Science
The Theory of Evolution and Charles Darwin: an Outline
Abstract
© Dr M D Magee
Contents Updated: Wednesday, 16 December 2009
Charles Darwin
Scott LaFee, a staff writer at the San Diego Union-Tribune, wrote an informative series paying tribute to Charles Darwin and his great discovery, the theory of evolution. While Darwin was traveling around the world on the Beagle on its five year scientific survey from 1831, nominally he was the ship’s naturalist, but actually was the captain’s companion. On Darwin’s 25th birthday, in 1834, Captain FitzRoy named the highest peak (8,163 feet) in Tierra del Fuego after him. Mountains in Antarctica, Tasmania and California (part of the Sierra Nevada range; 13,837 feet) have later been named after him. You have to be a man of some significance to have mountains named after you.
Back from his world travels on the Beagle, Darwin, as a man of means, did not have to work. Instead he spent his time looking, questioning and thinking. He never again traveled far from his home, Down House, near the village of Down in Kent, England. Instead, he conducted an enormous correspondence with people all over the world.
By then, he was an invalid, being months at a time in bed plagued by chronic poor health, with symptoms including headaches, heart palpitations, muscle spasms, pain, vomiting and vertigo. Many conditions have been blamed for Darwin’s poor health, including panic disorder, lactose intolerance and hypochondria. The best suggestion is that he had Chagas’s disease, a tropical parasitic illness (trypanosomiasis) contracted in South America, where Darwin described being bitten by an insect, a bug now known to be a vector of the disease. Being ill kept him home, away from spending time in London, giving lectures or having debates and arguments. He just stayed home thinking and writing. Darwin was an immensely deep thinker, and was extremely observant, a naturalist like Wallace. The explanation of evolution never occurred to any of the great philosophers or mathematicians.
He died of heart failure in 1882 at the age of 73. Darwin wanted to be buried without fuss near his home, but he was buried in state at Westminster Abbey, near the grave of Isaac Newton. Among his pallbearers were Wallace and famous naturalists, Joseph Dalton Hooker, and Thomas Huxley, both staunch defenders of Darwin’s work. Only five people who were not royal had such state burials in the 19th century.
Although he had trained at university to be a clergyman, after rejecting his father’s first thought of his becoming a medical doctor like others in the family, and began being a pious Anglican, Darwin ultimately became an agnostic—the term coined by Huxley for those who were unsure about God. When he died, a local evangelist said Darwin had renounced evolution and had again embraced Christianity. Darwin’s family refuted the claim. His last words, according to Emma, were, “I am not in the least afraid to die”. Emma remained herself devoutly Christian.
Variation
Unlike most natural scientists of his day, Darwin could not draw at all well, a severe drawback during the Beagle voyage, but he had written 368 pages of zoology notes, 1,383 pages of geology notes, a 770 page diary, and collected 1,529 specimens preserved in alcohol and 3,907 dried specimens. Life abounds, but no one knows how many species of animal, plant, fungus and microbe live on Earth. Almost two million have been described, but millions more exist. Thousands of new species are identified each year, mostly small or microbial, but also birds, fish and mammals.
In a study of just 19 trees in Panama, 1,000 of the 1,200 beetle species found were not previously known. An estimated 40 percent of South America’s freshwater fish have not yet been classified. According to the World Resources Institute, an environmental think tank:
- a single square meter of temperate forest can hold 200,000 mites
- a single square meter of tropical grassland may contain 32 million nematodes
- a single gram of its soil may harbour 90 million bacteria and other microorganisms.
Two decades after the end of the voyage of the Beagle, in 1859, Darwin was pushed by learning from fellow British naturalist, and contemporary and friend of Darwin, Alfred Russel Wallace (1823-1913), that he had independently formulated a similar hypothesis, into publishing On the Origins of Species. Not really being ready to publish, Darwin was not pleased with the result. Having envisioned a work five times longer, he thought the 500 page book to be just an abstract.
1,250 copies were printed in its first run, which was sold within a day or so of publication—an immediate best seller. The book went through six editions during Darwin’s lifetime. He made alterations and corrections in all of them. Darwin went on to write 16 books, including a treatise on emotions in humans and other animals. He thought blushing was a sign of deceit. His last book, published in 1881, was The Formation of Vegetable Mold, Through the Action of Worms, With Observations on Their Habits.
The Theory of Evolution
In Charles Darwin’s day, his theories of evolution and natural selection aroused plenty of controversy. Darwin loathed the fuss, preferring to cloister himself in his rural home, where he could study without distraction. It was left to Thomas Henry Huxley, a noted naturalist in his own right whose defense of Darwin was so unwavering that he was called “Darwin’s bulldog”. Herbert Spencer, a British philosopher and contemporary of Darwin, coined the phrase “survival of the fittest” in 1864. Darwin adopted the term in the fifth edition of Origins, giving Spencer full credit. Charles Lyell (1797-1875), who revolutionized the science of geology, greatly influenced Darwin and provided the vast timespan his ideas needed.
A century and a half later, Darwin’s work still arouses critics. Among determined defenders today is Richard Dawkins, the English biologist and best selling author, whose defense and explanations of evolution have earned him international admiration, and the enduring enmity of creationists. Does Darwin still need defending? Yes, mostly because of ignorance. There is a spectacular amount of ignorance among people who spread their opinions without having a clue about the theory of evolution.
Evolution was in the air when Darwin published his famous book, and the idea of evolution would have arisen even had Darwin never lived. Wallace had a similar idea contemporary with Darwin, and others probably would have done so as well. So, why is his theory of evolution by natural selection the most powerful concept ever? By it, Darwin explained our existence and that of all living things—no mean undertaking. Others had noticed how close some animals and plant were to each other, suggesting evolution, and some even thought of natural selection, but no one thought of a convincing explanation of it. None until Darwin and Wallace grasped that these ideas explained so much.
The power of a theory can be measured by how much it explains relative to how much that must be assumed for the explanation. A theory that makes a lot of assumptions is not much better than a description of what you observe. A good theory can explain lots of observations while making hardly any prior assumptions. Evolution explains the similarity and interrelationship of all life on earth with only three simple requirements. Darwin’s theory is huge because it explains everything about life but needs to assume only the extremely simple ideas of variation, heredity, and competition between inherited elements:
- Variation—Not all creatures are the same and even small differences can be important to survival.
- Inheritance—Creatures pass on their innate characteristics to their young.
- Overpopulation—Far more creatures are born than survive to adulthood. Those that fail to breed are either unlucky—a random factor—or are not suited to their environment in some respect, however small.
All of those things really follow naturally. If you have heredity, you can’t help but have errors in copying—mutations—so you can’t help getting competition. The assumptions Darwin needs to make are almost negligible. Of course, Darwin didn’t have the modern concept of heredity, particularly genetics. Darwin’s conception of genetics was that of his time, the notion of a mixing of paternal and maternal substances which, if it were true, could not have explained variation. Everything would become the same. That’s the opposite of Mendelian genetics, which says each generation is distinct.
Gregor Johann Mendel (1822-1884), an Austrian monk, made scientific experiments with pea plants revealing that inherited traits follow certain rules. The significance of Mendel’s work was not recognized until the twentieth century, but it is now considered fundamental to modern genetics. There’s no blending into uniformity. Rather it is s a shuffling of cards. Darwin came close to Mendel’s idea. He wrote to Wallace about experiments he had done with peas, as Mendel had, reporting results we can now see confirmed Mendel’s, and drawing parallels to other species. He noted that when you crossed a man and a female, you didn’t get an hermaphrodite. You got other males and females. So he was almost there in terms of understanding the genetics of reproduction. He just missed it.
Proposed in 1973 by Leigh Van Valen, the Red Queen hypothesis—named after the character in Alice in Wonderland, who explains to Alice that “it takes all the running you can do, to keep in the same place”—describes a view of nature in which species continually evolve through gradual changes to retain their ecological niche, but are never better adapted to compete with rival species or predators because they are co-evolving.
The environment of any species includes the other species that live in it, among them competitors for food and space, and predators. Each species evolves with these as aditional environmental pressures, and they must be able to respond to them or die out. It leads to “arms races”, most clearly between a predator and its prey. Speed is a weapon. A gazelle runs fast, but will be caught by a cheetah which can run faster. These two are therefore selected for speed, and over time they get faster without getting any advantage over the other.
Species and Speciation
In 1856, Darwin thought the word “species” was impossible to define:
It is really laughable to see what different ideas are prominent in various naturalists’ minds when they speak of ’species’. It all comes, I believe, from trying to define the indefinable.
“Species” has many definitions—more than two dozen have been published. Different definitions apply, depending upon the goals of the researcher. Someone interested in finding and naming new species might look for differences in shape, size or color. A geneticist would look for differences in genomes. The difficulty is that speciation is a gradual process, like aging. Where is the dividing line between youth, middle age and old age?
Nothing marks uniquely the transition over time or space of one species to another, but any observer of an evolving species at sufficient time intervals will see something different, depending on how far apart the intervals are. It happens by allopatric speciation. Two populations of the same species become geographically separated, perhaps by the formation of mountains, a shift in the course of a river, or by emigrating too far from their original home. One location may be richer in water and certain kinds of vegetation, the other may have different predators, and so on.
Animals and plants that live on earth at the same time, and once were the same species, but have become separated in some such a way, eventually will become different species. Living in different places, these organisms are exposed to different environments, and the differences in the environment each of the separated groups experience cause the organisms themselves to change differently and to different extents. The effect on their features in their responding to the environment is called environmental pressure.
In contemporary cases like these, members of the separated groups can be artificially brought together, to see whether they can reproduce. Some can, and some cannot, depending on how different they have grown to be. Horses and donkeys can reproduce to give hybrids called mules and hinnies when the stud is a male donkey or male horse respectively, but they are usually sterile themselves, so horses and donkeys are separate species now, but once were the same species.
Not that sexual reproduction is the only way that species can be determined. Bdelloid rotifers are microscopic aquatic animals in ponds, rivers, wet soils and mosses. They reproduce asexually, producing eggs that are genetic clones of the mother. Males do not exist. Bdelloid rotifers have been reproducing this way for at least 40 million years, yet they have evolved into multiple species. Tim Barraclough, a biologist at the Imperial College in London, says about rotifers:
One remarkable example is of two species living in close proximity on the body of another animal, a water louse. One lives around its legs, the other on its chest, yet they have diverged in body size and jaw shape to occupy these distinct ecological niches.
What causes rotifers and every other form of life to diverge, to speciate? There is no intention behind it. Slight changes in the genome of the separated groups accumulate by mutation—mainly accidental changes. Most mutations have little consequence for the organism—they are neutral—and, of the rest, most are bad for it. Occasionally, a mutation improves how the organism can respond in its own environment, and more commonly a previous neutral mutation becomes useful to it as its environment changes. Nature is not trying to make new species, but a system has evolved that actually does allow every living thing to change and therefore be able to survive in different—perhaps once hostile—environments. Different environments cause speciation, but normally, it takes a long, long time.
Over this long time, each population adapts to its particular slowly changing environmental pressure. Each population evolves. Once the adaptations are clear enough, the single species becomes two or more distinct species. But allopatric speciation is not the only method of natural speciation.
Organisms can diverge even when they come back into contact, or even when they never fully lose it. If species later come back into contact with each other, and if they reproduce sexually, they may no longer have the interest in interbreeding, or ability to successfully interbreed. Like horses and donkeys, their offspring might be sterile, or more likely, though offspring are still viable, the adults of the different species do not recognize each other as suitable mates. In human terms, they find each other repulsive.
The London underground mosquito is a variant of the species Culex pipiens. It entered the underground subway system in the 19th century and began to reproduce there. Though they can still meet their surface cousins, the transfer of genes is restricted—most of the subway variety never get out of the subway to meet the surface population, and the surface population rarely get into the underground. Although the insects share some genetic information, it is insufficient to prevent speciation, and the mosquitoes continue to evolve away from the other. Their genomes are getting increasingly dissimilar, and the types are beginning to behave differently. They are mating less and less, and are reluctant even when they get the chance.
In similar fashion, the Ensatina salamanders ringing the Central Valley in California have split into different subspecies. Individuals come into contact, like the ends of links in a chain, but each subspecies has become specifically adapted to its particular habitat. Such adaptations and differences can also occur within a single habitat. Cichlid fish in East Africa have diverged into multiple species within single lakes, probably to take advantage of different food sources or through sexual selection.
Therese Markow is a professor of biology at the University of California San Diego who specializes in speciation, looking for “populations that are different enough that they’re starting to diverge, to become different species, but not yet completely speciated”. She focuses upon four types of fruit fly (drosophila) found in the deserts of Baja California and across the Sea of Cortez on the Mexican mainland and elsewhere. The flies share a common ancestor, but each is on a continuum of evolutionary divergence, each moving toward becoming a distinct species:
All of these drosophila breed in decomposing cacti, but millions of years ago, they partitioned the niche. They live, feed and breed on different kinds of rotting cactus. One has abandoned the cactus altogether and lays its eggs in the soil beneath, its larvae feeding on the toxic juices that leach into the ground. The flies are becoming distinct species. In many ways, they remain much the same. They look alike. Adults will sometimes feed next to each other, but they also recognize the other fly is a different species. It’s not unlike humans and other primates.
Markow is not implying the recognition is conscious. The drosophilae simply do not recognize each other by their appearance, scent, or whatever, as suitable sexual partners.
What of the organisms that do not reproduce sexually? When the different species come into contact they do so as competitors, and one or the other will be more successful in their common environment, and will eventually push the other to extinction in that niche.
Geographic isolation and natural selection are clearly major factors driving speciation, but they are not the only ones. As researchers probe more deeply, employing scientific tools and knowledge Darwin never dreamed of, they reveal ever more complexity.
Revelations about genomic structures, how genes turn on and off and the remarkable ability of nature to use the same tools in different ways in different species have all dramatically boosted researchers’ knowledge. Ron Burton, a professor of marine biology at UCSD’s Scripps Institution of Oceanography studies a species of copepod—a small crustacean—found in tidal pools along the local coast:
The populations inhabiting the pools are genetically differentiated on various spatial scales. For example, we can easily distinguish Ocean Beach copepods from those in La Jolla.
In the lab, Burton can mate these different populations, producing hybrids. So, the populations are in an early stage of speciation, but the progeny do not live long. They are of “surprisingly low fitness” when brought together to interbreed. It might reflect environmental adaptations—that the parents are adapted to their respective environments, whereas the offspring no longer are—but Burton says there’s no evidence of that. Both species can live well enough in the same tidal pool, but the hybrids’ poor fitness is due to incompatibilities between their nuclear genome inherited from both parents and a handful of mitochondrial genes inherited from the mother:
These genes are crucial to the energy generating system of the cell, so small incompatibilities directly affect the fitness of the animals.
He believes the cause is random genetic mutations accumulating in isolated populations, and only when those different mutations are put together by hybridization are the incompatibilities apparent. Then the distinctiveness of the two species emerges.
A Changing Theory?
Can we ever come to a consensus in the debate about evolution? Among scientists there is a consensus. Only certain laymen oppose the theory of evolution, and the sort who still think it’s still at issue are the sort who don’t understand the issues but think they do without properly learning about them. There is no argument against evolution worth a dime. Nor is there any likelihood it will change in any significant way—any way that will radically change its consequences. The fact of evolution is certain, so, just as in Darwin’s time, there is only one question. How does it happen? Darwin answered it, and now we are refining his answer.
There are other ways beside natural selection by which genes can be selected, including pure chance—genetic drift—and that too could bring about progress, rather like a ratchet, when mutations cannot be reversed. Some recent work shows they cannot. Darwin thought evolution to be a long, gradual process, that selection occurred more or less constantly and continuously. No test that speciation occurs at a constant rate has ever been made against competing models that can predict virtually identical outcomes, nor has any mechanism been proposed that could cause the constant rate phenomenon.
A study carried out by Mark Pagel, Chris Venditti, and Andrew Meade at the University of Reading, in England, used 101 phylogenies of animal, plant and fungal taxa to test the constant rate claim against four competing models for predicting the distribution of evolutionary tree lengths, either by specifying how factors combine to bring about speciation, or by describing how rates of speciation vary throughout a tree. Phylogenetic branch lengths record the amount of time elapsed, or evolutionary change occurring, between two species branching off—successive events of speciation.
The team analyzed the lengths of branches in the evolutionary trees of thousands of species within the groups. Then they compared four models of speciation to determine which best accounted for the rate of speciation actually found. The basic Darwinian hypotheses that speciation follows the accumulation of many small events that act multiplicatively found support in 8% of the trees, and that act additively in none of them. A further 8% of trees hinted that the probability of speciation changes according to the amount of divergence from the ancestral species, and 6% suggested speciation rates vary among taxa.
The remaining 78% of the evolutionary trees fit the simplest model in which new species emerge from single events, each rare but individually sufficient to cause speciation. This model predicts a constant rate of speciation by rare stochastic events in the environment that cause reproductive isolation, such as genetic mutations, a shift in climate, or a mountain range rising up, but the constant rates are different for different groups of species. Why some groups have more or fewer species and species radiations require examination of the size of potential causes of speciation shared by a group of closely related organisms rather than how those causes combine.
The data are not compatible with the idea of speciation as a result of many small events. So continuous natural selection may not be the main cause of speciation, contrary to the Darwinian view of evolution, but rather it is random intermittent events—support perhaps for punctuated equilibrium.
In the 1970s, paleontologists, Niles Eldredge and Stephen Jay Gould, proposed an alternative to Darwin's steady process called punctuated equilibrium. From the fossil record, Eldredge and Gould argued that evolution consisted of long periods of stasis interrupted by brief bursts in which many new species appeared. The cause of the bursts was not known. Natural selection was believed to be a fine tuning mechanism, not a major force.
Some critics of punctuated equilibrium think it does not accurately reflect life’s revealed complexity, that species diverge and emerge on different timetables, each based on different sets of factors and influences. Several were noted in the Reading study, though at lower levels of incidence. David Reznick, a professor of biology at UC Riverside, studies evolution rates in natural populations of guppies in Trinidad. He has recorded change rates of the order of “10,000 to 10 million times faster than what Gould and Eldredge described as punctuations in the fossil record”.
Selective pressures like food and climate are far more powerful than Eldredge, Gould and other scientists had previously thought. They accelerate change. So, too, do random genetic mutations. Reznick said:
Organisms like ferns sometimes experience a doubling of chromosomes. This change can create a barrier to breeding with other members of what had been the same species; such a barrier is the prevailing definition for species. These rare events can, in theory at least, create species in a single step.
Fossils
In 1859, the fossil record dated back only to the time of the trilobites, hard-shelled marine arthropods that first appeared 540 million years ago. They were fascinating creatures, and the fossil record then was fraught with them. But for Darwin, they weren’t old enough to sufficiently support his argument that modern life had gradually evolved and diversified over the eons. In the century and a half since, the known history of life has been been pushed steadily back as ever-older fossilized organisms were found and identified. The fossil record now dates back to 3.5 billion years, only a million years after the earth formed. It’s microbial, sure, but it’s life.
Even so, the fossil record has never been entirely persuasive to people likecreationists, who point to gaps in the record as evidence that evolutionary theory is flawed. Where, they have asked, are the “transitional forms”, the creatures clearly evolving from one species to another? J David Archibald, a paleontologist at San Diego State University, says:
In fact, there are any number of well-studied and still-being-studied transitions. Dog-sized horses to modern horses, apelike forms to modern humans, terrestrial dinosaurs to birds, even hoofed ungulates to whales, protomammals to mammals, etc.
Moreover, as noted above, the idea of transitional forms assumes there is a normal form, but change in form is slow but continuous, so every generation is a transitional form, though that form might not show any observable change for millions of years. The trouble is that few of the many animals that live ever get fossilized, so when we find them they might have changed significantly, making it look as if one changed suddenly into the other.
Archibald says the existing record is compelling, even if it still is not complete. It isn’t complete because it is hard to become a fossil, and many, many animals never did. More than 90 percent of all of the animals and plants that have ever lived on Earth are extinct. Yet, for several reasons, only a fraction of them are preserved in the fossil record:
- Nature is good at recycling. Lots of extant organisms boast the ability to convert dead organic material back into the stuff of life and living tissue. From bacteria, worms, ants, beetles and flies to hyenas, vultures and scavenging fish, these organisms speed the process of decomposition, decay and disappearance. It takes just 10 days to reduce a dead, 330 pound adult male gorilla in the tropics to a pile of bones and hair. In three weeks, the carcass will consist of just a few small bone pieces.
- To have much chance at fossilization, an organism must perish under distinct circumstances and conditions and possess specific qualities. It should have hard body parts, bones, teeth or a shell that can be preserved through mineralization, a process in which surrounding inorganic minerals impregnate and replace the original biological matter, retaining its form.
- Where the organism dies is critical, too. To be fossilized, the dead animal or plant must be quickly covered by oxygen poor material that will deter scavengers and slow microbial decay. Even if an organism is properly buried, this is no guarantee that it will become a fossil. In some cases, rocks are heated up and fossils are destroyed—sedimentary rocks change to metamorphic rocks by melting.
Lake beds and deltas, rivers and streams are good places to fossilize, tropical forests and open plains are not. In tropical forests, decomposition happens too quickly. In open plains, other animals are likely to discover, consume and scatter the remains long before any fossilizing can occur. Some extinct species are known only by footprints left in fossilized mud flats and soft sand.
The necessities and realities of fossilization skew the record. Animals that lack substantial hard parts are much less common in the fossil record. The remains of soft bodied terrestrial organisms are sometimes preserved under extraordinary conditions, but for these groups the fossil record will never provide anything more than a meager understanding of past life.
On the other hand, advances in finding, extracting and studying fossils are constantly adding to the knowledge base. Recently, researchers at the University of Texas and Yale University reported that they had discovered color producing molecules in a 47 million year old feather fossil. By analyzing the molecules, they deduced that the long extinct bird sported a dark, iridescent sheen similar to modern starlings. They say the technique may help them reconstruct the colors of some feathered dinosaurs as well, from which birds evolved.
Skin impressions, leaves, tree rings and pollen can all become fossilized and thus retain their secrets for eventual discovery. Scientists have learned much from insects trapped in amber or fossilized resin.
New technologies allow scientists to extract additional information from old objects of interest. In Darwin’s time, fossils mainly provided only gross anatomical information. Today, even a single tooth can provide information about an animal’s diet, habitat and evolutionary relationships through studies of isotopes, the microscopic structure of the enamel, small scratches and pits left on the teeth, and macroscopic wear features.
CT scans have made it possible to examine the internal structures of even the most fragile fossils. Computers permit scientists to analyze large volumes of data to find new linkages, revelations and fields of inquiry. And advances in molecular biology and genomics have revised and refined Darwin’s increasingly bushy evolutionary tree.
Indeed, the growing list of extant animals with sequenced genomes shows humans are not so much different than most animals. We have, for example, only a few more genes than chickens and share roughly 50 percent of our genes with bananas. Some researchers have argued that the ability to fully sequence a species’ genome—that is, its genetic blueprint—renders fossils no longer necessary to the argument. Everything we need to know can be found and compared in DNA, they say.
Molecular studies have become paramount in unraveling evolutionary relationships of living species, and such studies are becoming increasingly important in reconstructing past genomes and how they worked. Only an ignorant person, however, would argue that molecular studies can replace fossils for understanding the past diversity of the other 90 to 99 percent of extinct species.J David Archibald
In these cases, said Archibald, nothing tells the tale quite so well as a bone, a tooth or other bit of tangible proof that something once walked, swam or flew upon the Earth.
What is Happening Now?
The process of speciation is ongoing and inevitable. It’s happening now, everywhere. The ultimate question is, what will happen next? In truth, there’s no way to know. In the fossil record, mass extinctions have been followed by periods of huge biological diversification. The rise of mammals, for example, after the dinosaurs died out 67 million years ago. You have to remember, though, that these adjustments take millions of years. A catastrophe, whether an impacting asteroid, or the ravaging of the earth by humanity, can be quick, even instantaneous, but the recovery is excruciatingly slow. Mike Arnold, an evolutionary biologist at the University of Georgia, says, according to Scott LaFee, that every mass extinction may not presage a subsequent blossoming of biodiversity:
The present mass extinction is humanly mediated. It’s largely caused by things like destruction of habitat, which may result in irreversible harm in terms of evolutionary diversification. Environments are being modified, but maybe not in any way that life can adapt to and evolve in. There’s not a lot of opportunity for diversification in a world of concrete.
LaFee makes it seem as if Arnold expects diversification to be instantly observable even as the mass extinction we are causing progresses. In fact, the damage we are causing will probably itself accummulate with a positive feedback that will kill off far more species indirectly than we kill off directly. By killing off an element of another animal’s food chain, we shall kill off all of the organisms that depend on that element. We chop down a tree in the Amazon and kill a thousand species that lived solely on that tree. Few people have noticed that we are on the list somewhere. Environmental damage we cause could kill us off ourselves. That is the danger of climate change. Life will still persist, adapt, evolve, as it has done for billions of years, but all large vertebrates are being driven to extinction. And we are among them.
Parts of evolutionary theory are, at present, rather like black boxes, notably the relationship between genes and phenotypes. A phenotype is the collection of observable traits that defines and describes an organism. You can do your Darwinism without any idea of the processes by which genes give rise to phenotypes, but it will be nice to dive inside that black box. We won’t just talk about a gene for long fingers. We’ll know how that gene produces long fingers. That’s embryology, not evolution, but it completes the picture.
