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Study Notes: Speciation, Extinction, and Systematics

Biological Species Concept

  • Defines a species as: members of populations that actually or potentially interbreed in nature, not according to similarity of appearance.

  • Appearance can help identify species but does not define them.

  • Organisms that look alike may be different species; example: Western meadowlark vs Eastern meadowlark are two different species.

  • Organisms that look different may be the same species; example: two workers and a queen ant of the species Pheidole barbata fulfilling different roles in the same colony.

  • Important implication: compatibility is a key criterion for species boundaries under the Biological Species Concept, but it has limitations (see below).

Extinction

  • Background extinction: extinction of species due to lack of survival over time; ongoing process in the history of life.

  • Mass extinction: almost simultaneous termination of organisms from closely and distantly related lineages (multiple taxa across various groups).

  • The study of extinction is linked to macroevolution and the fossil record, which helps infer patterns of diversity loss and recovery.

Fossilization and the Fossil Record

  • Fossilization is the process that preserves remnants or traces of organisms from the past.

  • The fossil record provides a historical archive of life’s changes and supports inference about evolution, speciation, and extinction.

  • Museums house fossil specimens and serve as reference collections for comparison with living organisms.

Speciation: Definitions and Key Concepts

  • Speciation: a lineage-splitting event that produces two or more separate species.

  • Species (definition recap): a group of related organisms that can interbreed and produce fertile, viable offspring; speciation leads to distinct lineages.

  • Importance of speciation in understanding biodiversity and the tree of life.

Causes of Speciation

  • Geographic isolation (allopatry): physical barriers prevent gene flow between populations.

  • Reduction of gene flow: barriers (geographic or behavioral) limit interbreeding and genetic exchange.

  • Reproductive isolation: mechanisms that prevent successful reproduction even if interbreeding occurs.

  • Together, these factors contribute to reproductive isolation and the formation of new species.

Ring Species

  • Ring species are distributed in a ring around a geographic obstacle; neighboring populations interbreed, but end populations at the ring’s closure may not.

  • Example: Ensatina salamanders around the Pacific Coast; interbreeding along the ring is possible, but the end populations in Southern California cannot interbreed.

  • Demonstrates how gradual variation can culminate in reproductive isolation across a ring, producing what can be interpreted as separate species at the ring’s ends.

Chronospecies

  • Chronospecies are different stages of physical changes in the same evolving lineage that exist at different points in time.

  • They illustrate how a single lineage may undergo gradual transformation, complicating species boundaries over time.

Timing and Morphology: Conceptual Diagram (Time vs Morphology)

  • Morphology changes over time within a lineage, illustrating how different morphologies can appear at different time points.

  • Used to visualize gradual evolutionary change in the fossil record (e.g., trilobite lineages).

  • Notation example: morphological states M1, M2, …, MO, etc., across time t1, t2, …

Sexual Reproduction and Speciation: Special Cases

  • l species: no sexual reproduction; they reproduce without sex, which challenges the Biological Species Concept (which relies on interbreeding).

  • Hybrids: offspring produced by combining traits of two organisms from different breeds, varieties, species, or genera; often sterile in many cases.

  • These concepts illustrate limitations of the Biological Species Concept and motivate alternative species concepts (e.g., phylogenetic, ecological).

Hybridization and Speciation

  • Hybrids can occupy evolutionary roles (e.g., creating novel genetic combinations) but may face sterility or reduced fitness.

  • Hybrid zones may form where related species meet and mate, revealing ongoing reproductive barriers.

Practical Examples of Reproductive Isolation

  • Behavioral isolation: differences in mating rituals, songs, dances, or pheromones prevent interbreeding.

  • Temporal isolation: differences in mating times or seasons prevent interbreeding.

  • Mechanical isolation: incompatibilities in reproductive organs prevent successful mating.

  • Postzygotic isolation: offspring viability or fertility is reduced or absent.

Sexual Selection and Speciation

  • Darwin proposed sexual selection as a driver of diversification in traits related to mating success.

  • Traits favored for mating success can lead to divergence between populations and contribute to speciation.

Cospeciation

  • Cospeciation: speciation events in two closely associated lineages occur in parallel (e.g., host and parasite or symbiont and host).

  • Example: gopher A and louse A; gopher B and louse B illustrate parallel speciation driven by host-diversification.

Systematics, Phylogeny, and Taxonomy

  • Systematics: study and classification of living organisms to determine their evolutionary relationships.

  • Phylogeny: the evolutionary relationships among a group of organisms.

  • Taxonomy: classification of organisms into a system that reflects degrees of relatedness.

Basic Assumptions and Goals

  • The basic assumption: all life on Earth shares a common origin; thus, any two different organisms share a common ancestor.

  • The goal of taxonomy today: produce a formal system for naming and classifying species to illustrate their evolutionary relationships.

Evidence for Evolutionary History

  • Existence and pattern of the fossil record.

  • Evidence of evolution in nature (observable phenomena such as trait shifts under environmental pressures).

  • Analogy with artificial selection (breeding for desired traits; mirrors natural selection processes).

  • Homology: shared ancestry reflected in similar structures or genes across taxa.

  • The universality of the gene code: all living organisms share a DNA/RNA system functioning similarly due to shared ancestry.

Fossil Record and Incremental Change

  • Fossil record shows ancient species with small, incremental changes leading to new species over time.

  • Change is incremental (gradual accumulation of changes).

Environmental Change and Natural Selection

  • Environmental change creates new selective pressures.

  • These pressures influence reproductive success and drive trait changes in populations over generations.

Analogies: Artificial Selection vs Natural Selection

  • Both involve selecting for certain traits, resulting in new forms over time.

  • The difference lies in the agent of selection (humans in artificial selection vs environment in natural selection).

Homology

  • Homology is the existence of shared ancestry between structures or genes in different taxa.

  • It underlies the inference of common descent from anatomical or genetic similarity.

The Linnaean Taxonomic System and Its Limitations

  • Linnaean classification is a hierarchical naming system that shows relationships from broad to specific (e.g., Vertebrates → Primates → Cetaceans → Bats → Mammals → Swifts → Penguins → Ducks → Birds).

  • Limitations: does not account for molecular evidence; based largely on physical similarity rather than evolutionary history.

  • Example: FAMILY: Canidae.

Classification is a Work in Progress

  • Systematics continually updates with new data; the tree of life represents our current understanding.

  • Historical shifts in kingdoms example:

    • Before 1866: two kingdoms (Animalia and Plantae) with many microorganisms grouped as Monera or Protista.

    • 1866: many single-celled organisms moved to Protista; Monera included bacteria and archaea? (text varies in history; the slide shows Protista as a separate group alongside Plantae and Animalia.)

    • 1938: prokaryotes moved to Monera.

    • 1959: fungi placed in their own kingdom.

    • 1977: Monera split into Bacteria and Archaea.

  • These shifts reflect advances in our understanding of evolutionary relationships and the limitations of earlier, morphology-based classification.

Phylogenetics and Cladistics

  • Phylogeny: the evolutionary relationships among a group of organisms.

  • Clades: monophyletic groups consisting of an ancestor and all its descendants.

  • Sister groups and outgroups: used to infer relationships within a clade.

  • Key terms:

    • Sister groups: two clades that are each other’s closest relatives.

    • Outgroup: a lineage outside the clade used for comparison.

  • The goal of phylogenetic classification: emphasize evolutionary relationships and minimize emphasis on arbitrary ranks.

Evidence for Common Descent and Ancestry

  • Most Recent Common Ancestor (MRCA): refers to the ancestry of groups of genes (haplotypes) rather than organisms; helps reconstruct evolutionary history at the genetic level.

  • Case studies in primates show deep relationships and divergence patterns consistent with phylogenetic analyses.

Primates: Classification Approaches and Traits

  • Over 600 species of primates.

  • Two approaches to classification:

    • Traditional (gradistic) classification.

    • Cladistic (evolutionary/phylogenetic) classification.

  • Traditional (Gradistic) Primates: based on grades and levels of anatomical complexity; splits into Prosimii (prosimians; lower primates) and Anthropoidea (anthropoids; higher primates).

  • Symplesiomorphy (primitive trait): shared widely; relatively ancient; not useful for tracing descent (e.g., lactation shared by all mammals).

  • Synapomorphy (derived trait): shared narrowly; relatively recent; useful for tracing descent.

Human-Chimpanzee-Gorilla Relationships and Chromosome Data

  • Present day chromosome counts:

    • Orangutan: 48 chromosomes (24 pairs)

    • Gorilla: 48 chromosomes (24 pairs)

    • Chimpanzee: 48 chromosomes (24 pairs)

    • Bonobo: 48 chromosomes (24 pairs)

    • Human: 46 chromosomes (23 pairs)

  • Divergence timeline (approximate):

    • 3 million years ago: divergence event leading toward orangutan lineage (illustrative on timeline)

    • 6 million years ago: divergence event toward gorilla lineage

    • 8 million years ago: divergence events related to human-chimp lineage

  • MRCA concepts: extant humans and the closest relatives share MRCA with chimpanzees and bonobos around ~5–7 million years ago; the gorilla lineage diverged earlier.

  • Notable conclusion: humans and chimpanzees share ~99% identical DNA, with about 5–7 million years since their lineage diverged.

  • Important nuance: the MRCA did not look like modern humans or chimpanzees; it was an intermediate form along the lineage that gave rise to both.

  • Miocene apes (23.5–5.3 million years ago) are often illustrated as the ancestor in reconstructions of human evolution.

Summary of Key Concepts and Relationships

  • Evolutionary relationships are best understood through phylogeny and systematics rather than appearance alone.

  • Speciation is driven by geographic isolation, gene flow reduction, and reproductive isolation, with additional influences from behavior and ecology (e.g., sexual selection).

  • Extinction shapes the history of life and interacts with macroevolution to produce the observed patterns in the fossil record.

  • The scientific naming/classification system has evolved with new data (molecular, fossil, and genetic evidence) and continues to adapt as our understanding deepens.

Key Formulas and Numerical References

  • DNA identity between humans and chimpanzees:

  • Divergence time between human and chimpanzee lineages:

  • Chromosome counts:

    • Humans: 46

    • Other great apes: 48 (per species, 24 pairs)

  • Genome-wide similarity implies deep homology and shared ancestry across taxa, supporting the unity of life and the branching pattern of evolution.