Evolutionary Biology Topics allow us to trace the history of life on Earth.
Factors Affecting Speciation and Evolution
A species is a population of organisms consisting of similar individuals which can breed together and produce fertile offspring. Species can be of plants or of animals. Wheat, paddy, sunflower, lotus, mango, neem, humans, tiger, dog and cats, etc., are all examples of various types of species. Human beings who look so different from each other in terms of size, colour, and looks are said to belong to the same species (Homo sapiens) because they can interbreed to produce fertile offspring (sons and daughters). The process by which new species develop from the existing species is known as speciation. In simple words, the formation of new species is called speciation. We will now explain how new species are formed from the existing species of various populations.
In most cases, new species are formed when the population of the same species splits into two separate groups which then get isolated from each other geographically by the barriers such as mountain ranges, rivers, or the sea. The geographical isolation of the two groups of population leads to their reproductive isolation due to which no genes are exchanged between them. However, breeding continues within the isolated populations producing more and more generations.
Over the generations, the processes of genetic drift (random change in gene frequency), and natural selection operate in different ways in the two isolated groups of population and make them more and more different from each other. After thousands of years, the individuals of these isolated groups of the population become so different that they will be incapable of reproducing with each other even if they happen to meet again. We say that two new species have been formed.
From the above discussion, we conclude that the important factors which could lead to the rise (or formation) of a new species are the following:
- The geographical isolation of a population is caused by various types of barriers (such as mountain ranges, rivers, and seas).
- The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of the population.
- Genetic drift is caused by drastic changes in the frequencies of particular genes by chance alone.
- Variations are caused in individuals due to natural selection.
It should be noted that geographical isolation is the major factor in the speciation of sexually reproducing animals because it interrupts the flow of genes between their isolated populations through the gametes. Geographical isolation, however, cannot be a major factor in the speciation of a self-pollinating plant species because it does not have to look to other plants for its process of reproduction to be carried out. Geographical isolation also cannot be a major factor in the speciation of an asexually reproducing organism because it does not require any other organism to carry out reproduction.
Evolution of Eyes
The eye is a very important organ for animals. The eye is a complicated organ that cannot be generated by a single DNA change. The complex body organs of animals such as eyes have been created in ‘stages’ over many generations. First of all the rudimentary eye (basic eye) like that of a flatworm (Planaria) was formed (see Figure). The eyes of flatworms are very simple that are actually just ‘eye spots’ which can detect light. Even these rudimentary eyes provide a survival advantage to flatworms. Starting from this basic design, more and more complex eyes then evolved in various organisms. Most of animals have eyes. For example, insects, octopuses, and invertebrates, all have eyes. The structure of eyes in each of these organisms is, however, different which suggests their separate evolutionary origins. The evolution of the eye is an example of evolution by stages.
A flatworm (Planaria) has very simple eyes called rudimentary eyes.
Evolution of Feathers
Sometimes an evolutionary change produced in an organism for one purpose, later on, becomes more useful for an entirely different function. For example, birds evolved feathers as a means of providing insulation to their bodies in cold weather but later on these feathers became more useful for the purpose of flying. Even some dinosaurs had feathers though they could not fly by using these feathers. Birds, however, adapted feathers for flying. The presence of feathers on birds tells us that the birds are very closely related to reptiles because dinosaurs (which had feathers) were reptiles.
(a) Birds evolved feathers as a means of providing insulation to their bodies in cold weather.
(b) Later on, feathers become more useful to the birds for the purpose of flying
Evolution of feathers.
Evolution by Artificial Selection
In the evidence for evolution, we have studied that very dissimilar-looking structures can evolve from a common ancestral body design. But that was all guesswork about what happened in history a long time ago. We will now give an example from the present time which will show that different-looking organisms can in fact be created from the same basic design of the ancestor.
The wild cabbage plant is a good example to prove that entirely different-looking organisms can evolve from the same organism by the process of evolution. The only difference is that here we are using artificial selection for evolution in place of natural selection. This will become clear from the following discussion. The farmers have been cultivating wild cabbage as a food plant for over two thousand years and have produced (or evolved) entirely different-looking vegetables like cabbage, broccoli, cauliflower, kohlrabi, and kale from it by artificial selection (see Figure).
The production of vegetables like cabbage, broccoli, cauliflower, kohlrabi, and kale from .wild cabbage by the farmers is a case of evolution by artificial selection.
- Some farmers wanted to have very short distances between the leaves of wild cabbage and produced the common variety of ‘cabbage’.
- When farmers opted for the arrested flower development of wild cabbage plants, it led to the production of another variety of cabbage called ‘broccoli’.
- Some farmers went in for sterile flowers of wild cabbage and developed another variety of cabbage called ‘cauliflower’.
- When farmers opted for the swollen parts of wild cabbage, it led to the evolution of yet another variety of cabbage called ‘kohlrabi’.
- And finally, the farmers wanted to grow large leaves of wild cabbage and ended up producing a leafy vegetable called ‘kale’ which is also a variety of wild cabbage.
Now, wild cabbage is the ancestor, and cabbage, broccoli, cauliflower, kohlrabi, and kale are all varieties that have been obtained by evolution ‘induced artificially’ by the farmers. The ordinary cabbage, broccoli, cauliflower, kohlrabi, and kale look so different from their ancestor wild cabbage that if people had not seen it being done with their own eyes, they would never have believed that vegetables having such different structures can be evolved from the same ancestral vegetable plant.
Evolution should not be Equated with Progress
There is no real progress in the concept of evolution. Evolution is just the production of a diversity of life forms and the shaping of this diversity by environmental selection. The only progress in evolution appears to be that more and more complex body designs of organisms have emerged over the ages. This will become clear from the following examples.
When a new species is formed, it is not necessary that the old species will disappear (or get eliminated) from Earth. It will all depend on the environment. Also, it is not as if the newly formed species are in any way better than the older ones. It is simply that genetic drift and natural selection processes have combined to form a population having different body designs which cannot interbreed with the older population. It is a common belief that chimpanzees are the ancestors of human beings. It is, however, not true that human beings have evolved from chimpanzees.
Actually, both chimpanzees and human beings had a common ancestor a long time ago. The two offspring of that ancestor evolved in their own separate ways to form modern-day chimpanzees and human beings. Again, it is not as if the body designs of older organisms were inefficient. This is because many of the older and simpler forms of organisms still survive on Earth. For example, one of the simplest and most primitive life forms called ‘bacteria’ still inhabit some of the most inhospitable (or unfavourable) habitats such as hot springs, deep-sea thermal vents, and the ice in Antarctica. Most other organisms cannot survive in such harsh environments.
Human beings have not evolved from chimpanzees. Actually, both human beings and chimpanzees had a common ancestor a long time ago.
Human evolution has been studied by using various tools of tracing evolutionary relationships like excavating (digging earth), carbon-dating, studying fossils, and determining DNA sequences. There is so much diversity in the human body and features on the earth that for a long time people used to talk about different ‘races’ of human beings. The human races were even identified on the basis of their skin colour and named white, black, yellow or brown. It is now known that the so-called human race has not evolved differently. In fact, there is no biological basis for dividing human beings into different ‘races’. All human beings (whether, white, black, yellow, or brown) are a single species (called Homo sapiens).
It has now been established by research that the earliest members of the human species (Homo sapiens) came from Africa. So, irrespective of where we have lived for the past few thousand years, we all come from Africa. In other words, our genetic footprints tell us that we have African roots. About a hundred thousand years ago, some of our ancestors left Africa while others stayed back. Those who left Africa slowly spread across the whole earth.
Mendel’s experiments tell us the mode of inheritance of traits from one generation to the next and Darwin’s theory of evolution tells us how organisms develop from simple to more complex forms. But neither tells us anything about how life originated on Earth (or began on Earth). We will now discuss the origin of life on Earth briefly.
Origin of Life on Earth
A British scientist J.B.S. Haldane suggested in 1929 that life must have developed from the simple inorganic molecules (such as methane, ammonia, hydrogen sulphide, etc.) which were present on the earth soon after it was formed. He said that the conditions on earth at that time (including frequent lightning) could have converted simple inorganic molecules into complex organic molecules which were necessary for life. These complex organic molecules must have joined together to form the first primitive living organisms. Haldane also suggested from theoretical considerations that life (or living organisms) originated in seawater.
Scientists have come to the conclusion that life (or living organisms) originated in seawater.
The theory of the origin of life on earth proposed by Haldane was confirmed by experiments conducted by Stanley L. Miller and Harold C. Urey in 1953. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like methane, ammonia, hydrogen sulphide, etc., (but no oxygen), over water. This was maintained at a temperature just below 100°C and electric sparks were then passed through the mixture of gases (to simulate lightning) for about one week. At the end of one week, it was found that about 15 percent of carbon (from methane) had been converted into simple compounds of carbon including ‘amino acids which make up protein molecules found in living organisms. This experiment provides evidence that life originated from inanimate matter (or lifeless matter) like inorganic molecules.