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Biology LibreTexts

Unit 18: Evolution

  • 18.1: Evolution and Adaptation
    Evolution involves two interrelated phenomena: adaptation and speciation. In adaptation, over the course of time, species modify their phenotypes in ways that permit them to succeed in their environment. In speciation, over the course of time, the number of species multiplies; that is, a single species can give rise to two or more descendant species. In fact, Darwin maintained that all species are related; that is, any two species on earth today have shared a common ancestor.
  • 18.2: Speciation
    A species is an actually or potentially interbreeding population that does not interbreed with other such populations when there is opportunity to do so.
  • 18.3: The Evolution of Body Form in Animals
    Advances in genetics - especially the sequencing of entire genomes from a wide variety of animals - has revealed an unexpected paradox. While the animal kingdom contains an extraordinary diversity of body types,  the great structural diversity of animals is not reflected in their genetic makeup. Throughout the animal kingdom, one finds thousands of orthologous genes; that is, genes that have similar sequences and encode similar products.
  • 18.4: Recapitulation
    The embryonic development of all vertebrates shows remarkable similarities. Recapitulation is the idea that embryonic development repeats that of one's ancestors. It is often expressed as "ontogeny recapitulates phylogeny"; that is, embryonic development (ontogeny) repeats phylogeny (the genealogy of the species).
  • 18.5: Mutation and Evolution
    So how can the small changes in genes caused by mutations, especially single-base substitutions ("point mutations"), lead to the large changes that distinguish one species from another? These questions have, as yet, only tentative answers.
  • 18.6: The Hardy-Weinberg Equilibrium
    The Hardy-Weinberg law argues that the gene frequencies and genotype ratios in a randomly-breeding population remain constant from generation to generation.  Evolution involves changes in the gene pool, while a population in Hardy-Weinberg equilibrium shows no change. Hence, populations are able to maintain a reservoir of variability so that if future conditions require it, the gene pool can change.
  • 18.7: Polymorphisms
    A polymorphism is a genetic variant that appears in at least 1% of a population. (e.g., the human ABO blood groups, the human Rh factor,  and the human major histocompatibility complex (MHC)). By setting the cutoff at 1%, it excludes spontaneous mutations that may have occurred in - and spread through the descendants of - a single family.
  • 18.8: Kin Selection
    In the discussion of natural selection, the emphasis was on how natural selection works on individuals to favor the more fit and disfavor the less fit in a population. The emphasis was on the survival (mortality selection), mating success (sexual selection), or family size (fecundity selection) of individuals. But what of the worker honeybee who gives up her life when danger threatens her hive? Or the mother bird who, feigning injury, flutters away from her nestful of young, thus risking death
  • 18.9: The Origin of Life
    To account for the origin of life on our earth requires solving several problems: How the organic molecules that define life, e.g. amino acids, nucleotides, were created. How these were assembled into macromolecules, e.g. proteins and nucleic acids, - a process requiring catalysts. How these were able to reproduce themselves. How these were assembled into a system delimited from its surroundings (i.e., a cell). A number of theories address each of these problems.
  • 18.10: Mars
    There have been efforts to identify signs of life on Mars.
  • 18.11: Endosymbiosis
    The endosymbiosis theory postulates that the mitochondria of eukaryotes evolved from an aerobic bacterium (probably related to the rickettsias) living within an archaeal host cell and the chloroplasts of red algae, green algae, and plants evolved from an endosymbiotic cyanobacterium living within a mitochondria-containing eukaryotic host cell.
  • 18.12: Geologic Eras
    Evolutionary changes coincide with geologic changes on the earth. But consider that changes in geology (e.g., mountain formation or lowering of the sea level) cause changes in climate, and together these alter the habitats available for life. Two types of geologic change seem to have had especially dramatic effects on life: continental drift and the impact of asteroids