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Investigating the barriers that lead to the creation of distinct biological species.
Imagine two groups of birds that look identical but can no longer understand each other's songs. How did they become strangers in their own home?
In biology, a species isn't just a group of animals that look alike. According to the Biological Species Concept, a species is a group of organisms that can interbreed in nature and produce fertile offspring. This means their children must also be able to have children. For example, while a horse and a donkey can mate to produce a mule, the mule is sterile (cannot reproduce). Therefore, horses and donkeys are considered separate species. This 'genetic barrier' ensures that the gene pool of one species remains distinct from another.
1. A male donkey and a female horse mate. 2. They produce a healthy offspring called a mule. 3. However, the mule has 63 chromosomes, making it unable to produce balanced gametes. 4. Because the mule is sterile, the horse and donkey remain classified as two different species.
Quick Check
If two organisms produce an offspring that is healthy but cannot reproduce itself, are the parents the same species?
Answer
No, because the offspring must be fertile for the parents to be considered the same species.
Allopatric speciation occurs when a population is split into two or more isolated groups by a physical, geographic barrier. This is known as geographic isolation. Imagine a river changing course or a mountain range rising. Once separated, the two groups can no longer exchange genes. Over time, different mutations and natural selection pressures in each environment cause the groups to diverge. If the barrier is eventually removed and they can no longer successfully breed, they have officially become two different species.
1. A single population of squirrels lived in a forest. 2. The Grand Canyon formed, creating a massive physical barrier. 3. The North Rim squirrels (Kaibab) faced colder winters, while the South Rim squirrels (Abert's) faced a warmer climate. 4. After thousands of years, their fur colors and ear shapes changed so much that they are now distinct subspecies/species.
Quick Check
What is the primary requirement for allopatric speciation to begin?
Answer
A geographic or physical barrier that prevents two populations from meeting.
Even without a canyon, species can stay separate through reproductive isolation. These are biological barriers that prevent different species from producing fertile offspring. Behavioral isolation occurs when two populations have different mating rituals; if a female bird doesn't recognize a male's song, she won't mate with him. Temporal isolation happens when species breed at different times (e.g., one in spring, one in fall). These mechanisms ensure that even if two species live in the same forest, their genes do not mix.
Consider three species of fireflies living in the same meadow: 1. Species A flashes its light in a 'dot-dot-dash' pattern at 9:00 PM. 2. Species B flashes in a 'long-glow' pattern at 11:00 PM. 3. Species C flashes a 'triple-blink' at 9:00 PM. Even though they are physically in the same place, Species A will never mate with B (temporal isolation) or C (behavioral isolation).
Which of the following is the best definition of a biological species?
A mountain range rises and divides a population of beetles. Over time, they become two species. This is an example of:
Two species of frogs living in the same pond that mate at different times of the year are experiencing behavioral isolation.
Review Tomorrow
In 24 hours, try to explain to a friend why a mule is not considered its own species and how a canyon could create two species of squirrels.
Practice Activity
Research the 'Ensatina' salamanders in California. Try to identify if they are an example of allopatric speciation or a ring species.