One of the most fascinating Biology Topics is the study of the brain and behavior.
Evidence of Evolution of Living Organisms – Definition and Its Types
A trait (or characteristic) of an organism that is ‘not inherited’ but develops in response to the environment is called an acquired trait. For example, if a beetle does not get sufficient food for a considerable time, its weight will be reduced due to starvation. The ‘low weight’ of this beetle is an acquired trait of the beetle which has been acquired in response to the environment that contained insufficient food. Again, suppose the tail of a mouse gets cut. The ‘cut tail’ of this mouse is also an acquired trait that has been brought about by some agent in its environment. A man may know how to swim or roller skate or speak French or may have a scar on the face from a cut he got in an accident. All these are acquired traits (or characteristics) that the man has picked up (or acquired) himself as he goes through life. The man is not bom with these traits and he cannot pass on these traits to his children. The acquired traits of organisms cannot be passed on to future generations. The reason for this is discussed below.
Swimming is an acquired trait (or characteristic). It is not present by birth. In other words, the technique of swimming is not inherited from parents. It is learned by the person himself (or herself).
We have already studied that the traits (or characteristics) of parents are passed to their offspring through genes in reproductive cells (or gametes) during the process of reproduction. So, for the trait of an organism to be passed on, it must have been caused by a change in the genes (or DNA) present in the reproductive cells of the organism. In other words, only those traits can be transmitted to future generations in which changes have occurred in the genes (or DNA) present in the reproductive cells (or gametes) of parent organisms. The changes in the non-reproductive body cells of an organism cannot be inherited by its offspring. This will become clear from the following examples.
When the weight of a beetle is reduced too much due to starvation, then though there is a change in the normal body cells of the beetle no change takes place in the genes (or DNA) present in its reproductive cells (or gametes). And since there is no change in the genes (or DNA) of gametes, this acquired trait (of low weight) of beetle cannot be inherited by its offspring. So, if some generations of beetles are low in body weight because of the availability of less food, then this is not an example of evolution because this change cannot be inherited over generations. Whenever these beetles will get sufficient food, they will become healthy again and the trait of Tow body weight’ will disappear.
Let us discuss the other example now. If we breed some mice, all the progeny of mice will have tails, just like their parents. Now, if we cut the tails of these first-generation mice surgically and breed them, we will get new mice, all with full tails. It has been observed that even after cutting the tails of mice for several generations, a tail-less mouse is never born. The cut tail of mice is an acquired trait that is never passed on to their progeny. This is because cutting the tails of mice does not change the genes of their reproductive cells (or gametes). And since the acquired trait of ‘cut tails’ does not bring about a change in the genes of mice, this trait cannot be passed on to the next generations. From this discussion, we conclude that the experiences of an individual during its lifetime (called acquired traits) cannot be passed on to its progeny, and hence cannot lead to evolution (because they are not caused by the change in genes).
(a) These are first-generation mice. All these mice have full tails.
(b) The tails of all the mice are cut surgically so that they become tail-less mice.
(c) All the second-generation mice produced by breeding cut-tail mice have full tails. No tail-less mouse is born
The cut-tail of mice is an acquired trait that cannot be passed on to their progeny in future generations through the process of reproduction.
A trait (or characteristic) of an organism that is caused by a change in its genes (or DNA) is called an inherited trait. Inherited traits can be passed on to the progeny of the organism because they have produced changes in the genes (or DNA) of the organism. Suppose there is a population of red beetles in the green bushes. Again suppose that a colour variation arises during reproduction so that there is one beetle that is green in colour (instead of red). This change of green colour in the beetle has been brought about by a change in the genes (or DNA) of the reproductive cells. The green colour of this beetle is an inherited trait that can be passed on to the next generations. The change from red beetle to green beetle can be considered to be an example of evolution because it helps in its survival by mixing with green bushes.
The colour change of beetle from red to green has been brought about by a change in the genes (or DNA) of its reproductive cells. So, the green colour of a beetle is an inherited trait that can be passed on to its progeny.
Inherited traits mean the characteristics which we receive from our parents. This point will become clear from the following example. Suppose a father has red curly hair, brown eyes, a snub nose, and a cleft chin [see Figure (a)]. Again suppose that the mother has straight black hair, blue eyes, a long thin nose, and a pointed chin [see Figure (b)].
This picture shows some of the characteristics which the children inherit from their parents. The development of these characteristics is controlled by the genes on the chromosomes.
The children in the family inherit some characteristics from each of their parents. For example, two children have red hair like their father but one of them has straight red hair while the other one has curly red hair. The two children have black hair like the mother. Again, two children have brown eyes like the father but the other two have blue eyes like the mother. And finally, two children have a snub nose and cleft chin like the father whereas the other two have long thin noses and pointed chins.
Evolution of Living Organisms
There are an enormous ‘number’ and ‘types’ of living organisms (plants and animals) on this earth. In addition to this wide variety of living organisms, the remains of the dead organisms which lived in the remote past (called fossils) are also known. An important question now arises: How and from where has such a great variety of living organisms come to exist on this earth? Also, how the human beings evolved on this earth? All these things are studied in the branch of biology called ‘evolution’. The word ‘evolution’ has been derived from the Latin word ‘evolvere’ which means to ‘unroll’ or ‘unfold’.
Evolution is a kind of gradual unfolding (or formation) of new organisms from pre-existing primitive organisms through slow and steady changes. We can now define evolution as follows: Evolution is the sequence of gradual changes that take place in primitive organisms over millions of years in which new species are produced. Since the evolution is of living organisms, it is also called ‘organic evolution’. It is through the constant process of evolution taking place in the organisms since the origin of life that such an enormous variety of plants and animals have come to exist on this earth at present. All the plants and animals (or organisms) which we see today around us have evolved from some or the other ancestors that lived on this earth long, long ago.
The process of evolution will become clear from the following example of ‘pterosaur’. Pterosaur is an ancient flying reptile that lived on the earth about 150 million years ago. The development of pterosaur is an example of evolution. It began life as a big lizard that could just crawl on land [see Figure (a)]. Over millions of years, small folds of skin developed between its feet which enabled it to glide from tree to tree [see Figure (b)], Over many, many generations, spread over millions of years, the folds of skin, and the bones and muscles supporting them grew to form wings which could make it fly [See Figure (c)]. In this way, an animal that crawled on the ground evolved into a flying animal. This evolution led to the formation of a new species (a flying reptile).
The development of ‘pterosaur’ (an ancient flying reptile) from a big lizard is an example of evolution.
Evidence for Evolution of Living Organisms
Various biological studies tell us that since their origin, living organisms have been changing their organization to evolve into new forms. Several common features of different kinds of organisms provide evidence in favour of evolution because they can be considered to have evolved from a common ancestor. The more characteristics (or features) two species have in common, the more closely they will be related. And the more closely they are related, the more recently they will have had a common ancestor. We will now give some of the evidence which indicates the occurrence of evolution. This evidence reinforces the view that living organisms have evolved from common ancestors. Some of the important sources which provide evidence for evolution are Homologous organs, Analogous organs, and Fossils. We will now discuss all this evidence for evolution briefly.
1. Homologous Organs Provide Evidence for Evolution
If we look at how living organisms are made, we can often see quite striking similarities in their construction. One of these is the presence of homologous organs. Those organs which have the same basic structure (or same basic design) but different functions are called homologous organs. The homologous organs of different animals provide evidence for evolution. This will become clear from the following examples.
There are many organs in different groups of animals (or plants) which all seem to be built from the same basic design but are used for many different purposes. These are called homologous organs. For example, the forelimbs of a man, a lizard (reptile), a frog (amphibian), a bird, and a bat (mammal) seem to be built from the same basic design of bones (as shown in Figure), but they perform different functions. The forelimbs of a human (man) are used for grasping; the forelimbs of a lizard are used for running; the forelimbs of a frog are used to prop up the front end of its body when at rest, and also act as shock absorbers when the frog lands back on the ground after a leap; whereas the forelimbs of a bird and a bat are modified for flying. Since the forelimbs of a human, a lizard, a frog, a bird, and a bat have similar structures (or designs) but perform different functions, they are homologous organs.
This diagram shows that the forelimbs of a human (man), a lizard, a frog, a bird, and a bat have the same basic design of bones. They are homologous organs.
The presence of homologous forelimbs in humans (man), a lizard; a frog, a bird, and a bat indicate that all these forelimbs have evolved from a common ancestral animal that had a ‘basic design’ limb. In other words, it tells us that a human, a lizard, a frog, a bird, and a man, all have evolved from a common ancestor. Thus, the presence of homologous organs in different animals provides evidence for evolution by telling us that they are derived from the same ancestor who had the ‘basic design’ of the organ on which all the homologous organs are based. Please note that the wings of a butterfly (which is an insect) and the wings of a bat cannot be considered to be homologous organs because they have different basic designs (though they are used for the same purpose of flying).
2. Analogous Organs Provide Evidence for Evolution
Those organs which have different basic structures (or different basic designs) but have a similar appearance and perform similar functions are called analogous organs. The analogous organs provide evidence for evolution. This point will become clear from the following discussion.
Many organs in different groups of animals seem to be built from different basic structures but appear to be similar in shape and perform similar functions. These are called analogous organs. For example, the wings of an insect and a bird have different structures (the insects have a fold of membranes as wings which are associated with a few muscles whereas a skeleton, flesh, and feathers support a bird’s wings) but they perform the same function of flying (see Figure). Since the wings of insects and birds have different structures (or different designs) but perform similar functions, they are analogous organs.
The wings of an insect and a bird have different structures but similar functions. They are analogous organs.
Now, since the analogous organs have different basic designs, so they do not indicate a common ancestor for the organism. The analogous organs provide evidence for the evolution in another way. The presence of analogous organs indicates that even organisms having organs with different structures can adapt to perform similar functions for their survival under hostile environmental conditions. Thus, the presence of analogous organs in different animals provides evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing ‘environment. The analogous organs provide a mechanism for evolution.
3. Fossils Provide Evidence for Evolution
The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other features, which are found in reptiles. This is because Archaeopteryx has feathered wings like those of birds but teeth and tails like those of reptiles. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide evidence that the present animals (and plants) have originated from the previously existing ones through the process of continuous evolution.
We will now describe how fossils are formed. Usually, when organisms (plants or animals) die, their bodies will decompose by the action of micro-organisms in the presence of oxygen, moisture, etc.
Archaeopteryx is a connecting link between reptiles and birds.
Sometimes, however, the conditions in the environment are such (like the absence of oxygen or moisture, etc), which do not let the body of the organism decompose completely. It is such body (or body part) of an organism that we get as fossils on digging the earth (see Figure). In many cases, the soft parts of the organisms get decomposed and what we get as a fossil is a skeleton of hard parts (like bones, etc). Even the soft parts of plants and animals (which usually decompose quickly) are sometimes preserved as fossils in the form of their impressions inside the rocks.
This animal fossil was found in the desert buried under the sand.
For example, if a dead leaf gets caught in mud, it will not decompose quickly. The mud around the leaf will set around it as a mould, gradually harden to form a rock, and retain the impression of the whole leaf. This forms a leaf fossil that can be dug out from the earth a long time later (see Figure). The fossil of a dead insect caught in mud is also formed in a similar way to a leaf fossil. All such preserved impressions of the body parts of the once-living organisms are also called fossils.
This leaf fossil was found in rocks.
Fossils are obtained by digging into the earth. The age of fossils can be estimated in two ways: by the relative method, and by the carbon dating method. The relative method works like this: When we dig into the earth; we find fossils at different depths. The fossils which we find in layers closer to the surface of the earth are more recent; the fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of the earth are the oldest ones. Fossils that we find today were once living objects. All living objects contain some radioactive carbon-14 atoms. When a living object dies and forms a fossil, its carbon-14 radioactivity goes on decreasing gradually. In the carbon dating method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.
The scientists who study fossils are called paleontologists. This picture shows the fossilized remains of a dinosaur being studied by paleontologists.
There are various kinds of fossils. Some of the important fossils which have been studied are those of ammonite, trilobite, and dinosaur. Ammonites were invertebrate animals (mollusks) with flat, coiled, spiral shells that lived in the sea [Figure (a)]. The estimation of the age of ammonite fossils has told us that they are about 180 million years old. This means that ammonites lived in the sea about 180 million years ago. Another invertebrate animal fossil that has been studied is that of trilobite [Figure (b)]. Trilobites were marine arthropods that were common between 400 to 600 million years ago. Dinosaurs are extinct carnivorous or herbivorous reptiles (The word ‘dinosaur’ means ‘terrible lizard’). The estimation of the age of dinosaur fossils [Figure (c)] has told us that they first appeared on Earth about 250 million years ago and became extinct about 65 million years ago. It is clear from the above discussion that we can even study those species which are extinct (no longer exist), by studying their fossils which are found during the digging of earth.
Various kinds of fossils.
Darwin’s Theory of Evolution
Charles Robert Darwin gave the theory of evolution in his famous book ‘The Origin of Species’. The theory of evolution proposed by Darwin is known as The Theory of Natural Selection. This theory is called the theory of natural selection because it suggests that the best-adapted organisms are selected by nature to pass on their characteristics (or traits) to the next generation. Darwin’s theory of evolution applies to plants as well as animals.
Darwin’s theory of evolution can be described as follows:
- Within any population, there is natural variation. Some individuals have more favourable variations than others.
- Even though all species produce a large number of offspring, populations remain fairly constant naturally.
- This is due to the struggle between members of the same species and different species for food, space, and mate.
- The struggle for survival within populations eliminates the unfit individuals. The fit individuals possessing favourable variations survive and reproduce. This is called natural selection (or survival of the fittest).
- The individuals having favourable variations pass on these variations to their progeny from generation to generation.
- These variations when accumulated over a long period, lead to the origin of a new species.
Charles Darwin: The scientist who gave the theory of evolution.
We will now understand Darwin’s theory of evolution by ‘natural selection’ by taking an example. No two animals are ever exactly alike. So some changes always appear when animals produce their progeny by sexual reproduction. For example, one of the progeny may be tall (having long legs) than the other progeny. Thus, there may be a variation in height in the progeny [see Figure (a)]. Now, the advantage of long legs to the progeny is that when no food (grass, etc.) is available on the ground, then this progeny having long legs can reach the leaves on tall trees, eat them as food and survive [see Figure (b)].
On the other hand, the progeny which has short height (due to short legs) cannot reach the leaves on tall trees, they will not get any food, they will starve, and hence die [see Figure (b)]. Thus, nature has selected the animal with long legs to survive (because it is the fittest animal under these circumstances). Now, since long legs help in survival, the long-legged animals will live long enough to produce their offspring. The offspring will inherit long legs. So, all the future generations will have long-legged animals [see Figure (c)], In this way, the animals having short legs have evolved into animals having long legs due to variation. This is an example of evolution.
An example to illustrate Darwin’s theory of evolution (by natural selection).
We can now define natural selection as follows: Natural selection is the process of evolution of a species whereby characteristics that help individual organisms to survive and reproduce are passed on to their offspring, and those characteristics which do not help are not passed on. Though Darwin’s theory was widely accepted, it was criticized on the ground that it could not explain ‘how the variations (which lead to .origin of new species) arise’. With the progress in genetics, the source of variations was explained to be the ‘genes’. Genes vary in a natural population. Genetic variation is the raw material of evolution. So, Darwin’s theory was modified accordingly. These days, the most accepted theory of evolution is the Synthetic Theory of Evolution in which the origin of species is based on the interaction of ‘genetic variation’ and ‘natural selection’.
Sometimes, a species (a type of animal or plant) may completely die out. It may become extinct. Dodo was a large flightless bird that has become extinct (see Figure). Once a species is extinct, its genes are lost forever. It cannot re-emerge at all. The small numbers of surviving tigers are a cause of worry from the point of view of genetics because if they all die out and become extinct, their genes will be lost forever (see Figure). Our coming generations will not be able to see tigers at all. We should, therefore, make all-out efforts to protect tigers (and other endangered species) to prevent them from extinction.
Dodo (a large flightless bird) found in Mauritius is no longer alive. It has become extinct.
Only a small number of tigers are alive today. Tigers are threatened with extinction shortly.