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Can We Call Human Evolution Adaptive Radiation? – Process, Characteristics, and Mechanism
Species are the basic taxonomic units of biological classification. Species are reproductively isolated, inter-breeding natural populations.
Speciation refers to the process by which new species form by branching off from ancestral populations. Modes of speciation are often classified as allopatric, sympatric, and speciation.
In 1963, Ernst Mayr proposed that speciation takes place through allopatry (or geographical isolation). In allopatric speciation, groups from an ancestral population evolve into separate species due to a period of geographical isolation, by a geographical barrier such as a mountain range, rockslide or river.
Case Study: The Grand Canyon was gradually curved out by the Colorado River over millions of years. Earlier, only one species of squirrel inhabited the area. As the Canyon got deeper over time, it became increasingly difficult for squirrels to travel, eventually, they diverged into different species.
Sympatric species originate by the instantaneous development of reproductive isolation between segments of species population without any physical separation.
E.g. Rhagoletis pomonella, the maggot fly has recently diverged into two subspecies due to the introduction of apple trees in the northeastern United States.
Adaptive radiation is an evolutionary divergence of members of a single phyletic line (a line of descent) into a series of different niches or adaptive zones.
It is a process in which organisms diversify from an ancestral species into a number of new forms with the change of environment resulting in the formation of new environmental niches. Starting from a single ancestor the process results in speciation causing an array of species exhibiting different morphological forms and physiological traits. The phenomenon of adaptive radiation is the result of natural selection, artificial selection, sexual selection, mutation, genetic drift, and migration. It is manifested through the development of variation quite adaptive to the specific environment.
Nature of Adaptive Radiation
Adaptive radiation is also known as macroevolution which operates above the species level and results in the establishment of new genera, families, and orders. Macroevolution is a genetic change sufficient to form a new species. Whereas, microevolution is a change of allele frequency in a population. When microevolutionary changes accumulate sufficiently to keep two fertile organisms of the opposite sex from successfully producing fertile offspring, macroevolution, the formation of a new species occurs.
Characteristics of Adaptive Radiation
There are four characteristics of adaptive radiation.
- A common ancestor of the component species.
- A phenotype-environment correlation. A significant association between environment and trail is noticeable in adaptive radiation.
- Trait Utility: Fitness or performance of the traits is observed in adaptive radiation.
- Rapid Speciation: Due to ecological and phenotypic divergence emergence of new species occurs.
Adaptive Radiation Process
The changes in the genotype occur on account of sud¬den mutations of large size, known as, macromutations or systematic mutations by Goldschmidt (1940). Macroevolu¬tion occurs in a group of individuals who have entered a new adaptive zone, free of competition. Macroevolution leads to specialization in a particular direction.
Adaptive Radiation Mechanism
In a new adaptive zone, the number of individuals is far less, and available new habitats are more. Therefore, the intraspecific struggle is virtually nil and the new zone is free from the enemies. Thus the newly entered population gets all the habitats of the adaptive zone and starts adapting themselves. As a result, one population which had acquired the new zone gets split up into several subpopulations, each of which accumulates mutations and evolves independently in different directions.
The evolution of reptiles represents the best examples of macroevolution in the fossil records. Class Reptilia first appeared in the Pennsylvanian period. Adaptive radiation in this group occurred between the Permian and Cretaceous periods. The divergent evolution or adaptive radiation of different reptilian groups from the earliest ancestral stock like cotylosaurs are examples of macroevolution. All living and extinct groups of reptiles representing six major radiating offshoots from the basal anapsid stock were evolved. These exhibit further diversification of their populations resulting in the formation of orders and then families and so on. Some conditions are applicable during the occurrence of the process of adaptive radiation and they are:
- The opening up of a new habitat.
- The new habitat is relatively isolated.
- The new habitat should have the availability of niche space.
Therefore, adaptive radiation is triggered by the above conditions as an ecological opportunity to create a new adaptive zone. The ecological condition creates an opportunity for dispersal causing the evolution of new species.
Adaptive radiation can also be seen in the Mammalian limbs. Ancestral mammals are believed to have been short-legged and five-fingered terrestrial animals showing slow locomotion. Their present living representatives are the insectivore and shrews. From them arose the arboreal forms (monkeys, squirrels) whose limbs were adapted for climbing trees. The same type of pentadactyle limbs are modified into wings in aerial forms for flying (bats). Horses and antelopes developed strong long limbs for running (cursorial forms). The fossorial forms (burrowing) like moles specialized their forelimbs for digging functions. In whales and seals limbs are modified for aquatic adaptation for swimming.
In the case of Parallel Evolution, two different classes of animals underwent, independently and separately, similar adaptive radiations. It leads to the formation of analogous structures. For example, horses and antelopes have evolved from pentadactylean ancestors. Both have slender and long limbs for fast running but horses have one toe and antelopes have two toes.
The Phytosaurs (or Triassic period) and Crocodiles (or Triassic to recent times) provide another example of parallelism. Phytosaurs were successfully adapted to aquatic life but became extinct. Later, the same aquatic realm was inhabited by crocodiles. The adaptations for aquatic life in both cases are similar.
As an example of adaptive radiation Darwinian finches are often cited. Darwinian finches are Galapagos endemics famously adapted for specialized feeding behaviour. Today there are about 15 species of finches and they are derived from a single ancestor that arrived in the Galapagos from mainland South America perhaps 3 million years ago. The finches showed a remarkable modification of their beaks which may be attached to their feeding habits. The small ground finches (Geospiza fuliginosa) have smaller beaks for eating smaller seeds. The medium ground finch (Geospiza fortis) has a medium-sized beak for eating intermediate size seeds. The other finches in the Gala¬pagos are uniquely adapted for particular niches. The warbler finches (Certhidea sp.) have short pointed beaks for eating insects. The woodpecker finches (Camarhyn- chus pallidum) have slender beaks for pecking at the wood in search of insects. Thus beak texture and form have been changed based on the food habits of the Darwin¬ian finches. The following diagram has made a display of such modifications.
In the vast living world of biodiversity, man appears as the most miraculous animal. Because of his superiority in the living world, man wants to dominate nature even. From the initial period of evolution, inquisitiveness in various aspects stroke the mind of man and with the help of his mental power a plethora of discoveries could be possible by man. For such an inquisitiveness man searched for his ancestors. Though a number of philosophical ideas as well as religious belief tried to quench the thirst, the reasonable mind of man could not be satisfied by that and therefore, the scientific mind of man have tried to find out plausible sources behind the origin and evolution of man. The paleontologists with their challenging endeavor could be able to reveal the history behind the appearance of man on this earth. Man has no disbelief now in their similarity with the monkeys and we now take apes as our forefathers.
Place of Man in the Living World
Famous taxonomist Linnaeus included man in the family Hominidae. The present scientific name of the man is Homo sapiens sapiens. This means that man belongs to the family Hominidae and species sapiens and as the present-day man is significantly superior to his immediate ancestor, he has been included within a sub-species called sapiens. The man exhibits many similar features with the monkey and ape and therefore, all of these animals belong to the order Primate. Scientists believe that the early primates probably evolved about 60 – 75 million years ago from the tree shrew. The primitive primates showed two lines of evolution. Along one line of evolution, the primates showed the evolution of man, whereas, on the other line, the prosimians gave the origin of man. It is to be mentioned here that the prosimians are the old-world monkeys and the monkeys of the present day are new-world monkeys.
Evidence as Available in Favour of Evolution of Man (Homo Sapiens Sapiens)
T. H. Huxley (1863) in his book “Man’s Place in Nature” stated the apes as the nearest relation to man. Subsequently in 1871 Charles Darwin in his book, “The Descent of Man” enlightened on the evolution of man and he stated that man and ape evolved from the same ancestor. But Darwin could not be able to give any supportive evidence in favour of his ideas. At this time only fossils of Neanderthal and Cromagnon man were discovered from Germany. All these come under the species Homo sapiens. In 1891 Eugene Dubois obtained a fossil from Java, which he named Pithecanthropus.
Later on, this was named as Homo erectus. In the contemporary period, Peking Man was discovered in China and he was named Homo erectus. Scientists believe that Homo erectus was the immediate forerunner of Homo sapiens. From the dating of fossils, it could be detected that Homo sapiens appeared about 5 million years ago. Raymond Dart in 1924 obtained a skull from South Africa and it was named the skull of Australopithecus africanus. In 1938 Robert Broom discovered the skull of Paranthropus. Subsequently, in 1970 the fossil of Homo habilis was discovered by Mary Leaky and Louis Leaky.
On analysis, they realized that H. habilis was the intermediate stage between Australopithecus and H. erectus. During the contemporary period, some more fossils were discovered as hominid existence. Among these 2.6-2.8 million years old fossil named Lucy obtained from Haden is the most significant one. Therefore, through the discovery of many fossil records, the steps in the evolution of man could be detected. A fossil named Ramapithecus, a fossil from about 12-14 million years ago, is taken as the first evolutionary stage of human evolution originated as a first division from the common ancestor of man and ape, representing a stage along the line of hominid development. Dryopithecus a stage of about 20-25 million years ago is taken as the common ancestor of man and monkey. The evidence in favour of the origin of present-day apes, namely Orangutan, Gorilla, and Chimpanzee from a common ancestor could be revealed from their DNA analysis. From such DNA analysis, Chimpanzee appears to be the immediate forefather of man. Though Orangutans and gorillas are somewhat distant by DNA features, they are also near relatives of man.
Place and Time of Origin
In the initial phase, Europe was taken as the site of the origin of man. However, this view was changed after the discovery of fossils from Asia and Africa. The oldest hominid fossils were discovered in Central Asian countries and therefore, it is presumed that the first human probably evolved in Asia. But Africa may also be a place of origin as suggested by many investigators. According to the measure of time scale probably during the first phase of Miocene, at about 15-20 million years ago, Ramapithecus (The first phase of human evolution) had probably originated.
Major Trends in Hominid Evolution
- Increase in volume of brain and expression of intelligence.
- Increase the carrying capacity of the skull.
- Bipedal locomotion.
- Conversion of forelegs into arms.
- Increase in length of hind legs.
- Decrease in the length of forelegs.
- Attainment of a straight posture.
- Increase the width of the pectoral girdle.
- The appearance of the chin.
- Sociality and culture.
Some Ancestors of Modern Man and Their Features
The records on the existence of Dryopithecus may be obtained as relics of the early Miocene period. From the analysis of the fossil of Dryopithecus africanus (common name ‘proconsul’), a lot of similarities could be observed. It is presumed that Dryopithecus was the ancestor of man and ape. The fossils of Dryopithecus were discovered in many regions of India and Europe.
The fossil of Ramapithecus was obtained from the Shivalik hills of India. The only remnant in this case represents the part of the jaw and a few teeth. It is presumed that about 14-15 million years ago Ramapithecus existed. Identical fossil records of Ramapithecus obtained from Africa have been named Kenyapithecus. The structure of the teeth of Kenyapithecus appears such that the food habit of it was identical with that of man. They probably could walk on their legs. It is presumed that in the line of evolution during the last part of the Miocene or in the early Pliocene, Ramapithecus appeared from Dryopithecus as the first Primate along the course of human evolution.
In 1924 Raymond Dart first discovered the relics of Australopithecus from South Africa. Subsequently, some more records were obtained from Java and Africa. They probably appeared in the last part of the Pliocene and the two important forms between man and ape were Australopithecus africanus and Australopithecus bisei.
Important features of Australopithecus related to the evolution of man were the followings:
- The capacity of the brain was about 450-600 cc.
- The face was of prognathous type.
- The eyebrow contained creases.
- The Arch of teeth was smooth parabola type.
- No existence of a simian gap.
- Absence of chin.
- The tooth and jaw were longer.
D. Homo habilis (Handyman or First toolmaker hominid):
Australopithecus gave rise to Homo habilis around 1.75 – 2 million years ago in the early Pleistocene.
- Homo habilis was about 1.5 to 1.8 meters tall and moved erect.
- He used his hands in making stone tools that were used in digging and killing small animals.
- The cranial capacity of Homo habilis reached 650-800 cc.
- He was maintaining a social life. Started living in caves and cared for young ones.
E. Homo erectus:
The fossil of Homo erectus was first discovered by Dubois in 1891. They are commonly known as Pithecanthropus. Similar fossils were also discovered in Peking. The name Homo erectus was given by Mayr. The fossil discovered from Java was named Homo erectus erectus or Java man; whereas the fossil obtained from Peking was named Peking man or Homo erectus pekinensis. It is presumed that during the intermediate part of Pleistocene from Australopithecus, Homo erectus was developed and they represent the stage immediately before the appearance of man.
The important features as could be noted in Homo erectus:
- The volume of the brain was on average 600-700 cc; though in the case of the Peking man, it was about 850-1200 cc.
- The face was a prognathous type.
- Jaws were massive and strong with well-set teeth.
- The forehead was low set and slanting downward.
- Their height was 5 feet and their weight was about 70 kg.
- They knew the techniques of preparation of weapons and also they had the knowledge of using these tools.
F. Homo Sapiens:
The hominid species developed after Homo erectus was named as Homo sapiens. Different Homo sapiens are known as Neanderthal man, Heidelberg man, Swan comb man, etc. Among them, the Neanderthal man was somewhat superior. The volume of their brain was about 1450 cc and that of man is about 2000 cc. The name of Neanderthal man has been given as Homo sapiens neanderthalensis. The Neanderthal man was present about 1,50,000 years ago. The Heidelberg man obtained from Heidelberg of Germany is considered as the forefather of man. Their jaws were broad and massive. The teeth of these forms were identical to those of the present-day man.
G. Cro-Magnon Man:
Immediately after the Neanderthal man, the Cro-Magnon man was developed. The Cro-Magnon man was more developed than the Neanderthal man and appeared to be similar to the modern man. Therefore, the Cro-Magnon man was named Homo sapiens sapiens. The height of this man was about 180 cm and the holding capacity of their skull was about 1600 cc. The other important features were a slender nose, round forehead, broad face, and prominent chin. Hunting was their only livelihood. They were also experts in art and the use of tools. It is presumed that about 30,000 years ago Cro-Magnon men appeared and they were identical with the modern man. In the line of evolution, no significant change in the body structure was observed, but only change occurred on the cultural side.
H. Modern Man and Cultural Evolution:
Besides the increased volume of the brain, several other important characteristics of modern man were a broad parietal area, a high forehead, and the presence of a chin. Though the actual time of the appearance of modern man is not known, it is taken that the Cro-Magnon man was the immediate forefather of the present-day man. The evolutionary modification as modern man showed in art and culture may be divisible into three ages namely the Palaeolithic age, the Mesolithic age, and the Neolithic age. Cro-Magnon man existed during the Palaeolithic age about 35 thousand years ago. At this period man used stones as their weapon and was house-dwelling. The Mesolithic age started at about 12000 years ago. The man in this period knew the techniques of building houses and they used to live by the lakes. On the other hand, the Neolithic age started about 10000 years ago and men during this period flourished in art, culture, and cultivation. The establishment of townships and cities are examples of this cultural development of man.
Change of Cranial Capacity in Human Lineage:
Salient Features of Origin and Evolution of Human Species
About the middle of the 19th century, Charles Darwin confirmed the theory that man had evolved from apes in the remote past and that both man and ape derived from a common ancestor. Now it is evident that modern man and the ape are both derived from a common ancestor, as is amply testified by anatomy, physiology, and embryology. One suggestion held that Propliopithecus might have been an ancestor of both ape and man, who lived 30,000,000 years ago. Another suggested the Egyptian ape, Aegyptopithecus, of the same antiquity. The genes are believed to have pursued two separate courses of evolution, of which one led to the great ape, Dryopithecus, from whom modern chimpanzee, the gorilla, and the orangutan developed. The other led modern man through the stages of Ramapithecus, Australopithecus, Homo erectus, and the early Homo sapiens. The difference between man and ape lies in the ability to make tools. From the known evidence, it was the genus of Australopithecus that began making tools. Thus the evolution of man may be roughly divided into three major stages as Australopithecus, Homo erectus, and Homo sapiens.