for a few weeks before they all turn white.
The seeds that are attached to the white fluffy part that helps them spread like wildfire are actually the result of parthenogenesis or virgin birth. That implies that all the seeds that a plant produces are clones —genetic copies of one another. Just last year biologists isolated the parthenogenesis (PAR) gene from the asexual dandelion that triggers embryo development in unfertilized egg cells. When the gene was deleted from dandelions by the CRISPR technique, they still produced the white sphere but one without seeds, as shown below in the picture to the right:
In my front yard, I enjoy having a variety of plants among my Kentucky bluegrass, but the dandelion is not my favorite. It grows at the expense of other plants that I prefer such as white clover, black medic, yellow clover, poppies and oregano. Since dandelions won’t naturally delete the PAR gene that allows them to multiply so efficiently, the key is to remove the plants before they flower. Even that can be a challenge. If you are like me and are aversive to using herbicides, then you have to pull them , making sure to pull out the entire root. Just like the lawn mower has acted as a selective force, which has shifted the dandelion population towards blooms with shorter stems, pulling weeds by hand has also had an impact. In my yard, it’s gradually given a selective advantage to plants that produce very long and deep-reaching roots. Those are more likely to leave a surviving fragment allowing the more common form of vegetative reproduction to keep propagating the weed
If dandelions don’t reproduce sexually, how do they create the variety that natural selection can then act upon? When pulling out dandelions, one observes a spectrum of root thickness among plants that are side by side in identical soil, reducing the likelihood of an environmental cause. When botanists checked for mutations among dandelions, they found a rate that ranged from 7 to 15 per 1000 individuals. A mutation is a change in the DNA code caused by either nature or by man-made products. The study from the same link concludes that the mutation rate in dandelions is directly proportional to concentrations of chromium, iron, manganese, and nickel in leaf-tissue. Another study reveals that some metals themselves cause dandelions to evolve. They don’t exclude the pollutants, but sequester them and then tolerate their presence, again in direct proportion to the amount of pollution they are exposed to.