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Imagine you could scan the earth with a life-detecting probe. You'd be stunned to find millions of living organisms, interacting in the most amazing ways. From leopards, lemurs and lice to ferns and flesh-eating plants, the variety of life forms that populate our planet is mind-boggling. Scientists refer to it as biodiversity. Biodiversity is more than just a lengthy list of the plants, animals, fungi and microorganisms found worldwide. It also encompasses the genes they contain, and the interactions that take place.
3.7 billion years ago, this planet was a tough place for life. Hot lava spewed from the surface of the earth and beneath the sea. Much of the land was dotted with boiling hot springs, and the atmosphere was thick with steam and carbon dioxide. So how did our planet become the hotbed for living creatures that we know today?
Scientists believe that electrical energy from lightning, heat from volcanoes and intense UV light allowed simple gases in the earth's primitive soup to react. The result of this grand-scale chemical reaction was life's building blocks: amino acids (the basic molecules that make up proteins), simple sugars, and nucleic acids (the basic molecules in DNA and RNA).
These molecules accumulated, and bumped into each other, until, several hundred million years later, the first rudimentary cells, surrounded by a membrane emerged. With this primitive life form, the stage was set for the drama of biological evolution to take place.
First came single-celled organisms, bacteria, that lived mostly in mud and water until they did something that radically changed the earth: they produced their food through photosynthesis. Cells could now remove carbon dioxide from the atmosphere and with the help of sunlight combine it with water to make sugars. This was a major breakthrough for life on earth. Why? Because the waste product of this photosynthetic reaction is oxygen.
Oxygen collected in the atmosphere allowing organisms that had evolved first in the seas to make their way onto dry land. This was a gigantic step for life, and it took ages. Microorganisms living in water dominated the planet between 3.7 billion and 570 million years ago, and the first life forms to emerge from a watery existence did so between 400 and 500 million years ago. At that time, scientists believe that UV levels might have been low enough for the first plants and animals to exist on land. 370 million years ago insects and amphibians invaded the land followed by the reptile reign roughly 225 million years ago.
Eventually, the age of mammals began about 65 million years ago. Humans arrived late on the scene, some 100,000 years ago.
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But how did the planet's 3.7 billion-year history spawn the amazing diversity that surrounds us today? The theory of evolution, proposed by Charles Darwin, has the answer. It says that all species living on the Earth trace their ancestors to a single origin of life.
Darwin's breakthrough was to unravel the puzzle of how evolution happens. In 1859, he wrote On the Origin of Species, where he proposed that evolution operates through natural selection, a system that allows species to change and give rise to new species.
In Victorian times, many people were horrified by Darwin's ideas. The concept that species might have changed over time not only defied religious orthodoxy, but it also went against their instinctive beliefs. They knew that bats, for example, remained bats generation after generation; they did not turn into anything different. The idea that all species descend from earlier, ancestral species, was even more shocking as it implied that humans might have evolved from something similar to an ape. (In fact, the theory of evolution does not claim that humans descend from apes, but that we descend from a common ancestor.) Even today some people believe that each species was made by God and has not changed.
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Natural selection is wonderfully simple: it is the inevitable result of competition between organisms. Take elephants, for example. Elephants face environmental pressures, such as a limited supply of resources. The trees and grass they eat cannot multiply as quickly as the elephants. So in every generation some elephants will starve to death. Indeed, in wild populations, there is a high death rate, evidence of a constant struggle for existence.
Because the offspring in each generation differ slightly from one another, when competing for resources, some inevitably do better than others. Those individuals with characteristics that boost their chances of survival are more likely to reproduce and pass on these advantageous characteristics to their offspring. Over time, more and more of the population will come to possess these beneficial characteristics.
The concept of genes was unknown to Darwin. He puzzled over what could be transmitting the information from generation to generation, and yet be open to change. Once genes were discovered, all the pieces of the evolutionary jigsaw fell into place. We can now summarise the process of natural selection as: genes mutate, successful individuals are selected, and populations evolve.
Let's follow an example of natural selection at work. Before the industrial revolution in the mid-1800s, the speckled light-grey form of the peppered moth was prevalent in England. When these moths rested on light-grey lichens on tree trunks, their colour camouflaged them from their bird predators. A dark-grey form also existed but was rare. During the industrial revolution, soot darkened the colour of tree trunks, and the dark form of moth became the common one, especially near industrial cities, because it blended in with the blackened trees. Through natural selection, the dark form began to survive and reproduce at a greater rate than the light-coloured ones.
As with these moths, natural selection leads to a remarkable fit between organisms and their environment. It is amazing to discover that species in similar environments growing in different parts of the world tend to evolve similar characteristics. For example, in deserts, shrubs tend to have the same set of traits that enable them to grow in hot, dry conditions. These adaptations include; deep roots to access water, small leaves to reduce heat loads, hairs or waxy coatings that protect against intense sunlight.
Under certain circumstances natural selection can transform one species into two different ones. The creation of a new species is called speciation. It may sound freaky, but it happens often in nature.
Two things need to happen for a species to split: the first is separation, geographic isolation. Say, two groups of the same population become physically separated by a physical barrier such as a mountain range, stream, lake or road, or by a volcanic eruption or earthquake for fairly long periods into areas with different environmental conditions.
The second phase of speciation is reproductive isolation. When populations become geographically isolated, the mutations that take place affect the genes of each group differently, in a process called divergence. If divergence continues long enough, members of the isolated populations may become so different in genetic makeup that they cannot interbreed. At that point, one species has become two, and speciation has occurred.
For instance, the early fox population became two species when some of them spread north in search of food and others migrated south and began living in another area with different environmental conditions. The result is the arctic fox, which is adapted to cold with heavier, white fur that matches snow for camouflage, and short ears, and the grey fox of the south, adapted to heat through lightweight fur and long ears, legs and nose which give off more heat.
Speciation can take place within only a few hundred years, if the organisms reproduce rapidly. However, with most species, speciation takes from tens of thousands to millions of years. Given this time scale, it is difficult to observe and document the appearance of a new species. Not surprisingly, there are many controversial hypotheses about how speciation actually takes place.
Discuss it...
With plentiful resources and medical advances, have humans overridden natural selection? |
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