Evolution of life on earth has not been a simple process.
Evolution of life on earth has not been a simple process. Some explain it as a process while some interpret it as a result of various natural occurrences. The evolutionary theory tries to convey the message of origin of the earth and how lives on earth evolved. Evolution as a process, explains how the world came into existence. It has resulted in biodiversity.
The process by which modern organisms have descended from their ancestors, with changes in their allele frequencies, is called evolution. Also Refer: Evolution and Hardy Weinberg Principle
The Big Bang Theory is the most accepted theory regarding the origin of life on earth and different forms of life in it. According to the theory, the universe is a result of a huge explosion, which occurred 20 billion years ago. It took another 10 billion years for the origin of the earth. Once earth originated, it grew, and the living conditions like the temperature and atmosphere stabilized. It is believed that the first life form came into existence on earth around 2000 million years ago (mya). Even though there is no clear picture of the evolution of the first living cell, it is assumed that the first cell originated from non-cellular molecules. This cell divided to form more of them. The evolving cells had few characteristic features which helped them to survive. These features include the ability to trap light energy and derived energy from it and the oxygen releasing capacity of the cells. Finally, single unicellular organisms evolved to form much more complex multicellular organisms. Once the multicellular organisms were evolved, evolution happened at a much faster pace. The further series of development and evolution of new organisms like invertebrates, plants, animals such as reptiles, fish, amphibians, mammals, etc., took place within a time range of 500-300 million years ago (mya). Later these organisms started to evolve in their own way. This commenced the branching of lineages. Environmental conditions like climatic changes, limited resources, predators, etc. played an enormous role in that. During each phase of evolution, one life form dominated over the other. Nature selected one over the other, which was called the fittest of all. The flow of evolution of animals began with jawless fish (350 mya) before plants (320 mya) invaded the lands. Later, fish which could live on both land and water replaced plants, they fed on them. Amphibians, which were derived from these lobefin animals later evolved into reptiles. Reptiles of different shapes, size, and life patterns flourished during that many mya. Certain ones lived in water, e.g., Ichthyosaurs (200 mya), others lived on land. Dinosaurs were a known land reptile which existed at that time. Due to some unknown reasons, they became extinct. Followed by reptiles, there was an era of mammals which dominated all other species. Various processes such as natural selection, genetic drift, gene flow, etc. had contributed a lot to develop the living conditions and organisms as we know them today. Also Refer: Darwin’s Contribution-The Theory of Evolution
The mechanism of evolution occurs due to the changes in the gene pool- the collection of genes . There are four key mechanisms that cause a population. These include:
Mutations are defined as the changes in the DNA sequence of a living organism. The mutations occurring in the germ cells, i.e., the egg or sperm cells only are passed on to future generations. These mutations are inherited from the parents and are present in every cell throughout a person’s life. These are the mutations that lead to evolution. The acquired mutations occur during a person’s life and are present only in some cells. These are caused due to environmental factors such as ultraviolet radiations and do not pass on to future generations. They are not a part of evolution.
The transfer of genes from one population to another is called gene flow.
For eg., pollen moving from one place to another by the action of wind or people moving to different cities or countries. When a person from one country moves to the other and mates with a person there, a transfer of genes occurs between the individuals. This is how the gene flows between different populations. Gene transfer can be horizontal, i.e., transfer of genetic material from one population to another by asexual means. This phenomenon is prevalent in prokaryotes.
It refers to the change in the allele frequency of a population as a matter of chance. It is a random event whose effect is larger in smaller populations and smaller in larger populations. The two examples of genetic drift are the bottleneck effect and founder effect.
Bottleneck Effect: It occurs when there is a sudden decrease in the population due to some environmental factors, such as an earthquake, tsunami, epidemics, etc. In this event, some genes are depleted from the population. This causes a drastic reduction in the genetic diversity of the original gene pool. That means that the genetic makeup of the surviving population becomes different from that of the original one.
Founder Effect: When a small number of individuals separated from a larger population make up a new population, there is a loss of genetic diversity. They do not carry the genetic diversity of the previous population. Due to this, some genetic traits become more prevalent than the others, which results in genetic diseases in future generations.
Some individuals with certain traits have higher survival and reproductive rate than others. They pass on these genetic features to their offspring which brings an evolutionary change in the future generations. This selection of the genetic qualities that prove beneficial for survival in future generations is known as natural selection.
Darwin’s Finches is one classic example of natural selection. Darwin’s finches have evolved into 15 different species depending upon their adaptation and feeding habits.
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