Human intervention in evolution

Chapter 16 – Part 1

Selective BreedingSelective breeding is the process of breeding

plants and animals for particular genetic traits which are desirable.

This is achieved in animals by using male animals with the desirable traits for a number of generations until the undesirable traits die out in the population.

Selective Breeding: Australian Sheep

Selective breeding is an integral part of the Australian wool industry.

Selective breeding has been used to increase the amount of wool a sheep produces and also to reduce the diameter of the wool fibres.

Selective breeding among sheep is achieved by breeding ewes with rams that have the desired traits.

Selective Breeding: Australian Cattle

Selective breeding has also been used with cattle in Australia in order to increase milk yields among dairy cows.

The modern dairy cow can produce about 35-50 litres of milk per day. This is about ten times more milk than a calf would ever need.

Reproductive technologiesThere are many reproductive technologies

associated with selective breeding.This includes:

Artificial Insemination (AI)Embryo transfer in livestock (MOET)Sex selection through sperm sortingMultiple ovulation and embryo transferOestrous synchronisation

Artificial Insemination (AI)Artificial Insemination involves collecting semen

from a selected male and then introducing this into the reproductive tract of a female.

The advantages of AI areA single volume of ejaculate from a male can be

sufficient to fertilise a number of eggs.Female can be fertilised by sperm that has come

from other regions and even other parts of the world.

A male’s ejaculate can be used long after it has died.

Genetic Impact of AI in animal breedingBy using semen samples from selected animals,

genetic variability is reduced among populations.If random mating was allowed to occur, then a

greater variety of traits would exist.The widespread use of AI could result in the loss of

undesirable traits and also expose animals to a greater susceptibility to diseases that they would otherwise have built up a resistance against.

Embryo transfer in livestock (MOET)

Multiple Ovulation and Embryo Transfer (MOET), allows more desirable females to contribute a greater amount of gametes.

Multiple ovulation refers to the process where the female animal receives injections of follicle stimulating hormone (FSH) in order produce a greater amount of eggs when she ovulates.

Embryo transfer involves transplanting fertilised embryos into surrogate mother.

Sex selection through sperm sorting

In some agricultural industries, being able to select the sex of the offspring is an advantage for farmers.

This can now be done by dying sperm with a fluorescent dye and separating out the sperm with X-chromosomes from the sperm with Y-chromosomes.

This is especially important in the beef industry where males are preferred and also in the dairy industry where females are preferred.

Manipulating breeding cyclesThis involves the synchronisation of the oestrus cycles of

female animals.NOTE: animals can only become pregnant if they are

inseminated during oestrus.This is important for the MOET procedure (discussed earlier). It also allows for higher rates of fertilisation and births.The synchronisation of oestrus is achieved by the introduction

of the hormone progesterone to the female. This can be introduced through: The animal’s diet Skin implants Vaginal implants

Once the source of progesterone is removed, the animal begins oestrus within 3 days.

Artificial pollination in plants

Artificial pollination has been used for centuries.This was the basis of Gregor Mendel’s pea

experiments. This process involves:

Removal of unripe stamens from plant to be fertilised.Protection of the stigma of the selected female plant

from stray pollen.Collection of pollenTransfer of pollen to the stigma of the female plant.

Creating HybridsArtificial pollination can be used to create hybrid

plants.For example wheat and rye can be crossed

together to create a hybrid (like a horse and donkey produces a mule).

This hybrid is infertile.

CloningCloning is a form of asexual reproduction.This involves the genetic information required

for a new individual originating from only one parent.

There are different types of cloning techniques. For example:Cloning by embryo splittingCloning by nuclear transfer

Embryo SplittingEmbryo splitting involves removing an embryo

from an individual. Individual cells from this embryo are removed and

then placed into Petri dishes that contain the correct nutrients and hormones for growth.

Each of the removed cells now divides to form a new embryo.

These embryos are then implanted into the uterus of surrogate mothers. These surrogate mothers will then give birth to identical offspring.

Cloning using nuclear transfer techniques

There are many different ways to manipulate cells and their nuclei. For example, you can:Remove the nucleus from a cell, enucleated.Transfer the nucleus from one cell to an

enucleated cell.Fuse a somatic cell with an enucleated cell

Dolly the SheepDolly the sheep was created using a somatic cell

from her mother and fusing this with an enucleated egg cell.

Once the egg cell showed evidence of early embryonic development, it was transferred into the uterus of a surrogate animal.

Cloning: the downsideSuccess rate of cloning is very low.Hundreds of embryos often need to be implanted into

females in order to successfully end up with a growing embryo.

Each time mammalian cells divide, the telomeres on the ends of chromosomes get shorter. It is thought that this is the cause of aging. So therefore cloned animals would age rapidly.

This has been evidenced in Dolly the sheep. She had to be put down at age 6 because of lung problems and severe arthritis.

Cloning PlantsThis can be naturally or artificially done.Examples of natural cloning are: cuttings,

runners, suckers, etc.Artificial cloning of plants involves the culturing

of a piece of adult plant.This can then be divided further to create a

large number of cloned copies of the plant.All of the resulting individuals are genetically

identical.