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4: Forces of Evolution

  • Page ID
    • Andrea J. Alveshere

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    Learning Objectives

    • Outline a 21st-century perspective of the Modern Synthesis.
    • Define populations and population genetics as well as the methods used to study them.
    • Identify the forces of evolution and become familiar with examples of each.
    • Discuss the evolutionary significance of mutation, genetic drift, gene flow, and natural selection.
    • Explain how allele frequencies can be used to study evolution as it happens.
    • Contrast micro- and macroevolution.

    It’s hard for us, with our typical human life spans of less than 100 years, to imagine all the way back, 3.8 billion years ago, to the origins of life. Scientists still study and debate how life came into being and whether it originated on Earth or in some other region of the universe (including some scientists who believe that studying evolution can reveal the complex processes that were set in motion by God or a higher power). What we do know is that a living single-celled organism was present on Earth during the early stages of our planet’s existence. This organism had the potential to reproduce by making copies of itself, just like bacteria, many amoebae, and our own living cells today. In fact, with modern technologies, we can now trace genetic lineages, or phylogenies, and determine the relationships between all of today’s living organisms—eukaryotes (animals, plants, fungi, etc.), archaea, and bacteria—on the branches of the phylogenetic tree of life (Figure 4.1).

    Branches lead off of a single celled universal ancestor to images of bacteria, archaea, and eukarya (represented by a mouse, mushroom, and fern, among others).
    Figure 4.1: Phylogenetic tree of life illustrating probable relationships between the single-celled Last Universal Common Ancestor (LUCA) and select examples of bacteria, archaea, and eukaryotes. Major evolutionary developments, including independent evolution of multicellularity, photosynthesis, and respiration, are indicated along the branches. Credit: Cladograma dos Dominios e Reinos by MarceloTeles has been modified (English labels replace Portuguese) and is under a CC BY-SA 4.0 License. [Image Description].

    Looking at the common sequences in modern genomes, we can even make educated guesses about the likely genetic sequence of the Last Universal Common Ancestor (LUCA) of all living things. Through a wondrous series of mechanisms and events over nearly four billion years, that ancient single-celled organism gave rise to the rich diversity of species that fill the lands, seas, and skies of our planet. This chapter explores the mechanisms by which that amazing transformation occurred and considers some of the crucial scientific experiments that shaped our current understanding of the evolutionary process.

    This chapter is a revision from “ Chapter 4: Forces of Evolution ” by Andrea J. Alveshere. In Explorations: An Open Invitation to Biological Anthropology, first edition , edited by Beth Shook, Katie Nelson, Kelsie Aguilera, and Lara Braff, which is licensed under CC BY-NC 4.0 .

    This page titled 4: Forces of Evolution is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Andrea J. Alveshere (Society for Anthropology in Community Colleges) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.