**Comprehensive Biology Notes: Biotechnology, Evolution, Health, and Diseases**
Unlock your academic potential with our meticulously crafted Biology notes, covering essential topics such as Biotechnology, Evolution, Health, and Diseases. These notes are designed to provide a deep understanding o...
BIOLOGICAL EVOLUTION
To explain evolution, Darwin talked about two things – (a) Branching
Decent (pattern of evolution) and (b) Natural Selection
According to Darwin, evolution is a branching process .Process of
developing a new species from a single common descendent is
branching descent. With time, new life forms appeared, and these new
life forms arise as branches from the previously existing life forms.
Hence, branching decent points towards the concept of common
ancestry.
Darwin emphasized that the selection of useful variations by nature is
the main mechanism for evolution to occur.
NEO-DARWINISM: The term was given by Romanes. Here variations are replaced by genetic
variations. Natural selection does not operate at the level of individuals but at the level of
population or gene pool. The various features of Neo-Darwinism are as follows.
1. Rapid multiplication. As in Darwinism
2. Limited food and space. As in Darwinism
3. Struggle for existence. As in Darwinism
4. Variations. They are caused by mutations, recombinations, migrations, non-random mating and
genetic drift.
5. Natural selections. It operates through differential reproduction. Repeated reproductive
selection of individuals with specific traits increases the frequency of their genes in the gene pool.
Individuals with harmful variations are eliminated at various stages.
6. Isolation. It is separation of a segment of population from the rest due to appearance of a
barrier.
7. New Species. Isolated population has no genetic contact with the parent population. Its gene
pool slowly changes due to mutations and differential reproduction. Ultimately this population
accumulates sufficient differences that its members fail to interbreed with individuals of the parent
population. This creates a new species.
MUTATION THEORY
In 1901, Hugo de Vries proposed the Mutation Theory on the basis of his observation on the wild variety of
evening primrose Oenothera lamarckiana.
According to mutation theory, new species originate as a result of large, discontinuous variation which appear
suddenly.
The main features of mutation theory are as follows:
1. Mutations arise from time to time amongst the individuals of a naturally breeding population.
2. Mutations are heritable and establish new forms or species.
3. Mutations are large and sudden and are totally different from fluctuating variations of Darwin, which are
small and directional.
4. Mutations may occur in any direction.
MECHANISM OF EVOLUTION
Speciation is an evolutionary process by which new species arise. According to Darwin, inheritable variations
are the cause as they keep on accumulating from one generation to other and, ultimately, give rise to new
species. But Hugo de Vries had different views.
According to him, ‘mutations’ were the cause of evolution and not the minor variations that Darwin talked
about. Mutations are the large changes arising suddenly in the genomic sequence (the DNA/RNA sequence) of
an organism. These mutations arise suddenly in a population and those mutations which arise in the germ cells
of an organism pass on to the next generation. The mutations are random and directionless.
Darwin said that evolution is a slow and gradual process which takes lot of time to proceed but deVries believed
that evolution occurs suddenly. The single step large mutation which can cause speciation was named saltation
by deVries.
, HARDY-WEINBERG PRINCIPLE
This principle states that a population is said to be in genetic equilibrium if it is not undergoing any kind of
evolutionary change. Main concepts of this principle are:
1) Allele frequencies are stable and is constant from generation to generation.
2) The gene pool (total genes and their alleles in a population) remains a constant. This is called a genetic
equilibrium.
3) Sum total of all the allelic frequency is 1.
Interpretation: Hardy-Weinberg principle can be used to mathematically interpret whether evolution has
occurred in a population or not. Disturbance in the genetic equilibrium or Hardy-Weinberg equilibrium, i.e.,
change in frequency of allele in a population, would be interpreted as resulting in evolution. When frequency
measured, differs from expected values, the difference indicates the extent of evolutionary change.
Factors affecting Hardy-Weinberg principle: Following five factors are known to affect Hardy-Weinberg
equilibrium. Therefore, if any of the following phenomenon occurs, change in frequency of allele takes place
that may result in the evolution:
1) Migration: The phenomenon of movement of alleles from one population to another is called gene
migration. It can either occur by
a) Migration of a section of population from one area to another, or by
b) Interbreeding between members of two populations resulting in interchange of alleles.
Gene pool: A total collection of all genes and its allele in a population is called gene pool. Thus, gene pool will
have all genotypes i.e., genes of the organisms.
Gene flow: If genes are exchanged between two different populations of a species, it is a gene flow.
2) Genetic Drift: It is random change in the allele number and allele frequency in a gene pool generally caused
by small size of the population due to destruction of a major part of population or separation of a segment from
the rest. Genetic Drift has two ramifications which are discussed below.
a) Bottle neck effect: It is the decrease in genetic variability in a population, e.g., cheetah population in Africa
decreased due to hunting. Their decreased numbers have limited cheetah genetic variability, with serious
consequences. The present cheetah population is susceptible to a number of fatal diseases. If any of these
diseases attacks the cheetah population, the path of extinction of cheetah cannot be reversed.
b) Founder’s effect: When one or few individuals are dispersed and become the founders of a new isolated
population at some distance from the place of origin, the alleles that they carry are of special significance. Even
if these alleles are rare in the source population, they will be a significant fraction of the new population’s
genetic endowment. This effect by which rare alleles and combination of alleles maybe enhanced in new
populations- is called the founder’s effect. For eg. evolution of Darwin’s Finches as in Galapogas Islands.
3) Genetic recombination: During gamete formation, the alleles present on the parental chromosome separate
and form new combinations. This results in a genetic recombination. The crossing over during meiosis is a major
source of variations in a population. The offsprings produced from these gametes show new combination of
characters and are called recombinants.
4) Natural selection: Nature selects those variants which are heritable and which make the survival better so
that the individuals bearing these useful variations are enabled to reproduce and hence produce more number
of progeny. Hence, natural selection also leads to change allelic frequencies. However the effects of natural
selection on different traits can be a. Stablising b. Directional or c. Disruptive
a. Stablising selection: If nature favours those individuals in the population which possess the mean character
value, then it is said that natural selection leads to the stabilisation of the traits. Let us take the example of
character human height. Tall, dwarf and medium – sized induviduals are the variants of this character. If nature
favours or selects the medium-sized induviduals, then the selection is said to be stabilizing. The peak of bell
shaped graph gets higher and narrower as nature tries to decrease the peripheral character values.
Sickle Cell Anaemia is an example of balancing selection
i. In few RBCs, 1-2% became sickle shaped during lack of oxygen.
ii. The heterozygotes [HbA/HbS ] who have one copy of sickle cell allele, coupled with one normal allele are
better survivors in the areas where malaria is endemic; because the malarial parasite spends a part of the life
cycle in the RBC; if they enter into the RBC which are sickle shaped, they will die.
iii. The women who are heterozygote have higher fertility; that’s why natural selection has not eliminated the
allele.
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