Speciation
Species
Definitions
- Morphological, wherein a species comprises individuals with similar characteristics that reflect relatedness.
- Biological, wherein a species comprises individuals that are capable of reproducing viable offspring.
- Evolutionary, wherein a species comprises individuals within a single lineage and shared ancestry.
- Ecological, wherein a species comprises individuals that are adapted to a particular environment.
Mechanisms
- Speciation occurs when a single population diverges into two or more reproductively isolated species.
- Allopatric and sympatric speciation are at extreme ends of a continuum; for example, peripatric and parapatric speciation are omitted here for simplicity.
Allopatric speciation:
- Occurs when a physical barrier, such as a river or mountain, creates a barrier to reproduction.
- Example: Single beetle species is divided by the formation of a new river; the two new populations continue to evolve independently until they form disparate species.
Sympatric speciation:
- Occurs when there are non-physical barriers to reproduction.
- Example: Non-physical disruptions in gene flow results in speciation.
- Disruptive selection, in which the phenotypes at the extreme ends of a continuum are selected for.
- Ecological isolation, in which portions of the population thrive in different ecological settings within the same geographic area (for example, some feed on apples and others on pears within the same orchard, and, thus, never cross paths).
- Genetic changes, such as polyploidy (the appearance of extra chromosomes).
Barriers to reproduction:
- Pre-zygotic, which includes barriers that block fertilization.
- For example, our first beetle species encountered a major pre-zygotic barrier to reproduction – the river separated them and lead to allopatric speciation.
- Post-zygotic, which includes barriers that reduce hybrid viability and/or fertility.
- For example, the sister species produced in our second beetle example may intermingle; however, they may encounter post-zygotic barriers, such as molecular incompatibilities, that prohibit development and/or fertility of hybrids.
Hybrid zones:
- Regions in which reproductive barriers are incomplete and the two species interbreed.
- Can result from process of sympatric speciation.
- The consequences of interbreeding vary, and include:
- Reinforcement of the reproductive barrier, which promotes further sympatric speciation; for example, if our sister beetle species interbreed, but post-zygotic barriers prevent the production of viable offspring, the species may continue to diverge.
- Fusion of species occurs when the reproductive barriers are relatively weak, and, eventually, speciation "reverses" – the two species fuse to form a single new species.
- Stabilization within the hybrid zone promotes the continued formation of hybrids.
Speciation Rates:
- Gradualism = evolutionary changes occur and accumulate at a slow, steady rate.
- Punctuated equilibrium = long periods of stability are "punctuated" by relatively rapid bursts of evolutionary changes and speciation.
These need not be mutually exclusive; it is likely that some species have evolved slowly and steadily, while others have passed through alternating periods of stability and rapid change.
Trait sorting based on evolutionary history:
Despite the vast number of species and the variation within and between them, several features, such as the eye and wing, show up in both closely related and unrelated species (be aware that this is relative; if we go back back far enough in history, all organisms share a common ancestor).
- Homologous traits, aka, homologies, are similar because they are derived from a common ancestor.
- Parallel traits are similar traits that evolve in separate species after divergence from a common ancestor.
- Parallel traits may appear in separate lineages due to similar ecological pressures.
- Analogous traits are those that arise independently long after divergence occurs.
- Products of convergent evolution.
- Similar features may arise in unrelated species due to similar ecological pressures.