Of the birds, ravens and crows,
Favor the darkest of clothes.
Known for great intelligence
As well as sartorial elegance.
Corvids are clever, not klutzes,
Use cars to crack edible nutses
Into fragments on which they dine.
Revealing a logical turn of mind.
Don’t believe what we’ve said?
Check it out on the web.
Of the birds, parrots and crows have reputations of being the most adaptable and intelligent. The Latin term for the branch of the tree of life in which crows find themselves is ‘Corvidae‘. In addition to the crows, this includes jays and magpies.
Many birds want to get hold of food that is inside a tough shell – walnuts and shellfish being two examples. In both cases, birds may fly up with the food in their beaks, and drop it from a great height in the hope that the tough outer wall will break when it hits the ground. Some crows have learned to drop nuts onto roads, and rely on cars to crush the nuts. In Japan, some crows learned to do this at pedestrian crossings. They wait for the lights to change, and when the cars have stopped, they waddle onto the road to collect their food without any risk of being run over. This is shown in this David Attenborough video.
Two types of peppered moth
One favors a lighter cloth.
The dark melanized form,
Years ago became the norm
Because in many industrial places,
Smoke blackens outside surfaces.
Harder for a hunter to see
Dark prey against a sooty tree.
Selection results from such variety
Leading,said Darwin,to nature’s diversity.
The peppered moth, Biston betularia, is significant in the story of how our understanding of evolution developed. The moth occurs in two forms, a light speckled gray, and a darker ‘melanized’ form. “Melanin’ adds blackness to organisms. In the middle of the nineteenth century, folk in England began to encounter the dark (melanized) form. By the late part of the nineteenth century, about 90% of the peppered moths were the dark variety. In about 50 years, the appearance of the species had changed.
Something else was changing at that time. The Industrial Revolution was transforming British society. All kinds of factories were springing up, especially where there was coal. Coal was burned for energy. Factory chimneys released smoke, sulfur, and tiny specks of soot into the air. The soot coated the surfaces of buildings and trees, turning them black. A soot blackened building below is in Manchester in England, one of the most industrial towns. To the left is the building darkened by the polluted air; to the right is the same building after it was cleaned.
Melanization occurred with the peppered moth, and with other insects as well. Some scientists, especially Bernard Kettlewell, speculated that the change to darker varieties happened because there were benefits of being darker. He said, if the lighter form came to rest on a blackened tree or building, birds would see them. But darker versions of the moths would be less obvious and were less likely to be eaten. They were more likely to survive, have little baby moths, and in that way the darker forms would become more abundant. The idea became the subject of lots of experiments that showed that the melanized and normal versions of the moth fared differently (as had been speculated) if they were on different colored surfaces. The significance is that the results confirmed Darwin’s argument that species changed when exposed to selection pressures.
An example of gradual change that affects people relates to the colors of our own skins. Different races have different amounts of melanin. Original populations had dark melanized skins, but as we moved into more northerly countries, our skins became lighter. Anthropologists have a reasonably good grasp of when different waves of migration and occupation of new areas of Earth took place. The demelanization process happened over tens of thousands of years.
Melanin in our skin blocks some of the ultraviolet radiation that can damage the genetic material in skin cells. Melanin reduces the risk of skin cancer. As this map of the world shows, the most intense radiation occurs around the equator, and is less towards the poles. Races that live in the highly radiated zones benefitted from added protection of more melanin – that is, to have dark skin. The need for protection reduced as we migrated north and south, because there was less damaging ultraviolet radiation. The result were drifts away from darker skin. Light-skinned folk still become tanned with melanin production when they spend a lot of time in the sun.
There is another benefit / disadvantage argument that bears on the amount of melanin in our skin. It is about vitamin D, something that is essential to our health. Vitamin D helps in the uptake of calcium for our bones. Folk with a vitamin D deficiency have weaker and more fragile bones. We can get vitamin D in food, but we also make it in our skin – but we need ultraviolet light to penetrate the skin to do this. As our far distant ancestors spread to cooler, more temperate, countries there was less ultraviolet radiation (see map). Less UV light was a disadvantage to us because we made less vitamin D. Those who had less melatonin in their skin (lighter colors) would make more vitamin D because there was less melatonin to block the ultraviolet. Dark skinned people living in temperate climates are more likely to be vitamin D deficient, and are advised to take dietary supplements of vitamin D.
Darwin’s ideas were quickly accepted by many of the scientists of his day, but others were horrified. They had been brought up with the idea that living things did not change. As that view often had religious foundations, disputes followed. The case of the peppered moth was subject to considerable debate. The debate was healthy. It led to more experiments, and confirmed that melanism in these moths did influence their survival. The ‘camouflage‘ page has other examples of colors and patterns that make organisms less easy to see.
These are ink-cap fungi, so called because they release spores in a black liquid that looks like ink. The blackness is caused by the spores themselves, as they are pitch black in color. There are quite a number of ink-cap species in the genera Coprinopsis and Coprinus. The mushroom itself should NOT be eaten, it has some bad side effects.
Disputes, debate, and evolution
Disputes and debate are good. Combined with a respect for observations and a readiness to explore uncertainties with fresh observations, then debate is part of science and the evolution of our understanding.
In 1859, Charles Darwin, published a book, The Origin of Species …’, in which he explained how he thought that the number of species and their characteristics had changed over time; and that change was a continuing process. At its time, this was revolutionary, because most people thought that all species had been created at the same time, all looked like they do today, and they did not change. That debate has not gone away.
Scientists are always evaluating their understanding. Are there weaknesses, inconsistencies, or other flaws. Darwin’s insights form one of the most solid foundations for biological science. Consequently, scientists have explored, tested, improved and expanded Darwinian thinking. The underlying argument that species change (evolve) has survived all of the scientific scrutiny. Darwin pointed to ‘variation’ within species. Some were bigger, or differently colored, or faster, lived longer under extreme conditions and so on. Darwin argued that variation would ensure that some organisms were more ‘fit’ when conditions got rough, and were more likely to survive. This was referred to as ‘natural selection’. The loss of species (their extinction), and threats of extinction to those that survive confirm that natural selection takes place. The peppered moth provided evidence of variation (there were dark and light forms), there was selection (the dark forms became more common over a 50 year period), and a reason could be given for this (the dark forms were less likely to be seen and eaten when associated with soot-blackened environments). But this runs counter to the view that the species do not change.
The dispute was not within science. Rather, it is a dispute about which approach we use to understand the world. Science is using observation and testing to improve our understanding. Scientific knowledge is always growing and changing. Others prefer fixed and unchanging views – they prefer non-scientific beliefs.
You may encounter these discussions. It is good to engage. Maple Ferryman Nature Readers take the position that observation and reflection are important, and that understanding evolves.
If you have more thoughts, please let us know below.