🐾General Biology II Unit 13 – Speciation and Macroevolution

Key Concepts

• Speciation involves the formation of new species from existing populations through reproductive isolation and genetic divergence
• Macroevolution encompasses large-scale evolutionary changes above the species level, such as the emergence of new taxa and morphological innovations
• Microevolution refers to small-scale evolutionary changes within a species, such as shifts in allele frequencies and adaptations to local environments
• Reproductive isolation prevents gene flow between populations, allowing them to diverge genetically and potentially form new species
• Allopatric speciation occurs when populations become geographically isolated, while sympatric speciation happens within the same geographic area
• Anagenesis is the gradual evolution of one species into another without splitting, while cladogenesis involves the splitting of lineages to form new species
• Adaptive radiation is the rapid diversification of a single ancestral species into multiple descendant species adapted to different ecological niches (Hawaiian honeycreepers)

Mechanisms of Speciation

• Genetic drift causes random changes in allele frequencies, potentially leading to speciation in small, isolated populations (founder effect)
• Natural selection drives speciation by favoring adaptations to different environments or niches, leading to divergence between populations
• Sexual selection can contribute to speciation through the evolution of distinct mating preferences and secondary sexual characteristics (peacock tail feathers)
• Hybridization between closely related species can sometimes result in the formation of new hybrid species (Heliconius butterflies)
• Polyploidy, the duplication of entire sets of chromosomes, can instantly create reproductive isolation and lead to speciation in plants (wheat)
• Habitat fragmentation can isolate populations and promote speciation by reducing gene flow and exposing them to different selective pressures
• Behavioral isolation, such as differences in mating rituals or courtship displays, can prevent interbreeding between populations (bird songs)

Types of Speciation

• Allopatric speciation occurs when populations become geographically isolated by physical barriers, such as mountains or rivers (Darwin's finches)
  • Vicariance is a type of allopatric speciation caused by the formation of a physical barrier that splits a population (Isthmus of Panama)
  • Peripatric speciation is a special case of allopatric speciation involving a small peripheral population that becomes isolated from the main population
• Sympatric speciation happens within the same geographic area without physical barriers, often driven by ecological or behavioral factors
  • Disruptive selection can lead to sympatric speciation by favoring extreme phenotypes adapted to different resources or niches (apple maggot fly)
  • Polyploidy can result in instant sympatric speciation, as the new polyploid individuals are reproductively isolated from their diploid progenitors (Tragopogon)
• Parapatric speciation occurs when populations have partial geographic separation with limited gene flow, often along environmental gradients (ring species)

Reproductive Isolation

• Prezygotic barriers prevent the formation of a zygote by acting before fertilization, such as habitat isolation, temporal isolation, and gametic isolation
  • Habitat isolation occurs when populations occupy different habitats or niches, reducing the likelihood of encountering potential mates (host plant preferences)
  • Temporal isolation happens when populations have different breeding seasons or times of sexual maturity, preventing interbreeding (periodical cicadas)
  • Gametic isolation involves incompatibilities between gametes, such as differences in sperm-egg recognition proteins or pollen-stigma interactions
• Postzygotic barriers act after fertilization to reduce the fitness of hybrids, including hybrid inviability, hybrid sterility, and hybrid breakdown
  • Hybrid inviability results in the death of hybrid offspring due to genetic incompatibilities or developmental abnormalities (liger)
  • Hybrid sterility renders hybrid offspring unable to produce viable gametes, preventing them from successfully reproducing (mule)
  • Hybrid breakdown occurs when first-generation hybrids are viable and fertile, but subsequent generations suffer reduced fitness due to the segregation of incompatible alleles

Macroevolution vs. Microevolution

• Macroevolution encompasses large-scale evolutionary changes above the species level, such as the emergence of new higher taxa and major morphological innovations
  • Macroevolutionary patterns are often studied through the fossil record, comparative anatomy, and molecular phylogenetics
  • Examples of macroevolutionary events include the origin of tetrapods from fish ancestors and the evolution of flight in birds and bats
• Microevolution refers to small-scale evolutionary changes within a species, such as shifts in allele frequencies and adaptations to local environments
  • Microevolutionary processes include mutation, genetic drift, gene flow, and natural selection
  • Examples of microevolution include the development of antibiotic resistance in bacteria and changes in beak size among Galápagos finches
• Macroevolution is the cumulative result of microevolutionary processes acting over long periods of time and across multiple speciation events

Patterns in Macroevolution

• Adaptive radiation is the rapid diversification of a single ancestral species into multiple descendant species adapted to different ecological niches (Galápagos finches)
  • Adaptive radiations often occur when a species colonizes a new environment with diverse resources and little competition
  • Key innovations, such as the evolution of flight or the ability to digest cellulose, can facilitate adaptive radiations by opening up new ecological opportunities
• Convergent evolution is the independent evolution of similar traits in distantly related lineages due to similar selective pressures (wings in birds and bats)
  • Convergent evolution can result in analogous structures that serve similar functions but have different evolutionary origins
  • Molecular convergence occurs when distantly related species independently evolve similar genetic or biochemical features (antifreeze proteins in Arctic and Antarctic fish)
• Coevolution involves the reciprocal evolutionary influence between two interacting species, such as predators and prey or hosts and parasites
  • Arms races are a type of coevolution in which species continually evolve adaptations and counter-adaptations to maintain a competitive advantage (cheetahs and gazelles)
  • Mutualistic coevolution occurs when species evolve adaptations that benefit both partners, such as the relationship between flowering plants and their pollinators

Evidence for Macroevolution

• The fossil record provides direct evidence of evolutionary changes over long periods of time, including transitional forms and the appearance of new taxa
  • Fossils demonstrate the sequential appearance of increasingly complex life forms, from simple prokaryotes to multicellular organisms and eventually to modern species
  • Transitional fossils, such as Archaeopteryx and Tiktaalik, showcase the intermediate stages between major evolutionary transitions (dinosaurs to birds, fish to tetrapods)
• Comparative anatomy reveals homologous structures that are shared among related species due to common ancestry, such as the pentadactyl limb in vertebrates
  • Vestigial structures, like the human appendix and whale hip bones, are remnants of functional structures in ancestral species that have lost their original function
  • Embryological development often recapitulates ancestral features, providing evidence for shared evolutionary history (pharyngeal arches in vertebrate embryos)
• Molecular evidence, including DNA and protein sequences, supports the evolutionary relationships among species and the timing of divergence events
  • The universal genetic code and shared metabolic pathways suggest a common ancestry for all life on Earth
  • Phylogenetic trees constructed from molecular data reveal the evolutionary relationships and divergence times among species
  • Endogenous retroviruses and transposable elements inserted at the same locations in the genomes of related species provide evidence for common descent

Real-World Examples

• The evolution of whales from land-dwelling mammalian ancestors is well-documented through a series of transitional fossils, such as Ambulocetus and Basilosaurus
  • Modern whales retain vestigial pelvic bones and hind limb rudiments, providing evidence of their terrestrial ancestry
  • Molecular studies have confirmed that whales are most closely related to even-toed ungulates, with hippopotamuses as their closest living relatives
• The Galápagos finches studied by Charles Darwin showcase adaptive radiation and the role of natural selection in driving speciation
  • The 15 recognized species of Galápagos finches evolved from a single ancestral species that colonized the islands approximately 2-3 million years ago
  • Each species has a unique beak shape adapted to exploit different food resources, such as seeds, insects, or cactus flowers
• The evolution of antibiotic resistance in bacteria is a prime example of microevolution in action, with important implications for public health
  • Bacterial populations can rapidly evolve resistance to antibiotics through mutations and horizontal gene transfer
  • The overuse and misuse of antibiotics in medicine and agriculture have accelerated the spread of antibiotic-resistant strains, leading to the emergence of "superbugs"
• The ongoing coevolution between humans and SARS-CoV-2, the virus responsible for the COVID-19 pandemic, illustrates the dynamic nature of host-pathogen interactions
  • As the human population develops immunity through infection or vaccination, the virus evolves to evade the immune response, leading to the emergence of new variants
  • The rapid evolution of SARS-CoV-2 highlights the importance of monitoring viral mutations and adapting public health strategies accordingly