Speciation Notes

Chapter 22: The Origin of New Species

Speciation

  • Speciation is the process by which one species splits into two or more species.

  • It is responsible for the diversity and similarities of life.

  • Speciation requires a reproductive barrier, preventing organisms from interbreeding.

Biological Species Concept

  • Defines a species as a group of populations whose members can interbreed in nature.

  • Interbreeding must produce viable and fertile offspring.

  • Reproductive isolation is necessary for speciation to occur.

  • Gene flow maintains a population’s phenotype; its absence can lead to new species.

Reproductive Isolation

  • Definition: the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring.

  • Hybrids are the result of mating between different species.

  • Reproductive isolation is classified based on when factors act: prezygotic or postzygotic barriers.

Prezygotic vs. Postzygotic Barriers

  • Prezygotic Barriers: Occur before fertilization.

    • Block fertilization from ever occurring.

    • Impede different species from attempting to mate.

    • Prevent the successful completion of mating.

    • Hinder fertilization if mating is successful.

  • Postzygotic Barriers: Occur after fertilization.

    • Prevent the hybrid zygote from developing into a viable, fertile adult.

Prezygotic Barriers

  • Habitat Isolation: Species in different habitats do not interact or rarely interact, even if they are in the same geographic area.

    • Example: Land vs. Water.

  • Temporal Isolation: Species breed during different times of day, different seasons, or different years.

    • Example: Late Summer vs. Late Winter.

  • Behavioral Isolation: Different courtship rituals or other behaviors prevent mate recognition.

  • Mechanical Isolation: Physical incompatibility prevents mating.

    • Example: Physical incompatibility

  • Gametic Isolation: Eggs and sperm are incompatible.

    • Example: Incompatible Gametes (Sperm/Egg)

Postzygotic Barriers

  • Reduced Hybrid Viability: Hybrid offspring fail to develop or survive.

    • Offspring fail to develop.

  • Reduced Hybrid Fertility: Hybrid offspring are sterile.

    • Example: Male Donkey + Female Horse = Mule (Sterile).

  • Hybrid Breakdown: First-generation hybrids are fertile, but subsequent generations are infertile or have low survivability.

    • Second generation fails.

Modes of Speciation

  • Allopatric Speciation: A population forms a new species while geographically isolated from its parent population.

  • Sympatric Speciation: A subset of a population forms a new species without geographic separation.

Allopatric Speciation

  • Gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations.

  • Colonization of a new area often leads to speciation.

  • Adaptive radiation: explosion of new species after colonization of a new area (e.g., Darwin's finches).

    • Example: Darwin finches = speciation due to geographical isolation.

Sympatric Speciation

  • Speciation takes place in geographically overlapping populations.

  • Less common than allopatric speciation.

  • Requires reduced gene flow within a subpopulation.

Mechanisms Reducing Gene Flow in Sympatric Speciation

  • Habitat Differentiation: Genetic variation allows a subpopulation to exploit a novel habitat/resource.

    • Example: Apple Maggot Fly.

  • Sexual Selection: Divergence of populations due to mate choice.

  • Polyploidy: Development of polyploidy (due to cell division error).

    • Example: Grey Tree Frog (Hyla versicolor), Rough Skinned Newt (Taricha granulosa).

Polyploidy

  • Definition: the presence of extra sets of chromosomes due to accidents during cell division.

  • More common in plants than in animals.

  • Polyploid individuals often cannot cross with diploid individuals, leading to a new species.

Comparing Allopatric and Sympatric Speciation

  • Allopatric: Geographic isolation restricts gene flow; reproductive isolation arises through natural selection, genetic drift, or sexual selection; interbreeding is prevented even if contact is restored.

  • Sympatric: Reproductive barrier isolates a subset of a population without geographic separation; can result from polyploidy, natural selection, or sexual selection.

Rate of Speciation

  • Punctuated Equilibria: Speciation occurs quickly, followed by long periods of stasis.

  • Gradual Pattern: Speciation occurs gradually over time.

  • The interval between speciation events varies:

    • 4,000 years (some cichlids).

    • 40 million years (some beetles).

    • Average of 6.5 million years.

  • Speciation might require the change of a single allele or many alleles.

Chapter 22: How New Species Arise (Simplified)

Speciation

  • Speciation: When one species splits into two or more.

  • Creates the variety and similarities of life.

  • Needs a barrier to prevent interbreeding.

Biological Species Concept

  • Species: Groups that can interbreed and produce fertile offspring.

  • Offspring must be able to reproduce.

  • Reproductive isolation is needed for speciation.

  • Gene flow keeps a population similar; lack of it can create new species.

Reproductive Isolation

  • Reproductive Isolation: Things that stop two species from creating fertile offspring.

  • Hybrids result from different species mating.

  • Isolation happens before (prezygotic) or after (postzygotic) fertilization.

Prezygotic vs. Postzygotic Barriers

  • Prezygotic Barriers: Before fertilization.

    • Stop fertilization.

    • Prevent mating attempts.

    • Block mating completion.

    • Hinder fertilization.

  • Postzygotic Barriers: After fertilization.

    • Stop a hybrid zygote from becoming a viable, fertile adult.

Prezygotic Barriers

  • Habitat Isolation: Species in different places don't interact.

    • Example: Land vs. Water.

  • Temporal Isolation: Species breed at different times.

    • Example: Summer vs. Winter.

  • Behavioral Isolation: Different mating rituals prevent recognition.

  • Mechanical Isolation: Physical differences prevent mating.

    • Example: Incompatible body parts.

  • Gametic Isolation: Eggs and sperm don't match.

    • Example: Incompatible sperm/egg.

Postzygotic Barriers

  • Reduced Hybrid Viability: Hybrid offspring don't survive.

    • Offspring fail to develop.

  • Reduced Hybrid Fertility: Hybrid offspring are sterile.

    • Example: Mule (Donkey + Horse).

  • Hybrid Breakdown: First-generation hybrids are fertile, but later generations aren't.

    • Second generation fails.

Modes of Speciation

  • Allopatric Speciation: New species forms due to geographic isolation.

  • Sympatric Speciation: New species forms without geographic separation.

Allopatric Speciation

  • Gene flow stops when a population is geographically divided.

  • Colonizing new areas can lead to speciation.

  • Adaptive radiation: Rapid new species after colonizing a new area.

    • Example: Darwin's finches.

Sympatric Speciation

  • Speciation in the same geographic area.

  • Less common than allopatric speciation.

  • Needs reduced gene flow within a subpopulation.

Mechanisms Reducing Gene Flow in Sympatric Speciation

  • Habitat Differentiation: Genetic variation allows using a new habitat.

    • Example: Apple Maggot Fly.

  • Sexual Selection: Mate choice causes divergence.

  • Polyploidy: Extra chromosomes due to cell division error.

    • Example: Grey Tree Frog, Rough Skinned Newt.

Polyploidy

  • Polyploidy: Extra sets of chromosomes from cell division errors.

  • More common in plants.

  • Polyploid individuals can't breed with diploid ones, creating a new species.

Comparing Allopatric and Sympatric Speciation

  • Allopatric: Geographic isolation stops gene flow; natural selection, drift, or sexual selection cause reproductive isolation; interbreeding is prevented.

  • Sympatric: Reproductive barrier isolates part of a population without geographic separation; caused by polyploidy, natural selection, or sexual selection.

Rate of Speciation

  • Punctuated Equilibria: Speciation is quick, then long periods of no change.

  • Gradual Pattern: Speciation happens slowly over time.

  • Speciation time varies:

    • Example: 4,000 years (cichlids).

    • Example: 40 million years (beetles).

    • Average: 6.5 million years.

  • Spe