The Path to the Chromosome Theory of Heredity
The notion of the chromosome theory of heredity is very important to the understanding of evolution and genetics. There were many ideas, some correct and others not, which influenced the revolutionary discovery of chromosomal heredity. The main stones in the path to the chromosomal theory are the pangenesis hypothesis, the germ-plasm theory, and Mendel’s Laws.
Pangenesis
Lamarck expressed the idea that by simply using or not using certain organs they may be developed or atrophied and their offspring can then inherit these acquired characteristics. (Milner p. 375, 1993) It should be noted that this theory was not widely accepted largely due to the fact that the French word that Lamarck used in the sense of “must” was translated as “wants to,” which makes it sound as though the organism decides to change its body. Furthermore, Lamarck provided no mechanism by which this could take place. (www.ridgenet.net/do_while/sage/v1i8f.htm) Darwin was the one to give a mechanism for Lamarck’s idea. This mechanism is known as pangenesis.
Darwin’s ideas of evolution were well founded in the beginning; however, as his theory progressed he reverted to Lamarckian thought to explain his observations. To begin his theory he started with the observations that there is variation in offspring. He wrote, “no one supposes that all the individuals of the same species inhabiting the same confined locality, are cast of the same mold…I am convinced that the most experienced naturalist would be surprised at the number of the cases of variability…as I have collected” (Chapter 2 of Origin of Species). He sees that there is a struggle for existence saying, “as more individuals are produced than can possibly survive, there must in every case be a struggle for existence” (Chapter 3 of Origin of Species). Finally, Darwin recognized that there is a survival of the fittest in that the organisms with the best variations for the continuance of the species live and those with variations that are not as useful to the continuation of the species die (Chapter 4 of Origin of Species). Darwin begins to diverge into Lamarckian thought at this point. Darwin wrote that, “ slightly different changes in the conditions of life add to the vigor and fertility of all organic beings…the crossing of forms which have been exposed to slightly different conditions of life or which have varied, favours the size, vigor, and fertility of their offspring” (Chapter 9 of Origin of Species).
No one knew how heredity (passing traits) worked until Gregor Mendel, an Austrian monk, crossbred pea plants in his monastery kitchen garden in 1865. In 1869, gene-hunters found rod-like shape in the nucleus of cells that turned red when dye was added. They named this “Chromosome”, from a Greek word, chroma for color. They also went deeper into the cell and discovered a wispy microscopic thread within the chromosomes, which they named DNA. There were several scientists who did not know about Mendel’s breakthrough at the time, but then in 1900, they rediscovered his experiment and old journal copies.
Any inherited, genetic change in a population which takes place over several generations is defined as Biological evolution. Darwin and Lamarck have contributed majorly to the evolutionary theory. Although Lamarck’s theory was unsupported, he probably provided inspiration to Darwin and many other enthused scientists.
Darwin theorized that nature selects those traits that best allow a species to reproduce and survive.
Darwin took copious notes, captured insects and animals and selected plants. These he preserved in jars and took them back to England where he thought about the implications of what he had seen. for almost three decades. What occurred to him was a simple notion: animals, plants, insects, fishes, etc., which were obviously related differed slightly and these differences seemed to be tied in with their ability to survive. Differences, which he called "adaptations," were often related to geographical factors. He also saw something similar in fossils: certainly some fish, sea shells, etc., that died and were covered up by sand, gradually turned to stone, and were caught forever in fossil form. There seemed to b...
Darwin has two theories on the key principles of theory of evolution. One is the natural selection, a species that attains characteristics that are adapted to their environments (Darwin, Charles). The other one is survival of the fittest, which is when an individual best adapts to their environment survive to reproduce, and their genes are passed to later generat...
Darwin discovered natural selection. This proclaims that all living beings did not arise, as they are, out of thin air. Each had adapted to their environment over time, allowing nature to choose those who were most fit for survival. Unfortunately, he failed to realize the error in the commonly held concept of heredity. This was that each trait from the parents was mixed together when generating offspring—similar to the process of mixing paint, as opposed to the correct analogy of mixing a deck of cards. Favorable traits would therefore never be properly passed down, revealing Darwin’s blunder.
However, Lamarck believed that an organism will keep on developing until it is finally most suited to its environment/surroundings. He thought that when an animal uses an organ extensively it becomes stronger and then will be stronger in its offspring. Lamarck believed that certain animals stretched their body parts to adapt to their surroundings. Take a giraffe for example. He believed that the giraffes stretched their necks to reach food which then later on in the offspring caused giraffes to have the resulting long necks. (See figure 3)
Gregor Mendel, an Austrian monk, introduced a new theory of inheritance based on is experiment with peas. Mendel’s genetic laws were called the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance. Mendel worked with peas that were yellow or green in color and smooth or wrinkled in shape. The characteristics in the traits were distinct and similar; therefore, they gave Mendel the conception that his experiments would be easily tested. He concluded that the yellow pea plants, which bred with the green pea plants, had yielded all yellow peas; as a result, all the hybrid children looked like only one of the parents. Mendel theorized that genes can be made up of three possible pairings of heredity units called ‘factors’: AA, Aa, and aa. In Mendel’s crosses, the starting plants were homozygous AA or aa, the F1 generation was Aa and the F2 generation was AA, Aa, or aa. While Mendel’s research was with plants, the principles also applied to people and other animals. After Mendel’s death, his work was recognized, and he was regarded by the name ‘father of genetics’.
When Mendel’s research was rediscovered in the early 20th century, many geneticists believed that his laws of inheritance conflicted with Darwin’s theory of natural selection.
...n a subject which the public had relatively no knowledge of. He described the way in which an individual of a species reproduced and genetically passed on variations. The species that adapted through variation was the one who survived. This is where the phrase “survival of the fittest” came from. As pointed out, Charles Darwin was a man ahead of his time, and his work laid the structural basis for how we now look at evolution. On the last page of Origin of the Species, Darwin summarizes his findings, “as Natural Selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection”.
Charles Darwin has five parts to his theory of natural selection, firstly the “Geometric increase” which claims that “all living things reproduce in great numbers”, meaning that species may survive but not all will survive because, the resources used for survival for instance ,food will not be enough for all living things. “The struggle for existence” because there is a limited number of resources and can only sustain some and not all, not all living things will survive, however the question lies in which living being will survive?. “Variation” is the third part of natural selection which claims that within those living things there are variations within them that will determine whic...
Darwin writes on how a species will adapt to its environment given enough time. When an animal gains a genetic edge over its competitors, be they of the same species or of another genus altogether, the animal has increased its chance of either procreation or adaptation. When this animal has this beneficial variance, the advantage becomes his and because of this, the trait is then passed on to the animals offspring.
Darwinian evolution is the theory of biological evolution stating that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual’s ability to compete, survive and reproduce. This theory was settled by Charles Darwin, a naturalist and geologist best known for this evolutionary theory. Darwinian evolution has been around since the 19th century (Darwin C. 1859 Descendant of Man), and has continued to now 2015. Some changes that are provided from Darwin until now includes the discovery of how genes have different biological or behavioral traits, and how genes are passed down to the offspring. Another idea that has changed is the natural selection, for example genes can be transferred from one population to another due to migration.
In conclusion, Lamarck’s theory involves adaptations to create new variations, followed by the inheritance of these characteristics, while Darwin’s theory involves random hereditary variation first, followed by the selection of the variation. Genetics has disproven Lamarck’s theory on the basis that characteristics acquired during the lifetime of a parent are not passed onto the offspring. On the other hand, Darwin’s theory failed in explaining why a beneficial change-the loss of functionality of the appendix, for instance- can be passed generation after generation. However, Lamarck and Darwin both believed that life is continuously changing and that organisms change to be better suited to their environment (Mills 2004:119-121).
On Darwin’s trip around the world he found something very interesting on the Galapagos Islands. On the isolated islands he found fourteen species of finches with very similar characteristics but they had some differences in their beaks, diet, body size and habitat. Darwin thought the birds had a common ancestor. He thought that some time back some finches arrived on the islands and the finches with the beaks that suited the islands conditions survived this happened on all the islands. When they had offspring the next generation would inherit the same beak. This is a great example of natural selection which was a contributor to how humans evolved. From this Darwin established his theory of natural selection and how slowly over time creatures...