Unit B

Module 4 ~ Lesson 2

 Biological and Geographical Barriers

Darwin observed finches in two isolated populations. In order for two separated populations to diverge into two distinct species such as Darwin's finches, the populations must be kept separate. Populations can be kept separate in a number of ways.

Geographical Barriers

Two populations can be prevented from interbreeding due to geographical barriers that keep them physically separate, such as mountains, oceans, or rivers. For example, the lava flow from an active volcano might create a physical barrier separating the two populations. If the populations cannot intermingle, they become reproductively isolated and are no longer able to mate. Over time, the populations will display distinct traits and possibly become distinct species. 

By Andrew Z. Colvin [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons

Biological Barriers

It is possible for two species to diverge even if they are not geographically isolated. There may be one or more biological characteristics displayed that create a reproductive barrier in a population. For example, in a population, one group of birds may nest on the ground while another group nests in the trees. The ground birds and tree birds, though the same population, will not interbreed. Over time, this biological barrier separates the two populations.

By Jason Thompson - Flickr: Long-tailed Broadbill (Psarisomus dalhousiae), CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=30899178

Other examples of biological barriers include, but are not limited to, pheromones (chemical signals), mating behaviours (such as bird songs or mating dances), or where eggs are laid.

By Andrew Z. Colvin [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons


Read "Keeping Populations Separate" and "Speciation Occurs in Reproductively Isolated Populations" on pages 137 to 139 of your textbook.


Watch the following videos explaining divergent speciation. One depicts the real-life example of the Galapagos finches and the other uses fictional species to simplify and explain the process.


A geographical barrier keeps populations physically separated from one another. Examples include canyons, mountain ranges, water between islands, large lava flows, and glaciers.

With a biological barrier, organism ranges may overlap, but biological barriers prevent reproduction. Courtship/mating rituals, pheromones, behaviour, niche/habitat preferences, and reproductive anatomy differences are examples of biological barriers.

Bird calls play a large role in mate selection. Variations in calls may affect the ability of males to attract female mates. Intraspecific variations in calls can lead to reproductive isolation.