10.3 Gene pools and speciation

Gene pool

Represents the sum total of alleles for all genes present in a sexually reproducing population

Large gene pool

High amounts of genetic diversity, increasing the chances of biological fitness and survival

Small gene pool

Low amounts of genetic diversity, reducing biological fitness and increasing chances of extinction

Allele frequency

The proportion of a particular allele within a population

Changes to allele frequency within a gene pool (evolution) can result from five processes

* Mutation
* Gene flow (movement of alleles into or out of a population)
* Sexual reproduction
* Genetic drift (Change in composition of a gene pool as a result of a chance or event)
* Natural selection

Genetic drift

The change in the composition of a gene pool as a result of chance or random events

Population bottlenecks

Occur when an event reduces population size by an order of magnitude (\~ > 50%)

How do population bottlenecks work?

* Are caused by natural occurences (fires, floods, etc.)
* Surviving population has less genetic variability than before

Founder effect

Occurs when a small group breaks away from a larger population to colonize a new territory.

How does the founder effect work?

As a small group breaks away from a larger population, it is subject to more genetic drift.

How can allele frequencies be compared?

Using Allele Frequency Databases

What are the three types of selection?

1. Stabilizing selection
2. Directional selection
3. Disruptive selection

Stabilizing selection

* When an intermediate phenotype is favored at the expense of both phenotypic extremes
* Operates when environmental conditions are stable.
* E.g. human birth weights (too large = birthing issues ; too small = risk of infant mortality)

Directional selection

* When one phenotypic extreme is selected at the cost of the other phenotypic extreme
* Operates in response to gradual or sustained changes in environmental conditions
* E.g. Development of antibiotic resistance

Disruptive selection

* Where both phenotypic extremes are favored at the expense of the intermediate phenotypic ranges.
* Occurs when fluctuating environmental conditions favour the presence of two different phenotypes.

Reproductive isolation

Occurs when barriers prevent two populations from interbreeding - keeping their gene pools separate

Prezygotic isolation

Occurs before fertilization can occur and no offspring is produced.

Behavioural isolation (prezygotic)

* When two populations exhibit different behaviors
* E.g. birds singing, not attracted to other bird songs

Temporal isolation (prezygotic)

* Occurs when two populations differ in their periods of activity or reproductive cycles.
* e.g. one animal breeds in spring but the other in fall

Geographic isolation (prezygotic)

Occurs when two populations occupy different habitats or separate niches.

Speciation

An evolutionary process that results in the formation of a new species from a pre-existing species.

Allopatric speciation

Occurs when a geographical barrier physically isolates populations of an ancestral species

Sympatric speciation

Divergence of species within the same geographical location, caused by genetic abnormalities.

Polyploidy

When meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete. (Diploid instead of haploid)

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This will result in offspring that have additional sets of chromosome = polyploidy

Fertile polyploid

When both parents are polyploid

Where is polyploidy most common and why?

* *Self-pollination* – many plant species possess both male and female reproductive parts (monoecious) and can hence self fertilise
* *Asexual reproduction* – infertile polyploids can still reproduce asexually via vegetative propagation

Speciation in allium

The genus Allium is comprised of monocotyledonous flowering plants and includes onions, garlic, chives and leeks

* In many of these species polyploidy has occurred, resulting in reproductively isolated populations with distinct phenotypes

Phyletic Gradualism

When speciation occurs uniformly

Punctuated Equilibrium

Speciation remains still for a long time and then undergoes abrupt rapid changes.