Outline a 21st-century perspective of the Modern Synthesis.
Define populations and population genetics.
Identify and distinguish between the forces of evolution and become familiar with examples of each.
Explain how allele frequencies can be used to study evolution as it happens.
The Modern Synthesis
Darwin understood natural selection, but he didn’t know about genes or DNA. Gregor Mendel worked out the basic rules of inheritance, but his work was largely ignored during his lifetime and not widely known until Darwin’s death. Scientists brought these two bodies of knowledge together into what is now called the Modern Synthesis.
The Modern Synthesis united Darwin’s theory of natural selection with Mendelian genetics and, eventually, population genetics. It produced our current understanding of evolution: a change in allele frequencies in populations over time, driven by the four forces of mutation, genetic drift, gene flow and natural selection. Each force was rigorously tested and retested through decades of experimental and observational research. That process of testing and refinement is what makes the theory of evolution one of the most thoroughly supported explanatory frameworks in all of science.
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 5.1).
Figure \(\PageIndex{1}\): 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.
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. And that brings us to the central question Darwin was trying to answer: How do new species originate? In the grand scheme of life, how did a single microscopic organism give rise to all of the amazing diversity of life that now fills every corner of our planet today? The answer lies in a set of processes called the forces of evolution.
Chapter Overview
This chapter explores those forces: mutation, genetic drift, gene flow and natural selection. Together, they explain how genetic variation arises, how it spreads or disappears within populations, and how over vast stretches of time, they can lead to new species. These forces are what shape the bacteria that become resistant to antibiotics, the insect pests that cause farmers to search for ever more potent pesticides, and even our own human history as a species.
