Bio lec 14
The sources cover topics related to how biological diversity evolves, starting with a description of the Hickory Horned Devil, the caterpillar of the Regal Moth. This insect is found across much of the eastern US. The caterpillars feed on hickory, walnut, and pecan tree leaves, while the adults do not eat. Moths lay eggs on leaves, which hatch into larvae that feed, grow, and molt four times. The final stage caterpillar burrows into the ground to pupate overwinter and then metamorphoses into the Regal Moth, which lives for only one or two weeks. The Hickory Horned Devil is described as the largest caterpillar in North America, often compared to hotdogs due to its size.
The sources then delve into the processes of evolution that generate diversity. Microevolution is defined as the generation-to-generation change in a population’s frequencies of alleles. Macroevolution refers to major biological changes, including the formation of new species. The sources explain that branching evolution is responsible for the multiplication of species.
A central concept is what defines a species. The Biological Species Concept defines species as groups of interbreeding natural populations that are reproductively isolated from other such groups. Reproductive isolation is key to keeping closely related species from interbreeding. Mechanisms preventing interbreeding are categorized as pre-zygotic isolating mechanisms, which prevent the formation of a zygote, and post-zygotic isolation, which prevents hybrid offspring from developing or reproducing. Examples of pre-zygotic mechanisms include temporal, geographic, behavioral, mechanical, and gametic isolation. Post-zygotic isolation can result in hybrid inviability or hybrid sterility. Examples of sterile hybrids include the liger (male lion + female tiger), tigon (male tiger + female lion), and mule (male donkey + female horse).
The sources describe two main modes of speciation, the origin of new species. Allopatric Speciation occurs when a geographic barrier divides a population, preventing gene flow. Examples include a river carving a canyon that separates populations (like the two different species of squirrel on either side of the Grand Canyon) or individuals being blown from a mainland onto an island and isolated. Islands are highlighted as excellent examples where new species can arise due to isolation from parent populations, such as the Galapagos finches with their beak shapes adapted to different food sources. It is noted that genetic isolation of two populations may or may not result in speciation; speciation requires the evolution of reproductive barriers. Sympatric Speciation is the origin of new species without geographic isolation. This is less common in animals but fairly common in plants. It often results from a genetic mutation leading to the formation of a new species in one generation, usually due to an accident during cell division that causes an extra set of chromosomes, a condition called polyploidy. The increased number of chromosomes makes the individual reproductively incompatible with the parent species. Examples of polyploid species include the gray treefrog and the Chinese hibiscus.
The tempo of speciation is discussed through two concepts: Gradualism and Punctuated Equilibrium. Gradualism proposes a continuous, slow accumulation of changes over a very long time. Punctuated Equilibrium suggests that most species-defining characteristics change over a relatively short period in the species' overall lifetime. This involves change occurring in relatively short "bursts" followed by long periods of stasis. Punctuated Equilibrium is presented as likely a better fit for how species actually evolve, with species evolving in response to changes in the environment.
The sources also cover Taxonomy, the system by which organisms are classified based on their evolutionary relationships. This system was developed by Carolus Linnaeus in the mid-1700s. His system uses "Binomial nomenclature", a two-part scientific name for each species consisting of the Genus name and the Specific Epithet, both written in italics with the genus name capitalized (e.g., Homo sapiens, Rana pipiens). Organisms are organized into a hierarchical classification system: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Classification is based on characteristics, particularly homologous structures (derived from a common ancestor). The more homologous characters organisms share, the more closely related they are considered to be.
A brief history of Earth and life is provided via the Geological Time Scale. Key events include Earth formation (~4.6 billion years ago), first prokaryotes (~3.5 BYA), oxygen accumulation (~2.7 BYA), oldest eukaryotes (~2.2 BYA), oldest animal fossils (~700 MYA), invertebrate and algae diversification (~600 MYA), the Cambrian Explosion (~540 MYA) where most modern animal phyla evolved, origin of true plants (~500 MYA), first vertebrates (fishes ~430 MYA), extensive land forests, dominant amphibians, and origin of reptiles (~300 MYA), and the extinction of dinosaurs (~65 MYA) followed by diversification of mammals, birds, and pollinating insects. The first Homo sapiens, the Neanderthals, appeared about 130,000 years ago.
The sources explain the impact of Plate Tectonics and Biogeography. The Earth's crust moves over the mantle, divided into giant plates. Plate movement causes geological activity like volcanos, mountain uplift, and earthquakes. About 250 million years ago, plate movements formed a supercontinent called Pangea. The formation and subsequent breakup of Pangea during the mid-Mesozoic era had dramatic impacts on species distributions, forcing adaptation to new conditions. Biogeography is defined as the study of past and present distribution of organisms. Examples of the impact of continental drift include Madagascar, whose unique animal and plant species diversified from ancestral populations after the island broke away from Africa and India. Over 50 species of lemurs evolved from a common ancestor there over 40 million years. The separation of Australia also resulted in over 200 species of marsupials that are mostly endemic (found nowhere else). The distribution of marsupials is explained by continental drift: they originated in what is now Asia, dispersed to South America (connected to Antarctica), and reached Australia before Pangea broke apart. Marsupials thrived in isolation in Australia, while placental mammals became dominant elsewhere.
Finally, Mechanisms of Macroevolution are explored, particularly at the interface of evolutionary and developmental biology (evo-devo). This field studies how slight genetic changes can become magnified into major structural differences. Homeotic genes are master control genes that program development, affecting the rate, timing, and spatial pattern of organisms' development. Changes in the rate of developmental events explain changes in homologous limb bones of vertebrates. Examples include increased growth rates producing the extra-long "finger" bones in bat wings and slower growth rates of leg and pelvic bones leading to the loss of hind limbs in whales. Paedomorphosis is another mechanism, involving the retention in the adult of body structures that were juvenile features in an ancestral species. This has occurred in the evolution of axolotl salamanders and humans. In human evolution, genetic changes slowed the growth of the jaw relative to the skull, resulting in an adult with head proportions similar to a child and a larger brain case and bigger brain because brain growth continued for several more years.
The sources also discuss the adaptation of old structures for new functions. Bird feathers are given as an example. Fossils of Archaeopteryx, an early bird, showed reptilian features. Thousands of fossils of feathered dinosaurs exist with feathers that could not have been used for flight. Their initial utility may have been for insulation. Once flight became advantageous, natural selection gradually remodeled feathers and wings for this additional function. Structures that evolve in one context but are co-opted for another function are called exaptations.
The evolution of complex structures is understood as occurring in gradual stages from simpler versions. This is a process of refinement rather than a sudden appearance of complexity. The evolution of complex eyes is a frequently cited example, traceable from a simple patch of photoreceptor cells through a series of incremental modifications (eyecup, simple pinhole eye, eye with primitive lens, complex camera lens-type eye) that benefited their owners at each stage. The sources emphasize that step-by-step, small genetic changes can result in increasingly complex organs for perceiving light.