Human Intervention in Evolution

Next


Artificial selection is the intentional selection of certain traits, or combinations of traits, over others 

It was first postulated by Charles Darwin as a contrast to natural selection (whereby uncontrolled environmental influences select for traits)

Desired traits can be selected for (or against) via repetitive breeding programs or by genetic intervention using a number of molecular biology techniques

Eugenics is a social philosophy which advocates the improvement of human hereditary traits through various means of intervention


Selective Breeding

Selective breeding of domesticated animals is an example of artificial selection, which occurs when man directly intervenes in the breeding of animals to produce desired traits in offspring

As a result of many generations of selective breeding, domesticated breeds can show significant variation compared to the wild counterparts, demonstrating evolutionary changes in a much shorter time frame than might have occurred naturally

Examples of selective breeding include:

  • Breeding horses for speed (race horses) versus strength and endurance (draft horses)
  • Breeding dogs for herding (sheepdogs), hunting (beagles) or racing (greyhounds)
  • Breeding cattle for increased meat production or milk
  • Breeding zebras in an attempt to retrieve the colouration gene from the extinct Quagga


Cloning

A clone is a group of genetically identical organisms or a group of cells derived from a single parent cell

Somatic Cell Nuclear Transfer (SCNT) is a method of reproductive cloning using differentiated animal cells

  • A female animal (e.g. sheep) is treated with hormones (such as FSH) to stimulate the development of eggs
  • The nucleus from an egg cell is removed (enucleated), thereby removing the genetic information from the cell
  • The egg cell is fused with the nucleus from a somatic (body) cell of another sheep, making the egg cell diploid
  • An electric shock is delivered to stimulate the egg to divide, and once this process has begun the egg is implanted into the uterus of a surrogate
  • The developing embryo will have the same genetic material as the sheep that contributed the diploid nucleus, and thus be a clone


Different Uses of Cloning


Transformation

Because the genetic code is universal, genetic information can be transferred between species to create new transgenic organisms

The process of gene transformations occurs in four key steps:

  • Digestion:  The gene of interest is cut out of a section of DNA using restriction endonucleases
  • Ligation:  The gene of interest is inserted into a vector (either a plasmid or a virus) using DNA ligase
  • Transformation:  The vector (with the gene of interest) is introduced into a host organism
  • Expression:  The host organism will hopefully begin to express the desired trait


Creating a Transgenic Sheep that Produces Human Clotting Factors


Stem Cell Differentiation

Stem cells can be derived from embryos or the placenta / umbilical cord of the mother; also minimal amounts can be harvested from some adult tissue

Stem cells can be used to replace damaged or diseased cells with healthy, functioning ones

This process requires:

  • The use of biochemical solutions to trigger differentiation into desired cell type
  • Surgical implantation of cells into patient's own tissue
  • Suppression of host immune system to prevent rejection of cells
  • Careful monitoring of new cells to ensure they do not become cancerous


Differential Gene Expression Leading to Specialisation of Cell Structure and Function


Examples of therapeutic uses of stem cells:

1.  Retinal cells:  Replace dead cells in retina to cure diseases like glaucoma and macular degeneration

2.  Skin cells:  Graft new skin cells to replace damaged cells in severe burn victims

3.  Nerve cells:  Repair damage caused by spinal injuries to enable paralysed victims to regain movement

4.  Blood cells:  Bone marrow transplants for cancer patients who are immuno-compromised as a result of chemotherapy


Genetic Screening

Genetic screening involves testing an individual for genetic defects (via karyotyping, DNA microarrays, sequencing, etc)

  • This information can be used in the managment of diseases and for planning pregnancies
  • In vitro fertilisation allows couples to potentially select desirable traits, by fertilising egg and sperm with the appropriate alleles
  • There are several ethical and legal issues involved with genetic screening, including the implications of designer babies and the ramifications for health insurance


In Vitro Fertilisation


Gene Therapy

Gene therapy is the insertion of genes into an individual's cells or tissues to treat heriditary diseases by replacing defective alleles with healthy ones

It differs from standard transgenic practices in that it is intended to transform existing organisms rather than create new ones

Typically viral vectors are used, due to their ability to integrate their DNA into the host's genome

Although the technology is still in its infancy, it has been used with some success (e.g. treating ADA deficiency in SCID patients)