Evolution
is the sum total of all the adaptive changes taking place over a long period of
time, it is a progressive change that gives birth to new species from earlier
ones.
Evolution
teaches that all living organisms has a common ancestors who are the
descendants of previous forms of life and are therefore related to each other.
Types
of Evolution
There
are two types of evolution
1)
Organic
evolution:
Organic evolution is the evolution of living multicellular organisms from non
living unicellular ones such as algae.
This type of evolution explain that the first living organism came from
atmospheric gasses, (methane, ammonia, hydrogen molecules and water vapors),
which reacted together in the presence of ultra violet radiation from the sun
and electric lighting from the atmosphere to form the first living thing (unicellular
organism).
2)
Inorganic
evolution:
This is the evolution of non living things,
it involves deposits in lakes, sea beds and sedimentary rocks over
millions of years, this formation is then exposed or raised to the surface
above sea level by disturbance in the
earth crust (earth movement), the oldest being at the bottom and the newest at
the top.
Theories of
evolution
1.
Lamarck’s theory
of inheritance of acquired characters: French zoologist, Jean Baptiste de Lamarck (1744 - 1820), in his theory stated that
environment plays a significant role in the change of parts of an organism.
Lamarck noted that when individuals of the same species grow under different
environmental condition, they exhibits mark difference. For example, when leave
grow in the dark, they fail to develop chlorophyll, they also develop weak
stem. Also, in animal adaptation is brought about by the use and disuse of
parts. He believe that new characters acquired in each generation are preserved
and passed on to offsprings, an example is the long neck of Giraffe, which is
as a result of want of exercise to feed on leaves on top of tree which lead to
the degeneration of short neck .
Lamarck’s theory
of evolution
can be summarized as follows,
· Environmental
effect on living organisms.
· Use and disuse
of parts of organism.
· Inheritance of
useful characters.
2.
Darwin’s theory
of natural selection: Darwin’s theory of natural selection is based
on three major factors.
· Most organisms
(Plants and animals) reproduce large numbers of offsprings more than can
survive and reproduce (Overproduction).
· The manageable
size of any given population remains constant, although, it may be fluctuating.
· Some variations
may occur due to environmental conditions and organisms will adapt to these
harsh conditions and transmit their characteristics to their offsprings
(Inheritance of adaptive characters).
· Organisms with
favorable conditions may continue to exist while those with unfavorable
characters will die off hence survival of the fittest (Survival of the fittest).
· Accumulation of
favorable variation over a long period of time will lead to divergence from the
original stock leading to the formation of new species (Formation of new
species)
He deduce the followings from his observation,
· All living organisms
are constantly in a state of struggle for existence
· Those that
survive and reproduce have variations with competitive advantages over others.
· These fittest survivors’
best adapt to the environment and transmit the variation to their offsprings.
Darwin believes that natural selection acting
via environmental pressure is the force causing the process of evolution to
take place.
Hungo
Devries theory of mutation: Devries
in his theory stated that there is a sudden and spontaneous variation among
offsprings in one generation leading to evolution. This spontaneous variation
was what he called mutation. Mutant may have certain favorable
characteristics in the struggle for existence and may become distributed in the
population within the generation.
Patterns of Evolution
The
patterns of evolution are as follows,
·
Many
unrelated organism evolve similar body form due to adaptation to the same
environment or ecological niche, this is called convergent evolution.
·
Group
of organisms from the same ancestral stock accumulate difference giving rise to
new species, a process known as Divergent evolution. It happens when structures having a common
origin are adapted in different ways in different lineages. For example the
forelimb in vertebrate which has been modified to form paddle in whale, arms in
man, wings in bate and legs for running in horses
·
Species
that interact evolve together with adaptive changes in one producing
complimentary change s in other (Co evolution). In co evolution there
is mutual relationship between two species, an examples is seen in the
relationship between predator and prey.
In this case, as the prey is evolving strategies to avoid being capture,
the predator is also changing to become better captors.
The
followings are the sources of evidences
of evolution.
1. Sex and
evolution:
Many scholars especially Darwin, attributed great important to variations of
sexual significant as seen in the Plumage of birds, the female birds chooses only the best male
for mating in other to reproduce the best offsprings with beautiful feathers.
2. Fossil record: Fossil record is all about the remains of
organisms preserved in sedimentary rocks. The age in which fossil occur is
estimated using radioactive isotope dating. The fossil can be arranged
according to age and evolutionary trend from very simple life forms to complex
ones. For example, the evolution of modern day horse, New World Stilt Legged horse
(NWSLH) Equus from Dinohippus…..
3.
Biochemical
similarities:
The chemical building and biological process of all living organisms are similar.
All organisms are made up of the same complex molecules which include carbohydrates,
lipids, proteins and nucleic acids. They also use ATP bonds as their immediate
source of energy for metabolic and anabolic processes. Living organisms also
use genetic materials which are stored, expressed and transmitted in the same
way; this shows that all organisms have the same ancestor.
4. Evidence from
cytology:
Electron microscope has shown that such cell structures as the Golgi bodies,
Membrane, Mitochondria, Ribosomes, and
Lysosomes, are all similar in living
organisms. This proves that all living organisms come from a common ancestor
with similar cell structures.
5. Comparative Anatomical: Anatomical
similarities are found in many group of vertebrates, this is seen in the body
plan with a brain case(skull), back bone (Vertebral column) and a pair of
girdles to which the fore and hind limbs are attached(homologous structure).
The fore limbs
of vertebrates are modified into wings for flying in birds and bats, and
flippers for swimming in water. The hind
limbs are use for walking and swimming in some vertebrates, while the human
arms are used for grasping and holding. There is also a complex change in the
structure of vertebrate from Pisces to mammalian; this is very obvious in the
evolution of vertebrate’s heart from two chambers in Pisces to four chambers in
mammal. This means that the circulatory system evolve from single in fishes to
double in mammals.
Also, the presences
of vestigial organs found in human body are evidence of evolution as listed
below,
· The appendix
found in man is vestigial
· The muscles of
the ears found in human are more elaborate than those found in horse.
· Some stage in
human embryonic development shows the presence of gill slits, this is also
found in fishes and tadpoles, the gill in adult stage develop into lung for
respiration, except in fishes where it persist.
· Human has fewer
hairs on their body when compared to the fur on other animals.
· The tail in man
is a vestigial organ; it is very prominent in embryonic stage. Tail is also
seen in fishes, reptiles, amphibians, birds and other mammals.
6. Molecular record: All living
organisms irrespective of speciation store hereditary character in nucleic acid in form of DNA (DEOXYRIBONUCLEIC ACID), The DNA of closely related species shows more similarities than those
of distance species, for example, the DNA of human and Chimpanzee are 98%
alike.
7.
Behavioral
similarities:
Organisms with the same behavioral similarities are more likely to be closely related
than those with different behavioral patterns, Jane Goodall, a prominent
Tanzanian scholar studied the behavioral patterns of Chimpanzee and found that
they were similar to those of man. Goodall observations which pointed out that
Chimpanzee and human has a common ancestor
are summarized as follows,
· Chimpanzees
usually employ the use of simple tools like sticks for attacking, exploring,
poking and teasing, they use twigs and grass blade to collect ants, and they
also use stones to crack nuts and leaves to wipe themselves clean.
· The play
patterns of behavior of adults and young ones resemble that of human.
· The way the
parents wean the offspring is similar to that of human
8. Embryological similarities: The Embryo of
some organisms resembles that of its ancestor at some stages of development,
for example, human embryo possesses gill slit like fish at early development.
Also, the young embryo of human is similar to that of other vertebrates, there
by pointing to the fact that vertebrates have a common ancestor.
9.
Geological
evidence: Many fossil remains of human has been
unearthed in geological strata called sedimentary rocks. The remains of
organisms are preserved in these rocks as fossils.
Modern Views of
Evolution
Modern
views of evolution are based mainly on mutation and isolation.
Mutation: The Theory of
mutation was postulated by Hungo de vries , a Dutch botanist in 1901, after
many years of researching on evening primos plant, he, observed that the offsprings of the plant
showed variation that was not present in
their parent. De Vries, then concluded that
there is a sudden and spontaneous variation among offsprings in the same
generation leading to formation of new species (evolution).
Thus,
Mutation is a sudden change in the genetic constituent of an organism leading
to introductions of new and stable characteristic. Mutants may have certain
characteristics that will benefit them in the struggle for existence; this
trait is usually inherited by the offsprings and may become distributed in the
population within a generation.
Knowledge
of gene and chromosomes can be used to explain the source of genetic variations
on which natural selection depends.
There are two natural forces responsible for evolutionary change, they include,
(a) gene drift and (b) gene flow.
Questions
What is Gene drift and Gene flow?
How
best can you explain the term vestigial?
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