Part 5
Chapter 3 – The Primrose Path to Macro-Evolution
In his “Origin of Species” Darwin first drew his readers attention to the dramatic variety that was possible through the artificial selection of domestic animals.
This was the theme of chapter 2 of Dawkins book where he showed how humans, through selective breeding, have “sculpted and kneaded dog flesh to assume a bewildering variety of forms, colours, sizes and behaviour patterns.”
In this third chapter he leads his readers through some intermediate stages on the path from artificial to natural selection, or as he calls it “a step by step seduction of the mind.”
Probably every flower that are coloured anything but green and whose smell is anything more than just vaguely plant-like owes it’s form to generations of selection, but unlike the journey from wolf to Chihuahua the selector was not human but animal.
In order to pass their genes on to the next generation flowers need to pollinate other specimens of the same species. Some achieve this by producing huge quantities of pollen that is randomly carried on the wind. Others are equipped to employ birds and insects to do the task more efficiently. By offering the reward of energy-rich nectar, bees, butterflies, hummingbirds and even bats can be bribed to transport pollen on behalf of the flower. These creatures have nervous systems capable of being attracted by particular colours shapes and patterns, which increases the chance that their load will reach its intended destination.
Some flowers have simply evolved a general colour that is attractive to insects or birds. Flowers that may look plain to us often have spots or lines that are visible only to insects who see the ultra-violet end of the spectrum. Although the insect may then go and visit a number of other species the chances of the pollen reaching its intended destination is still greatly increased. A meadow full of flowers is like natures Times Square, a slow-motion neon sign that changes from week to week through the year.
Other flowers have gone further and become totally dependant on one particular bird or bee to do it’s bidding. Both Darwin and Wallace commented on the amazing orchid Angraecum sesquipedale which has nectaries more than 11 inches long. In 1862 Darwin predicted that there must be an, as yet undiscovered, moth capable or exploiting this. It was not until after his death that a new species, Darwin’s Hawk Moth was discovered in Madagascar in 1903 with a proboscis that fulfilled his prediction.
Insect pollination represents a huge advance in economy over the wasteful scattergun of wind pollination. To the insect the flower is a reliable source of nectar; to the flower the insect is like a well-paid Federal Express service. Each side could be said to have domesticated the other, selectively breeding them to do a better job than before. Human gardeners have simply taken the process a step further, selecting from existing specimens for colour and perfume.
Animals also select traits in one another. Peacocks would escape predators better if they had drab feathers and small tails but generations of peahens have selected the gaudiest peacocks to mate with thus selecting genes for big bright tails to pass on to the next generation of males and the reference for the same onto the next generation of female. The same is true for fish (we will come to an astonishing example involving guppies in a later chapter) reptiles, amphibians and mammals.
It is not only the eyes that have selected traits in flora and fauna; ears have played their part too. Generations of female canaries have selected males with the best voices to mate with. It is now known that the sound of the male call (even a tape recording) causes the females’ ovaries to swell and produces hormones that bring them into reproductive condition. Males are in effect giving female canaries a hormone injection when they sing. At the same time females are selectively breeding males who have the best voices; it is two sides of the same coin.
Unlike female creatures who positively breed males for certain features some predators shape the populations of their prey buy avoiding certain features. There is a caterpillar whose butt looks exactly like a snake. Not surprisingly these caterpillars get left alone by birds. Generations of birds have unknowingly been selectively breeding caterpillars who most closely resemble the snake.
Deep sea angler fish lure their prey in the almost total darkness of the deep with a spine that has become elongated and which has at it’s tip a receptacle that contains bacteria that is bioluminescent. Like little neon signs that say, “eat me” they attract unsuspecting prey. By being attracted to the most convincing lures the victims are selectively breeding anglerfish to become increasingly effective at their deception. Anglers with less attractive lures are more likely to starve to death and it’s the prey that is doing the selecting.
At this point Dawkins provides a short summary of the path so far.
1. Humans choose features of flowers and animals through ARTIFICIAL selection. This is powerful enough to turn wolves into Chihuahuas and stretch maize cobs from inches to feet in length.
2. Many female birds and animals choose attractive mates for breeding in a process Darwin called sexual selection
3. Small prey fish choose attractive anglerfish for survival by feeding the most attractive specimens with their own bodies, thereby inadvertently choosing them for survival and passing on their attractive genes to the next generation. This is NATURAL selection and was Darwin’s greatest discovery.
4. Now without any kind of choosing agent, individuals who are “chosen” for survival by the fact that they happen to posses superior equipment to survive than their peers. They will then pass on their superior genes to the next generation and so every gene pool in every species tends to become filled with genes for making superior equipment for survival and reproduction.
Artificial selection is then a special case of natural selection.
The general case is the non-random survival of randomly varying hereditary equipment.
The ultimate test for a scientific hypothesis is experiment and Dawkins book contains numerous examples where the power of selection has been illustrated in the lab. The chapter concludes with three such examples....
