Theory of Evolution
Theories of Evolution (Lamarck, Darwin and The modern synthetic theory of Evolution and human evolution)
Lamarckism (Lamarck’s Theory of Evolution)
Having accepted the fact that new species have arisen from pre-existing species with modification, a number of scientist have explained their opinion about the mechanism by which this might have occurred. The first scientific theory concering this came from lamarck. His ideas, written in his book, philosophy of Zoology (1809) are known as Lamarckism. Based on his observation, Lamarck proposed that variations among organisms originate because of response to the needs of the environment. Moreover, this ability to respond in a particular direction guides to a trait’s adaptation. Thus Lamarck placed fossils in the evolutionary context and stressed on adaptation as means for evolutionary modification.
His theory is often called the theory of inheritance of acquired characters or the theory of use and disuse of organ. Lamarck tried to explain the origin of long neck and high shoulders of giraffe on the basis of this principle. According to Lamarck, as the giraffes continually strained to stretch their necks for browsing leaves of higher levels of plants, their necks grew longer and shoulders grew higher in response to their needs. But Lamarckism was very strongly criticized by August weismann and he discarded the idea of inheritance of acquired Characters. He showed that even after cutting the tails of mice continuously for let generations, a tail-less mouse was never born. Lamarck’s theory of evolution is now considered as an erroneous and faulty assumption since the acquired characters are not inherited.
●Jean Baptiste de Lamarck was a French naturalist who born in 1744. He proposed Theory of Inheritance of Acquired Characters.
●Lamarck’s theory regarding evolution was published in 1809 in ‘Philosophie Zoologique’. It has the following postulates known as Lamarckian doctrine.
(i) Living organisms and their components tend to increase in size as a result of internal forces of life.
(ii) Reproduction of a new organ results from a new need and from the new movement, which this need starts and maintains. .
(iii) The development and degeneration of organs will be based on use and disuse of organs respectively.
(iv) Inheritance of acquired characters All changes acquired by the organism during its life are transmitted to offsprings by the process of inheritance.
●Theory of Continuity of Germplasm was proposed by August Weismann in 1886. He criticised the theory of inheritance of acquired characters by removing the tail of mice continuously for 22 generations though, he was failed to obtained tailless mice till 22nd generation.
●August Weismann (1834-1914) suggested that the changes occurring in germplasm are inherited by offsprings Whereas in somatoplasm are not transmitted to next generations.
●A group of scientists has further studied Lamarck’s theory of evolution and some scientists like Spencer, Giard, Lawrence, Mc Dougall Cope, Rechard, Wells Giard, Dali, etc., supported the modified form of Lamarckism which is termed as ‘Neo-Lamarckism’.
●Neo-Lamarckism explains that only those acquired characters are inherited by offsprings which influence the germ plasm or germ cells.
Darwinism (Darwin’s Theory of Evolution) and New Darwinism
Darwinism and neo-darwinism
In 1859 Charles Darwin published his book On the Origin of Species by means of natural selection. In time it became one of the most influential books ever written. Many of the ideas in the book were not new.Influenced by many other scientists including his grandfather Erasmus Darwin. Charles organized ideas already present in the scientific community into a coherent theory
Darwin’s theory had two main claims
1. The theory of universal common descent
2. Natural selection
1. The theory of universal common descent
States that every creature descended from a single celled common ancestor somewhere in the distant past. Although the idea of common ancestry had been discussed by others before, Darwin was the first to put it together with a mechanism to explain the evolutionary process. The mechanism he suggested was natural selection, which included four parts
It refers to the idea that more creatures are born than can survive on the available resources.
There is variation in any population of living creatures
some traits are adaptations increase in organisms fitness this means the traits enable an organism to survive and reproduce better than others.
2. Natural selection
Traits that increase in organisms Fitness are passed on to their offspring replacing traits that do not and changing populations over time
Essentially Darwin believed that evolution is gradual. Small changes acted on by natural selection accumulate over long periods of time. It is important to realize that natural selection happens to individual creatures but evolution happens gradually to populations over time. According to Darwinism small evolutionary changes within populations millions of years add up to major change, in other words the theory claims that lots of micro evolution over lots of time equals macro-evolution.
Throughout origins 101 we will explore why we disagree with this claim. Within a couple decades after Darwin’s book was published and most scientists had accepted the first of Darwin’s two claims the idea of universal common descent. The second claimed natural selection was not accepted for many decades because at the time of Darwin little or nothing was known about the inner workings of the cell. Scientists didn’t know about mutations so they didn’t know how variation was generated. They didn’t know about genetics so they didn’t understand how traits were passed on to offspring.
In addition a scientist named Lord Kelvin had recently estimated that the earth was 15 to 20 million years old. Scientists doubted that natural selection could have produced enough change in that amount of time to account for the diversity of life we see today. During this period of many decades sometimes called the eclipse of Darwin, scientists suggested other mechanisms to explain evolution. It was not until about 1940 that the majority of biologists accepted the main tenets of Darwinism and this is where neo-darwinism comes in.
In the 1930s and 40s more than 70 years after the original publication of Darwin’s book scientists synthesized the newest information from molecular biology and genetics with Darwin’s theory of evolution by natural selection. Using mutations to explain the source of variation and using genetics to explain how traits were passed from one generation to the next. Many scientists became convinced of the validity of Darwin’s second main claim. There are several different names for the modern version of evolutionary theory established by scientists in the 1930s and 40s. it is sometimes called neo-darwinism.
The neo-darwinisn synthesis or the modern synthesis. Although it took over 70 years to be accepted by the scientific community. It has been the reigning scientific paradigm for many decades since that time. Neo-darwinism or the modern synthesis is summarized by these two propositions. Gradual evolution results from small genetic changes that are acted upon by natural selection. The Origin of Species as well as macro evolution can be explained in terms of natural selection acting on individuals. In other words like Darwinism it says that lots of small changes accumulated over lots of time amount to macro evolution.
Darwin’s four postulates were restated as follows:-
- Because of mutation and the recombination of alleles as reproduction takes place individuals in a population vary in the traits they possess.
- Individuals pass their alleles on to their offspring.
- In every generation some individuals are more successful at surviving and reproducing than others.
- The individuals that survive and reproduce most have the best adaptations this idea has sometimes been called survival of the fittest.
If neo-darwinism sounds a lot like Darwinism that’s because it is what makes neo-darwinism different is that because of the advances in science since Darwin’s time. Neo-darwinism is able to offer explanations for how variation occurs in populations and how organisms pass on their traits to the next generation. Even though it took many decades for scientists to accept all of Darwin’s theory, it turned out to be a very successful theory.
A theory is considered to be successful
- if it organizes and explains groups of data
- if it can suggest research to test those explanations
- if it can predict the outcome of experiments
Darwin’s theory has been very successful at doing these things. It has been successful in organizing and explaining a broad range of biological data. It provides explanations and suggests ways to test them and in many cases the very successfully predicts the outcome of experiments.
For these reasons many scientists have great confidence in the theory. It is more widely believed by scientists today after 150 years of research, than it was in Darwin’s lifetime.
However after being the reigning paradigm for several decades neo-darwinism is again being questioned even by some evolutionary scientists. In his book Darwin’s doubt Stephen Meyer states that “leading figures in several sub disciplines of biology now openly criticize key tenets of the modern version of Darwinian theory in the peer-reviewed technical literature”.
- To summarize both Darwinism and neo-darwinism include the idea of universal descent from a common ancestor.
- Both also include the idea that the mechanism for evolution is natural selection
- Neo-darwinism uses the advances in science since Darwin’s time to explain variation and how individuals pass on their genes to their off spring.
- In spite of doubts in the scientific community about natural selection is the mechanism for evolution. Neo-darwinism remains the dominant explanation for the diversity of life we see around us.
It is published without qualification and textbooks and is affirmed and disseminated freely by the media. We have described the claims of Darwinism in this presentation but we do not accept either the theory of universal common descent or the idea that lots of micro evolution adds up to macro evolution in later origins 101 presentations. We will explore the evidence but first we will look at some common misconceptions that people have about the claims of evolution.
Some key point of Darwin’s theory of Evolution:
● Darwin started a travel in 1831 by H M S Beagle for a voyage of world exploration planned by British Government With Dr Henslow.
●Darwin found a living laboratory of evolution in Galapagos Island during his journey and found closely related species of finches known as Darwin’s finches.
●In 1838 Darwin found an essay of T R Malthus on population entitled ‘The Principles of Populations’.
●Darwin and Wallace jointly propounded the ‘Theory of Natural Selection’ and published a joint paper titled ‘Origin of species’ in 1858.
● Darwin’s theory of Evolution is based on the following five principles
(ii) Struggle for existence
(iii) Variations and their inheritance
(iv) Survival of fittest
(v) Natural selection and species formation
●Continuity of existence of the fittest was called survival of the fittest by Herbert Spencer, While Darwin called it natural selection.
●Later in November 1859, Darwin published his observation and conclusion in a book On the Origin of Species by means of natural selection.
●The theory proposed by Darwin and Wallace has been modified in the light of modern evidences from Genetics, Molecular Biology, Palaeontology, Ecology and Ethology, and known as Neo-Darwinism. Weismann, Mendel and de Vries were the supporters of Neo-Darwinism.
Synthetic Theory of evolution
●The modern synthetic theory of evolution was born in 1937 With publication of Dobzhansky’s Genetics and The Origin of Species Which was supported by Huxley (1942), Mayr (1942), Simpson (1944) and Stebbins (1950).
●According to synthetic theory of evolution there are five basic factors involved in the process of organic evolution. These are
(i) Gene mutations
(ii) Change in chromosome structure and number (iii) Genetic recombinations
(iv) Natural selection
(v) Reproductive isolation
●Stebbins (1950) provided an account of variations, heredity, isolation and natural selection.
●Humans and human like ancestors are called homonins or hominid.
●The study of human evolution and culture is known as anthropology.
●Carolus Linnaeus (Father of Taxonomy) coined the scientific name (Homo sapiens) to human which stands for Man who is wise. He placed human along With monkeys and apes.
●T H Huxley (1863) explained origin of humans scientifically in his book ‘Man’s Place in Nature’.
Dryopithecus (Common ancestor of man and apes)
(i) Fossil of Dryopithecus africans was obtained from rocks of Africa and Europe.
(ii) Dryopithecus afn’cans shows close similarity to Chimpanzee.
(iii) It’s cranial capacity was 400 cc.
Ramapithecus (Earliest man like primate or oldest of man’s ancestor or forest hominid)
Edward Lewis (1932) obtained fossil of Ramapithecus from Pliocene rocks of Shivalik hills of India.
Australopithecus (First ape man)
(i) Prof. Raymand Dart (South African anthropologist) discovered the skull of 5-6 year old baby which appeared about 5 million years ago. This baby fossil was termed as Australopithecus africanus (African ape man or tuang baby).
(ii) It was the ancestor of man who first stood erect,i.e., fully bipedal hominid.
(iii) It’s cranial’capacity was 500-700 cc, i.e., equivalent , to modern gorilla.
Java man (Pithecanthropus erectus or Homo erectus erectus)
(i) In 1891 Eugene Dubois obtained fossils from Pleistocene deposits in central Java.
(ii) It was named Pithecanthropus erectus by Eugene Dubois and Homo erectus erectus by Mayer (1950).
(iii) Java man was the first pre-historic man who began the use of fire for cooking, defence and hunting.
(iv) It’s cranial cavity was 940 cc which is about intermediate between Australopithecus (600-700 cc) and modern man (1400-1600 cc).
Peking man (Homo erectus pekinensis or Pithecanthropus pekinensis or Sinanthropus pekinensis)
(i) The fossils of Peking man were discovered by WC Pai (1924) from the lime stone caves of Choukoutien near Beijing during middle Pleistocene period.
(ii) WC Pai named Java man Sinanthropus whereas Davison Black (1927) named it Sinanthropus pekinensis and Mayer (1950) called it Homo erectus pekinensis.
(iii) The cranial capacity of Peking man was 850-1200 cc that is more than Java man.
Neanderthal man (Homo sapiens neanderthalensis)
(i) Fossils of Neanderthal man was discovered by C Fuhlrott (1856) from Neander valley in Germany.
(ii) Neanderthal man arose about 1,50,000 years ago and flourished in Asia, Europe and North Africa.
(iii) Neanderthal man existed in the Late Pleistocene period.
(iv) The cranial capacity of Neanderthal man was about 1400 cc.
(v) It is also called ceremonial burial of the dead.
Cro-magnon man (Homo sapiens fossilis)
(i) Mac Gregor discovered the fossils of Cro-magnon man from Cro-magnon rocks of France in 1868.
(ii) Cro-magnon arose about 3,40,000 years ago and lived in Europe and other parts of world.
(iii) Cranial capacity was about 1650 cc i.e., much more than modern man (1450 cc).
(iv) They could run fast and lived in families and considered as the direct ancestor of living modern man.
Living Modern Man (Homo sapiens sapiens)
(i) It is believed that living modern man first appeared about 10,000 years ago in the regions of Caspean sea and Mediterranean sea.
(ii) Its cranial capacity is average 1450 cc which is lesser than Cro-magnon.
(iii) Human species (sapiens) have white or caucaroid, mongoloid and black or negroid races.
Future man (Homo futuris)
(i) It is believed that in future, human could be change as a result of gene mutation, gene recombination and natural selection.
(ii) According to H L Shapiro, an american anthropologist, the Future man will contain following characters.
(a) Height will be higher.
(b) Hair will be reduced and skull become dome shaped.
(c) Body and cranium will be more developed.
(d) The fifth finger probably will be reduced.
(e)The age will increase.
These are the theory of Evolution