The Nature of Species

•Any concept of species must account for:

–Distinctiveness of species occurring together in a single locality.

–Connection among populations of the same species that are geographically separated.

The Nature of Species

•Distinctiveness of Sympatric Species

–Sympatric species are distinctive entities that are phenotypically different, utilize different parts of the habitat, and behave separately.

–Two species that occur together and appear to be nearly identical are termed sibling species.

The Nature of Species

•Geographic Variation Within Species

–Populations within a species that occur in different areas and are distinctive may be classified as subspecies or varieties.

•Biological Species Concept

–If sympatric species commonly exchange genes, their gene pools should become homogenized.

Biological Species Concept

•Ernst Mayr

–“groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.”

•Individuals that cannot produce fertile offspring are termed reproductively isolated, and thus members of different species.

–Hybridization

Biological Species Concept

•Application Problems

–Difficult to apply the concept to populations that do not occur together in nature (Allopatric populations).

–No meaning for asexual species.

Prezygotic Isolating Mechanisms

•Prezygotic Isolating Mechanisms prevent the formation of zygotes.

•Ecological Isolation

–Even if two species occur in the same area, they may utilize different portions of the environment and thus do not hybridize because they do not encounter each other.

•Lions and Tigers in India

Prezygotic Isolating Mechanisms

•Behavioral Isolation

–Many birds differ in courtship rituals which tends to keep species distinct.

•Temporal Isolation

–Varying breeding seasons.

•Mechanical Isolation

–Reproductive structural differences.

•Prevention of Gamete Fusion

–Broadcasting of gametes.

Postzygotic Isolating Mechanisms

•Postgotic Isolating Mechanisms prevent the proper functioning of zygotes after they form.

–Hybrid inviability

–Hybrid sterility

–Hybrid breakdown

Reproductive Isolation and
Evolutionary Change

•Most reproductive isolating mechanisms initially arise for some reason other than to prevent reproduction.

•Random Changes May Cause Reproductive Isolation

–Given long enough periods of time, any two isolated populations will diverge due to genetic drift.

Reproductive Isolation and
Evolutionary Change

•Adaptation and Speciation

–Adaptation and Speciation are probably related in many cases.

•As species adapt, they accumulate many differences that may lead to reproductive isolation.

Geography of Speciation

•Speciation is a two-part process:

–Identical populations must diverge.

–Reproductive isolation must evolve to maintain the differences.

•Allopatric Divergence is Primary

–Geographically separated populations appear much more likely to have evolved substantial differences leading to speciation.

Problems With the Biological Concept

•Extent of Hybridization

–In recent years, greater amounts of hybridization than previously realized between populations has been found.

•Seem to be neither experiencing reinforcement nor losing specific identities.

•Recent survey indicated about 10% of world’s 9,500 species are known to have hybridized in nature.

Darwin’s Finches

•Adaptive Radiation - Process producing a cluster of species, occupying a series of similar habitats, all evolving from a recent ancestor.

•Galapagos finches comprise four groups:

–Ground finches

–Tree finches

–Warbler finch

–Vegetarian finch

Hawaiian Drosophila

•At least 1,250 species of Drosophila in the world; more than a quarter found only in the Hawaiian islands.

–Native Hawaiian flies are closely associated with native plants.

•When ancestors first reached the islands, they probably encountered many empty niches, thus facilitating diversification and speciation.

Diversity of Life Through Time

•Trends in Species Diversity

–There have been 5 major mass extinctions interspersed within relatively consistent extinction patterns.

•Most Famous - End of Cretaceous Period (65 mya)- Dinosaurs went extinct.

–Mammals quickly experienced evolutionary radiation.

»Biological diversity tends to rebound after mass extinctions.