4.2.1: Species Defined

Species Defined

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Learning Objectives

• Confirm whether two organisms belong to the same species using the biological definition of species as a guide
• Explain how reproductive isolation can result in speciation

What is a Species?

A biological species is defined as a group of individuals that, in nature, are able to mate and produce viable, fertile offspring. There are other definitions of species but, according to the biological definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile, living offspring.

Members of the same species share both external and internal characteristics which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin’s or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and, therefore, share characteristics and behaviors that lead to successful reproduction.

Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce.

Figure \(\PageIndex{1}\): Dogs of different breeds still have the ability to reproduce. The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo.

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group. If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile: unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development; therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Figure \(\PageIndex{2}\): Species that appear similar may not be able to reproduce. The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species.

Gene Pools

Populations of species share a gene pool: a collection of all the variants of genes in the species. The basis of any change in a group or population of organisms must be genetic; it is the only way to share and pass on traits. Variations within a species can only be passed to the next generation via asexual reproduction or sexual reproduction. Changes will be passed on asexually if the reproducing cell already possesses the changed trait and the offspring will be a genetic copy of the parent.

In sexual reproduction, unlike sexual reproduction, only the changed traits in gametes can be passed down to offspring. In other words, sexually-reproducing organisms can experience genetic changes in their body, or somatic, cells but if these changes do not also occur in a gamete, the changed trait will not appear in the next generation from the parents. Only heritable traits can evolve. Therefore, reproduction plays a large role for genetic changes to take root in a population or species. In summary, organisms must be able to reproduce with each other to pass new traits to offspring.

Video

In this 10.5-minute video explains the concepts of species and speciation.
Questions after watching: There is a colony of green aphids on a tomato plant. On the tomato plant that is next to the first one, there are red aphids. The two colonies started from a third one, that was green, on another nearby plant. The green and the red aphids are distinct species because the green aphids are attracted to green counterparts; and red ones only have interest in other red ones. Which does this example represent: Allopatric or sympatric speciation? Pre-zygotic and post-zygotic speciation?

Reproductive Isolation

Reproductive isolation, through mechanical, behavioral, and physiological barriers, is an important component of speciation. Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction. In other words, if individuals of the two populations were to be brought together, mating would be improbable, but if mating did occur, offspring would be non-viable or infertile. Many factors may influence this process known as reproductive isolation. Reproductive isolation is the inability of two populations to interbreed with one another.

There are two different types of barriers that prevent the members of two different species from mating, to producing offspring, or having fertile offspring: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of the development of an organism that reproduces sexually. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place; this includes barriers that prevent fertilization when organisms attempt reproduction. A postzygotic barrier occurs after zygote formation; this includes organisms that do not survive the embryonic stage and those that are born sterile.

Optional Activity \(\PageIndex{1}\)

Which condition is the basis for a species to be reproductively isolated from others?

a. It does not share its habitat with related species.
b. It does not exist outside of a single habitat.
c. It does not exchange genetic information with other species.
d. It does not undergo evolutionary changes for a significant amount of time.

Answer

c. It does not exchange genetic information with other species.

Prezygotic Barriers

There are several different types of prezygotic barriers, including temporal, habitat, behavioral, gametic, and mechanical. Some of these prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. 

Temporal isolation

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 \(\PageIndex{3}\): These two related frog species exhibit temporal reproductive isolation. (a) Rana aurora breeds earlier in the year than (b) Rana boylii.

Habitat Isolation

In some cases, populations of a species move to a new habitat and take up residence in a place that no longer overlaps with other populations of the same species; this 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.

Figure \(\PageIndex{4}\): Speciation can occur when two populations occupy different habitats. The habitats need not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, while the cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity, but because of their different soil preferences, they became genetically isolated.

Behavioral Isolation

Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species 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.

Gametic and Mechanical Barriers

Other prezygotic barriers work when differences in their gamete cells prevent fertilization from taking place; this is called a gametic barrier. Similarly, in some cases, closely-related organisms try to mate, but their reproductive structures simply do not fit together (mechanical barrier). For example, damselfly males of different species have differently-shaped reproductive organs. 

Figure \(\PageIndex{5}\): The shape of the male reproductive organ varies among male damselfly species and is only compatible with the female of that species. Reproductive organ incompatibility keeps the species reproductively isolated.

In plants, certain structures aimed to attract one type of pollinator simultaneously prevent a different pollinator from accessing the pollen. The tunnel through which an animal must access nectar can vary in length and diameter, which prevents the plant from being cross-pollinated with a different species.

Figure \(\PageIndex{6}\): Some flowers have evolved to attract certain pollinators. The (a) wide foxglove flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet creeper flower is adapted for pollination by humming birds.

Postzygotic Barriers

There are different types of postzygotic barriers as well, including hybrid inviability and hybrid sterility. When fertilization takes place and a zygote forms, postzygotic barriers can prevent reproduction.

Hybrid individuals in many cases cannot form normally in the womb and simply do not survive past the embryonic stages; this is called hybrid inviability.

In another postzygotic situation, reproduction leads to the birth and growth of a hybrid that is sterile and unable to reproduce offspring of their own; this is called hybrid sterility.

Video

This 3-minute video provides an overview of pre-zygotic and post-zygotic barriers to reproduction, which lead to speciation.
Question after watching: In most cases, which do you hypothesize arises first: pre-zygotic or post-zygotic barriers? Why?

Optional Activity \(\PageIndex{2}\)

Which situation is not an example of a prezygotic barrier?

a. Two species of turtles breed at different times of year.
b. Two species of flowers attract different pollinators.
c. Two species of birds display different mating dances.
d. Two species of insects produce infertile offspring.

Answer

d. Two species of insects produce infertile offspring.