Evolution: A Grand Unifying Theory

Evolution - A Grand Unifying Theory of Life

• Evolution is the change in heritable traits of biological populations over generations
• Evolution emphasizes the relatedness of all life, rather than its differences
• The guide question is: Why are there so many similarities in different types of organisms?
• One theory proposes a common ancestor that changed gradually to adapt to different environments

Evidence for Evolution

• Direct Observation: Small-scale evolution can be observed in organisms with short lifecycles, such as antibiotic-resistant bacteria and pesticide-resistant insects.
• Homologous Features: Shared physical features (like bone structure or body plan) in different species indicate a common ancestor. Forelimbs of whales, humans, birds, and dogs are an example.
• Embryology: Vertebrate embryos have similar features early in development, such as gill slits and a tail.
• Vestigial Structures: Structures that are no longer useful function in an organism but are remnants of ancestral features, like the vestigial pelvic bone in whales or the appendix in humans.
• Analogous Features: Similar physical features that evolved independently in different organisms due to similar environments or selective pressures, exemplifying convergent evolution
  • An example is the arctic fox and the ptarmigan (bird), both with white coloration for camouflage in the Arctic.
• Molecular Biology: Similarities in molecules like DNA, genetic codes, gene expression processes, and amino acids suggest a shared ancestry among all living organisms.
• Homologous Genes: Genes in different species with similar sequences (often called homologous or orthologous genes) reflect shared ancestry. Humans and chimpanzees possess similar insulin genes.
• Biogeography: The geographic distribution of organisms is influenced by evolutionary history and continental drift (movement of tectonic plates over geological time). Shared ancestry often results in unique organisms being found in those regions. Isolated islands support unique species due to geographic isolation and evolution.
• Fossil Records: Fossils document extinct organisms and their relationships to current species; fossils are remains of organisms found in rocks, tar pits, frozen in ice or trapped in amber. Layers of rocks called strata record the evolutionary history of organisms over time. Radiometric dating (e.g., using Carbon-14) helps estimate the fossil's age.

Mechanisms of Evolution

• Natural Selection: Organisms better suited for their environments are more likely to survive and reproduce, passing on those advantageous traits to their offspring.
• DNA Mutation: Changes in the DNA sequence of an organism can be harmful, neutral, or beneficial, creating variation.
• Gene Flow/Gene Migration: Genes move between populations, altering the gene pool of the receiving population.
• Genetic Drift: Random changes in allele frequencies within populations, especially prominent in small populations. Bottleneck effects in a population can result from a drastic reduction in the population size. This effect significantly alters the gene pool.