Pre-Darwinian and Darwinian Thoughts on Evolution
Pre-Darwinian Thoughts on Evolution
Throughout the Middle Ages, there was one predominant component of the European world view: stasis.
- All aspects of nature were considered as fixed and change was unconceivable.
- No new species had appeared, and none had disappeared or become extinct.
The social and political context of the Middle Ages helps explain this world view:
- shaped by feudal society - hierarchical arrangement supporting a rigid class system that had changed little for centuries
- shaped by a powerful religious system - life on Earth had been created by God exactly as it existed in the present (known as fixity of species).
This social and political context, and its world view, provided a formidable obstacle to the development of evolutionary theory. In order to formulate new evolutionary principles, scientists needed to:
- overcome the concept of fixity of species
- establish a theory of long geological time
From the 16th to the 18th century, along with renewed interest in scientific knowledge, scholars focused on listing and describing all kinds of forms of organic life. As attempts in this direction were made, they became increasingly impressed with the amount of biological diversity that confronted them.
These scholars included:
- John Ray (1627-1705) - put some order into the diversity of animal and plant life, by creating the concepts of species and genus.
- Carolus Linnaeus (1707-1778) - added two more categories (class and order) and created a complex system of classification (taxonomy) still used today; also innovated by including humans in his classification of animals.
- Georges-Louis Leclerc (1707-1788) - innovated by suggesting the changing nature of species, through adaptation to local climatic and environmental conditions.
- Jean-Baptiste Lamarck (1744-1829) - offered a comprehensive system to explain species changes; postulated that physical alterations of organic life would occur in relation to changing environmental circumstances, making species better suited for their new habitat; also postulated that new traits would be passed on to offspring (the theory known as inheritance of acquired characteristics).
Therefore, the principle of "fixity of species" that ruled during the Middle Ages was no longer considered valid.
In the mid-19th century, Charles Darwin offered a new theory which pushed further the debate of evolutionary processes and marks a fundamental step in their explanation by suggesting that evolution works through natural selection.
Charles Darwin (1809-1882)
Charles Darwin's life as a scientist began when he took a position as naturalist aboard HMS Beagle, a ship charting the coastal waters of South America. As the ship circled the globe over a five-year period (1831-1836), Darwin puzzled over the diversity and distribution of life he observed. Observations and collections of materials made during these travels laid the foundation for his life's work studying the natural world.
As an example, the Beagle stopped five weeks in the Galapagos archipelago. There Darwin observed an unusual combination of species and wondered how they ended up on this island.
Darwin's observations on the diversity of plants and animals and their particular geographical distribution around the globe led him to question the assumption that species were immutable, established by a single act of creation. He reasoned that species, like the Earth itself, were constantly changing. Life forms colonized new habitats and had to survive in new conditions. Over generations, they underwent transmutation into new forms. Many became extinct. The idea of evolution slowly began to take shape in his mind.
In his 1859 publication On the Origin of Species, Darwin presented some of the main principles that explained the diversity of plants and animals around the globe: adaptation and natural selection. According to him, species were mutable, not fixed; and they evolved from other species through the mechanism of natural selection.
Darwin's theory of natural selection
In 1838, Darwin, at 28, had been back from his voyage on the Beagle for two years. He read Thomas Malthus's Essay on Population, which stated that human populations invariably grow until they are limited by starvation, poverty, and death, and realized that Malthus's logic could also apply to the natural world. This realization led Darwin to develop the principle of evolution by natural selection, which revolutionized our understanding of the living world.
His theory was published for the first time in 1859 in On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.
The theory of adaptation and how species change through time follows three postulates:
- Struggle for existence: The ability of a population to expand is infinite, but the ability of any environment to support populations is always finite.
- Example: Animals require food to grow and reproduce. When food is plentiful, animal populations grow until their numbers exceed the local food supply. Since resources are always finite, it follows that not all individuals in a population will be able to survive and reproduce.
- Variation in fitness: Organisms in populations vary. Therefore, some individuals will possess traits that enable them to survive and reproduce more successfully (producing more offspring) than others in the same environment.
- Inheritance of variation: If the advantageous traits are inherited by offspring, then these traits will become more common in succeeding generations. Thus, traits that confer advantages in survival and reproduction are retained in the population, and traits that are disadvantageous disappear.
Examples of adaptation by natural selection
During his voyage on the HMS Beagle, Darwin observed a curious pattern of adaptations among several species of finches (now called Darwin's finches) that live on the Galapagos Islands.
Several traits of finches went through drastic changes in response to changes in their environment. One example is beak depth:
- There was huge variation in beak depth among finches on the island; it affected the birds' survival and adaptation to local environmental changes.
- During a drought, finches with deeper beaks were more likely to survive than finches with shallow beaks (which were at a disadvantage because it was harder for them to crack larger and harder seeds).
- Parents and offsprings had similar beak depths.
Through natural selection, average morphology (an organism's size, shape and composition) of the bird population changed so that birds became better adapted to their environment.
Benefits and disadvantages of evolution
Adaptation results from competition among individuals, not between entire populations or species.
Selection produces adaptations that benefit individuals. Such adaptation may or may not benefit the population or species. In the case of finches' beak depth, selection probably does allow the population of finches to compete more effectively with other populations of seed predators. However, this need not be the case. Selection often leads to changes in behavior or morphology that increase the reproductive success of individuals but decrease the average reproductive success and competitive ability of the group, population, and species.
- Example of conflict between individual and group interests: All organisms in the population produce many more offspring than are necessary to maintain the species. A female monkey may, on average, produce 10 offspring during her lifetime. In a stable population, perhaps only two of these offspring will survive and reproduce. From the point of view of the species, the other eight are a waste of resources. The species as a whole might be more likely to survive if all females produced fewer offspring.
The idea that natural selection operates at the level of the individual is a key element in understanding adaptation.
Instead of a completely random selection of individuals whose traits will be passed on to the next generation, there is selection by forces of nature. In this process, the frequency of genetic variants for harmful or maladaptive traits within the population is reduced while the frequency of genetic variants for adaptive traits is increased.
Natural selection, as it acts to promote change in gene frequencies, is referred to as directional selection.
Finches' beaks (Example)
Large beaks have benefits as well as disadvantages. Birds with large beaks are less likely to survive their juvenile period than birds with small beaks, probably because they require more food to grow.
Evolutionary theory prediction:
- Over time, selection will increase the average beak depth in a population until the costs of larger-than-average beak size exceed the benefits.
- At this point, finches with average beak size in the population will be the most likely to survive and reproduce, and finches with deeper or shallower beaks than the new average will be at a disadvantage.
At this point, the population reaches equilibrium with regard to beak size. The process that produces this equilibrium state is called stabilizing selection.
Even though average characteristics of the beak in the population will not change in this situation, selection is still going on. The point to remember here is that populations do not remain static over the long run; if so, it is because a population is consistently favored by stabilizing selection.
Rate of Evolutionary Change
In Darwin's day, the idea that natural selection could change a chimpanzee into a human, much less that it might do so in just a few million years (which is a brief moment in evolutionary time), was unthinkable.
Today, most scientists believe that humans evolved from an apelike creature in only 5 to 10 million years. In fact, some of the rates of selective change observed in contemporary populations are far faster than necessary for natural selection to produce the adaptations that we observe.
- The human brain has roughly doubled in the last 2 million years (rate of change of 0.00005% per year); that is 10,000 times slower than the rate of change observed among finches in the Galapagos Islands.
Therefore the real puzzle is why the change in the fossil record seem to have been quite slow.
The fossil record is still very incomplete.
It is quite likely that some evolutionary changes in the past were rapid, but the sparseness of the fossil record prevents us from detecting them.
In On the Origin of Species, Darwin proposed that new species and other major evolutionary changes arise by the accumulation of small variations through natural selection.
This idea was not widely embraced by his contemporaries.
- Many accepted the idea that new species arise through the transformation of existing species.
- Many accepted the idea that natural selection is the most important cause of organic change.
- But only a few endorsed Darwin's view that major changes occur through the accumulation of small variations.
Darwin's critics raised a major objection to his theory: The actions of selection would inevitably deplete variation in populations and make it impossible for natural selection to continue.
Yet Darwin couldn't convince his contemporaries that evolution occurred through the accumulation of small variations because he could not explain how variation is maintained, because he and his contemporaries did not yet understand the mechanics of inheritance.
For most people at the time, including Darwin, many of the characteristics of offspring were thought to be an average of the characteristics of their parents. This phenomena was believed to be caused by the action of blending inheritance, a model of inheritance that assumes the mother and father each contribute a hereditary substance that mixes, or "blends", to determine the characteristics of the offspring.
The solution to these problems required an understanding of genetics, which was not available for another half century. It was not until well into the 20th century that geneticists came to understand how variation is maintained, and Darwin's theory of evolution was generally accepted.