EEB Bioogy 3/3

Polyploidy

• Definition: Polyploidy refers to the condition in which an organism has more than two complete sets of chromosomes.

• Function: This can occur via self-fertilization in organisms such as some species of toads, resulting in diploid sperm fertilizing a diploid egg to form tetraploid offspring.

• Hybrid Formation: Hybridization between closely related species can occur, and typically the resulting hybrid is infertile, similar to mules due to chromosomal misalignment, preventing effective reproduction.

Hybrid Zones

• Definition: A hybrid zone is a specific region where two populations coexist, allowing for interbreeding, and can be used to study the dynamics of hybridization.

• Graphical Representation: The frequency of hybrid alleles can be plotted to show distribution; for instance, yellow-bellied toads show a frequency shape where fewer individuals carry specific alleles as one moves toward the boundaries of the hybrid zone.

• Stability and Isolation: These hybrid zones can indicate stable genetic boundaries between species but also highlight the degree of genetic isolation and barriers to gene flow, influenced by environmental factors.

Secondary Contact and Divergence

• Secondary Contact: This refers to the scenario when previously isolated populations come into contact again. Outcomes can include reinforcement, fusion, or stasis.

• Examples of Divergence: During divergence, populations may change morphologically or genetically, a process influenced by both genetic drift (particularly in small populations) and environmental selection pressures.

Outcomes of Hybridization

• Reinforcement: If hybrids have low fitness, strong selective pressures can enhance barriers to mating, resulting in more distinct populations.

• Fusion: If hybrids are viable and fit, the two populations may merge back into one larger population.

• Stable Hybrid Zones: Occasionally, stable hybrid zones arise, where different populations coexist without significant gene flow outside the hybrid zone.

Speciation Models

• Punctuated Equilibrium: Proposed by Stephen Jay Gould, this model suggests that species remain relatively unchanged (stasis) for long periods, with rapid bursts of speciation occurring, seen in the fossil record.

• Gradualism: In contrast, gradualism holds that species evolve through a slow accumulation of small changes over time. This view aligns more with Darwin's theories on evolution.

• Fossil Record Gaps: The fossil record tends to be incomplete, which can lead to misconceptions about the pace and nature of evolutionary changes.

Evolutionary Debate

• Contrast of Views: The debate on the mechanisms of speciation has persisted between paleontologists and evolutionary biologists, with significant implications on how to interpret evolutionary processes in both modern and ancient contexts.