3Evolution, Species Concepts, and Speciation Notes

Foundations of Evolution: Darwinian Principles and Selection Types

  • Theodosius Dobzhansky Quote: "Nothing in biology makes sense except in light of evolution."

  • The Four Components of Evolution (Darwin, 1859):     In "The Origin of Species," Charles Darwin outlined the fundamental factors driving evolution:

    1. Variation: Differences exist between individuals in terms of morphology, physiology, and behavior.

    2. Heredity: At least some parts of these variations are heritable (passed to offspring).

    3. Competition: Individuals compete for the opportunity to reproduce.

    4. Differential Reproductive Success: Certain individuals are more successful at reproducing than others due to their heritable variations.

  • Types of Selection:

    • Directional Selection (Gerichtete Selektion): A shift in the population mean toward an extreme phenotype.

      • Example: Predators like coyotes (Canis latransCanis\,latrans) catch the slowest roadrunners (Geococcyx veloxGeococcyx\,velox). Over time, the escape speed in the roadrunner population increases.

    • Stabilizing Selection (Stabilisierende Selektion): Selection against extreme phenotypes, favoring the average.

      • Example: Wing length in birds. Too short wings are inefficient and lead to starvation; too long wings reduce maneuverability, making birds easy prey. The population stabilizes around an optimal medium length.

    • Disruptive/Destabilizing Selection (Destabilisierende Selektion): Selection against the average, favoring both extremes.

      • Example: Beak width. Narrow beaks are excellent for catching insects, while wide beaks are ideal for cracking seeds. An intermediate beak is poorly suited for either task, leading to a bimodal distribution.

Neutral Evolution and Genetic Drift

Evolution is not always driven by adaptive selection; random processes also play a significant role.

  • Genetic Drift: Random changes in allele frequencies.

    • Impact: Small populations experience massive fluctuations and lose diversity quickly. In large populations, the effects are negligible.

  • Bottleneck Effect (Flaschenhalseffekt): A drastic reduction in population size due to an environmental event, leaving a small number of survivors with reduced genetic diversity.

    • Case Study: Cheetahs (Acinonyx jubatusAcinonyx\,jubatus):

      • Population crashed approximately 10,00010,000 years ago after the last ice age.

      • The modern genome is 95 %95\,\% homozygous (compared to 24 %24\,\% in domestic cats).

      • Consequences: Low sperm viability, high deformity rates, and the ability to perform skin transplants between unrelated individuals without rejection.

  • Founder Effect (Gründereffekt): Occurs when a small group of individuals colonizes a new area, carrying only a fraction of the parental population's genetic variation.

    • Case Study: Amish Population in Pennsylvania:

      • Founded c. 17201720 by approximately 3030 individuals from Europe.

      • The founders happened to carry mutations for dwarfism and polydactyly.

      • Due to being a closed population with limited gene flow, these traits occur significantly more frequently than in the general population.

Evolution Within a Species: Ecotypes and Adaptation

  • Ecotypes (Biotypes): Populations within a species that have adapted to specific local environmental conditions.

    • Example: Biston betularia: The melanistic (dark) form is an ecotype adapted to industrially polluted, soot-covered areas.

    • Example: Boechera pulchra: Populations at higher altitudes show significantly better water-use efficiency (p=0.009p = 0.009) compared to those at lower altitudes.

  • Experimental Evolution: Guppies (Poecilia reticulataPoecilia\,reticulata):

    • Observation: In streams without predators, males have many bright spots to attract females. In streams with predators, they have fewer spots to remain camouflaged. In turbid (cloudy) water, spots increase again.

    • Experiment (Endler, 1980): Guppies moved from predator-rich to predator-free environments developed significantly more spots within just two years.

    • Concept - Trade-offs (Kompromisse): Traits often balance competing pressures (sexual selection vs. predator avoidance).

  • Identifying Ecotypes:

    • Countergradient Variation (Gegengradientvarianz): Observed in the common frog (Rana temporariaRana\,temporaria), where geographical patterns might be hidden by environmental factors.

    • Common-Garden Experiment: Growing different populations in a controlled environment (e.g., at 14 ∘C14\,^{\circ}C, 18 ∘C18\,^{\circ}C, and 22 ∘C22\,^{\circ}C) to see if differences persist.

    • Reciprocal Transplantation: Swapping individuals between their native and non-native habitats to test for local adaptation.

    • Local Adaptation Meta-analysis (Hereford, 2009): A study of 7474 reciprocal transplant experiments found local adaptation in 48 %48\,\% of cases, while 52 %52\,\% did not show clear adaptation (where home fitness was lower or equal to away fitness).

Speciation: Mechanisms of New Species Formation

  • Anagenesis: Evolution of a taxon over time without the creation of sister species (linear evolution: Species A becomes Species B).

  • Cladogenesis: Splitting of a lineage into two or more sister species, increasing biodiversity.

    • Process of Cladogenesis:

      1. Genetic isolation (spatial, ecological, or sexual).

      2. Divergence through drift or selection.

      3. Reproductive isolation as a byproduct or via selection against hybrids.

  • Modes of Speciation:

    • Allopatric: Geographic separation.

      • Isthmus of Panama (3.73.7 to 3.03.0 million years ago): Separated snapping shrimp (AlpheusAlpheus) populations into Pacific and Caribbean species.

      • Ice Age Refugia: Carrion Crows (Corvus corone coroneCorvus\,corone\,corone) in the West and Hooded Crows (Corvus corone cornixCorvus\,corone\,cornix) in the East were separated by glaciers. They now meet in a hybridization zone where hybrids have low fitness.

      • Grand Canyon: Separation of Ground Squirrels (Ammospermophilus harrisiiAmmospermophilus\,harrisii vs. Ammospermophilus leucurusAmmospermophilus\,leucurus).

    • Peripatric: A small population at the edge of the range becomes isolated (e.g., Drosophila on Hawaiian islands, where older islands host ancestors and younger islands host derived species).

    • Parapatric: Speciation across a shared border with limited gene flow.

    • Sympatric: Speciation without geographic barriers.

      • Example: Cichlids (Amphilophus citrinellusAmphilophus\,citrinellus complex) in Crater Lake Apoyo. In a small, homogenous lake, a new species (A. zaliosusA.\,zaliosus) emerged due to shifts in feeding behavior (eating algae vs. other sources).

      • Cryptic Species: Zhang & Wiens (2023) suggest that for every morphological vertebrate species, there are roughly two cryptic species.

  • Ring Species: A series of neighboring populations that can interbreed with close neighbors, but for which "end" populations are too distantly related to interbreed (e.g., EnsatinaEnsatina salamanders or LarusLarus gulls around the Arctic).

  • Adaptive Radiation: Rapid speciation to fill empty ecological niches, often on islands (e.g., Hawaiian honeycreepers).

  • Convergent Evolution: Different species evolving similar solutions for the same niche (e.g., long-billed nectar feeders in Hawaii and Madagascar).

Reproductive Isolation and Species Concepts

  • Ecological Speciation: Driven by selective forces (e.g., beak shapes for seeds vs. insects). Hybrids often suffer Postzygotic Isolation because they are unfit for either niche.

  • Prezygotic Isolation Mechanisms:

    • Mating Signals: Dances, calls, songs, pheromones.

    • Temporal/Spatial: Different mating times or locations.

    • Mechanical/Gametic: Incompatible reproductive organs or gametes (e.g., crab taxonomy often focuses on the morphology of the male gonopods/penises).

  • Species Concepts:

    1. Morphospecies Concept: Based on physical traits. Problem: Phenotypic variation (e.g., dog breeds) and cryptic species.

    2. Ecospecies Concept: Based on ecological niches. Problem: Niches are hard to define; risk of circular reasoning (defining the niche by the species and vice versa).

    3. Biospecies Concept (Mayr & Dobzhansky): A group that can interbreed and produce fertile offspring.

      • Problem 1: Fertile hybrids (e.g., Lions and Tigers produce Ligers in captivity).

      • Problem 2: Asexual organisms (E. coliE.\,coli, certain lizards like Aspidoscelis neomexicanusAspidoscelis\,neomexicanus via parthenogenesis).

      • Problem 3: Extinct species (fossils like ArchaeopteryxArchaeopteryx) cannot be tested for breeding.

      • Problem 4: Hybridization in plants (e.g., TragopogonTragopogon species).

Biological Systematics

  • Linnaean Binomial System: Developed by Carl Linnaeus.

    • Hierarchy: Domain $\rightarrow$ Kingdom (Animalia) $\rightarrow$ Phylum (Arthropoda) $\rightarrow$ Class (Insecta) $\rightarrow$ Order (Diptera) $\rightarrow$ Family (Muscidae) $\rightarrow$ Genus (MuscaMusca) $\rightarrow$ Species (Musca domesticaMusca\,domestica).

    • Includes intermediate levels (Subfamily, Tribe).

Questions & Discussion

  • The effect of exercise breaks on learning (Fenesi et al., 2017):

    • Experiment: Comparison of three treatments: 1) Exercise breaks (5×50 s5 \times 50\,s calisthenics like jumping jacks), 2) Non-exercise breaks (55 minutes on a smartphone), 3) Sports breaks (Sportpausen).

    • Recommendation: Take brief active breaks (77 minutes) to improve learning retention.