Reproductive Isolation (HL) (DP IB Environmental Systems & Societies (ESS))

Reproductive Isolation

• Reproductive isolation occurs when different populations of a species can no longer interbreed

  • This prevents gene flow between the populations, i.e. they no longer interchange genes with each other during the production of offspring

  • This leads to the development of new species over time (speciation)

• It can occur due to physical barriers (geographical isolation) or differences in behaviour or ecology (behavioural or ecological isolation)

Types of reproductive isolation

Geographical isolation

• Geographic isolation occurs when physical barriers like rivers, mountains, or oceans separate populations

• This prevents mating between individuals from each population

  • Speciation happens because, over time, these populations evolve differently due to their unique environments

  • Random mutations within each population also change the alleles present in each population

  • When the genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring, speciation has occurred

• Example: bonobos and common chimpanzees

  • Bonobos and common chimpanzees were once part of the same species

  • The Congo River in Africa separated the two populations, preventing interbreeding

  • Over time, the two populations developed different physical and behavioural traits, eventually becoming distinct species

Ecological and behavioural isolation

• Populations living in the same area (no geographic barriers) can become reproductively isolated if they occupy different ecological niches or develop different behaviours

• These differences prevent the populations from interbreeding, even though they live close together

  • In the example below, the insects often mate in the same location they feed

  • As some individuals start to change their food source, mating between the two groups of insects starts to decrease

  • Over time, two distinct populations form which no longer interbreed

• Example: apple maggot fly (Rhagoletis pomonella)

  • Originally, the apple maggot fly laid its eggs on hawthorn trees in North America

  • When apple trees were introduced, some flies started laying eggs on apples

  • The two groups of flies have become behaviourally isolated because they now mate on different plants

  • This has led to the early stages of speciation

• Other examples of ecological or behavioural changes that can lead to reproductive isolation include:

  • Seasonal changes:

    • Some individuals in a population may develop different mating or flowering seasons (becoming sexually active at different times of the year) to the rest of the population (i.e. their reproductive timings no longer match up)

  • Mechanical changes:

    • Some individuals in a population may develop changes in their genitalia that prevent them from mating successfully with individuals of the opposite sex (i.e. their reproductive body parts no longer match up)

  • Behavioural changes:

    • Some individuals in a population may develop changes in their courtship behaviours, meaning they can no longer attract individuals of the opposite sex for mating (i.e. their methods of attracting a mate are no longer effective)

High rates of endemism on isolated islands

• Endemism refers to species that are found only in one specific geographical area, like an island

  • Islands, especially those far from the mainland, often have high rates of endemic species due to geographical isolation

  • For example, pygmy three-toed sloths are only found on a small island off the coast of Panama

• Example: Hawaiian honeycreepers

  • Hawaiian Honeycreepers are a group of birds found only in the Hawaiian Islands

  • There are many different species of honeycreepers, and each one has adapted to a specific niche within the environment

  • Some species are found on specific islands, while others may be spread across multiple islands, but all are endemic (found only in Hawaii)

  • The isolation of the islands created unique conditions for these birds to evolve separately, leading to many different species that exist nowhere else in the world