Mechanism of Organic Evolution

1. MECHANISM
OF
EVOLUTION

2. MUTATION
Hugo De vries (1901) coined the term mutation
and defined it as sudden and discontinuous
variations in living beings.
It contributes for biological evolution by
favouring the origin of a new species.
It occurs in nature with time, the organism or
plant bearing any mutation is called as a
mutant, who is distinct from its parents.
Mutations are heritable.
They may be advantageous or disadvantageous
but, disadvantageous mutations are eliminated
by natural selection.

3. Mutations may be of two types as gene/point
mutation and chromosome mutation.
Point mutation is any change in a particular
gene or nucleotide base.
Chromosome mutation is change in number or
structure of chromosomes or chromosomal
aberration.
Chromosomal Aberration is any change in
chromosomal structure that change number,
position or sequence of genes is called as
chromosomal aberration or chromosomal
mutation.
It may be of following types :-

4. Deletion / Deficiency :- loss of a part or
more.
Duplication :- addition of genes or lost
part of another chromosome.
Inversion :- lost parts reunite in reverse
order.
Translocation :- transfer of a part of a
chromosome to different location of same
or different chromosome.
Reciprocal Translocation :- exchange of
parts between two chromosomes.

6. VARIATION IN CHROMOSOME NUMBER
Aneuploidy :- addition or loss of few
chromosomes to the usual diploid sets.
Monosomy :- loss of one
chromosome (2n – 1)
Nullisomy :- loss of two / one pair of
chromosome (2n – 2)
Trisomy :- addition of one
chromosome (2n + 2)
Tetrasomy :- addition of one pair of
chromosomes (2n + 2)

7. VARIATION IN CHROMOSOME NUMBER
Euploidy :- addition or loss of complete
set of chromosomes to the usual diploid
sets.
Haploidy / Monoploidy :- loss of one
set (2n – n = n)
Triploidy :- addition of one set (2n +
n = 3n)
Tetraploidy :- addition of two sets
(2n + 2n = 4n), and so on

8. RECOMBINATION
It is the process of formation of
recombinant chromosomes due to
crossing over. The offspring having
such chromosome(s) is called as a
recombinant offspring. As we got
new varieties in dihybrid cross
described earlier. (see dihybrid cross
and crossing over).

9. VARIATION
These are differences between members of
same/closely related species. It is of 6 types in 3
sets
 Based on body parts
Meristic variation
Substantive variation
 Based on range of variation
Continuous / Plus-minus variation
Saltatory variation
 Based on cells of occurrence
Somatic variation
Germinal variation

10. Meristic variation :- Variation in number of
body parts. e. g. 6 fingers in hand
Substantive variation :- Variation in shape, size,
colour, form of body parts. e. g. blue eye colour.
Continuous / Plus-minus variation :- Slight
variation on either sides of an average
condition. e. g. Skin colour of human beings,
cow milk yield.
Saltatory variation :- Sudden and large
variation. e. g. origin of a new species.
Somatic variation :- Occur in somatic or body
cells and not herritable. e. g. accidental losses.
Germinal variation :- Occur in germ cells and
inherited. e. g. gene mutation.

11. Gene pool :- It is the collection of all the genes of a
particular population in a particular area.
Gene flow / Gene migration :- It is the addition or
removal of some genes to / from a gene pool on entry
and interbreeding (immigration) or exit and
outbreeding (emigration) of individuals to / from
that population. It results in genetic recombination
and increase chances of evolution with a tendency to
form a new species.
Genetic drift :- It is the process of random change in
genotype of in members of a species when they
migrate to an isolated or distant place to overcome
some natural or artificial calamities. Gradually they
differ from original species and become a new one

12. under natural selection. It contributes to evolution.
Hybridisation is process of mating between two
different species, it may form fertile hybrid or
offspring when mates are from closely related species
and it in turn transfer both parental characters to its
offsprings. Founder effect is a drastic variation in
genotype of a small group of members of a species on
their isolation or distant migration. Some times
during the decline of a population, a small group of
population may survive and isolated or under
restricted distribution, after random genetic drift,
they somehow regain past richness, this is called as
bottle neck effect, it may help in evolution.

13. HARDY WEINBERG’S PRINCIPLE / HARDY
WEINBERG’S EQUILIBRIUM - 1908
It states that, other factor remaining same,
frequency of particular genes remain constant in a
population. Changing gene frequency indicate
change in other factors and evolution.
So evolution occur on failure of H – W equilibrium.
Frequency is ratio of number of individuals of
specific character to that of total number of
individuals in a population.
Example of 100 rats in a rat population of an area.
If 84 of them are black and 16 are white,
frequencies are 0.84 and 0.16 respectively. Let B be
the dominant allele of black colour forming gene, b
is its recessive.

14. GENERATIONS CROSSMALE
FEMALEPARENTS
B bGAMETES
BbF 1 GENERATION
BB Bb
Bb bb
B
B
b
b
F 2 GENERATION
BLACK WHITE
ALL BLACK
1 – BLACK
2 – BLACK
3 – BLACK
4 - WHITE

15. Monohybrid Ratio = 3 Black : 1 White
Frequency of genotype =
Homozygous dominant = 0.36
Heterozygous dominant = 0.24 each or 0.48
together,
Homozygous recessive = 0.16
If p is frequency of allele B and q is frequency of
allele b, as there are only two alleles as one is B
and other is b, (p + q) is always equal to 1, then,
H – W equation may be given as :-

16. HARDY WEINBERG’S EQUATION
(p + q)2 = p2 + q2 + 2pq = 1
p2 = No. of homozygous BLACK
q2 = No. of homozygous WHITE
2pq = No. of heterozygous BLACK
IN EQUATION
 q2 = 0.16 (as from above two tables it is frequency of
white cats) ⇒ q = √0.16 = 0.4
 (p + q)2 = 1 ⇒ p + q = 1 ⇒ p = 1 – q = 1 – 0.4 = 0.6
 Frequency of homozygous black would be = p2 x 100
= 0.62 x 100 = 0.36 x 100 = 36%
 Frequency of heterozygous black would be = 2pq x
100 = 2 x 0.6 x 0.4 x 100 = 0.48 x 100 = 48%
 Frequency of white would be = q2 x 100 = 0.16 x
100 = 16%

17. ADAPTIVE RADIATION
It is the process of evolution
of different species in a given
area from one species and
forming other species. Gilled
fishes and lung fishes from a
common ancestor is an example.