Speciation and Macroevolution

17.1 How New Species Evolve

  • Macroevolution

    • Definition: Evolution occurring on a large scale, best observed within the fossil record.

    • Involves the origin of species (speciation).

  • Speciation

    • Definition: The splitting of one species into two or more species.

    • Final result: Changes in the allelic and genotypic frequencies of a gene pool.

What Is a Species?

  • Background on Taxonomy:

    • Linnaeus, the father of taxonomy, once grouped birds and bats together because of their wings.

    • Current understanding recognizes birds and bats as distinct organisms on different evolutionary branches.

  • Evolutionary History

    • Every species possesses a distinct evolutionary history.

    • The diversity of life on Earth arises from the evolution of new species.

Species Concepts

  • Definition: Various methods for defining a species.

Morphological Species Concept

  • Definition: Species identified based on distinctive physical characteristics.

    • Method: Analyzed anatomical traits.

    • Historical Context: Used by Linnaeus and held for 200 years.

    • Limitations: Does not apply well to bacteria and microorganisms with few measurable traits.

Evolutionary Species Concept

  • Definition: Distinguishes species based on shared evolutionary pathways.

    • Application: Used to explain speciation in the fossil record.

    • Limitation: Fossil records do not provide information on color, soft tissue anatomy, or behavioral traits.

Phylogenetic Species Concept

  • Definition: Identification of species through their common ancestor.

    • Analogy: For example, for humans and their cousins, the common ancestor is the grandmother.

Biological Species Concept

  • Definition: Populations that breed exclusively among themselves, exhibiting reproductive isolation from other populations.

    • Testing: Very few species have been tested for reproductive isolation.

    • Example: Flycatchers, although similar in appearance, do not reproduce with each other, indicating different species.

    • Example: Leopard frogs differ in habitat, courtship songs, and species distinctions.

Reproductive Isolating Mechanisms

  • Definition: Mechanisms that inhibit gene flow between species.

  • Types:

    • Prezygotic Isolating Mechanisms: Prevent mating or fertilization.

    • Habitat Isolation: Species occupy different habitats even within the same locale.

    • Temporal Isolation: Species reproduce at different times (seasons or times of day).

    • Behavioral Isolation: Different courtship behaviors or signals (like songs and pheromones) lead to mate recognition issues.

    • Mechanical Isolation: Anatomical incompatibilities prevent successful mating.

    • Gamete Isolation: Sperm and egg do not fuse, preventing fertilization.

    • Postzygotic Isolating Mechanisms: Occur after fertilization.

    • Zygote Mortality: Fertilization occurs, but zygote does not survive.

    • Hybrid Sterility: Hybrid organism survives but is sterile (e.g., mules from horse-donkey pairings).

    • F2 Fitness: Hybrid is fertile, but its offspring (F2 generation) have reduced fitness.

17.2 Modes of Speciation

  • Definition of Speciation: The splitting of one species into two or the transformation of one species into a new species over time.

  • Modes of Speciation:

    • Allopatric Speciation:

    • Description: Independent alterations of gene pools in geographically isolated populations can lead to reproductive isolation and speciation.

    • Factors: Different selection pressures in distinct environments can accelerate process.

    • Sympatric Speciation:

    • Description: A single population diverges into multiple reproductively isolated groups without geographic isolation.

    • Example: Midas cichlid evolves into arrow cichlid in an open water habitat.

    • Mechanism in plants: Polyploidy, where chromosome number exceeds diploid (2n).

    • Outcomes: Tetraploid hybridization yields self-fertile species that cannot reproduce with the parental populations due to chromosomal incompatibility during meiosis.

Polyploidy

  • Types of Polyploidy:

    • Autoploidy:

    • Description: Occurs when diploid plants produce diploid gametes via nondisjunction.

    • Result: Fusion of diploid with haploid gametes yields a triploid (3n) plant, which is sterile due to mismatched chromosomal pairing during meiosis.

    • Alloploidy:

    • Description: More complex; involves hybridization between two different, but related species followed by chromosomal doubling.

Adaptive Radiation

  • Definition: The rapid diversification of a single ancestral species into multiple new species each adapted to specific environments.

  • Contexts: Often occurs after the removal of a competitor, predator, or due to environmental change.

  • Relation: Allopatric speciation can lead to adaptive radiation.

Convergent Evolution

  • Definition: The evolution of similar biological traits in unrelated species due to similar environmental pressures.

  • Characteristics: Traits that share similar function but originated from different ancestors are termed analogous traits.

    • Example: Bird wings vs. bat wings.

    • Opposite: Homologous traits, which are similar due to a common ancestor (e.g., wings of butterflies and moths).

Genetic Basis of Beak Shape in Finches

  • Example: Darwin's finches adapted from a common ancestor, showcasing variations in beak shape and size related to diet.

  • Genetic Factors:

    • Gene activity regulates beak morphology, with BMP4 and calmodulin (CaM) genes influencing beak length and depth.

    • Specifics:

    • Cactus finch: Low BMP4, high CaM leads to a shallow, long beak.

    • Ground finch: Opposite pattern resulting in a short, deep beak.

17.3 Principles of Macroevolution

  • Definition of Macroevolution: Evolution occurring at the species or higher classification levels.

  • Models of Evolution:

    • Gradualistic Model: Suggests gradual evolution at the species level with speciation following population isolation. Difficulty in indicating precise points of speciation.

    • Punctuated Equilibrium: Characterizes evolution by periods of stasis interspersed with rapid speciation, with some species emerging unexpectedly in the fossil record.

  • Developmental Genes and Macroevolution:

    • Genes can induce drastic changes in morphology through variable expression.

    • Specific genes involved in body development include Pax6, Tbx5, and Hox genes—all hinting at a common genetic ancestry among animals dating back over 600 million years.

    • Example: Pax6 affects eye development; Tbx5 influences limb formation; Hox genes determine structural layout in vertebrates.

    • Findings indicate random mutations in developmental genes contribute to variability; natural selection favors advantageous traits in certain environments.

  • Evolution of Horses (Equus):

    • Early horse family members appeared roughly 57 million years ago, with significant evolutionary trends including increased size, toe reduction, and changes in tooth morphology.

    • Current understanding recognizes many diverse ancestors and the complexities involved in the evolution of modern horses (Equus).

Simplified Family Tree of Equus

  • Timeline of evolution:

    • Various species existed at different times, indicating a non-linear, branched path to modern horses (Equus). Fossils provide insight but reveal a complicated lineage with numerous species evolving varying traits, illustrating speciation, diversification, and extinction throughout the fossil record.