Population GeneticsChapter 13 – Part 2

Selection: Two KindsThere are two types of selection:

Natural Selection

Artificial selection

Natural SelectionCharles Darwin’s theory on Evolution is based on

the concept of natural selection. Although he used may examples of artificial selection in his book Origin of Species

This theory involves heritable traits becoming more or less common in a population over successive generations.

Artificial SelectionArtificial selection (also known as selective

breeding) is the intentional breeding of certain (often desirable traits).

E.g.: Dairy cows that produce larger amounts of milk are selectively bred. Cat, pigeon, chicken and dog breeds

Loss of Genetic Variation

In the event of a natural selection pressure, the individuals which display a higher fitness (i.e. produce more viable offspring) to the circumstances will survive.

However, if there is no variation between the individuals, the entire species may be affected.

This can be devastating on a population as it can lead to the extinction of an entire species.

Saving Genetic VariationThere are many ways in which genetic diversity

is preserved today.

E.g.: commercial and private fishing companies are restricted by regulations as to where, when, the species, the size and number of fish they’re allowed to catch.

As fishing companies prefer larger fish, they are selecting against these larger fish and hence reducing the genetic variation among fishes for size. The fish are getting smaller.

MigrationMigration of populations can change genetic

diversity.

Migration of individuals can introduce new alleles into a population. If this new allele provides a fitter phenotype, then it is likely that this allele will be retained in the population.

Human MigrationScientific evidence indicates that early humans

(Homo erectus then Homo sapians) migrated from Africa to the Middle East then to Europe and Asia.

This is called the ‘Out of Africa’ hypothesis.

Chance EventsChance events can also change the genetic

diversity of a population.

This is called Genetic Drift.

Genetic Drift is more likely to occur in smaller populations. This is known as the Bottleneck Effect and Founder Flush.

Evolution Within a SpeciesOver time, a population containing one species

can evolve into two different sub-species.

These sub-species can retain their ability to successfully reproduce with one an other. In this case they are still one species.

As soon as a sub-species loses their ability to reproduce with individuals of another sub-species, they become a species in their own right.

Studying Populations Using mtDNA

Populations can be studied and compared using mitochondrial DNA.

Remember, mtDNA can only be inherited from an individual’s mother, not their father.

Over time mutations occur in the mtDNA.

Mutations in mtDNA have been important in the study of the evolution of humans (Homo sapiens).

It is through the mutations in mtDNA that we have been able to map out the migration of early humans from Africa.

mtDNA Variation Between Populations

As populations become isolated, they develop and maintain different mutations in their mtDNA.

The different mtDNA found in different populations are known as Haplogroups.