Genetics and Evolution

Genetics and Evolution

Table of Contents

Definition of Evolutionary Genetics

“Evolutionary Genetics is the broad field of studies that resulted from the integration of genetics and Darwinian evolution, called the ‘modern synthesis’, achieved through the theoretical works of R. A. Fisher, S. Wright, and J. B. S. Haldane and the conceptual works and influential writings of J. Huxley, T. Dobzhansky, and H.J. Muller.” This field of study focuses on the changes of genes and genotype frequencies within populations and the process that result in variation within population. Evolutionary Genetics comprise of four main concepts, i.e. mutation, natural selection, gene flow and random genetic drift.

Definition of Genetics

The term Genetics refer to the study of genes, genetic variation and heredity in living organism.

In other words, genetics is the branch of biology which deals with the study of genes – what they are, what they do and how they work. These genes plan an important role in shaping our appearance and physical properties.

Genes

“A gene is the basic physical and functional unit of heredity.” These are made of DNA and act as the instructions to form molecules called proteins. In case of human beings, these genes vary in size from few hundred DNA bases to more than 2 million bases.

Definition of Genes

A gene is a locus of DNA that is made of nucleotides and is the molecular unit of heredity.

Definition of Chromosomes

“In the nucleus of each cell, the DNA molecule is packaged into thread – like structure called chromosomes.”
Each chromosome is made of DNA tightly coiled many times around proteins called histones that support its structure.
Following image shows presence of chromosomes in nucleus and gene as its functional unit –

gene as its functional unit

History of Genetics

The work of Augustinian Friar Gregor Johann Mendel on pea plant resulted in the start of history of genetics. His theory was published in the year 1866, with the name Mendelian Inheritance. The rediscovery of Mendel was carried out in the year 1900 by Hugo de Vries, Erich von Tschermak and Carl Correns when they applied Mendelian genetics to wide variety of organisms. After the establishment of basic pattern of genetic inheritance, biologists started investigating physical nature of genes and in 1940s and early 1950s; experiments were carried out that pointed to DNA as an important portion of chromosomes. This resulted in the discovery of helical structure of DNA in 1953.

After this, chemists started developing techniques for sequencing both proteins and nucleic acid and some focused on finding the relationship between the forms of biological molecules, i.e. the genetic code. In 1960s, regulation of gene expression became the central issue and in 1970s, scientists were able to control and manipulate gene expression via genetic engineering.

Evolution

“Evolution is the process that results in heritable changes in a population spread over many generations (change in allele frequencies over time) leading to diversity of organisms on earth.” Evolution results in genetic change in species or population over the generations.

Origin of Life

Origin of life is a unique event of history of universe. Universe is very old and vast, i.e. 20 billion years old. It includes huge clusters of galaxies which contains stars and clouds of dust and gases. The Big Bang Theory explains the origin of universe which include huge explosion that is unimaginable in physical terms. The universe expanded and gradually temperature reduced. Later hydrogen and helium formed and gases condensed under gravitation and formed galaxies of present day universe.

Earth is supposed to have formed 4.5 billion years back and the early earth was not having any atmosphere. Carbon dioxide, methane, water vapor and ammonia released from molten masses were released and the ultra violet rays broke water into Hydrogen and Oxygen. This oxygen combined with methane and ammonia to form carbon dioxide, water vapor and others. It resulted in the formation of ozone layer and as it cooled down, water vapor started falling as rain. This formed oceans and life was formed 500 million years back.

According to some scientists, life came from outside the earth. As per Greek thinkers, the unit of life is spores which are transferred to different planets including earth. It is also believed that life came from the decaying and rotten matter such as mud, straw, etc. This theory is referred as theory of spontaneous generation. This theory was dismissed by the experimentation of Louis Pasteur as per whom life came from some pre – existing life.

Haldane of England and Oparin of Russia proposed that first form of life could have come from non – living organic molecule life RNA and protein and the formation of life was the result of formation of diverse organic molecule from inorganic constituents.

Evolution of Life Forms – A Theory

Theory of Evolution of Life Forms has three connotations –

  1. All living organisms (different types and species) we are able to see today are created as such.
  2. The diversity is always the same since creation and will continue to be same in future.
  3. The earth is 4000 years.

All these ideas were strongly challenged in 19th century. According to the conclusion of Charles Darwin, existing living forms share several similarities to varying degrees with themselves and other life forms that exist millions of years ago. Added to this, there have been extinction of different life forms and there has been gradual evolution of different forms of life.

Evidences of Evolution

Comparative anatomy and morphology indicates several similarities and differences among different organisms. This comparison helps in identifying common ancestors like bats, whales, Cheetah and human share a similarity in the forelimb pattern. Although, these forelimbs perform different function but are anatomically similar. Comparative anatomy also shows that evolution is a remodeling process.

What is Adaptive Evolution?

“In evolutionary biology adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches.”

Example of Adaptive Evolution

The finches of Galapagos Island are the classical example of Adaptive evolution. Darwin realized that there are many varieties of finches in the same island that evolved on island itself.

Following figure shows, adaptive evolution, in which every bird has seed – eating features and many others forms with altered beak arose that helped them to become insectivorous. This process of evolution in a particular geographical area is referred as adaptive evolution.

adaptive evolutionBiological Evolution

“Biological Evolution is defined as organisms reproducing but experiencing changes with each generation.” It can happen in small as well as large context and we can observe small genetic change over multiple generations.

Following image shows the biological evolution of man and Gibbon, to Orangutan, then chimpanzee, gorilla and finally the present man.

biological evolution of man and GibbonDarwin Theory is based on natural selection. According to Darwin, the rate of appearance of new forms is related with life cycle of the organism. For instance, microbes divide fast and we can observe variation within hours. A colony of particular bacteria (Consider A) grows on a given medium and develops the new population (Consider B) which has the ability to survive under new and different conditions.

Prior Darwin, French naturalist Lamarck also focused on evolution of living forms. He proposed an example of giraffe, as per which their attempt to forage leaves of taller trees have resulted in an elongation of their neck.

Following diagram shows the elongation of neck of Giraffe as described by Lamarck:

elongation of neck of Giraffe

Mechanism of Evolution

mutation where DNA is changedAccording to Hugo DeVries, it is the mutation that results in evolution. Mutation is random and directionless. It is the change in the nature of DNA and gives rise to new alleles.
Following diagram shows mutation where DNA is changed into two, i.e. original copy and mutant copy:

Definition of Mutation – “Mutation, is the driving force of evolution, is a random change in an organism’s genetic makeup, which influences the gene pool of the population.”

Hardy – Weinberg Principle

“The Hardy–Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.”
In a given population, we can find out the frequency of allele occurrence and this frequency is supposed to remain fixed and even same through generations. The principle can be applied in two ways. Firstly, a population is assumed to be in Hardy–Weinberg proportion, where it is easier to calculate genotype frequencies and secondly, if the genotype frequencies are known and can be tested for deviations which are statistically significant.

This principle can be stated via algebraic equation. According to the principle, gene pool remains constant and is referred as genetic equilibrium. Sum total of all these allele frequencies is Individual Frequencies (say p, q etc.) In a diploid, p and q are frequency of allele a and allele A.

Frequency of individuals AA in a population is p2. Similarly, frequency of individuals aa is q2.
Thus, p2 + 2pq + q2 = 1

This is a binomial expansion of (p + q)2

From the above expression we can conclude that, when frequency is measured, differs from expected values, the direction (difference) indicates “the extent of evolutionary change.”

Factors that affect Hardy–Weinberg Equilibrium

  1. Gene flow or gene migration
  2. Mutation
  3. Genetic drift
  4. Natural selection
  5. Genetic recombination

Assumptions underlying Hardy–Weinberg Equilibrium

  1. Only sexual reproduction occurs
  2. Organisms are diploid
  3. Mating is random
  4. Non overlapping generations
  5. Allele frequencies are equal in their sexes
  6. Population size is infinite
  7. There is no selection, mutation or migration

Violations to these assumptions may cause deviation from expectations.

Origin and Evolution of Man

Around 15 million years ago, Ramapithecus and Dryopithecus were present. They were hairy and used to walk like Chimpanzees and gorillas. Ramapithecus were man – like while Dryopithecus were ape - like. Discovering fossils of man – like bones in Tanzania and Ethiopia revealed several hominid features that resulted in a belief that around 3 – 4 million years ago, man like primates walked eastern Africa. They walked upright and were not taller than 4 feet. Around 2 million years ago, Australopithecines probably lived in the grasslands of East Africa. As per the evidences, they used to hunt via stone weapons and ate fruits. These creatures are referred as first human – like being and were named Homo habilis. The brain size of Homo habilis was 650 – 800 cc.

Fossils discovered in the year 1891 in Java revealed the next stage of development and was referred as Homo erectus. These existed 1.5 million years ago. They had larger brain and the size was 900cc. They ate meat.

Homo sapiens aroused in Africa and they travelled to different continents resulting in the development of different races. Agriculture came around 10,000 years back and human beings started to settle down at one place.

Following diagram shows the comparison of skulls of adult modern human being, baby chimpanzee and adult chimpanzee. We can clearly conclude that the skull of baby chimpanzee is more like adult skull of human as compared to adult chimpanzee skull:

skulls of adult modern human being

Frequently Asked Question

Q1: Who is the father of genetics?

Answer: Gregor Mendel is the father of genetics. He studies “trait inheritance” pattern which describe the transfer of traits from parents to offspring.

 

Q2: What is genetic in science?

Answer: “Genetic is the study of heredity or the study of how traits are passed from parents to offspring.”  This study is based on genes and these genes are the structure found in every single cell. It contains information about an organism and intersects with many life sciences.

 

Q3: Why it is important to study genetics?

Answer: It is important to study genetics, so that doctors and scientists are able to utilize its information in order to treat, diagnose, prevent and cure many illnesses. Genes are instructions that help the body in making proteins required to grow and survive. Understanding these proteins helps the scientists understand how the body works and possible reasons of non – working. Added to this, its knowledge will help in adopting more effective methods of treatment.

 

Q4: Which ape is genetically closed to humans?

Answer: Orangutans are the closest living relative to humans. This is so because humans share at least 28 unique physical characteristics with them.

 

Q5: Are chimps closer to humans than gorillas?

Answer: Humans are closer to chimpanzees but there is lot of similarities of humans with gorillas too. Researchers have found that 15% of the gorilla’s DNA is closer to humans than chimps. The “shared” pool includes genes present in brain, hearing and development, challenging the theory that humans listening ability evolved when we developed language. Yes, chimps are closer to humans than gorillas. British scientists sequenced human genome with chimps and orangutan and found 97% of similarities. According to further studies, chimps are closer to humans in 70% of their genes while gorillas are closer to us by just 15%.

 

Q6: How many years ago did humans and gorillas diverge evolutionarily?

Answer: Humans and gorillas diverged evolutionarily around 10 million years ago and humans and chimps diverged from each other about 6 million years ago.

 

Q7: What is the difference between homologous and analogous organs?

Answer:

Homologous Organs

Analogous organs

Same basic structural plan and origin but these organs perform different functions. Different structural plan and origin but perform same function.
It shows common ancestry. It does not show common ancestry.
It indicates divergent evolution. It indicates convergent evolution.
For example – Thorn of Bougainvillea and Tendril of Cucurbits. For example – Thorn of citrus and spine of Opuntia, Tendril of pea and tendril of cucumbers.
Wings of bat, paws of cat, flipper of seal and human hand. Wings of birds and wings of insects.

Following diagram shows the forelimbs of higher vertebrates, i.e. human, cat, whale and bat. These limbs are used to carry out different functions but have similar anatomy:

forelimbs of higher vertebrates

Bougainvillea and CucurbitaIn plants as well, the thorn and tendrils of Bougainvillea and Cucurbita represent homology as shown in the figure of Plant.

 

Q8: What is founder effect?

Answer: “Founder effect is defined as the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.” This effect was completely outlines by Ernst Mayr in the year 1942.

 

Q9: How is genetics related to evolution?

Answer:   Genetics and evolution are interrelated as it is evolution that results in the change of genetics of organisms. It is important to study genetics because with evolution of organisms we are able to understand genetics in better manner.

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