The Origin of Species Review Flashcards

Defining and Classifying Species

• Taxonomy

  • Defined as the science of naming and classifying species.
  • The modern science of taxonomy was established in the 1600s.
  • Carl Linnaeus significantly advanced the field in the 1700s.
  • Drawing lines between species can be controversial; for example, Northern, Baltimore, and Bullock’s Orioles are treated as a single species by some and as two separate species by others.

• The Multiplicity of Species Definitions

  • There are currently 25 competing definitions of "species."
  • Different definitions are best suited for different branches of evolutionary biology, such as paleontology versus molecular research.

• Major Species Concepts

  • Phylogenetic Species Concept: Useful for systematists; focuses on the phylogenetic history of organisms. Under this concept, species are recognizable geographic forms with unique evolutionary histories.
  • Biological Species Concept (Mayr, 1942): Defines species as "actually or potentially interbreeding populations which are reproductively isolated from other such groups."
    • This concept is helpful because it focuses on how species form rather than just identifying them once they exist.
    • Limitations: It only works for extant (living), sexually reproducing species. A problem arises when hybrids are created in overlapping ranges.
  • General Lineage Species Concept (de Queiroz, 2005): Defines species as metapopulations of organisms that evolve independently from other metapopulations.
    • Metapopulation: A group of spatially separated populations of the same species with some level of gene flow between them.
    • Frequent allele exchange (gene flow) allows these populations to be treated as members of the same evolutionary lineage.

Barriers to Gene Flow

• Mechanism of Distinctions

  • Maintaining species distinctions requires isolating barriers that prevent gene flow.
  • A key step in the evolution of new species is the splitting of an original population.

• Types of Barriers

  • Geographic Barriers: This is the most common factor preventing interspecific breeding. This leads to allopatry (geographic separation).
    • Example: Elk (North America) and Red Deer (Europe and Russia). They are closely related (same genus) and can interbreed in zoos. They were separated approximately $9,000$ years ago due to rising sea levels. Because they can still interbreed, $9,000$ years was insufficient for complete reproductive isolation.
  • Reproductive Barriers: These can be effective even in sympatry (the same geographic area).

• Thermal Tolerance and Allopatry

  • Stenothermal Species: Species with a narrow temperature tolerance.
  • Warm Stenothermal Species: Ranges contract during glacials (ice ages) and expand during interglacials.
  • Cool Stenothermal Species: Ranges expand during glacials and contract during interglacials. If they expand north far enough during expansion, they can be split into allopatry during the next interglacial contraction.

• Pre-zygotic Reproductive Barriers (Before Mating)

  • Temporal Isolation: Timing of reproduction; for example, different coral species release gametes into the water at different times.
  • Pollinator Isolation: In plants, if an insect only visits a specific flower species, it will not transfer pollen between different plant species. Examples include sympatric monkeyflowers (Mimulus) and painted petal irises (Lapeirousia) with different patterns pollinated by long-tongued flies.
  • Behavioral Isolation: Courtship rituals, such as the specific flashing patterns of fireflies.
  • Mechanical Isolation: Anatomical barriers; for example, similar species of ground beetles (Carabus) have incompatible anatomy that prevents cross-fertilization.

• Post-mating/Post-zygotic Reproductive Barriers

  • Post-mating, Pre-zygotic: Occurs after mating but before fertilization due to gametic incompatibility (sperm or pollen fails to penetrate and fertilize the egg).
  • Post-zygotic: Occurs after fertilization.
    • Interspecific embryos may fail to develop.
    • Hybrids often have lower fitness (weakness or deformities).
    • Sterility (e.g., horse $\times$ donkey crosses).
    • Hybrid behavioral/anatomical mismatch: They cannot find mates because their rituals/genitalia are not compatible with either parent species.

• Genetic Incompatibilities

  • Species may share the same alleles but they cannot work together effectively.
  • Some alleles are fixed (frequency = $100\%$) within a species but cause problems when expressed as a heterozygote.
  • Chromosome Rearrangements: Incompatible chromosomes may fail to align during meiosis.

Models of Speciation

• Allopatric Speciation

  • Evidence includes dispersal by rafting (e.g., green iguanas traveling from Guadeloupe to Anguilla on vegetation mats following hurricanes).
  • Geographic separation can be caused by glaciers, avalanches, mountain peaks (terrestrial islands), rivers changing course, or sea level changes joining/separating land.
  • Processes after separation: accumulation of mutations, natural selection for different adaptations, and sexual selection/conflict.

• Secondary Contact

  • If barriers are strong: Complete speciation occurs with no interbreeding.
  • If barriers are weak: Populations interbreed, genetic differences disappear, and no speciation occurs.
  • Reinforcement: If hybrids have lower fitness, selection favors individuals that can discriminate between types, leading to the evolution of additional reproductive barriers to complete speciation.
  • Example: Ice sheets split North American birds into eastern and western populations. Many forms (e.g., Bullock’s vs. Baltimore Oriole; Spotted vs. Eastern Towhee) now coexist in the Great Plains and sometimes interbreed.

• Sympatric Speciation

  • Harder to observe; occurs within the same area through non-random mating (individuals mating with those most similar to themselves).

• Ecological Speciation

  • Evolution of reproductive barriers due to adaptation to different environments or niches (biotic or abiotic factors like soil pH, precipitation, or sunlight).

Testing Speciation Models and Case Studies

• Isthmus of Panama

  • $15$ million years ago (mya), the isthmus rose; by $3$ mya, it joined North and South America, isolating the Pacific and Atlantic Oceans.
  • Shrimp studies show species P1 (Pacific) is more closely related to C1 (Caribbean) than C1 is to C2 (same ocean), confirming allopatric speciation after closure.

• Lord Howe Island Palms

  • Small volcanic island eastern of Australia. Glacial cycles exposed coral reefs, creating calcium carbonate ($CaCO_3$) dust.
  • Two soil types formed: acidic volcanic slopes and alkaline ($CaCO_3$) lowlands.
  • One species colonized volcanic soil, then a group adapted to alkaline soil. The lowland species now flowers $6$ weeks earlier, preventing gene flow. This is an example of sympatric and ecological speciation.

• Hawaiian Swordtail Crickets (Laupala)

  • Colonized older islands then expanded to newer ones (Oahu to younger islands). Range movements between islands are rare.
  • Isolated populations diverged rapidly ($6$ species on the Big Island in the last $430,000$ years, over $10\times$ faster than the arthropod average).
  • Speciation is driven by male song and female song preference, not food or habitat.

• Apple Maggot Flies

  • Originally fed on native hawthorns; apples introduced $400$ years ago.
  • Flies show high fidelity to the tree species of their birth. Currently, they are considered different races with accumulating genetic differences (potential sympatric/ecological speciation).

• Galápagos Finches

  • Different beak shapes evolved for food; different shapes influence songs, creating incidental reproductive barriers.

• Polar Bears

  • Diverged from brown bears approximately $479,000$ to $343,000$ years ago during a warm interval.
  • Isolation occurred as they moved north and climate changed; brown bears in the north faced strong selection to adapt to the arctic (seal diet, high cholesterol processing, white coats).
  • Transformations occurred in as little as $230,000$ years ($20,500$ generations). They show unidirectional gene flow from polar bears to brown bears but remain separate species through reinforcement.

The Speed of Evolution

• Slow Speciation

  • In fruit flies (Drosophila), reproductive isolation increases with genetic distance. It can take hundreds of thousands of years to complete.

• Fast/Instant Speciation

  • Occurs in plants through hybridization and asexual reproduction.
  • Aneuploidy: A change in chromosome number by one (breaking or fusion).
  • Polyploidy: Additional sets of chromosomes ($70\text{--}80\%$ of flowering plants are polyploids).
    • Autopolyploidy: Duplicated sets from the same species (self).
    • Allopolyploidy: Duplicated sets resulting from adding chromosomes of a different species.
    • Polyploid hybrids can breed with each other but not the parent species, creating instant isolation.
  • Example: Gray Treefrog (Hyla versicolor) formed from three biotypes ($4n$) merging.

Uncovering Hidden and Cryptic Species

• Giraffes: DNA studies revealed that giraffes diverged into six lineages approximately $1$ mya with distinct coat patterns. Because hybrids in overlapping regions account for less than $1\%$ of individuals, they are considered six separate species.
• Cryptic Species: Groups that look identical but are genetically distinct.
  • Neotropical Skipper Butterfly (Astraptes fulgerator): Mitochondrial DNA (mtDNA) sequencing revealed many species among what was thought to be one.
  • Genetic Logic: Non-coding regions evolve faster because mutations there often don't affect fitness, making them useful for studying closely related species. Coding regions evolve too slowly to distinguish recent common ancestors.

The Puzzle of Microbial "Species"

• Prokaryote Definition Challenges

  • Traditional definitions use morphology and biochemistry, but modern science relies on the Phylogenetic Species Concept and DNA.
  • 16S rRNA gene: Evolves slowly; used for universal comparison across the tree of life. If sequences are $\ge 97\%$ similar, they are typically assigned to the same species.

• Genomic Variation in Bacteria

  • Core Genome: Small fraction of genes found in every strain of a species (e.g., E. coli).
  • Pan-Genome: All genes found across all strains of a species.
  • Horizontal Gene Transfer: Transfer of genetic material (often plasmids) within the same generation, not involving sexual reproduction.

• Stable Ecotype Model

  • Argues microbes undergo ecological speciation (e.g., selection for specific temperature or light in hot springs). Clusters remain distinct even while incorporating helpful genes through horizontal transfer.

Questions & Discussion

• What is the definition of "taxa"?
  • Taxa is the plural of taxon; it is a general term referring to a group of organisms belonging to any taxonomic rank.
• How does gene flow occur in animals vs. plants?
  • In animals: Immigration and migration; sessile species have motile larvae. In plants: Movement of seeds and pollen.
• What is interspecific?
  • It refers to interactions "between different species."
• What is the effect of genetic drift on speciation after gene flow stops?
  • If one separated population is small, genetic drift can rapidly make it genetically distinct from the other, even without environmental differences.
• Do humans mate randomly?
  • Generally non-randomly (based on religion, race, etc.), though exceptions maintain gene flow between groups.
• Could ecological speciation occur in sympatry or allopatry?
  • It can occur in both.
• Is speciation via chromosome number change evolution without natural selection?
  • Yes, it is often considered "instant speciation."