Sources of Diversity: Hybridization

Overview of Hybridization

  • Hybridization involves interactions between different species, resulting in mixed offspring.

  • It is primarily the result of incomplete reproductive barriers and occurs in designated "hybrid zones."

Learning Objectives

  • Understand and describe hybrid zones and hybridization events as they pertain to speciation and evolution.

  • Discuss various examples of hybridization events corresponding to different species concepts:

    • Biological

    • Morphological

    • Phylogenetic

    • Ecological

  • Apply theoretical concepts from speciation, hybridization, and hybrid zones to real-world examples.

Key Concepts in Hybridization

  • Hybridization:

    • Occurs due to incomplete reproductive barriers, resulting in hybrid offspring across two species or subspecies.

    • Can be classified into two categories:

    • Intra-specific hybridization: Occurs between different subspecies within a single species (e.g., two subspecies of a bear).

    • Inter-specific hybridization: Occurs between different species (e.g., Grizzly bear and Polar bear).

  • Hybridization is less common in animals than in plants, where cross-pollination is more prevalent.

  • The limitations of the concept of species are highlighted by the existence of hybrid networks which demonstrate that species are not discrete entities but part of a continuum.

Speciation through Hybridization

  • In the context of speciation, hybridization can lead to different possible outcomes:

    • Reinforcement: Strengthening of reproductive barriers to prevent hybridization.

    • Fusion: Merging of two distinct species back into one as barriers weaken.

    • Stability: Continued existence of hybrid zones with ongoing gene flow.

Visual Representation of Speciation through Hybridization

  • Diagram showcasing the relationship between isolated populations, gene flow, hybrid zones, and barriers to gene flow in speciation processes.

Case Study: Nanulak / Grolar Bear

  • Grolar Bear (Nanulak or Pizzly Bear):

    • Hybridization between Grizzly bear (Ursus arctos) and Polar bear (Ursus maritimus) under changing environmental conditions in the Canadian Arctic.

    • Mixed offspring have been documented both in zoos and in the wild, with eight individuals observed in nature and verified through DNA testing over the last 20 years.

  • Geographic distribution: Observations based on data from the International Union for Conservation of Nature (IUCN), highlighting various bear species' distribution within North America:

    • Ursus maritimus (Polar bear)

    • Ursus arctos (Grizzly bear)

    • Ursus americanus (American black bear)

    • Ursus malayanus (Malayan sun bear)

    • Ursus ursinus (Sloth bear)

    • Tremarctos ornatus (Spectacled bear)

    • Ursus thibetanus (Asian black bear)

Summary of Grolar Bear Dynamics

  • Grizzly and Polar bears diverged only recently.

  • The reproductive barrier exists primarily due to habitat isolation, with Grizzlies favoring land-based habitats, while Polar bears are adapted to water and ice.

  • Climate change prompts Grizzly bears to expand their range northward, increasing contact and potential hybridization with Polar bears.

  • This dynamic raises concerns regarding the long-term viability of Polar bear populations as climate shifts continue.

Additional Examples of Hybridization and Speciation

  • Wood Bison:

    • Example of intra-specific hybridization between two subspecies of Bison:

    • Bison bison athabascae (Wood Bison)

    • Bison bison bison (Plains Bison)

  • Coywolf:

    • An example of inter-specific hybridization involving the Eastern Coyote (Canis latrans), Red Wolf (Canis rufus), and Western Coyote (Canis latrans).

    • The expansion of Western Coyotes into Eastern North America due to decimation of native wolves.

Assessment of Eastern Coyote / Coywolf Dynamics

  • In North America today, there are two primary roaming canine species:

    • Canis lupus (wolf)

    • Canis latrans (coyote)

  • The western Coyote's migration eastward into habitats previously occupied by wolves has led to hybridization events.

  • This instances of hybridization do not align with the biological species concept due to the presence of gene flow, contradicting the idea of reproductive isolation.

  • The term "Coywolf" remains a subject of debate among scientists and does not correspond to a recognized species.

Key Points on Speciation and Hybridization

  • Evolution is an ongoing and dynamic process; speciation may be observed in various stages.

  • Some species do not conform neatly to the biological species concept due to hybridization events.

  • Diverging populations do not always result in the creation of a new species.

  • Notable hybridization events include:

    • Grolar Bears in the Canadian Arctic.

    • Wood Bison in Alberta.

    • Coywolves along the East Coast of North America.