Population Genetics, Selection and Speciation Mechanisms
Population Genetics and Genetic Drift
When a small group founds the next generation, fewer individuals represent the genetic diversity of the original population. This situation can lead to the founder effect, where the founders' different characteristics are disproportionately represented in their progeny. Likewise, populations that suffer drastic reductions due to drought, pests, or catastrophic incidents undergo a bottleneck effect.
Natural Selection
Natural selection is the elimination of less fit individuals: organisms with lower biological efficiency contribute fewer genes to the next generation. Selection acts on phenotypes and thereby produces changes in genotypic frequencies and adaptations. Selection and mutation have antagonistic actions; observed gene frequencies result from a balance between mutation and selection.
Types of Selection
- Directional selection: An extreme phenotype is favored. The trait mean can shift in one direction as individuals with that extreme value have higher fitness.
- Stabilizing selection: Favors an intermediate phenotype; the population mean is maintained and extreme variants are selected against.
- Disruptive selection: Favors both extreme phenotypes over the intermediate. For example, very small individuals may escape predator detection while very large individuals may resist predation or exploit other resources.
Speciation and Evolutionary Lineages
Speciation is the evolutionary process by which a new species arises from a preexisting species. A single line (lineage) represents phyletic evolution, while branching lines represent cladogenesis. Causes of speciation include isolation mechanisms.
Speciation by Isolation
Speciation by isolation occurs when an isolated population diverges and becomes a distinct species. It is typically a slow process in which populations accumulate features—often by random mutation and selection—that represent adaptations to the local environment.
Types of Isolation Leading to Speciation
- Allopatric speciation: Occurs due to geographical isolation. A population is divided into subpopulations by a geographic barrier, preventing gene flow and allowing divergence.
- Sympatric speciation: Occurs without geographical isolation; divergence happens within the same area. Barriers are not geographical but biological, arising from reproductive isolation mechanisms.
Types of Reproductive Isolation
Reproductive isolation mechanisms prevent gene flow between groups and contribute to speciation:
- Ecological (habitat) isolation: Species or populations specialize on different habitats or resources; for example, some parasites or phytophagous insects specialize on a single plant species.
- Seasonal (temporal) isolation: Populations reproduce at different times or seasons.
- Behavioral isolation: Differences in mating behavior or courtship prevent interbreeding; for example, distinct mating displays attract only conspecific mates.
- Mechanical isolation: Physical differences in reproductive structures prevent successful mating (an example could be compatibility differences between dog breeds or more generally between species).
- Gametic isolation: Male gametes cannot survive or fertilize eggs in the reproductive environment of the other population or species.
Quantum Speciation and Polyploidy
Quantum speciation (instant speciation) can occur due to major mutations such as polyploidy, especially in plants. Two main types are:
- Autopolyploidy: Involves a single species. A mutation produces diploid (2n) gametes instead of haploid (n), and fertilization can produce polyploid offspring that are reproductively isolated from the parent population.
- Allopolyploidy: Involves two species. If a species with 2n = 12 crosses with another with 2n = 16, their gametes (n = 6 and n = 8) can form a hybrid with 2n = 14. Subsequent chromosome doubling or stabilization can lead to a new, reproductively isolated species.