Evolution:
Evolution is a process by which different
kinds of life forms are believed to have developed from earlier life forms
during the history of the earth.The characteristics that are modified during
evolution are inherited. Evolution is a gradual process. Evolution has occurred
in three patterns namely,
i.
Divergent
evolution:
The species have their ancestor in common.
To avoid competition, they moved away from a particular geographical area and
isolation for a long period of time with some modifications(may be due to
environment) made them develop as new distinct species as there was no gene
flow(mating).
Ex:
Finches in GalapagosIsland
ii.
Convergent
evolution:
It
takes place when different species with different ancestors develop similar
traits as they tend to develop in similar geographical area(environment).
Ex:
Humming bird and Humming butterfly.
iii.
Parallel
evolution:
It
is the evolution of geographically seperated individuals with a common
ancestor. These species showsome morphological resemblance.
Ex:
Litopterns
and Horses
Evidences for evolution:
There are many evidences to prove that evolution has taken place. Some of them are,
1. Paleontology-
It is defined as the studies that is concerned with fossils.
2. Comparative anatomy and
morphology- It shows both similarities and
differences between the present day organisms and their ancestors. It can be
studied under two aspects,
a) Homology-
Organisms have similar basic structure but evolve to perform different
functions. Ex: forelimbs of birds and mammals.
b) Analogy-
Organisms have different basic structure but evolve to perform similar
functions. Ex: Wings of birds and butterflies.
3. Vestigial organs-
These are reduced and non functioning organs in a species but were well
developed and functional in its ancestral forms. Ex: rudimentary body hairs in
man that were well developed in his ancestors.
4. Molecular homology-
It refers to similarities in nucleotide sequences of a species and its
ancestor. Ex: Human and apes
5. Resistance to chemicals- Organisms evolve to become resistant to the environmental and human hazards. Ex: Resistance shown by mosquitoes to DDT chemical.
Darwin’s theory of evolution:
Darwin was an English naturalist, geologist and biologist best known
for his contribution in science of evolution. He made a voyage to Galapagos
island with his ship HMS Beagle(His/ Her Majesty Ship Beagle) and documented
some observations in the book "The Voyage of the Beagle". His another main contribution is
the book "Origin of Species". Darwin's theory of evolution has five
major principles. They are,
1)
Over
production
The organisms which produce more
off-springs have more chance to survive and evolve.
2)
Competition
Different species present in a
population compete with other life forms for their survival when the resources
are limited.
3)
Variation
Individuals in a population vary
greatly in their characteristics. This may be due to mutation.
4)
Adaptation
Individuals that adapt to the existing
environment are more likely to survive than non adapted ones. Adaptations may
be
a. Structural(locomotion
and food capture)
b. Chemical(production
of venom)
c. Behavioral(camouflage,
mimicry)
5)
Natural
Selection
Individuals having favorable traits
survive and are more fit(reproductively) than that of individuals with
unfavorable
traits.
Speciation:
The process by which new, genetically
distinct species evolve gradually as a result of genetic isolation from the
main population over time. Isolation refers to separation of individual life forms from the main
population. It favours evolution of new species.Three ways by which speciation
takes place are,
a.
Allopatric
speciation
Occurs
when a population splits and becomes isolated from each other due to physical
barriers to an extent that prevents gene flow i.e. mating does not occur. Ex:
Galapagos finches.
b.
Peripatric
speciation
Small groups of individualsseperate from the large population and evolve. It follows divergent evolution. It prevents gene flow.
c.
Paraptric
speciation
There
is no seperation of species by geographical barriers but an extreme change in
habitat. Gene exchange takes place.
d. Sympatric speciation
New species evolving from surviving
ancestral species. Both continue to inhabit the same geographical region. Ex:
Polyploids occupying same place with parents.
Cladogenesis:
Cladogenesis is
an evolutionary splitting of a parent species into two
distinct species, forming a clade.This event usually occurs when a few
organisms end up in new, often distant areas or when environmental
changes cause several extinctions, opening up ecological niches for the
survivors and causing population bottlenecks and founder
effects changing allele frequencies of diverging populations
compared to their ancestral population.
Population
bottleneck:
Sharp reduction in the size of
a population due to environmental events (such as famines,
earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide).
Founder
effect
Loss of genetic variation when a new
population is established from a very small number of individuals of a larger
population.
Reproductive
Isolation:
Reproductive
isolation is a mechanism that keeps species from mating with others.Reproductive
isolationis defined as the inability of a species to breed successfully with
related species due to geographical, behavioral, physiological, or genetic
barriers or differences. It is of two types
§
Pre-zygotic
§
Post-zygotic
Pre-zygotic:
Pre-zygotic isolation occurs before the
formation of a zygote. In most cases mating does not even occur. Forms of
pre-zygotic isolation include
ü
Spatial(occurs when members of two populations never
encounter each other, they will never mate and no gene flow will occur. Spatial
isolation occurs between populations that are separated by great distances)
ü
Behavioral(due to difference in mating or sexual behavior
between related species)
ü
Mechanical(due to incompatibility of reproductive organs
between different species)
ü
Temporal(due to physical barriers or different time of
sexual maturity between two species)
ü
Gametic(the gametes (egg and sperm) come into contact, but
no fertilization takes place)
Post-zygotic:
Post-zygotic isolation occurs after members of
two different species have mated and produced a zygote. The offspring of such a
mating is called a hybrid. Forms of post-zygotic isolation includes,
a. Zygote
mortality and non-viability of hybrids
Occurs when the egg or ovule is fertilized
but the zygote does not develop, or it develops and the resulting
individual has a reduced viability.
b. Hybrid
sterility
A hybrid has normal viability but is typically
deficient in terms of reproduction or is sterile. Sterility is due to the
interaction between the genes of the two species involved; due to chromosomal
imbalances because of the different number of chromosomes in the parent
species; or due to nucleus-cytoplasmic interactions.
Genetic
Polymorphism:
Polymorphism is
diversity in the shapes or forms of individuals within a species.
Occurrenceofdifferent forms of individuals among the members of a single
species due to discontinuous genetic variation.According to the theory of
evolution, polymorphism results from evolutionary processes and isheritable.
Causes for genetic polymorphism are genetic variation, mutation,natural
selection and random mating.
Reference:
www.darwins-theory-of-evolution.com
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