Natural selection is a powerful yet extremely subtle process. It is constantly at work. Variations among living things, and favouring any that enable them to leave their offspring. Unlike deliberate, or artificial, selection, it does not have any preferred outcomes or ultimate aims, and it cannot encourage features that might be useful in the future unless they are already advantageous now. That means that every current feature of living things has evolved through an almost infinite series of steps, each of which helped its owner to survive.
Natural Selection is often summed up as “survival of the fittest”- a phrase invented by Herbert Spencer, a 19thh century philosopher. It is a neat summary of Darwin’s key idea, but in evolutionary terms; it does not have quite the same meaning as it does in everyday life. That is because the fittest individuals are not necessarily the strongest or fastest. In some cases, fitness can come through what look like negative qualities, such as being small and timid rather than big and bold.
Another feature of Darwinian fitness is that is does not necessarily stay the same. Sometimes that is fit today may not be quite so fit tomorrow, because the environment has changed. Natural selection does not linger over past favourites: if they are at a disadvantage, they are sidelined without delay. Persistent losers leave fewer and fewer offspring, so their features become more and more rare. Eventually, they become extinct. It is easy enough to imagine hoe natural selection can favour changes that produce immediate benefits, such as human eyes.
Nevertheless, he as convinced that eyes evolved in a series of small steps- a view shared by biologist today. Eyes certainly look as if they have been created intact, because all their parts work together as a whole. But a close look at the animal kingdom shows that there is a wide spectrum of eye “design”: some eyes are simple, others are complex, but crucially, they all work. By using any one as a springboard, natural selection could make one that is till more sophisticated- a powerful argument for evolution step-by-step.
Natural selection emphasizes features that help in the survival stakes, but it also minimizes ones that are no longer pulling their weight. A striking example involves the wings of birds. For birds, flying involves a major investment in muscle power- and it is one that normally pays off in the struggle to survive. But if a bird evolves that feeds and nest on the ground, and has few significant predators, it no longer needs to fly. In these conditions, natural selection favours smaller flight muscles and wings, eventually creating species that can no longer fly.
This process is the same one that produces vestigial organs*- ones that no longer have a useful function at all. A special form of selection occurs in species where individuals of one sex- usually the males- compete with each other to attract mates. In this contest, any adaptation that successfully attracts females is an advantage, because it allows a male to father the young. This process, known as sexual selection, explains why some male animals have eye-catching “accessories”, such as flamboyant plumage or extra-large antlers.
Sexual selection can favour these adaptations even if they are a handicap in daily life. Over many generations, it tends to make males and females more and more different- a situation called sexual dimorphism. HOMINIDS Hominids came into being when our ancestors finally parted company with the apes, a evolutionary split that is now dated at between 6-8 million years ago. A host of fossil remains makes it clear that hominids originated in Africa, and then later spread to other parts of the world.
The first members of the family- belonging to the genus Ardipithecus- had many ape-like features, but the australopithecines, which followed them, had an upright stance, together with slightly larger brains, Homo habilis, which appeared about 2. 4 million years ago, marked the start of the line directly to us. It was the first known toolmaker, and probably the last of our ancestor to live exclusively in Africa. By contrast, it successor, Homo erectus, spread right across Europe and Asia as well. Appearing about 2 million years ago, this species had better tool making skill; and ash from fossil sites suggest that it also used fire.