2.2 What is a Species?

Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, called temporal isolation, can act as a form of reproductive isolation. For example, two species of frogs inhabit the same area, but one reproduces from January to March, whereas the other reproduces from March to May (Figure 2.2.3).

In some cases, populations of a species move or are moved to a new habitat and take up residence in a place that no longer overlaps with the other populations of the same species. This situation is called habitat isolation. Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. For example, a cricket population that was divided after a flood could no longer interact with each other. Over time, the forces of natural selection, mutation, and genetic drift will likely result in the divergence of the two groups.

Behavioural isolation occurs when the presence or absence of a specific behaviour prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species of fireflies display their lights differently. If a male of one species tried to attract the female of another, she would not recognize the light pattern and would not mate with the male.

In some cases, closely related organisms attempt to mate, but their reproductive structures simply do not fit together. This is known as mechanical isolation, a type of reproductive barrier that prevents successful mating due to incompatible anatomical structures. For example, damselfly males of different species have differently shaped reproductive organs. If one species tries to mate with the female of another, their body parts simply do not align properly, preventing fertilization (Figure 2.2.4).

In plants, certain structures are aimed at attracting one type of pollinator while simultaneously preventing a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary widely in length and diameter, which prevents the plant from being cross-pollinated with a different species (Figure 15).

Gametic isolation is where the sperm of one species is unable to fertilize the egg of another species. This can occur due to chemical incompatibilities or differences in reproductive proteins. For example, many aquatic species (e.g. many fishes, mollusks, jellyfish, sea urchins) release their gametes into the water. Fertilization only occurs if the sperm and egg are from the same species because their surface proteins must match precisely for fusion to happen.

Hybrid inviability is when the hybrid offspring fails to develop properly or dies at an early stage. This is often due to genetic incompatibilities between the two parent species. For example, when certain species of frogs interbreed, the resulting embryos may not survive past early developmental stages, preventing the continuation of the hybrid line.

Hybrid sterility is where the hybrid offspring is healthy and reaches adulthood but is sterile, meaning it is unable to reproduce. A well-known example is the mule, which is the offspring of a horse and a donkey. Although mules are strong and capable animals, they are generally sterile because of differences in the number of chromosomes between their parents, which disrupts normal gamete formation.

Hybrid breakdown occurs when the first-generation hybrids are fertile and viable, but their offspring are weak, sterile, or otherwise less fit. This is seen in some species of cultivated rice, where the initial hybrid plants can reproduce, but their descendants often show reduced fertility or poor growth.