Modern Evolutionary Synthesis: Definition & Formation

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we will discuss the history of evolutionary theory. We will focus on Darwin and Mendel, discussing why some scientists initially believed these two revolutionaries were incompatible, and how these problems were resolved.


What is a theory? In casual speech, you might say 'I have a theory' to indicate that you have an unsupported hunch about something. But in science, a theory is something that is very well supported with evidence. What makes theories fascinating to watch is how they change and grow to assimilate new information.

Let's watch the theory of evolution evolve over time!

Early History of Evolution

It's important to remember that though we associate Charles Darwin with evolution, he didn't invent the idea. Scientists have been amassing evidence that the Earth was very ancient since fossils were first discovered, in the 1660s. Jean-Baptiste Lamarck suggested that animals somehow 'strove' to become more complex. In 1844, still 15 years before Darwin's On the Origin of Species, science enthusiast Robert Chambers published a wildly popular book about evolution. The book reported on a few dubious experiments, but it still solidified evolution in the public imagination, and was read by luminaries ranging from Abraham Lincoln to Queen Victoria.

Charles Darwin

So what did Charles Darwin do that was so great? Charles Darwin proposed a plausible mechanism by which evolution could occur: natural selection - the differential survival and reproduction of individuals based on traits. It was a way to explain evolution that didn't require mysterious forces like Lamarck's animals 'striving' to improve.

Darwin recognized the importance of variations between animals. Though the variations themselves are random, some of them will make an organism better suited or ill suited to survival and reproduction. The organism that is better suited to its environment will live longer and have more offspring who in turn will pass on those traits to become more common in the population.

However, what Darwin did not know was where this variation came from.

Charles Darwin in 1880

Gregor Mendel

Some people imagined that a mother's traits and a father's traits blended in the offspring, much like two different colors of paint will blend together.

Gregor Mendel, a monk who lived in what is today the Czech Republic, is considered to be the father of modern genetics. His work breeding pea plants, published in 1866, showed that traits were inherited as discrete packages rather than blending.

To simplify his work, he observed that when he hybridized smooth peas with wrinkled peas, the offspring wouldn't be half-wrinkled; they would all be smooth. However, one-quarter of the offspring of the next generation would be wrinkled. He observed several other discrete Mendelian traits; flowers were either purple or white, but not half-purple, and so on.

Modern Evolutionary Synthesis

Some early 20th century thinkers thought that Darwin's ideas could not be reconciled with Mendel's, and that one of them must be wrong. In short, we can observe that most traits aren't simple Mendelian traits. For example, you aren't simply short or tall; you're somewhere on a spectrum of height. We know now that this is because many different genes and environmental factors control height. But at the time, it appeared that most traits were blended rather than discrete.

Modern evolutionary synthesis was the merging of Darwin's discoveries with Mendel's. One of the first major leaps was the invention of population genetics, the study of the change in frequency of different gene types within a population, by the scientists R.A. Fisher, J.B.S. Haldane, and Sewall Wright.

  • Fisher first described how the continuous, blending variation that people had observed could be the result of discrete Mendelian traits on many different loci. Mendel's observations were no longer incompatible with our observations of Darwinian traits.
  • Haldane used mathematics to quantify how natural selection could occur from combinations of Mendelian traits.
  • Wright developed the theory of genetic drift, showing how random changes in the numbers of gene variants in a population could have a large effect over time.

Variation and New Species

In the early 20th century, like today, many geneticists studied fruit flies (they are pretty easy to study). Most of these geneticists believed that all fruit flies had almost the same genes.

Why did they believe that? Because all of their fruit flies had almost the same genes - they were using laboratory fruit flies that were taken from a small initial population and bred over many generations. Then we figured out that there was a good deal more genetic variation in wild fruit flies, and in other wild species, than was previously believed.

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