Speciation: Videos & Practice Problems

Speciation is the formation of new species through the process of reproductive isolation. This can occur via prezygotic mechanisms, which prevent mating before a zygote forms, or postzygotic mechanisms, which affect the viability or fertility of the offspring. For example, prezygotic mechanisms include differences in mating behaviors or physical incompatibilities, while postzygotic mechanisms can result in sterile offspring, like mules. Speciation can be allopatric, involving geographic barriers that separate populations, or sympatric, where reproductive isolation occurs without physical barriers, often due to changes in behavior or resource use. Understanding these processes is crucial for grasping evolutionary biology and the biological species concept, which defines species based on reproductive capabilities.

Allopatric speciation occurs when a geographic barrier, such as a mountain or river, physically separates a population into two groups, preventing gene flow between them. Over time, these isolated populations accumulate genetic differences, eventually leading to the formation of distinct species. In contrast, sympatric speciation happens without geographic barriers. Instead, reproductive isolation arises from factors like changes in behavior, mating preferences, or resource use. For example, one group might adapt to feeding on a different food source, leading to reproductive isolation even though they live in the same area. Both processes result in the formation of new species, but the mechanisms driving them differ.

Prezygotic mechanisms are barriers that prevent mating or fertilization before a zygote forms. Examples include temporal isolation (different mating times), behavioral isolation (different mating rituals), mechanical isolation (incompatible reproductive structures), and gametic isolation (incompatibility of sperm and egg). Postzygotic mechanisms occur after fertilization and affect the viability or fertility of the offspring. For instance, hybrid inviability results in offspring that do not develop properly, while hybrid sterility, like in mules, results in offspring that cannot reproduce. Both types of mechanisms contribute to reproductive isolation, which is essential for speciation.

The biological species concept defines a species as a group of organisms that can interbreed and produce viable, fertile offspring. This concept emphasizes reproductive isolation as the key criterion for distinguishing species. If two populations cannot mate successfully or produce fertile offspring, they are considered separate species. However, this concept can be challenging to apply in cases where hybridization occurs, such as with horses and donkeys producing sterile mules. Despite these complexities, the biological species concept remains a fundamental framework for understanding speciation and the diversity of life.

An example of speciation in a laboratory setting involves fruit flies. Researchers can start with a homogeneous population of flies and separate them into two groups, each given a different food source, such as starch and maltose. Over several generations, these flies adapt to their specific food sources, leading to genetic and behavioral differences. Eventually, when reintroduced, the two groups may no longer mate with each other, demonstrating reproductive isolation. This experiment illustrates how environmental factors and selective pressures can drive speciation, even in controlled settings.