Origin of Species and Macroevolution

Origin of Species and Macroevolution

References and Copyright

  • Textbook: BIOLOGY Sixth Edition by Robert J. Brooker, Eric P. Widmaier, Linda E. Graham, Peter D. Stiling

  • © 2023 McGraw Hill, LLC. All rights reserved.

  • Authorized only for instructor use in the classroom.

  • No reproduction or further distribution permitted without prior written consent of McGraw Hill, LLC.

Key Concepts

  • Identification of Species

  • Reproductive Isolating Mechanisms

  • Allopatric and Sympatric Speciation

  • The Pace of Speciation

  • Evo-Devo: Evolutionary Developmental Biology

Identification of Species

  • Macroevolution: Refers to evolutionary changes that create new species and groups of species, typically occurring through the accumulation of microevolutionary changes.

  • Species Definition: A group of organisms maintaining a distinctive set of attributes in nature.

Number of Species on Earth

  • About 2 million species currently identified.

  • Estimates for total number of species range from 5 to 50 million.

  • Difficulty in Defining a Species:

    • Subspecies: Groups of the same species with different traits, insufficiently different to be considered separate species.

    • Ecotypes: Genetically distinct populations of bacterial species adapted to local environments.

Characterization of Species

  • Species Identification Factors: Varies depending on the species, with common characteristics including:

    • Morphological traits

    • Ability to interbreed

    • Molecular features

    • Ecological factors

    • Evolutionary relationships

Morphological Traits

  • Definition: Physical characteristics of an organism used for species identification.

  • Drawbacks:

    • Number of traits to consider may vary.

    • Traits may exhibit continuous variation.

    • Dissimilarity thresholds may be ambiguous.

    • Members of the same species may appear very different.

    • Members of different species may appear very similar.

Reproductive Isolation

  • Explanation: Similar-looking species may be different due to inability to interbreed in nature, which constitutes reproductive isolation.

  • Importance: Prevents one species from interbreeding with others.

  • Four Problems:

    • Difficult to determine reproductive isolation in nature.

    • Some species interbreed yet maintain distinct characteristics.

    • Concept does not apply to asexual species.

    • Cannot be applied to extinct species.

Molecular Features

  • Comparison: Features such as DNA sequences, gene order, chromosome structure, and number used to identify similarities and differences among populations.

  • Challenges: Difficulty in delineating separate groups due to similarities.

Ecological Factors

  • Definition: Factors related to an organism’s habitat that can differentiate species.

  • Examples: Many bacterial species categorized as distinct based on ecological information.

  • Drawback: Different bacteria may show similar growth characteristics; variability within the same species may exist.

Evolutionary Relationships

  • Discussed further in Chapter 25, where evolutionary trees depict relationships between ancestral and modern species, incorporating fossil records and DNA sequences.

Species Concepts

  • Definition: Methods to define the concept of a species and approaches to distinguish between them.

  • Biological Species Concept: Defines species as a group of individuals that can interbreed to produce viable, fertile offspring among themselves, but not with other species.

  • Evolutionary Lineage Concept: Species defined based on separate evolutionary pathways.

  • Ecological Species Concept: Each species occupies a specific ecological niche based on habitat requirements and its environmental interactions.

  • General Lineage Concept: Each species is an independently evolving lineage, defined by morphology, reproductive isolation, DNA sequences, and ecology.

Reproductive Isolating Mechanisms

  • Definition: Mechanisms that prevent interbreeding between different species, arising due to genetic differences as species adapt to their environments.

  • Interspecies Hybrid: Occurs when two species do produce offspring.

Prezygotic Isolating Mechanisms

  • Prezygotic Mechanisms: Prevent formation of a zygote.

    • Habitat Isolation: Species occupy different habitats, preventing contact.

    • Temporal Isolation: Species breed at different times.

    • Behavioral Isolation: Limited sexual attraction due to differences in behavior.

    • Mechanical Isolation: Morphological differences prevent successful mating.

    • Gametic Isolation: Gametes may meet but fail to unite due to incompatibilities.

Postzygotic Isolating Mechanisms

  • Block development of viable, fertile individuals post-fertilization.

  • Examples:

    • Hybrid Inviability: Fertilized egg does not progress past the early embryo stage.

    • Hybrid Sterility: Hybrid species may be viable but sterile (e.g., mules).

    • Hybrid Breakdown: First-generation hybrids are viable and fertile, but subsequent generations exhibit genetic abnormalities.

Allopatric and Sympatric Speciation

  • Speciation: Formation of new species through the accumulation of genetic changes leading to distinct population differences.

  • Reasons for Speciation: Abrupt events like chromosome number changes causing reproductive isolation or adaptation to different ecological niches.

  • Patterns of Speciation:

    • Cladogenesis: Division of a species into two or more, necessitating interrupted gene flow.

    • Allopatric Speciation: Most prevalent form; occurs when members geographically separate.

Allopatric Speciation Details

  • Occurs when a small population moves to a separate location; results in rapid changes due to natural selection adapting to new environments, linked to Adaptive Radiation, where a single species evolves into multiple markedly different descendants.

Hybrid Zones

  • Regions where two populations can interbreed despite geographical separations being incomplete; reduced gene flow leads to reproductive isolation.

Sympatric Speciation Mechanisms

  • Occurs within the same range as species diverge without physical barriers; involves:

    • Polyploidy: More than two chromosome sets, more common in plants than animals.

    • Hybrid Speciation: New species forming from interspecies hybrids.

    • Adaptation to Local Environments: Variation in habitat informs divergence among members.

    • Sexual Selection: Preference differences in mate display or characteristics.

The Pace of Speciation

  • Gradualism: Continuous evolutionary change over long times; large phenotypic changes are a result of numerous minor genetic alterations.

  • Punctuated Equilibrium: Sporadic changes; extended periods of equilibrium interrupted by brief periods of rapid evolution.

  • Species Evolution Rates: Larger animals with longer generation times evolve slower compared to microbial species; new bacterial species can emerge rapidly, having significant environmental impacts.

Evo-Devo: Evolutionary Developmental Biology

  • Examines developmental processes across organisms to discern:

    • Ancestral relationships.

    • Mechanisms of developmental evolution.

  • Investigates genes regulating development; differences in roles among species can influence evolution.

Key Developmental Genes

  • Influences of Developmental Genes:

    • Control processes such as cell division, migration, differentiation, and apoptosis, helping form specific body patterns.

    • Important for determining phenotypes.

Case Study: Chicken vs. Duck Feet

  • Nonwebbed (chicken) vs. webbed (duck) patterns caused by differences in BMP4 and gremlin expression.

    • BMP4: Induces apoptosis in developing webbing.

    • Gremlin: Inhibits BMP4, allowing for survival of webbing.

  • Mutations altering these genes allowed adaptation to different environments.

  • Natural Selection: Maintains advantageous traits in specific habitats, suggesting geographical isolation stimulates speciation.

Hox Genes

  • Universal in animals; variety in Hox gene arrangement correlates with animal body plan complexity.

  • Evidence supporting their evolution includes:

    • Control over developmental axes.

    • The trend of more complex animals exhibiting more Hox genes.

    • Similarities between Hox gene evolution and general animal evolution trajectories.

Study of the Pax6 Gene and Eye Evolution

  • Investigates evolutionary origins of diverse eye types from a common primitive structure.

  • Pax6: Master control gene influencing the development of diverse eye forms.

    • Cross-species function indicates shared ancestry in eye evolution.

Conclusion

  • Ancestral Eye Structure: Assumed to be a simple two-cell eye structure; subsequent divergence retains the Pax6 homolog across species.

  • Evolutionary modifications led to diverse and complex eye forms governed by variations in Pax6 activity and its associated genes, reflecting a rich palette of eye development across the animal kingdom.