Biology Today — January 2018

Sickle cell anaemia

The sickle cell gene produces a variant form of the protein haemoglobin, which differs from the normal haemoglobin by a single amino acid. Sickle cell

anaemia is caused by the substitution of glutamic acid by valine at sixth position of beta chain of normal haemoglobin. In people, homozygous for this

abnormal haemoglobin, the red blood cells (RBCs) become sickle-shaped. The people affected by this disease usually die before reproductive age, due

to a severe haemolytic anaemia. Inspite of its disadvantageous nature, the gene has a high frequency in some parts of Africa, where malaria is also

in high frequency. The heterozygotes for the sickle cell trait are exceptionally resistant to malaria. Thus, in some parts of Africa, people homozygous

for the normal gene tend to die of malaria and those homozygous for sickle cell anaemia tend to die of severe anaemia; while the heterozygous

individuals survive and have the selective advantage over either of homozygotes.

Examples of Natural Selection

Resistance of insects to pesticides

When DDT was introduced as an insecticide, it was effective

against pests. But within two to three years of the introduction

of this insecticide, new DDT resistant mosquitoes appeared in the

population. These mutant strains, which are resistant to DDT, soon

became well established in the population and to a great extent,

replaced the original DDT-sensitive mosquitoes.

Industrial melanism

In Great Britain, Biston betularia (peppered moth) existed in two forms,

light coloured (white) and melanic (black). Before industrialisation, barks

of trees were covered by white lichens, so white moths escaped unnoticed

from predatory birds. After industrialisation, barks got covered by smoke,

so population of white moths were selectively picked up by birds and black

moths increased as they escaped unnoticed.

SPECIATION

• Formation of one or more new species from an existing species is called speciation.

Sympatric speciation

A small segment of the original population becomes isolated reproductively.

As the isolating mechanism comes into force, a new subspecies emerges

and new species is formed. Thus, sympatric speciation is the formation of

species within a single population without geographical isolation. E.g.,:

Pig frog and Gopher frog occur in different habitats.

Allopatric speciation

In this type of speciation, a part of the population becomes geographically

isolated from the main population and becomes entirely separated and

finally constitutes a new species. Thus, geographic isolation brings about

allopatric speciation, for example: formation of Darwin’s finches that

formed separate species in the Galapagos Islands.

Quantum speciation

The budding off a new and very different daughter species from

a semi-isolated peripheral population of the ancestral species is

quantum speciation. This is based on the observation of H.L. Carson

on Drosophila inhabiting Hawaii islands. It is a sudden and rapid

speciation where genetic drift or chance plays a major role in

quantum speciation.

Parapatric speciation

It takes place when a population of a species enters a new niche or habitat.

It occurs only at the edge of the parent species range. Although there is

no physical barrier between these populations, yet the occupancy of a new

niche results as a barrier to gene flow between the population of new

niche. Two species are produced due to reproductive isolation from single

one. E.g.,: speciation in flightless grasshoppers, snails and annual plants.

Types of Speciation

Table : Differences between allopatric and sympatric speciation

Allopatric speciation Sympatric speciation

• A new species arises because a physical barrier separates
  it from other members of an existing species.
  
  
  • A new species arises from an existing species that is living
    in the same area.
  
  
  
  


• Physical barriers may include mountain ranges, valleys
  of water bodies or human-made features such as roads,
  canals of built-up areas.
  
  
  • Temporal and behavioural isolations produce significant
    changes in the genetic make-up within a species so that a
    new species is formed.
  
  
  
  

HARDYWEINBERG PRINCIPLE

• Hardy-Weinberg’s principle describes a theoretical situation in which a population is undergoing no evolutionary change. It
  explains the stability of population and species over a number of generations.


• “The relative frequencies of various kinds of genes in a large and randomly mating sexual panmictic population tend to
  remain constant from generation to generation in the absence of mutation, selection and gene flow.” This is called Hardy-
  Weinberg principle or Hardy-Weinberg equilibrium. This principle is an expression of the notion of a population in ‘genetic
  equilibrium’ and is the basic principle of population genetics.


• In a population at equilibrium, for a locus with two alleles, D and d having frequencies of p and q, respectively, the genotype
  frequencies are: DD = p^2 , Dd = 2pq and dd = q^2.