Old and new theories of inheritance and species perpetuation: An outline by a student

May, 2012

The origin of life has always been a subject of profound philosophical interest to man.

Let me pass through all your flock today, removing from it every speckled and spotted sheep and every black lamb, and the spotted and speckled among the goats, and they shall be my wages. So my honesty will answer for me later, when you come to look into my wages with you. Every one that is not speckled and spotted among the goats and black among the lambs, if found with me, shall be counted stolen.” Laban said, “Good! Let it be as you have said.” But that day Laban removed the male goats that were striped and spotted, and all the female goats that were speckled and spotted, every one that had white on it, and every lamb that was black, and put them in the charge of his sons. And he set a distance of three days’ journey between himself and Jacob, and Jacob pastured the rest of Laban’s flock. Genesis: Jacob 30:32-43

Perhaps the biblical text above represents the first documented exposition on the theories of inheritance. In it, we are introduced to the story Jacob who pastured the flocks of Laban in exchange for speckled sheep arising there from, as his reward. Realizing he could actually obtain speckled by crossing white sheep, effectively controlling reproduction, Jacob devised a plan to increase his flocks by putting the striped rods in front of his sheep in order that they may give birth to spotted offspring. It is said that he believed that the visual effects of the stripped rods influenced the phenotype of the offspring and by this Jacob probably set the foundation of genetics. Although this is scientifically untenable, as observed when Mongolian white rats are bred in streaked cages, we can at least posit that the observation may have contributed in inspiring later research about the effect of environment on genetics, artificial selection and recessiveness. Thus artificial selection, as an empherical process based on the subtle capacity of humans in perceiving certain differences amongst different organisms, is as an old practice.

In the Quaran, we learn about the creation of man describing organogenesis during the development of embryo in the verses

“Man We did create From a quintessence (of clay); Then We placed him As (a drop of) sperm In a place of rest, firmly fixed; Then We made the sperm Into a clot of congealed blood; Then of that clot We made A (foetus) lump; then We Made out of that lump Bones and clothed the bones With flesh; then We developed Out of it another creature. So blessed be Allah, The Best to create!”[Al-Qur’aan 23:12-14]

Regarding the role of water in the creation of life and lineage, we read in the Quran thus:

“It is He Who has Created man from water: Then has He established Relationships of lineage And marriage: for thy Lord Has power (over all things).” [Al-Qur’aan 25:54]

And on speciation

“There is not an animal (That lives) on the earth, Nor a being that flies On its wings, but (forms Part of) communities like you.” [Al-Qur’aan 6:38]

Earlier theories

Jean Baptiste de Lamarck (1744-1829)

Born in Bazentin-le-Petit in the north of France, Jean Baptiste de Lamarck is perhaps the least celebrated scientists of our time. His Scientific theories have largely been ignored or discredited and mocked with satirical sketch of giraffe stretching its neck, in allusion to his theory of “inheritance of acquired traits”. Yet his classical book Philosophie Zoologique, published in 1809 contains only two sentences specifically about giraffes. His contribution to science and medicine are rarely cited. For example, he coined the word “invertebrates“ and was the first to separate Crustacea, Arachnida and Annelida from insecta and anticipated the work of Schleiden and Schwann in cell theory by saying:

Nobody can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue”.

The underlying principle behind Lamarck´s theory of inheritance is based organism’s tendency to progression. He argued that organisms are not passively altered by their environment, instead, a change in the environment causes changes in the needs of organisms living in that environment, which in turn causes changes in their behavior. This progression is too slow to be perceived but observable in fossil record. Mankind is at the top of this chain of progression, having passed through all the previous stages in prehistory. This change necessitated the principle of spontaneous generation such that as a species transformed into a more advanced one, altered behavior leads to greater or lesser use of a given structure or organ and use would cause the structure to increase in size over several generations, whereas disuse would cause it to shrink or even disappear. Based on this, Lamarck put forward two laws; the first states that use or disuse causes structures to enlarge or shrink while the second states that these changes are heritable.

The result of these laws was the continuous, gradual change of all organisms as they became adapted to their environments based on their physiological needs. This work made Lamarck the first man whose conclusions on the subject of evolution excited much attention inspiring Darwin. Examples drawn from Lamarckian theory include, but are not limited to, giraffes stretching their necks to reach leaves high in trees which caused their necks to strengthen and gradually lengthen. This is transferred to their offspring who often have slightly longer necks (also known as “soft inheritance”). A blacksmith, through his work, strengthens the muscles in his arms and may be expected to pass similar muscular development to his children when they mature. People who travel tend to acquire new cultural traits that they often pass to their offspring even if the offspring do not travel along with them.

The argument against this view is that experiments simply do not support the second law—purely “acquired traits” do not appear in any meaningful sense to be inherited. For example, a human child must learn how to catch a ball even though his or her parents learned the same feat when they were children. In 1859 Lois Pasteur challenged the theory of spontaneous creation in an experiment in which he boiled meat broth in a flask that had a long neck which curved downward, like a goose and observed no growth. Cutting the tails of dogs over several generations apparently has not led to the emergence of tailless dogs. Finally, if a mother constantly paints her hair, no child of hers will be born with the same color of hair, debunking the theory of acquired characteristics.

Charles Darwin (1809-1882)

In his quest to understand how individuals of the same species vary, Charles Darwin developed what is today acclaimed as the most sensible explanation to life from a biological point of view, prompting Theodosius Dobzhansky, a Russian biologist to write:

“Nothing in biology makes sense except in the light of evolution.”

Darwin himself would probably be surprised at the biological and philosophical dimensions his theory of evolution has taken since he published his book, The Origin of Species in 1859. His powerful theory has become the central organizing principle of modern biology.

Central to Darwin´s theory is that all species have descended over a period from common ancestors which gave rise to branching pattern of evolution arising from a process called natural selection. Using compelling evidence, and rising above scientific rejection, Darwin´s scientific discovery is today regarded as the unifying theory of life sciences that explains diversity of life. The geographical distribution of wildlife and fossils collected during his 1831 voyage on HMS Beagle led to the detailed investigations that culminated in the theory of natural selection. At about the same time, Alfred Russel Wallace was articulating an article on the same subject and contacted Darwin, prompting the joint publication of their theories in a publication “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection” in the Linnean Society of London in July of 1858.

Borrowing from the idea of Lamarck, Darwin posited that species are highly plastic in form and behaviour. He concluded that variation was naturally produced according to some unknown laws and that the agent of selection, which in some cases could be man himself or other natural means, had no hand in the variation with which it could work and therefore had to make do with what nature threw up.

Complication in Darwinism arises because the theory needed to demonstrate that heritable and selectable variation exists in order to prove that evolution by natural selection occurs. However, it was necessary to explain how traits were inherited. To address this, Darwin put forward the theory of describing the process of Pangenesis which posits that each organ in the body, throughout an individual’s life produces small particles called ‘gemmules’ which contain information about the organ. When gemmules are released from an organ, they travel through the body to the sperm and eggs in the reproductive organs where they stick together. In this way the information could be passed on to the next generation, thereby explaining the heritability of variation. Although Pangenesis did not ultimately solve the mystery of inheritance, its historical importance is the fact that it did not disprove the theory of evolution. One of the most important arguments in support of Darwinian inheritance is an experiment in which guinea pigs presented epilepsy when the lumbar spinal cords of their parents were surgically removed.

 

Grego Mendel (1822 1884)

As is often the case in the history of science, important experiments are only uncovered by serendipitous turn of events. Indeed the modern theory of inheritance as we know it today was developed by Grego Mendel and, Charles Darwin, of all people, was never aware of it even though it was developed during his life time. Armed with his sound mathematical background, Mendel conceived and conducted elegant breeding experiments with garden pea which paved way for a realistic picture of inheritance. In addition to luck, Mendel had the fortune of selecting discrete and easily distinguishable traits like morphology of pea, flower colour etc.

Although Mendel´s findings were published in Records of the Brünn Association for Natural Research in 1866, the work was unnoticed and completely ignored by over 40 leading botanists of his time only to be rediscovered in 1900. Meanwhile, a German biologist, August Weisman presented the germ plasm theory which states that organism’s cells are divided into somatic cells (the cells that make up the body) and germ cells (cells that produce the gametes). The work of Mendel led to the postulation of two important laws of inheritance. The first law, also known as law of segregation states that every individual possesses a pair of alleles for any particular trait and that each parent passes a randomly selected copy of only one of these to its offspring. The second, also known as the law of independent assortment, or inheritance law, states that separate genes for separate traits are passed independently of one another from parents to offspring. That is, the biological selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait.

Until his death, Mendel never obtained the same results in subsequent attempts he made. However, other scientists like Lineu (1761) and John Goss (1822) conducted experiments that concurred Mendel´s findings. Perhaps amongst the shortcomings of Mendelian theories is the omission of the effect of external environment as well as evolution.

Today, despite the excellent idea of how inheritance works and the origin of species we have, made possible by breakthroughs consolidated by the genomic and post- genomic era, several questions remain unanswered. Starting from the pioneer work of Thomas Morgan on flies in Columbia University and the deciphering of the three dimensional structure of DNA by Watson and Crick, perhaps the 20th century represents the blossoming era of genetics.

Credits

Bodmer, W and Mckie, R (1993) The Book of Man: The Human Genome Project and the Quest to Discover our Genetic Heritage Oxford University Press, London, UK

Clark, W (1996) Sexo e as Origens da Morte (Sex and the origins of death) Editora Record Ltda, Rio de Janeiro, Brazil

Dawkins R (1976) The Selfish Gene Granada Publishing Ltd, London

Garret and Grisham Biochemitry 2nd Ed pdf

Wikipaedia

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3 thoughts on “Old and new theories of inheritance and species perpetuation: An outline by a student

  1. thanks for the reminder. i used to think genetics was simple until when i had to study gen 303. i was totally lost!

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