Heredity and Evolution

Speciation

Species are a group of actually or potentially interbreeding natural population which can be reproductively isolated from other such groups. The phenomenon of formation of new species from pre-existing one is called speciation. The origin of new species symbolizes the dynamics of organic evolution.

Minute changes occurring below species level is called microevolution, while evolution of new species and genera is known as macroevolution. The example of beetles represents microevolution in which simply change in characters had occurred within a species. It does not explain the origin of new species. In order to explain the mechanism of speciation, let us consider that the bushes on which the beetles feed are spread over a wide range of area Beetles as a habit do not travel far, but when the population of beetles increases in a particular place and food becomes scarce, they migrate by forming sub-population into different areas of their feeding range. This results in the formation of several such species of beetles.

Speciation may be summarized in the following way. A species may have different populations. The different population of a species may inhabit or migrate to different geographical and environmental conditions. Through the elemental force of mutation and natural selection, they try to adopt themselves to their environment. In doing so, they become different from one another. They belong to the same species till they can interbreed with each other. When the difference between difference between different populations become so large that they fail to interbreed with each other, they are said to constitute new species.

Evolution and classification

The old classification of plants and animals was based entirely on superficial resemblances and behaviours. After some time it was realized that without estabilising the relationship between two groups of organisms? their placement at particular place in the classification tree is not possible. This led to the certain of a new branch of biological science, i.e. systematic. Thus, systematic is the branch of taxonomy in which classification of living beings is done on the basis of their evolutionary relationship and not merely on the some superficial resemblances.

In taxonomy, the individuals showing certain similarities are put together in one group. The smaller groups resembling one another in some features are combined into a larger group and so on. The similarities within a group and between the various groups suggest that there is a definite relationship between the various forms of life.

The vertebrates, such as fish, frog, lizard, pigeon and rabbit look very different forms one another, but they all resemble in having dorsal tubular never cord, central nervous system, a ventral heart, a hollow- notochord and gill slits. It indicates common ancestry. There are some basic characteristic that are shared by most organisms. For instance, all living beings are made up of microscopic units called cells. Organisms can be classified into two broad groups, i.e. eukaryotes and prokaryotes on the basis whether their cell possess well- defined nucleus and the cell organelles or not.

Similarly eukaryotes can be grouped into unicellular and multi-cellular organisms on the basis of whether they are made up of single cell or several cells.

Organisms can be classified into plants and animals on the basis of photosynthetic activity.

Multicellular organisms can further be grouped on the basis of levels of body organization. For example, in sponges, cells are not organized into tissues hence, they have cellular level of organization.

In coelenterates (Hydra), cells arranged to form tissues but organs are no found, hence they have tissue level of organization.

From Platyhelminthes (flatworms) ownwards to the most complex chordates, i.e. mammals, all have orgain system level of organization.

Evidences of Evolution

Evolution is a very slow and gradual process. It is not possible for human beings to see step by step how evolution takes places. To prove the occurrence of evolution, many kinds of evidences have been drawn from various fields of biological science, like morphology, anatomy, cytology, genetics, embryology, palaeontology and animal distribution.

Evidence from comparative anatomy

Anatomy is the study of internal structures of an organism. When organisms are grouped together on the basis of the comparative anatomical studies, they exhibit their relationship with each other and also their line of descent.

There are two types of organs: Homologous and Analogous

Homologous organs

Homologous organs are those organs which are similar in origin and fundamental structures but are different in function. Examples- Forelimbs of frog and lizard, wing of bird, and arm of man have similar origin and bear the same anatomical structure. They are formed of similar type of bones i.e., humerus, radio-ulna, carples and metacarpels. But they are adapted to perform different functions. The forelimbs of frog are meant for jumping, lizards for crawling, birds for flying and human beings for catching and holding.

Analogous organs

Analogous organs are those organs which have different origin and anatomical structures but appear similar in look and perform same function. For example- wings of on insects and wings of birds are analogous organs. Both are meant to perform a common function, i.e. flying, but they differ in the embryological development and structure. Wings of an insect are membranous, double layered outgrowths of cuticle, while the things of a bird are made up of bones, muscles and feathers.

Evidence from Vestigeal Organs

Vestigeal organs are nonfunctional organs. It is believed that through vestigial organs are not functional in the present-day. Organisms they were functional and fully developed in the ancestral forms. The animals possessing vestigial organs are believed to have descended from the form in which these organs were functional. It means evolution is taking place and form one form another form is evolving.

There are large numbers of vestigial organs present in human body itself. The few notable ones are vermiform appendix, nictitating membrane, pinna muscles, coccygeal(tail) vertebrae, wisdom teeth, hairs on body, etc.

Evidence from embryology

Embryology is the study of development of an organism from egg to adult. Comparative embryology of animals shows a remarkable inter-relationship between various groups and provides an evidence of evolution. Ernst Haeckel in 1856 proposed Recapitulation Theory or Biogenetic law in which he said that during the development period, every organism repeats its evolutionary history.

Biogenetic law says that all multicellular organisms show similar early developmental features, that is zygote (single cell) → morula(group of cells) → blastula(2 layered embryo) → gastrula(embryo with 3 germinal layers).

Early stages of development in the embryos of vertebrates were found to be same. All of them posses gill slit and tail during the embryonic stages. It indicates that all vertebrates have evolved from a common ancestor.

Fossil

Fossil are the most reliable evidences of evolution. From the study of fossil records, the following facts have been established.

The fossils present in the bottom rocks are simple, while the most-recent fossils found in the upper strata are highly complex. This geological succession completely agrees with the concept of evolution.

Some fossils provide a connecting link between two groups of organisms. These are called connecting links. For example, Archaeopteryx lithographic is a connecting link between reptiles, birds as it possesses both reptilian as well as avian features. The reptilian features of Archaeopteryx are presence of teeth, claws and a long tail while avian features are presence of wings feathers, beak and keel (flat sternum).

The ancestry of certain mammals like horse, camel, elephant, man, etc. is all based on fossil records. Fossil history of horses indicates that horses have descended from five- toed ancestors. Consequently, four-toed, three-toed and single-toed horses have evolved. The present day houses are single- toed.

Theories of Evolution

Evolution is a very slow and gradual process. Nobody in his lifetime can visualize the exact mechanism or processes through which evolution takes place. Hence to explain the mechanism of evolution there are two main theories.

Lamarckism

Effect of environment

All living beings are continuously surrounded by environment. Environment is not always static, it may change. Changing environment produces changes in the needs of organisms and to fulfil the new demands of environment, living beings use certain parts of their body more in preference to the other parts. Lamarck believed that environment is supreme and it exerts a pressure on the organism to change in accordance with the changing situation.

Inheritance of acquired characters

Acquired characters are those characters which an organism develops or loses due to use and disuse. Lamarck cited the example of giraffe. Giraffe is a long- nacked mammal inhabiting the hot climatic regions of Africa. He believed that when the climate of Africa was mild, the progenators of giraffe were short- necked grazing mammals but when the climate became warm, grasses and small plants disappeared. At this time, the progenitor of giraffe started feeding on tree leaves for which they had to stretch their necks. This habit led to the gradual lengthening of neck of giraffe in subsequent generations and after hundreds of generations, present day long ? necked giraffe evolved.

Darwinism

Charles Robert Darwin (1809-1882), an English naturalist, gave his concept about the mechanism of evolution. He described his theory of evolution in his famous book origin of species. Darwin?s theory is scientifically known as origin of Species by Natural selection.

Over population

Living beings are enormously fertile, yet their number remains almost constant. For example elephant is a very slow breeder. In its 90 years of lifespan, it produces a maximum of six young ones. If we start with a single pair it will produce 19,00,000 elephants in a period of 750 years.

A single salmon fish produces 25,000,000 eggs in a season. It all of them develop into new fish and they also reproduce in the same manner, then in a very short period of time, the whole sea will be crowded with this single species of fish.

These examples indicate that the living beings have enormous power to increase their number but, due to certain causative factors, their number remains almost constant.

Struggle for Existence

The space provided and food available is limited. The earth can support only a sizeable number of individuals. Due to over- production, the number of individuals increases enormously. These individuals struggle to acquire more space and food in order to maintain their existence. In this struggle, only those individuals survive who are strong and have more adaptive values. Darwin believed that struggle for existence may occur among the members of the same species or between the members of different species or between living beings and their environment Thus, struggle for existence is the causative factor which keeps the population of living beings under control.

Origin of New Species

If a particular species is selected by nature, due to continuous occurring variations in different directions at different places, after a long period of time; new species arises which are markedly distinct from their ancestors. Origin of a new species symbolizes the occurrence of evolution.

Darwin believed that variations and natural selection are the two key factors without which evolution cannot take place.

Human Evolution

Man is the best and most precious creation of evolutionary process. Some thirty million years ago, he was something like that of an ape and about 75 million years ago, he was like a tree shrew. Naturally, man's evolution has been gradual and through several traits and errors. The evolutionary history of man and their evolutionary relationship is based on fossil history. Age of fossils and their other details have been studied through different methods like carbon dating system, potassium- argon method and more recently through DNA sequencing.

Important human evolutionary changes are:

1. Erect posture

2. Shorter arms

3. Perfection of thumb opposability

4. Loss of jaw power

5. Reducing in the size of teeth

6. Development of chin prominences.

7. Increases in cranial capacity of skull.

8. Increase in size and complexity of brain.

9. Development of articulated speech

Origin of present form of human beings took place about 10,000 years ago. Some of our ancestors left America while others are continued to living there. The migrant moved from Africa to west Asia,, then to central Asia, Eurasia, south Asia and east Asia. They further moved down to the island of Indonesia, Philippines, Australia and they crossed the being land bridge to the America. In this way, they spread to the whole planet. While moving they separated into several groups migrating into different directions.

Most of the earliest members of Homo sapiens have been traced back to be the inhabitant of Africa. While moving from place to place, the different groups of human beings acquired some morphological differences to change into races, like white race, black race, yellow and brown races, etc. For a long time it was considered that the different human races have separated and independent origin. Through the paleontological and cytological studies, it is now proved that all human beings belong to a single species, Homo sapiens.