Biology Today — January 2018

• The two formulae are – p^2 + 2pq + q^2 = 1, p + q = 1

where, p = Frequency of the dominant allele in the population

q = Frequency of the recessive allele in the population

p^2 = Percentage of homozygous dominant individuals

q^2 = Percentage of homozygous recessive individuals

2pq = Percentage of heterozygous individuals

1 = Sum total of all the allelic frequencies

No genetic recombination

There should be absence of new combinations of

characteristics as a result of recombination of alleles.

No mutation

Absence of both gene or chromo-

somal mutations is essential to

maintain equilibrium.

No genetic drift

Any change in the population of alleles in

gene pool, causing elimination of certain

alleles must be absent in a population.

No natural selection pressure

There should be absence of natural selection

pressure with respect to alleles in question.

No gene flow

There should not be any movement or

flow of alleles from one population to

another, so as to maintain stability.

Factors affecting

Hardy-Weinberg

principle

• Constant gene frequencies over several generations indicate that evolution is not taking place whereas change in gene
  frequencies indicate progress/onset of evolution. Thus, evolution occurs when genetic equilibrium is upset.

BRIEF ACCOUNT OF EVOLUTION

Evolution of Plants

• It is considered that first cellular forms of life originated about 2000 million years ago. Some of these cells developed pigments
  to capture solar energy and release oxygen by employing water as hydrogen donor during photosynthesis. The prokaryotes
  originated in archaeozoic era. Gradually prokaryotes became eukaryotes. These eukaryotic cells diversified to form green
  algae and early invertebrates. Each of which evolved and gave rise to plants and animals respectively.

Cenozoic

Mesozoic

Paleozoic

Quaternary

Tertiary

Bryophytes

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Angiosperms (flowering plants)

Monocotyledons

Dicotyledons

Seed ferns

Progymnosperms

Conifers Cycads

Gnetales

Ginkgos

Ferns

Sphenopsids

(horsetails)

Herbaceous

lycopods

Arborescent

lycopods

Zosterophyllum

Psilophyton

Rhynia-type plants

Tracheophyte ancestors

Chlorophyte ancestors

Fig.:A sketch of the evolution of plant forms through geological periods