Chapter 24 - original of species and macro evolution

Chapter 24: Origin of Species and Macroevolution

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:

  • Evolutionary changes that create new species and groups of species

  • Typically occurs through the accumulation of microevolutionary changes

• Species:

  • Defined as a group of organisms that maintains a distinctive set of attributes in nature

Number of Species on Earth

• Approximately 2 million species identified

• Estimates of total species range from 5 to 50 million

• Challenges in identifying species:

  • Presence of subspecies (groups with different traits but not distinct enough to warrant separate species status)

  • Ecotypes (genetically distinct populations adapted to local environments)

Characterization of Species

• Characteristics used to identify a species depend on the organism in question:

  • Morphological traits

  • Ability to interbreed

  • Molecular features

  • Ecological factors

  • Evolutionary relationships

Morphological Traits

• Physical characteristics of organisms

• Drawbacks:

  • Varying traits (e.g., weight, size) can complicate species identification

  • Members of the same species can appear very different

  • Members of different species can appear very similar

Reproductive Isolation

• Similar-looking species may be different due to inability to breed

• Mechanism preventing interbreeding among species includes:

  • Distinctions in nature

  • Cases where species interbreed yet maintain distinct characteristics (e.g., yucca plants)

  • Inapplicability to asexual or extinct species

Molecular Features

• Identifying species through:

  • Analyzing DNA sequences

  • Evaluating gene order, chromosome structure, and chromosome number

• Challenge in determining cutoff points for genetic differences

Ecological Factors

• Distinguishing species may involve various factors related to habitat

• Common in bacteria based on ecological distinctions

• Drawback: variability in growth characteristics complicates classification

• Example: Cyanobacteria response to nutrient environments

Evolutionary Relationships

• Evolutionary trees illustrate relationships between species

  • Built from fossil record and DNA sequences

Species Concepts

• Definitions and approaches to distinguish species include:

  • Biological species concept:

    • Groups of interbreeding individuals capable of producing fertile offspring

    • Example: Horse and donkey can breed but produce infertile mule

  • Evolutionary lineage concept:

    • New species result from lineage evolution

  • Ecological species concept:

    • Species occupy unique ecological niches

  • General lineage concept:

    • Independent evolving lineages based on morphology, reproduction, DNA, and ecology

Reproductive Isolating Mechanisms

• Mechanisms preventing interbreeding include:

  • Prezygotic: prevent zygote formation

  • Postzygotic: hinder viable offspring development

Types of Prezygotic Isolation

• Habitat Isolation:

  • Different habitats prevent contact

    • Example: Eastern and Western meadowlark

• Behavioral Isolation:

  • Differences in mating behaviors

    • Example: Differences in bird songs

• Temporal Isolation:

  • Species breeding at different times

    • Example: Field crickets maturing in spring vs. fall

• Mechanical Isolation:

  • Incompatibility of reproductive organs

    • Example: Male dragonflies have specialized appendages for females

• Gametic Isolation:

  • Failure of gametes to fuse during fertilization

Types of Postzygotic Isolation

• Hybrid inviability: offspring fail to develop properly

• Hybrid sterility: viable hybrids but sterile

  • Example: Mules

• Hybrid breakdown: hybrids fertile in initial generation but offspring have abnormalities

Speciation

• Definition: Formation of new species through genetic changes

• Drivers:

  • Chromosomal changes leading to reproductive isolation

  • Adaptations to different ecological niches

Patterns of Speciation

• Cladogenesis: Division of one species into multiple

  • Requires interruption of gene flow

Forms of Speciation

• Allopatric Speciation:

  • Geographical isolation leading to speciation

• Sympatric Speciation:

  • Divergence without physical barriers

Example of Allopatric Speciation

• Geological changes separated populations, leading to distinct species

  • Example: Evolution of Panamic Porkfish due to the emergence of the Isthmus of Panama

Adaptive Radiation

• Rapid evolution into diverse forms due to adaptation

  • Example: Cichlids in the Great Lakes

  • Example: Hawaiian honeycreeper species evolving from a common ancestor

Hybrid Zones

• Areas where different species can interbreed

• Gene flow diminishes over time leading to reproductive isolation

  • Example: Baltimore oriole hybrid zone

• Possible outcomes:

  • Reinforcement: Increased isolation

  • Fusion: Reduced separation

  • Stability: Persistent overlap despite reduced fitness of hybrids

Sympatric Speciation Mechanisms

• Includes:

  • Polyploidy: Increased chromosome sets

  • Hybrid Speciation: New species arise from hybridizations

  • Adaptation to local environments: Divergence within geographical locales

  • Sexual Selection: Preference for certain traits leads to divergence

Polyploidy

• Organisms can have multiple chromosome sets, common in plants

• Autopolyploidy: Derived from species' own chromosomes

• Allopolyploid: Derived from the fusion of chromosomes from different species

Hybrid Speciation and Local Adaptation

• Hybrids can become a new species which may also adapt to their local environments

  • Example: Cactus finch mating and producing larger offspring

• Adaptation: Variability in growth niches leads to divergence

Sexual Selection

• Preferences in males and females can drive divergence

  • Example: Cichlids showing color pattern preferences

Speed of Speciation

• Gradualism:

  • Slow, continuous evolution

• Punctuated Equilibrium:

  • Rapid bursts of evolution followed by long periods of stability

Factors Affecting Speciation Pace

• Organism size influences pace:

  • Larger species with long generation times evolve more slowly

  • Microbial species demonstrate rapid speciation

Evo-Devo: Evolutionary Developmental Biology

• Compares organisms' development for insight into evolutionary relationships

• Investigates genes controlling development that contribute to variation in different species

Importance of Developmental Genes

• Control processes like cell division, migration, and differentiation

• Developmental genes significantly influence phenotypes

Case Study: Chicken vs Duck Feet

• Differences arise from gene expression variations

  • BMP4: Causes apoptosis, leading to non-webbed feet

  • Gremlin: Inhibits BMP4, allows survival of cells leading to webbed feet

Natural Selection and Habitat

• Different habitats select for specific foot adaptations

  • Non-webbed feet selected for terrestrial life

  • Webbed feet selected in aquatic environments

Hox Genes and Complexity

• Hox gene variations contribute to body plan diversification

• Number and arrangement of Hox genes varies among organisms, connected to evolutionary complexity

Hox Genes and Evolution

• Evidence supporting the link between Hox gene complexity and animal evolution

• Correlation between Hox gene duplication events and major diversification epochs in vertebrate evolution

Developmental Genetic Changes

• Heterochrony refers to changes in growth rates affecting morphology

• Comparisons demonstrate how changes in developmental timing can lead to diversity

Pax6 Gene and Eye Evolution

• Pax6 acts as a master control gene influencing eye development

• Common ancestry of eyes in various species derived from simpler forms

Conclusion on Eye Evolution

• Independent evolution of eyes is improbable; evidence suggests common origin

• Pax6 controls eye formation across many species, demonstrating genetic conservation

Summary Activity

• Reproductive isolation mechanisms:

  • Prezygotic/different barriers before mating or postzygotic after mating, e.g., hybrid sterility in mules

  • Adaptive radiation occurs in niches after sudden environmental changes

  • Speciation can be gradual or punctuated.