Biology X | Heredity and Evolution | EVOLUTION SHOULD NOT BE EQUATED WITH ‘PROGRESS’ and Human Evolution

EVOLUTION SHOULD NOT BE EQUATED WITH ‘PROGRESS’

In an exercise of tracing the family trees of species, we need to remember certain things. Firstly, there are multiple branches possible at each and every stage of this process. So it is not as if one species is eliminated to give rise to a new one. A new species has emerged. But that does not necessarily mean, like the beetle example we have been thinking about, that the old species will disappear. It will all depend on the environment. Also, it is not as if the newly generated species are in any way ‘better’ than the older one. It is just that natural selection and genetic drift have together led to the formation of a population that cannot reproduce with the original one. So, for example, it is not true that human beings have evolved from chimpanzees. Rather, both human beings and chimpanzees have a common ancestor a long time ago. That common ancestor is likely to have been neither human or chimpanzee. Also, the first step of separation from that ancestor is unlikely to have resulted in modern chimpanzees and human beings. Instead, the two resultant species have probably evolved in their own separate ways to give rise to the current forms.

In fact, there is no real ‘progress’ in the idea of evolution. Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection. The only progressive trend in evolution seems to be that more and more complex body designs have emerged over time. However, again, it is not as if the older designs are inefficient! So many of the older and simpler designs still survive. In fact, one of the simplest life forms – bacteria – inhabit the most inhospitable habitats like hot springs, deep-sea thermal vents and the ice in Antarctica. In other words, human beings are not the pinnacle of evolution, but simply yet another species in the teeming spectrum of evolving life.

Human Evolution

The same tools for tracing evolutionary relationships – excavating, time-dating and studying fossils, as well as determining DNA sequences – have been used for studying human evolution. There is a great diversity of human forms and features across the planet. So much so that, for a long time, people used to talk about human ‘races’. Skin colour used to be the commonest way of identifying these socalled races. Some were called yellow, some black, white or brown. A major question debated for a long time was, have these apparent groups evolved differently? Over recent years, the evidence has become very clear. The answer is that there is no biological basis to the notion of human races. All humans are a single species.

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Figure 9.14

Evolution — Ladder versus Tree

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Biology X | Heredity and Evolution | Evolution by Stages

Evolution by Stages

A question that arises here is – if complicated organs, such as the eye, are selected for the advantage they provide, how can they be generated by a single DNA change? Surely such complex organs will be created

bit-by-bit over generations? But how can each intermediate change be selected for? There are a number of possible explanations. Even an intermediate stage (Fig. 9.11), such as a rudimentary eye, can be useful to some extent. This might be enough to give a fitness advantage. In fact, the eye – like the wing – seems to be a very popular adaptation. Insects have them, so does an octopus, and so do vertebrates. And the structure of the eye in each of these organisms is different – enough for them to have separate evolutionary origins.

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Also, a change that is useful for one property to start with can become useful later for quite a different function. Feathers, for example, can start out as providing insulation in cold weather (Fig. 9.12). But later, they might become useful for flight. In fact, some dinosaurs had feathers, although they could not fly using the feathers. Birds seem to have later adapted the feathers to flight. This, of course, means that birds are very closely related to reptiles, since dinosaurs were reptiles!

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