Speciation (Microevolution)

What is a Species? The morphological species concept expresses the following: Species, in its simplest interpretation, means "kind." Attempting to determine whether similar, yet different, animals are the same species by appearance (phenotype) is not reliable due to the subtle variations that are displayed.

The biological species concept relies on reproduction to define relatedness of species. – Ernst Mayer says, "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." – As good as it is, this definition is troublesome for organisms that are non-sexually reproducing and those known only from fossils

Genetic Change and Speciations Speciation is the attainment of reproductive isolation, but genetic changes between populations of the same species can be countered by gene flow. Genetic divergence is the process whereby local units of a population become reproductively isolated from other units and thus experience changes in gene frequencies between them.

Speciation in Geographically Isolated Populations Allopatric Speciation Defined (Extrinsic Model) In this model some physical barrier arises and prevents gene flow between populations. Allopatric refers to the "different lands" the two species occupy on either side of the barrier, geographical areas that have different selection regimes. Reproductive isolating mechanisms evolve in the genetically diverging populations and will result in complete speciation when the two species can no longer interbreed.

The Pace of Geographic Isolation Isolation may be sudden as in an earthquake that results in separation of species. Glaciers, on the other hand, may produce conditions that yield separate over vast periods of time. Studies of enzymes from fishes on the Atlantic and Pacific sides of the Isthmus of Panama reveal molecular differences.

Sympatric species are those that are separated by a reproductively isolation mechanism Speciation occurs among three populations of fish even though the different species occupy the same region There are several ways for subspecies to become reproductively isolated

Reproductive Isolating Mechanisms (Intrinsic Mechanisms) – Reproductive isolating mechanisms are any heritable features of body form, functioning, or behaviour that prevent interbreeding between genetically divergent populations.

Prezygotic Isolation (mechanisms take effect before or during fertilization) – Ecogeographic Isolation: populations may become so specialized for different environmental conditions that they cannot survive under the others conditions. – Habitat Isolation: potential mates may be in the same general area by not in the same habitat where they are likely to meet (for example: different species of manzanita shrubs live at different altitudes and habitats). – Temporal (Seasonal) Isolation: different groups may not be reproductively mature at the same season, or month, or year (for example: periodical cicadas). – Behavioural Isolation: patterns of courtship may be altered to the extent that sexual union is not achieved (for example: albatross courtship rituals). – Mechanical Isolation: two populations are mechanically isolated when differences in reproductive organs prevent successful interbreeding (for example: floral arrangements in sage plants discriminate between different bee pollinators). – Gametic Isolation: incompatibilities between egg and sperm prevent fertilization or the sperm is killed outright by the female reproductive tract.

Postzygotic Isolation (mechanisms take effect after fertilization) – Developmental Isolation: Fertilization may occur, but development of the embryo is irregular and development is not completed. – Hybrid Unviability: Fertilization does occur between different species, but the hybrid embryo is weak and dies. – Hybrid Sterility: In some instances the hybrids are vigorous but sterile (example: mule produced by a male donkey and a female horse). – Post-zygotic isolating mechanisms may appear first since the two populations are recently diverged and may still be similar. However, these are costly in terms of time, energy, lost reproductive opportunities, and fitness. Therefore, selection should ultimately operate for pre-zygotic mechanisms. – Most species are separated from one-another by more than one pre-zygotic mechanism.

Parapatric Speciation Daughter species form from a small proportion of individuals along a common border between two populations. Literally means "near another homeland." Interbreeding individuals produce hybrid offspring in this region called a hybrid zone.