4.1.1: The Modern Synthesis

Historical Framework

When learning about biological sciences today, we always recognize the contributions of Charles Darwin and Gregor Mendel, so it may be surprising to learn that for a time, before we arrived at today’s understanding of genetics and inheritance, both Darwin’s and Mendel’s work fell out of favor. Neither Darwin’s theory of natural selection, nor Mendel’s particulate inheritance were individually sufficient to fully explain all the phenomena being observed in the natural world. It would take many decades, and many careful scientific experiments to solve the puzzle of evolution.

Rethinking Darwin

Figure \(\PageIndex{1}\): The Modern Synthesis perspective: The ancestral population had a range of variation in neck length. Those individuals with the longest necks would be the most likely to survive to pass on their longer-neck alleles to future generations.

As noted in Chapter 2, Darwin’s 1859 book On the Origin of Species made a big splash;

Figure 4\(\PageIndex{2}\): The Lamarckian hypothesis: If a short-necked parent often stretched its neck to reach higher branches, each generation of offspring would be born with somewhat longer necks.

however, as other researchers began doing what scientists do—testing whether or not the concept of natural selection could consistently account for the variation seen in organisms—they began to find many exceptions. One reason for this is that, as we now know, natural selection is only one of the forces of evolution. Another challenge was a general lack of understanding about how variation is initiated and how inheritance works. Many scientists of the day subscribed to the concept known as Lamarckian inheritance, which posited that offspring would inherit characteristics that were acquired during their parents’ lifetimes (Figure 4.2).Darwin himself, in 1868, promoted an idea called pangenesis, which combines the Lamarckian idea of inheriting acquired characteristics with the idea that particles from different parts of the body make their way to the sex cells. Alfred Russell Wallace, evolution. Another researcher, August Weismann, also rejected the idea that acquired characteristics could be passed on. Weismann (1892) devised an experiment to directly test whether offspring inherited acquired characteristics: he cut the tails off mice, bred them, and then waited eagerly to find out if the offspring had tails. All the baby mice were born with tails intact, demonstrating Lamarckian inheritance of acquired characteristics to be incorrect (Figure 4.3).

Figure \(\PageIndex{3}\): Weismann’s mouse-tail experiment showing that offspring do not inherit traits that the parents acquired during their lifetimes.who had arrived at the concept of natural selection independently of Darwin, rejected Lamarckian

Rediscovering Mendel

Figure \(\PageIndex{4}\): The continuous range of variation observed in hooded rat coat patterns after five generations.

In 1900, two scientists named Hugo de Vries and Carl Correns, who were independently studying the mechanisms of inheritance, rediscovered Gregor Mendel’s work. Mendel’s pea plant experiments provided the concepts of dominant and recessive traits, which explained retention of certain characteristics in a way that Darwin’s idea, blending inheritance, didn’t. The debate that unfolded was between the Mutationists, who believed that variation was caused by mutations in distinct, inherited cells, and Biometricians, who believed that individual mutations of discrete hereditary units could never account for the continuous spectrum of variation seen in many traits. One set of experiments that helped resolve this debate was a five-year study carried out by William Castle and John Phillips on laboratory rats (Castle and Phillips 1914). The dominant coat color was the gray wild type, and the piebald or “hooded” color was recessive. He cross-bred the rats multiple ways for five generations and proved that he could achieve a continuous range of variation; in fact, he even achieved coat pattern variations that were more extreme than the original maximums of the parent groups (Figure 4.4).

Another scientist, Thomas Hunt Morgan, conducted studies in which he induced genetic mutations in populations of the fruit fly, Drosophila melanogaster (Figure 4.5). His work demonstrated that most mutations merely increased variation within populations, rather than creating new species (Morgan 1911).

Figure \(\PageIndex{5}\): Examples of mutations producing phenotypic variation in a single species of fruit fly.

Tying It All Together

While the biggest leap forward in understanding how evolution works came with the joining (synthesis) of Darwin’s concept of natural selection with Mendel’s insights about particulate inheritance, there were some other big contributions that were crucial to making sense of the variation that was being observed. R.A. Fisher (1919) and John Burdon Sanderson Haldane (1924) developed and tested mathematical models for evolutionary change that provided the tools to study variation and became the basis for the study of population genetics. Sewall Wright (1932) and Theodosius Dobzhansky (1937) performed studies that revealed the existence of chromosomes as carriers of collections of genes. Edmund Brisco Ford (1949) conducted studies on wild butterflies that confirmed Fisher’s mathematical predictions and also led to his definition of the concept of polymorphisms to describe alternative phenotypes, or multiple forms of a trait. Ford (1942) also correctly predicted that human blood type polymorphisms were maintained in the population because they were involved in disease resistance. Julian Huxley’s 1942 book, Evolution: The Modern Synthesis, provided an easy-to-read summary of the evolutionary studies that had come before. It was with this book that the term Modern Synthesis was first used to describe the integration of Darwin’s, Mendel’s, and subsequent research into a unified theory of evolution. In appealing to the general public, Huxley’s book also found new success establishing a wide acceptance of the process of evolution.