Mechanisms of Speciation and Genetic Evolution

Introduction to Speciation and Reproductive Isolation

• Reproductive Isolation as a Precursor: In order for speciation to occur, reproductive isolation must first exist between populations to prevent gene flow.

• Classifying Speciation by Geography:

  • Allopatric Speciation: Occurs when populations are in two different geographical distributions separated by a physical barrier.

  • Sympatric Speciation: Occurs within the same geographical distribution where no physical barrier exists, yet speciation still takes place.

Allopatric Speciation: Mechanisms and Examples

• Allopatric speciation occurs through two primary mechanisms: Geographical Barriers (Vicariance) and Dispersal.

• 1. Vicariance (Geographical Barriers):

  • Definition: The fragmentation of a once-continuous population into two or more subpopulations by a physical catastrophe or geographical phenomenon.

  • Examples of Vicariance:

    • River Course Changes: A massive flood or natural event causes a river to carve a new pathway, splitting a single population.

    • Human-Made Structures: The definition of a barrier is relative. A four-lane highway built in the middle of a forest is not a barrier for a species like a deer, which can cross it. However, for a small worm or small reptile, that same highway acts as a definitive geographical barrier.

    • Biogeography and Plate Tectonics: This is described as the "ultimate vicarious event." Continents act as floating plates on the planet's molten core, coming together and breaking apart over the last $715$ million years.

  • Impact: Once physical separation occurs, populations are impacted by their local microclimates and specific environmental pressures.

• 2. Dispersal:

  • Definition: The movement of a subset of a population to a new, isolated location.

  • The Galapagos Iguana Case Study:

    • Ancestors of the Galapagos iguanas (specifically land iguanas) originated from Continental Ecuador in South America, where the species exhibits high diversity.

    • Marooning Hypothesis: It is believed that ancestors survived a storm, became marooned on floating vegetation, and eventually ended up on the Galapagos Islands.

    • Evolutionary Consequences:

      • Isolation: This created a physical barrier that ceased gene flow between the island group and the mainland.

      • Mechanisms at Play: Genetic drift, mutation, and natural selection began affecting the two populations differently.

      • Adaptive Differences: Today, iguanas in South America eat normal vegetation. In contrast, the Galapagos iguanas have adapted to find food in the ocean, eating algae underwater and becoming marine/aquatic specialists.

    • White Chasers Example: Within South America, rivers changing courses have also led to the evolution of different species of iguana (referred to as "white chasers") within the continent itself.

• The Outcome of Allopatric Speciation: Regardless of whether the barrier arose via dispersal or a geographical event, the result is the cessation of gene flow. This allows both populations to diverge along different pathways driven by genetic drift, mutation, and natural selection.

Sympatric Speciation: Speciation Without Geography

• Historical Context: Charles Darwin originally did not have a clear understanding of how speciation could occur without geographical separation. While he proposed the idea of sexual selection, he did not initially connect it to speciation; he used it to explain why males and females of some species look dramatically different.

• Definition: The formation of a new species without the presence of a geographical barrier.

• Primary Mechanisms:

  • Genetic Polyploidy.

  • Natural Selection (specifically Resource Partitioning).

  • Sexual Selection.

Natural Selection and Resource Partitioning

• The Competitive Exclusion Principle: This is an ecological principle stating that when members of the same or different species compete for the same resource, there are only two possible outcomes:

  1. Extinction: The less competitive member goes extinct.

  2. Resource Partitioning: Members partition the resource to exploit slightly different niches in the same area to avoid direct competition.

• Example: The Weevil Beetle in North America:

  • Pre-European Settlement: Different natural seed variations existed, and weevil beetles possessed natural variations in the length of their proboscis.

  • Introduction of Apple Trees: When Europeans brought apple trees to North America, they introduced a new resource with very short seeds.

  • Divergence: Beetles with naturally short proboscis began selectively colonizing apple trees. Beetles with long proboscis continued to exploit traditional plant seeds.

  • Result: Today, these beetles are partitioned onto two different types of trees in the same forest. They do not cross these "ecological" barriers, leading to divergence within the same geographical region.

Polyploidy and Chromosomal Errors

• Definitions of Ploidy:

  • Ploidy: The number of sets of chromosomes in a cell.

  • Haploid ($n$): One complete set of genetic traits.

  • Diploid ($2n$): Two complete sets of genetic traits.

  • Polyploid: Multiples of two or more than two sets of chromosomes.

  • Aneuploidy: An abnormal number of chromosomes involving an error of $2n + 1$ (Trisomy) or $2n - 1$ (Monosomy).

• Non-disjunction:

  • Description: The error during cell division where homologous chromosomes or sister chromatids fail to separate.

  • Occurrences: Non-disjunction can occur in Meiosis I, Meiosis II, and Mitosis.

  • Meiotic Error Rates: If non-disjunction happens in Meiosis I (homologous pair failure), all final gametes are abnormal ($n + 1$ or $n - 1$).

  • Mitotic Example (Turner Syndrome): Identical twin sisters were born; one was normal and the other had Turner Syndrome ($Monosomy\,X$). Because they were monozygotic twins (one zygote), the error must have occurred during mitosis in the early embryo stages (before the $8$-cell stage, which is indeterminate). One daughter cell experienced non-disjunction, resulting in one monosomic twin; the trisomic cell did not survive.

  • Mechanism of Error: Spindle fibers, controlled by centrioles/microtubule organizers, choreograph chromosome movement. If a spindle fiber on one side dissolves, the whole pair is pulled to a single side.

  • Demographics: In humans, the error of non-disjunction increases with maternal age. For women aged $35$ and above, the probability of conditions like Down Syndrome ($Trisomy\,21$) doubles or triples. In the US, Down Syndrome occurs in approximately $1$ in $800$ births.

Polyploidy as an Evolutionary Mechanism

• Tolerance Levels:

  • Animals: Generally cannot tolerate polyploidy. The condition is too severe, and the zygote typically fails to develop.

  • Plants: Highly resilient and capable of tolerating polyploidy. This is the fastest mechanism for evolution, potentially producing a new species in a single generation.

• Commercial Examples:

  • Strawberries: Commercial strawberries are an octaploid species, containing $8$ copies of their genome. They are reproductively isolated from wild strawberries (which are tiny and thorny) and cannot interbreed with them.

  • Food Security: Most commercial varieties of food are polyploids; without them, grocery stores would be largely empty.

• Types of Polyploidy:

  • Autopolyploidy (Self-polyploidy): Non-disjunction occurs starting with a diploid germ cell, resulting in a diploid gamete. When joined, it becomes tetraploid. As long as chromosomes are balanced, it becomes a separate species.

  • Allopolyploidy: Involves two different but closely related species.

    • Process: Species A ($2n = 6$) and Species B ($2n = 4$) mate. An initial error in non-disjunction (specifically noted as occurring in Species B) leads to unbalanced chromosome numbers in the hybrid.

    • Plant Resilience: Even with unbalanced chromosomes, plants can survive and reproduce asexually via Parenchyma cells.

    • Parenchyma Cells: These are totipotent stem cells capable of producing every part of the plant, allowing an unbalanced hybrid to grow into a mature plant that may later undergo a second non-disjunction to produce a balanced, fertile polyploid species.

• Case Study: Wheat Hybridization Attempts: Researchers attempted to use the principles of allopolyploidy to create a hybrid between two types of wheat. However, because the species were too far removed in their adaptive gene pools, the attempts resulted in hybrid breakdown.

Questions & Discussion

• Question: What is the word for the error during cell division in the middle of Anaphase when chromosomes don't separate?

• Response: The term for that error is non-disjunction. It is the failure of separation of homologous chromosomes or sister chromatids.

• Question (regarding Turner Syndrome): Is it affecting…? (Incomplete)

• Response: Turner Syndrome is a female born with only one copy of the X chromosome, known as monosomy X.

• Question: So the second part where the hybrid ($7$ chromosomes) becomes an unreduced gamete, that’s not an error? It’s just asexual reaction?

• Response: The initial error is the non-disjunction in the parent (Species B). The plant’s ability to survive with those chromosomes is due to its asexual reproduction capabilities using totipotent parenchyma cells.

• Dialogue on Strawberries:

  • Speaker: "You guys love strawberries? You know, the commercial strawberries?"

  • Student: "I like the taste, hate the texture."

  • Speaker: "What does that mean? You put it in your smoothie then. … Who doesn't like strawberry? Come on. … Commercial strawberries are an octaploid. They are reproductively isolated from wild strawberries today. They cannot interbreed."