### KEY TOPICS

star Adaptive Radiaton in Galapagos Island
star Adaptive Radiaton in Australia
star Evolution by Natural Selection
star Lamarck's Theory of Evolution
star Darwin's Theory of Evolution

● During his journey color{Brown}"Darwin" went to color{Violet}"Galapagos Islands".

● There he observed an color{Violet}"amazing diversity" of creatures.

● Of particular interest, color{Violet}"small black birds" later called color{Violet}"Darwin’s Finches" amazed him.

● He realised that there were color{Violet}"many varieties" of finches in the same island.

● All the varieties, he conjectured, color{Violet}"evolved on the island" itself.

● From the original color{Violet}"seed-eating features", many other forms with color{Violet}"altered beaks arose", enabling them to become color{Violet}"insectivorous and vegetarian" finches.

● This process of evolution of color{Violet}"different species" in a given geographical area color{Violet}"starting from a point" and literally color{Violet}"radiating to other areas" of geography (habitats) is called color{Brown}"adaptive radiation".

● color{Violet}"Darwin’s finches" represent one of the best examples of this phenomenon.

● Another example is color{Brown}"Australian marsupials".

● A number of color{Violet}"marsupials", each different from the other evolved from an color{Violet}"ancestral stock", but all within the color{Violet}"Australian island" continent.

● When more than color{Violet}"one adaptive radiation" appeared to have occurred in an color{Violet}"isolated geographical area" (representing different habitats), one can call this color{Brown}"convergent evolution".

● color{Violet}"Placental mammals in Australia" also exhibit adaptive radiation in evolving into varieties of such color{Violet}"placental mammals" each of which appears to be ‘similar’ to a color{Violet}"corresponding marsupial" (e.g., Placental wolf and Tasmanian wolf marsupial).

### EVOLUTION BY NATURAL SELECTION

● Evolution by color{Brown}"natural selection", in a true sense would have started when color{Violet}"cellular forms of life" with differences in metabolic capability originated on earth.

● The essence of color{Brown}"Darwinian theory" about evolution is color{Violet}"natural selection".

● The color{Violet}"rate of appearance" of new forms is linked to the color{Violet}"life cycle" or the color{Violet}"life span".

● Microbes that color{Violet}"divide fast" have the ability to multiply and become color{Violet}"millions of individuals" within hours.

● A colony of color{Violet}"bacteria (say A)" growing on a given medium has color{Violet}"built in variation" in terms of ability to color{Violet}"utilise a feed" component.

● A change in the color{Violet}"medium composition" would bring out only that part of the color{Violet}"population (say B)" that can survive under the new conditions.

● In due course of time this color{Violet}"variant population outgrows" the others and appears as new species.

● This would color{Violet}"happen within days".

● For the same thing to happen in a color{Violet}"fish or fowl" would take color{Violet}"million of years" as life spans of these animals are in years.

● Here we say that color{Violet}"fitness of B" is color{Violet}"better" than color{Violet}"that of A" under the new conditions.

● color{Violet}"Nature" selects for color{Violet}"fitness".

● One must remember that the color{Violet}"so-called fitness" is based on characteristics which are color{Violet}"inherited".

● Hence, there must be a color{Violet}"genetic basis" for getting selected and to evolve.

● Another way of saying the same thing is that some organisms are color{Violet}"better adapted" to survive in an color{Violet}"otherwise hostile environment".

● color{Violet}"Adaptive ability" is inherited. It has a color{Violet}"genetic basis".

● color{Violet}"Fitness" is the end result of the color{Violet}"ability to adapt" and get color{Violet}"selected by nature".

● color{Violet}"Branching descent" and color{Violet}"natural selection" are the two key concepts of color{Brown}"Darwinian Theory" color{Brown}"of Evolution".

### LAMARCK'S THEORY OF EVOLUTION

● Even before Darwin, a color{Brown}"French naturalist Lamarck" had said that evolution of life forms had occurred but color{Violet}"driven by use" color{Violet}"and disuse of organs".

● He gave the examples of color{Violet}"Giraffes" who in an attempt to color{Violet}"forage leaves on tall trees" had to adapt by elongation of their necks.

● As they passed on this acquired color{Violet}"character of elongated neck" to succeeding generations, Giraffes, slowly, over the years, came to color{Violet}"acquire long necks".

● color{Violet}"Nobody believes" this conjecture any more.

● color{Brown}"Is evolution a process" color{Brown}"or the result of a process"?

● The world we see, color{Violet}"inanimate and animate", is only the success color{Violet}"stories of evolution".

● When we describe the story of this world we describe color{Violet}"evolution as a process".

● On the other hand when we describe the color{Violet}"story of life on earth", we treat evolution as a color{Violet}"consequence" of a process called color{Violet}"natural selection".

● We are still not very clear whether to regard color{Violet}"evolution and natural selection" as processes or end result of unknown processes.

### DARWIN'S THEORY OF EVOLUTION

● It is possible that the work of color{brown}"Thomas Malthus1" on populations influenced color{brown}"Darwin".

● color{brown}"Natural selection" is based on certain color{Violet}"observations" which are color{Violet}"factual".

● For example, color{Violet}"natural resources" are limited, color{Violet}"populations" are stable in size except for color{Violet}"seasonal fluctuation", members of a population color{Violet}"vary in characteristics" (in fact no two individuals are alike) even though they look color{Violet}"superficially similar", most of variations are inherited etc.

● The fact that theoretically color{Violet}"population size" will color{Violet}"grow exponentially" if everybody reproduced maximally (this fact can be seen in a color{Violet}"growing bacterial population" and the fact that population sizes in color{Violet}"reality are limited", means that there had been color{Violet}"competition for resources".

● Only color{Violet}"some survived" and grew at the color{Violet}"cost of others" that could not flourish.

● The color{Violet}"novelty and brilliant insight" of Darwin was this: he asserted that color{Violet}"variations", which are heritable and which make color{Violet}"resource utilisation better" for few (adapted to habitat better) will enable only those to reproduce and leave more progeny.

● Hence for a period of time, color{Violet}"over many generations", survivors will leave color{Violet}"more progeny" and there would be a change in color{Violet}"population characteristic" and hence color{Violet}"new forms" appear to arise.