Chapter 24 - The Origins of Species

The biological species idea places a premium on reproductive isolation.

The word species comes from the Latin word meaning "kind" or "appearance." In everyday life, we often differentiate between distinct "kinds" of organisms—for example, dogs and cats—based on variations in their appearance. But are creatures actually classified into the distinct components known as species?

Biologists examine not only the morphology (body form) of various groups of species, but also less visible variations in physiology, biochemistry, and DNA sequences to answer this issue.

The findings usually show that physically different species are really separate groupings that differ in numerous ways other than their body shapes.

The biological species idea is the fundamental definition of species utilized in this work. A species, according to this idea, is a collection of populations whose members have the capacity in nature to interbreed and create viable, fertile children—but do not produce viable, fertile offspring with members of other such groups.

Thus, individuals of a biological species are linked by the fact that they are potentially reproductively compatible.

All humans, for example, are members of the same species. A businesswoman in Manhattan is unlikely to meet a dairy farmer in Mongolia, but if the two do meet and mate, they may produce healthy offspring that mature into fruitful adulators.

Because biological species are characterized by reproductive compatibility, the emergence of a new species is dependent on reproductive isolation—the presence of biological characteristics (barriers) that prevent individuals of two species from interbreeding and generating viable, fruitful offspring.

Such barriers obstruct gene flow between species and limit the development of hybrids, or offspring produced by interspecific mating.

Although a single barrier cannot block complete gene flow, a collection of obstacles can successfully isolate a species' gene pool.

In allopatric speciation (from the Greek allos, which means "other"),

When a population is split into geographically isolated subpopulations, gene flow is disrupted. For instance, the water level in a lake may drop, resulting in two or smaller lakes that are now home to different species populaces (as shown in the image attached).

Alternatively, a river's course may alter and split a group of animals that are unable to cross it.

Allopatric speciation can also occur in the absence of geologic remodelings, such as when people inhabit a remote location and their offspring evolve.

The descendants are geographically separated from the parent population.

In mosquitofish populations from ponds with and without predators, different body forms have developed.

These variations influence the fish's ability to flee rapidly and their survival rate when confronted with predators. The images with the graphs attached represent differences in body shapes.

A laboratory population of the fruit fly Drosophila pseudoobscura was split, with some flies raised on a starch medium and others on a maltose medium.

Natural selection resulted in divergent evolution after one year (approximately 40 generations): populations reared on starch digested starch more effectively, whereas populations raised on maltose digested maltose more efficiently.

After that, the researcher placed flies from the same or distinct populations in mating cages and assessed mating frequencies. All of the flies used in the mating preference experiments were raised on a normal cornmeal medium for one generation.

Numerous studies show that speciation may occur in allopatric populations.

Laboratory studies, for example, demonstrate that reproductive barriers can form when populations are segregated artificially and subjected to various environmental circumstances.

Allopatric speciation has been observed in nature, according to field research.

Consider the 30 species of Alpheus snapping shrimp that dwell off the Isthmus of Panama, the land bridge that links South and North America. 15 of these species are found on the Atlantic side of the isthmus, while the remaining 15 are found on the Pacific side.

Gene flow between the Atlantic and Pacific populations of Mexico might have occurred prior to the formation of the isthmus.