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What is Gene Flow?

• In its simplest form, gene flow is the unidirectional movement and incorporation of alleles at a single locus between two populations.

• It can also be termed as the movement of alleles from one population to another population via individuals or gametes.

• It is an essential component to animal evolution and is the basis of many animal adaptations.

• Movements of animals can be characterized as either dispersal or migration.

What is Migration?

• Migration is the movement of alleles from one population to another population due to the movement of individuals or gametes.

• It can also be defined as "the relatively long distance movements made by large numbers of individuals in approximately the same direction at approximately the same time and is usually followed by a regular return migration" 1 (As in seasonal migrations).

What is Dispersal?

• Dispersal is the continuous, non-directional, random, and small-scale movement of individuals instead of groups which results from their daily activities.

• There are two types of dispersal:

(1) Natal Dispersal = the continuous, non-directional, random, and small-scale movement of individuals instead of groups which results from their daily activities.

(2) Breeding Dispersal = The permanent movement made by an individual from its birth site to the place where it reproduces or would have reproduced if it had survived and found a mate.

How Does Gene Flow Effect Populations?

• The effect of gene flow on the genetic structure of a population depends on the rate of immigration and the amount of genetic difference between the recipient population and the immigrants.

• The effect of gene flow also depends on the amount of genetic difference between the recipient and the immigrants. If there is no genetic difference, there is no gene flow. As the amount of genetic difference increases, the effect of gene flow also increases.

• There are mathematical models to measure gene flow, but we will not go into them here.

What is the Evolutionary Importance of Gene Flow?

• The primary effect of gene flow is to maintain the genetic continuity throughout the range of a species! This is known as genetic population structure.

• Gene Flow can be viewed at two levels:

(1) As the evolutionary glue that holds together the fragments of a species that does not occur continuously over it's range OR

(2) As a force preventing local adaptation through the constant inclusion of foreign alleles.

If gene flow ceases and populations become genetically isolated (for example through the emergence of some sort of geological, reproductive, morphological, or behavioral barrier), differential selection pressures can act on populations for them to evolve new genotypes or phenotypes. This eventually leads to the creation of a new species if gene flow does not resume before the species have differentiated substantially enough.

Speciation

• Speciation is the creation of new species. There are three main types of speciation:

(1) Allopatric Speciation

This is the most common form of speciation. It occurs when the 2 populations are not in contact with one another. A & B are the resultant species in all 3 examples

(2) Parapatric Speciation

This form of speciation occurs when the ranges of the 2 populations contact one another

(3) Sympatric Speciation

This form of speciation occurs when the ranges of the 2 populations greatly overlap

Allopatric Speciation

There are 2 models which try to explain allopatric speciation: Parapatric Speciation

(1) Dumb Bell Model

A barrier arises that separates 2 populations of the ancestor species of C & D. Over time, C & D change slowly because they both begin with large population sizes. It is a very gradual process.

(2) Peripheral Isolate Model

A small population breaks off of the ancestral population. Over time, the small population develops into a new species. D changes very quickly because it started with a small population size in an extreme part of the ancestors range - it is subjected to founder effect, inbreeding, & lack of gene flow.

• Parapatric speciation occurs when there is a zone of contact (where the 2 different populations meet) where hybrids are produced.

• Character displacement occurs (the individuals closest to the zone of contact are the most different from each other while the individuals farthest away from the zone of contact are the most similar to each other). This causes selection to favor individuals that are not hybrids.

Sympatric Speciation

• Sympatric speciation occurs when two populations occupy the same habitat, but there is a selective force preventing them from mating. These can be behavioral adaptations, morphological adaptations, genetic adaptations, etc.

• Possible causes of sympatric speciation are:

(1) Disruptive selection - this is when selection favors the extremes in a populations. For example if really tall and really short people had greater survivability than people of average height.

(2) Hybridization & Back crossing - This occurs in plants and some fish species. It is when both parental species of an individual hybrid possess different chromosome numbers. For instance if Species A with 4n chromosomes mates with Species b with 6n chromosomes, they will produce hybrid offspring with 5n chromosomes. Since the hybrids posses an odd chromosome number, they can not mate with either parental species and they have automatically become a new species.

(3) Chromosome Mutations - Certain mutations can confer an advantage to either homozygous form while selecting against the heterozygous form.

What is a Species?

• There are several definitions for a species depending on the criteria that you want to use.

• A general definition is a group of interbreeding individuals that are evolutionarily independent of other populations.

• Biological Species = A species must be reproductively isolated and it can not interbreed with close relatives

• Phylogenetic Species = A species population must have at least one unique derived feature & is a monophyletic unit.

• Morphological Species = A species has a unique morphological feature that identifies it from other species. This definition of species is generally used for identifying fossils.

References

• Cooke, F. and Buckley, P.A. 1987. Avian Genetics A Population and Ecological Approach. Academic Press. Chapter 6.

• Freeman, S. and Herron, J. C. 2001. Evolutionary Analysis Second Edition. Prentice-Hall, Inc. New Jersey.