Orthogenesis is the theory that organisms evolve in a definite direction due to some internal mechanism, rejecting natural selection. Allometry describes the relationship between an organism's size and its body parts, such as brain size increasing with body size. Adaptive radiations occur when environmental changes open new niches, causing rapid speciation and phenotypic adaptation in a relatively short time, as seen with Hawaiian honeycreepers adapting to different island environments.

1. Yousaf Rafique | Evolution | November 4, 2018
Orthogenesis, Allometry &
Adaptive Radiations
Evolution and principles of Systematic Zoology

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Orthogenesis
 According to the theory of orthogenesis,” The biological hypothesis that organisms
have an innate tendency to evolve in a definite direction towards some goal
(teleology) due to some internal mechanism or "driving force”.
 Evolutionary changes produced in straight line.
 It is also known as orthogenetic evolution, progressive evolution, evolutionary
progress, or progressions.
 The term orthogenesis was introduced by Wilhelm Haacke in 1893 and
popularized by Theodor Eimer five years later.
 This theory rejects the theory of natural selection i.e. survival of the fittest.
 Orthogenesis meant literally "straight origins", or "straight line evolution".
 This theory tells that motivational behavior present in individuals or organisms
have power to cause variation or evolutionary changes in forward direction or in
straight line.
 For example, different species of Heliconius butterfly have independently evolved
similar patterns, apparently both facilitated and constrained by the available
developmental-genetic toolkit genes controlling wing pattern formation.
Allometry
 The relation between size of an organism to it’s body part is termed as allometry. It
is an important method for describing morphological evolution.
 For example, there is allometric relation between the size of brain with it’s
individual body in such a way that animals with larger or greater body size have
larger or greater brain.
 This method was developed by Huxley. The allometric relations can be studied;
1. During the growth of a single organism.
2. Between different organisms of the same species.
3. Between the organisms of different species
 Plotting the typical graphs of allometric relations, the body size is taken along the
x-axis while it’s body parts like brain, eye-stalk are taken along the y-axis. The
points on these graphs can be used for;
1. The same individual measured at different ages.
2. For different individuals of same species.
3. For different species in a higher taxon.
 This method works easily for two characters, but it is clumsy for complex shapes.
For example, stalk-eyed flies from Malaya, which have an eye span longer than their
body. There is an allometric relationship between eye span and body length.

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Adaptive Radiations
Radiations or adaptive radiations can be defined as,” the evolution of an organism or
plant as ancestral species into wide variety of adapted lineages or species with the
change in environment with specialized modes of life”
 Adaptive radiations are best exemplified in closely related groups that have
evolved in a relatively short time.
 Radiations occur when a change in the environment makes new resources
available, creates new challenges, or opens new environmental niches.
 The process of speciation and phenotypic adaptation shows different
morphological and physiological changes.
 The individuals should have evolutionary, ecological and geological access for their
survival.
 An example of adaptive radiation would be the avian species of the Hawaiian
honeycreepers. Via natural selection, these birds adapted rapidly and converged
based on the different environments of the Hawaiian Islands