What Darwin Didn't Know: The Modern Science of Evolution

Retrace Darwin’s path to his theory of evolution by natural selection, which appeared in his masterpiece The Origin of Species, published in 1859. Encounter collector Alfred Russel Wallace’s astonishing, almost identical, key insight. Detail the types of evidence, not known to Darwin, that have accumulated in the century and a half since his time, extending his ideas to a remarkable degree.

Missing from On the Origin of Species is any account of how traits pass from one generation to the next. Explore the work on genetic inheritance by Gregor Mendel, whose pioneering rules of heredity remained essentially unknown for 35 years. Follow up with 20th-century pioneers, including Thomas Hunt Morgan and Theodosius Dobzhansky, who established the “modern synthesis” of evolutionary biology.

The arrival of genetics in the early 20th century addressed what Darwin did not know about inheritance, but there was more to uncover: How do genes function, and where do variations come from? Trace the discovery of DNA as the carrier of genetic information and the realization that mutations and other structural changes in DNA are a source of the modifications that underlie natural selection.

Natural selection is not the only mechanism driving evolution. In this episode, discover how the movement of individuals leads to gene flow between populations. Travel to the Galapagos Islands and neighboring Cocos Island to see how finches evolved into multiple species in the Galapagos archipelago but stayed a distinct species on isolated Cocos. Consider the implications for human evolution.

Trace the importance of geology in Darwin’s thinking and his many observations that make sense only in light of the theory of plate tectonics, which was not developed until the 1960s. Chart the breakup, movement, and reassembly of continental plates that dispersed related flora and fauna all over the planet. Also look at the Wallace Line in Indonesia, which separates Asian from Australian species.

Explore how population bottlenecks and the founder effect lead to random changes in the frequency of genes, an independent mechanism of evolution known as genetic drift. Darwin had an inkling of this process when he proposed that “spontaneous variations” play a role in evolution. But genetic drift has proved far more significant than he ever envisioned.

Darwin thought evolution was an imperceptibly slow process, but it can happen remarkably quickly. Review Peter and Rosemary Grant’s famous studies of Galapagos finches, along with the work of other scientists on guppies in Trinidad, moths in England, and foxes in Siberia. These show evolution playing out in real-time as creatures adapt to changing conditions within a few generations.

One thing Darwin never anticipated was that evolution would be observed in the laboratory. In this episode, analyze lab experiments that shed light on the minute details of evolution, helping to settle a long-standing debate: Is the outcome of evolution random or predictable? Also cover digital life simulations, which inspire new ideas that can be tested with living populations.

Despite its title, On the Origin of Species does not fully address how new species arise. Delve into this complex problem by investigating what a species is. Consider definitions based on morphological, biological, phylogenetic, and genomic distinctions. Then examine the reproductive barriers, both before conception and after, that can lead to the origin of new species.

Darwin was puzzled by the sudden appearance of complex, diverse flora and fauna in the fossil record roughly 540 million years ago, a period known as the Cambrian explosion. And Darwin had no idea that the history of life on Earth has included five big mass extinction events followed by accelerated periods of evolution that often took life in radically new directions.

Darwin envisioned the history of evolution as a great Tree of Life, in which all the branches are connected by ancestry. Explore the modern version of this idea, which has been revolutionized by DNA sequencing. Investigate the concept of phylogenetics and the surprisingly close link between single-celled microorganisms, plants, and animals. Also probe the phenomenon of “jumping” genes.

Zoom in on the branch of the Tree of Life that gave rise to our species. Fossil discoveries and insights from DNA have led researchers to abandon the iconic image of a linear progression from hunched apes to upright humans. In its place is a much more intertwined tree for humans and their closest living and extinct relatives, including Neanderthals and the recently discovered Denisovans.

Convergent evolution occurs when natural selection causes different species to evolve in similar ways. Does this mean that evolution follows a predetermined path? Focus on the recent debate between scientists Stephen Jay Gould and Simon Conway Morris. Gould perceived contingencies and unpredictability, but Conway Morris saw repetition and consistency. How do these views relate to human evolution?

Life is even more adaptable than Darwin could have known. In this episode, investigate extremophiles - organisms that flourish in extreme conditions. These have made biologists rethink the limitations of life on Earth. From bacteria existing miles underground that divide once every 10,000 years to creatures thriving next to superheated undersea volcanoes, life is programmed to adapt and survive.

While Darwin knew of inefficient anatomical features of humans and other animals, he didn’t consider these a distinct category of evidence for natural selection. Explore ad hoc body designs - from our imperfect eyes and sexual anatomy, to the bizarre faces of flounders and the false thumbs of pandas. Each adaptation shows evolution devising a solution that is “good enough,” even if it's not ideal.

Why was Darwin afraid that ants might undermine his theory of natural selection? Delve into the sterile worker paradox: the puzzle of why ants and other “eusocial” species evolved to have large numbers of non-reproducing offspring. Since the ability to reproduce is central to natural selection, this feature, which is common among insects and also present in other animals, demands explanation.

Darwin saw that natural selection not only leads to species that evolve to their mutual advantage, but to enemies that wage an evolutionary arms race that ends up benefiting both sides. Study coevolutionary cases - from the yucca plant and its symbiotic partner, the yucca moth, to the fastest animal on Earth, the cheetah, and its prey the springbok antelope, which has evolved to be almost as fast.

On the Origin of Species failed to account for a major part of the Tree of Life, namely bacteria and other microorganisms. These represent the original forms of life, and they have played a central role in the evolution of every species since. Study the symbiotic role of microbes in the functioning of plants and animals, and consider the view that all organisms are, in part, microbial.

In Darwin’s lifetime, comparisons between the brains of different species were restricted to examinations of anatomy alone. Today, researchers use genetic tools to gain deep insights into how behaviors and sensory abilities evolve. Study behavior in creatures from fire ants to crows to humans, asking how did human brains get so large - and why are big brains so useful anyway?

Darwin devised his theory of sexual selection to explain many traits that can’t be understood through natural selection alone - from the peacock’s gaudy tail to the elaborate constructions of bowerbirds. Probe deeper to discover why sexual reproduction exists at all, what causes individuals to develop into males versus females, and why some males take on the role of raising the young.

Darwin’s writings seem to imply that evolution through natural selection should always favor longer lifespans. So why don’t we live forever (or at least for several centuries)? Consider ways that evolutionary processes account for aging and death. Weigh factors such as accumulated mutations, programmed cell death, and genes whose multiple effects are antagonistically at odds with one another.

Explore one of the ultimate applications of evolutionary principles: harnessing evolution to benefit human health. Study diseases such as malaria, AIDS, influenza, and cancer that evolve rapidly to outmaneuver the body’s changing defenses. Also contrast our modern lifestyle with the physiology we inherited from our prehistoric ancestors, who evolved to compete in a far different world.

Darwin contrasted natural selection with artificial selection - the time-tested techniques for selective breeding that promote desired traits in plants and animals. See how far we’ve come with 21st-century tools such as CRISPR, which allows precise edits to the DNA sequence of any species. Evaluate the promise and perils of this technology, which lets us take evolution into our own hands.

What does the future hold? Will we evolve into new species? Or have we reached an optimum state that will see minimal evolutionary changes? Weigh the impact of our ever-more-sophisticated technology and consider what will happen to humans who leave Earth for another planet with new physiological challenges. As you learn in this course, evolution isn’t just possible; it’s inevitable.