MODES OF SPECIATION

Introduction

Species

According to wikipedia, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined.

Reproductive Isolation

• Reproductive isolation is an isolating mechanism that prevents gene exchange among the population.

TYPES OF REPRODUCTIVE ISOLATION :
1. PREZYGOTIC ISOLATING MECHANISM

• ECOLOGICAL ISOLATION
• TEMPORAL ISOLATION
• BEHAVIORAL ISOLATION
• MECHANICAL ISOLATION

2. POSTZYGOTIC ISOLATING MECHANISMS

• HYBRID INVIABILITY
• HYBRID STERILITY

Speciation

HOW DO NEW SPECIES ARISE ?

• New species arise through a process called speciation. In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.
• Reduced gene flow probably plays a critical role in speciation. Various models have been presented whereby a single evolutionary lineage splits into two or more genetically independent lineages due to reduced gene flow .
• Darwin envisioned speciation as a branching event. In fact, he considered it so important that he depicted it in the only illustration of his famous book, On the Origin of Species. A modern representation of Darwin’s idea is shown in the evolutionary tree of elephants and their relatives.

For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists often divide the ways that speciation can occur into two broad categories:

Allopatric speciation —allo meaning other and patric meaning homeland—involves geographic separation of populations from a parent species and subsequent evolution.

Case study: squirrels and the Grand Canyon

• The Grand Canyon was gradually carved out by the Colorado River over millions of years. Before it formed, only one species of squirrel inhabited the area. As the canyon got deeper over time, it became increasingly difficult for squirrels to travel between the north and south sides.

Sympatric speciation —sym meaning same and patric meaning homeland—involves speciation occurring within a parent species remaining in one location.

Conclusion

• The biological species concept defines a species as a group of individuals living in one or more populations that can potentially interbreed to produce healthy, fertile offspring. Other species concepts exist and may be more useful for certain types of organisms.
• Species are kept distinct from one another by prezygotic and postzygotic barriers. These barriers keep organisms of different species from mating to produce fertile offspring, acting before and after the formation of a zygote, respectively. These barriers maintain the reproductive isolation of species.
• New species form by speciation, in which an ancestral population splits into two or more genetically distinct descendant populations. Speciation involves reproductive isolation of groups within the original population and accumulation of genetic differences between the two groups.
• In allopatric speciation, groups become reproductively isolated and diverge due to a geographical barrier. In sympatric speciation, reproductive isolation and divergence occur without geographical barriers—for example, by polyploidy.