|
Bill Kelso
An Introduction to Evolutionary Theory
As noted earlier evolution has never been that popular a doctrine in the US. Presently only about a third of Americans think it explains life on our planet. To explain why people both reject and accept Darwin’s notion of evolution I want to 1) first look at why Pre Darwinian ideas rejected evolution, and 2) secondly see why subsequently Darwin developed his notion of descent with modifications.
(1)
Pre Darwinian Views of Evolution
If you look at the recent history of mankind, it is easy to understand why the public tends to dismiss notions of evolution. After all humans had never seen any human let alone any animals changing their appearance. Their only experience with the evolution of animals was the proliferation of different dog breeds in 19th century England. But that change had been a result of artificial selection rather than impact of some mysterious biological process called evolution.
a. Every day View of Humans and Animal Life
While humans had witnessed many changes in society, in all cases they seemed to be the result of human actions rather than biological evolution. Ever since humans abandoned their nomadic life of hunters and gathers, people have seen society becoming more technologically sophisticated and complex. Given their relative brief period of life, especially compared to the history of the planet, they witnessed no noticeable changes in the appearance of their fellow human beings.
It was true that change was a constant feature of modern life. But if any change occurred, it was because of human ingenuity and intelligence rather than some vague notion of natural selection. To all appearance there was no hidden force called evolution that was shaping our lives.
b.Greek and Christian views of Natural History
If everyday experience seemed to undermine the doctrine of evolution, the idea of evolution was also rejected by two philosophical traditions that came to underlie western civilization.
The first doctrine was developed by the Greeks 2500 years ago. Unlike many early civilizations the Greeks developed a naturalistic and philosophical rather than a religious history of the world. As part of their naturalistic view of the world, the Greeks insisted there was a Great Chain of Being that linked all segments of life together. The Greek Philosopher Aristotle went one step further and argued every element in that great Chain of Being followed what he called its teleology, which was its specific or final goal.
According to this classification system each species occupied a link in a chain of ever increasing complexity. Each item in life also reflected its final purpose. While this chain explained the diversity of life, it had no room for evolution. After all the Greeks believed that life had an inherent order that was static, immutable and reflected some larger philosophical purpose.
Four hundred years after the Greeks, Christianity becomes the dominant outlook of most Europeans. While Christians tended to adopt the Greek idea of a Great Chain of Being, they made one significant change in the doctrine. They insisted that man, who occupied the top position in the Chain of Being, was also made in the image of God.
This view is best expressed in Michelangelo’s famous picture of God giving life to Adam in the fresco of the Sistine Chapel. If man reflected the image of God, it is hard to see why any divine power would tolerate the idea of evolution.
God Giving Life to Adam
c.Doubts About the Great Chain of Being
However, by the 17th to the 19th century, Europeans starting finding fossils of animals that no longer seemed to exist. The most notable of those fossils were the large bones of animals that we today called dinosaurs. If there was a Great Chain of Being, it was hard to explain why these massive animals no longer existed.
An even more dramatic find occurred in 1856, three years before Darwin publish his book on evolution, when quarry workers discovered the first Neanderthal skull. Despite the primitive nature of paleontology at that time, the skull appeared to be an earlier more primitive version of mankind.
In light of these new findings, many began to question the idea that all examples of life had always existed in their present form. There were too many life forms that seemed to have no place in the Great Chain of Being. Despite these doubts, the people who had misgivings about the Great Chain of Being, initially lacked an alternative doctrine to explain the existence of life on earth.
(2)
The Rise of Darwin’s Theory of Evolution
a. Mathus Offers a New Insight
While Darwin was one of those with misgiving about the Great Chain of Being, he soon found that the writings of Thomas Malthus, a famous British demographer and political economists, provided a possible alternative explanation for the existence of life on earth.
Malthus’ major contribution has been his idea that human populations faced a potentially life threatening dilemma. He pointed out that human populations tended to grow geometrically in time while the growth of food production increased at best arithmetically. In light of this situation Malthus predicted that humans would eventually outstrip the available resources and thus die of either war, disease or starvation. Darwin soon realized that Malthus findings would apply to animal and plant populations as well as human populations.
b. The First Source of Change
While Malthus ideas were suggestive, they were not a complete answer or substitute for the idea that life was static and non-changing. To achieve that goal Darwin modified Malthus findings by suggesting that the various organism in the world would soon adjust to their potentially dire situation by enhancing their ability to obtain food. Instead of dying by famine, as suggested by Malthus, animals and plant organisms would compete in a kind of arms race to find some ecological niche where they could isolate themselves from the efforts of competing members of their species. If the present food supply was inadequate for a population, various groups within that population might try to develop new living arrangements in the existing environment. By changing their anatomy, or digestive track, or area of residence, one species might alter its current features or even evolve into an alternative species that could then exploit different sources of food and survive in a competitive environment.
Darwin’s major insight was that both the Greeks and Malthus had failed to see that the world was constantly changing. In a variable world in which environments constantly evolved, organisms had to mimic the changes in the environment if they wish to flourish and reproduce. Among other factors, climate might change, food supplies might become more or less abundant and populations might increase. As the world developed or evolved, plants, animals and even humans had to adjust if they wanted to avoid Malthus’ pessimistic predictions about the future.
c. Darwin’s Second Source of Change
While Darwin initially focused on the competition among animals to explain the development of new features in a species or even the rise of totally new species, he soon expanded the idea of natural selection to include changes in the natural environment as a source of evolutionary change. For instance, as noted earlier in our discussion of human evolution, people run the risk of dying if they are exposed for too long to the dangers of solar radiation. To limit that risk, natural selection created different races of humans to either mitigate or enhance their ability to handle either too much or too little sunshine. Likewise humans evolved tall lanky physiques to dissipate heat in hot climates like east Africa and shorter more compact bodies to preserve heat in very cold climates like Siberia.
Other examples of organisms adjusting to their environment are too numerous to mention. But an obvious example is either the one humped camel of Saudi Arabia or the two humped camel of Central Asia. While human races evolved races to handle the problem of sunshine, other animals like camels evolved anatomical humps to cope with an environment that had a limited supply of water.
The source of evolutionary change was thus two fold: the creation of new species arose from either competition from other plants and animals or adverse developments in the environment which could pose a potential danger to the health or well being of an organism.
d. Darwin’s Descent from a Common Ancestor
But in an even more controversial point, Darwin also speculated that every form of life had all evolved from a single species. Darwin was thus not only insisting that all animals and plants including mankind, have changed over time, but that they were all related. Whether it was a rather lowly house plants, a minor insignificant insect, or a megafauna like an elephant, humans shared a common ancestor with all of the life forms that came before it. It appeared that life on earth consisted of one gigantic family, who had all originated from the same ancestors.
e. Darwin’s Descent with Modifications
Surprising enough, Darwin did not call his new doctrine a Theory of Evolution but rather a Theory of Descent with Modifications. Darwin argued that as nature changed, or what he came to call Natural Selection, life forms would have to evolve new modifications if they wanted to survive and flourish in a competitive harsh world.
(3)
The Search for Evidence.
After developing his theory Darwin began a search for evidence to support his speculations. As we shall see the evidence that all life forms evolved from a single ancestor has to wait for the development of a new subdivision in biology called Evo Devo.
But evidence for the fact that species had dramatically changed over time was quickly forthcoming. Fortunately Darwin was one of the first to gather data in support of his theory. When Darwin was a young man he was hired on the HMS Beagle to travel round the world to study the environment.
How Evolution has shaped the anatomies and physiologies of animals
a.Darwin’s Finches
Before he knew it, Darwin hit paydirt. In exploring the Galapagos islands off the coast of South America, Darwin noted that the sources of food varied from one island to the next. He also noted that on every island the finches, who were originally one species, seemed to have evolved different types of beaks. The more he studied the birds the more he realized that to avoid the fate Malthus had predicted for larger populations, the species had evolved into different types of birds.
Darwin's Finches
In some cases, if the birds had chosen to eat the seeds of plants they found on the ground, they had develop robust beaks which could break apart the casings on the seed. In other islands the finches, who had decided to eat insects, had developed pointed and rather sharp beaks which they could used to spear flying insects. Yet other birds had sharp beaks for extracting blood from other fnches.
In place of all the birds eating the same food source, the various birds had evolved different specialties and become 17 different species of finches.
In place of an immutable Chain of Being, the birds on the Galapagos island had evolved to take advantages of the different kinds of environment in which they lived.
b. The Appearance of Herbivores and Carnivores
While Darwin’s pioneering work with birds seemed to confirm his theory of evolution, his initial studies were somewhat limited and parochial in nature. After all they merely examined how one species of birds had splintered or evolved into numerous separate species of finches.
As the theory of evolution became more popular among scientists, biologists started studying larger and more significant developments in the evolution of life on the planet. Whereas Darwin’s studies had merely shown how finches had modified their beaks as food supplies became limited, biologists now wanted to study how our earliest relatives had undertaken even more radical strategies in modifying their appearance in their quest for supplies. In particular they wanted to trace how our earliest diminutive mammalian ancestors had eventually evolved into the large herbivores, carnivores and primates who dominate the planet today
Before undertaking this endeavor, biologists started identifying all forms of life on earth. As we will find out when we study biological communities, they identified four types of organisms on earth including:
1) Providers such as grasses and bushes,
2) Consumers such as herbivores who graze on the above plants,
3) Carnivores who attack and eat herbivores and the
4) Decomposers such as vultures, termites and alga who recycle the earth’s minerals.
In this post we will focus on the second and third form of organisms cataloging the traits which enhanced their ability to flourish in different environment.
Biologists had noted that since herds of herbivores had very different life styles or eating habits from that of carnivores, the two groups must have evolved very different physical anatomies.
1.Eyes of Prey, Predators and Primates
For instance, in the process of giving animals the gift of sight, evolution had to select from two distinct options. First it had to decide if it wanted to create a sight system that stressed either field of vision or secondly binocular vision. In binocular vision an animal uses both eyes to create a single clear three dimensional image of an object. This process involves the brain combining the different images received by each eye to create a unified image of the object the eye is trained on. But in achieving a three dimensional view of an object, an organism may have a limited view of objects or creatures on its periphery.
The other option is to develop animals with a much wider field of vision. Besides a central vision or what is directly in front of it, a wider field of visions enables an animal to detect animals on its periphery. An animals can thus see very early on if any predator is approaching it from either side.
A.Herbivores as Prey Animals
Generally animals who are usually prey animals or the hunted want to possess a wide field of visions and thus they tend to have their two eyes positioned on opposite sides of their heads. In such animals the eyes often move independently of one another to expand their field of vision.
Among animals with the widest field of vision we would include the large number of herbivores on the planet including rabbits, buffalo, antelope, zebra and wildebeest.
Picture of a Herbivore
Since most herbivores are only slightly faster than their predators, the key to their survival is a quick start in eluding their carnivore enemies. To hold their predators at bay, they evolved eyes on the sides of their heads to alert them to possible attacks from different directions.
b. Carnivores as Predators
In contrast, carnivores who wanted to attack herbivores, evolved a very different system of vision. To enhance their ability to hunt herbivores, they have eyes on the front rather than the side of their head which enables them to hone in on their target. They also wanted a three dimensional view of their prey in order to maximize their strategy for attacking a distant target.
c. Primates and Humans
If binocular vision is of benefit to carnivores, then the inevitable question is why do primates including man also have a similar form of vision? After all, since humans survived as a hunted prey on the African savannah for several million years before acquiring the ability to make spears and bow and arrows only 70,000 years ago, why do they have binocularl vision.
The answer is found in their initial environment. Because our earliest ancestors lived and traveled in the canopy of the rain forest, they needed to have a three dimensional view of their environment to safely move from tree to tree. If you are going to jump from limb to limb on either the same or different trees, a three dimensional picture of the object you want to grab is essential. In addition, by living in the forest canopy, our primate ancestors had little reason to fear an attack from their periphery. Unlike most land herbivores, our earliest relatives lived in a relatively secure environment of the rainforest which held few predators.
In fact today, the only animals that possess binocular vision are carnivores, primates including humans, and carnivore birds commonly knowns as raptors such as hawks, eagles and owls.
2. Teeth
To acquire their food, the different types on animals on the planet have also evolved separate and specialized types of teeth. Presently all animals have four types of teeth. At the front are incisors used for grabbing food, next are large canines used for tearing apart animals flesh, next are molars for grinding up difficult to digest food and finally sub molars which are something of a cross between incisors and molars.
Because carnivores like lions attack herbivores, they have large incisors and very sharp canines to tear open the flesh of their prey. In contrast, herbivores have smaller incisors, and canines but much larger molars that help them grind up the tough vascular plants that make up their diet.
Picture of a lion's canine teeth
Even when carnivores and herbivores share the same type of teeth, they have evolved variations of those teeth to accommodate their separate diets. For instance, since carnivores are primarily meat eaters, their molars tended to have a serrated edge like that of a knife to assist in their slicing meat. In contrast the molars of herbivores have a pronounce ridge on their molars which helps them grind up more effectively difficult and tough plant material.
Similarly because herbivores like zebra and elephants eat rough foliage, they had thick layers or enamel to protect their teeth while carnivores have a much thinner layer of enamel on their teeth because their meals are easier to digest.
3. Digestive Track
Since carnivores and herbivores consume different types of food, the have also evolved very different types of digestive systems. Because most herbivores eat low quality food, they have developed elaborate digestive systems to break down tough fibrous plant material. For instance, some herbivores are ruminants which means they have several stomachs which use fermentation to digest their food. That means that they have special microorganisms in a specialized stomach called a rumen to break down the complex carbohydrates in their stomach.
Since many herbivores are not ruminants, they have evolved much larger colons or large intestines, which also rely on fermentation, to help them digest their food. Elephants, in particular, are noted for their reliance on hindgut fermentation to obtain nutrients from their foods. Because this process is very inefficient, elephants are well known for having one of the least efficient and effective digestive systems of any animal on the planet.
To satisfy their metabolic needs, elephants are known to spend anywhere from 60% to 80 % of their awake hours feeding. Because of their large size and inefficient means of obtaining nutrients from their food supply they have to spend an inordinate amount of time searching for something to eat.
Pictures of Elephants Constantly Eating
Ironically enough, the poor digestive system of elephants is a bonanza to many other animals on the African plains. Because elephant droppings are rich in partly digested food, many other animals search out their droppings to feed themselves. For many insects like dung beetles, as well as butterflies, birds, baboons and even some larger mammals, have come to rely on elephant dung as a major source of food. In this case the ineffective elephant digestive system works to the benefits of many other animals on the African savannah.
Animals Eating Elephant Dung
In contrast lions and other carnivores have a smaller digestive system with a smaller colon and a proportionately larger small intestine. Since they eat high quality meat, they do not need to elaborate kind of digestive systems found in the various species of herbivores. Finally, in the case of humans, they also have a relatively large small intestine which absorbs nutrients because they eat a higher quality of diet than herbivores. In contrast their large colon is primarily used to get rid of water and stool rather than to further ferment plant material.
4. The Size of the Animals
Finally we have to realize that the largest animals on the planet are herbivores. There seem to be three reason why that is the case. First, since the greatest source of food consists of the grasses, plants and trees that characterize our planet, herbivores have a greater source of food than carnivores. There is more plant material on earth than there are cadavers of dead herbivores.
Secondly, as mentioned above, once herbivores start to get big, they need a large digestive system to process their food which only adds to their overall size.
Thirdly, it is a simple rule of ecology that many herbivores get big as a way of limiting predators from attacking them. For example, elephants are one of the few animals that continue to grow throughout their lives, a process which has significantly improved their safety on the African savannah.
The benefits to elephants are clearly evident in their main feeding grounds. As we shall see when we discuss the notion of community, large herbivores like the elephants and giraffes on the African savannah are rarely attacked by carnivores. In most cases carnivores primarily attack animals that are comparable or slightly smaller than themselves. The main reasons carnivores do not become too big, is that the bigger they get, the slower they become. To maximize speed and strength most carnivores are of medium size.
(4)
The Limits of Evolution I
How Rapid Changes in the Nature Affects Evolution
Despite the successful track record of evolution in modifying plants and animals to better survive and prosperous in their environment, there are times when evolutionary pressures fail. First, there are often times in which evolutionary change cannot keep up with rapid changes in the environment.
a. Death of Dinosaurs
If there are very rapid developments in nature, animals often lack enough time to modify their anatomy or diet to cope with a changing environment.
For example, when a meteorite hit the earth around 66 million years ago, the debris from the explosion blocked the sunlight hitting the earth for years. As a result, large dinosaurs who had to constantly consume large amount of plant material to sustain their massive size, quickly died out. There was not enough time for dinosaurs to downsize to survive in this more hostile environment.
The reasons our ancestors, the early mammals survived, is that our earliest ancestors were shrew like creatures not bigger than a few centimeters. Given their small size they could survive on the few plants that survived the initial impact of the meteorite while their larger counterparts perished. When the earth finally recovered our small mammalian ancestors evolved into the large carnivores, herbivores and primates that dominate the planet today.
Our earliest mammalian ancestors who survived the meteorite attack on earth
(5)
The Limits of Evolution II
Non Functional By Products of Evolution
A second case in which evolution fails, occurs when it creates non functional body parts for either animals or plants. In contrast to the above case, when an organism lacks time to cope with a major challenge, this second failure is generally neither fatal nor even harmful to the organism. In this situation the failure of evolution is rather minor and certainly not life threatening. However the recognition of this point has been a relatively recent development in history of biology.
Among biologists there has been a recent school of thought which insists that that while Darwinism thought is an important development, it does not explain all aspects of evolutionary history. The advocates of this approach, which are part of the Punctuated Theory of Descent insist that factors beyond what Darwin called natural selection often shaped the evolutionary process. For example they insist that not all changes in the makeup of plants and animals actually helps an organisms better survive and adjust to its environment. They argue that many changes are neutral or have little or no impact on the evolutionary fitness of plants or animals.
a.How Closely related Genes sometime Produce Non Functional Body Parts.
A classic case of this phenomenon is the existence of tits in human males. While such human features are functional for women who choose to breast feed their children, the same anatomy serves no functional role in males. However, because the gene that creates breasts and tits in females is closely linked to the genes shaping the male anatomy, evolution sometimes creates non useful or functional body parts. Male tits are thus a byproduct of larger trends in the nature of human evolution.
b. Spandrels
Finally in other cases evolution fails because of some unintended consequence of a desirable evolutionary change. This process which is called a spandrel is the development of a particular trait that is the byproduct of another functional change in the anatomy of an animal or plant. The term comes from the field of architecture and is the byproduct of architects creating a certain kind of window. To be more exact, when architects create arch windows, which serve a functional purpose, they have to fill in the space between the windows.
If you look at the following post, you will see that the area between the arches, which is called a spandrel, serves no major architectural purpose. While the arch is functional, the spandrel is not. It is just an architectural side effect of spaces that result from creating two important functional arches.
An area that is a byproduct of a functional arch
An example of a biological spandrel would be the human chin. Our human chin may not have evolved for any functional or evolutionary reason. Rather it may exist due to changes in our modern human face and diet. As we modern humans began to cook our meat and eat softer diets, we got a reduction in our jaw size and teeth. As a byproduct of these changes in the makeup of our jaws we also acquired a chin. Since this process of cooking was more pronounced among modern humans than early version of ourselves such as Neanderthal, our earliest ancestors has much smaller or even insignificant chins. If you look below at a skull of a Neanderthal face, you will see that they lack the large chin that characterizes the face of modern Homo Sapiens.
A Neanderthal man with little or no Chin
(6)
Evolution: A Controversial Topic
While today evolution is a controversial topic in American life, and many people reject it, they may nonetheless find the doctrine appealing because of its interesting and provocative insights into the appearance of animals and humans alike.
To appreciate this point of evolution, we shall look at how evolution has also shaped the makeup of our plant kingdom in the next post on evolution. As we shall soon see, many of the same principles that have shaped the ways animals and humans look, have also affected the development of our trees and bushes.
a.Are human races and the shape of Christmas Trees and oak Trees related.
While we often tend to think that appearance of humans and plants may be unconnected, we will see that the impact of natural elements such as sunshine, heat and cold which shaped the creation of different races in humans, also shaped the design and makeup of trees. For instance have you ever wondered why a Christmas tree with its pyramid shape and its major branches at its base, looks so different from a tall oak tree with a rounded canopy and a naked wooden trunk. Perhaps the factors that explain why one human race differs from another, also explains why we have different looking trees with unique shapes. But this is a topic we will explore in the next section on biology and evolutionary change.
Introduction to Evolution
(1)
Pre Darwinian Views of Evolution
Every day Views of Life
Greek and Christian Ideas of Life
(2)
Darwinian Views of Evolution
Two Key Ideas
a.
Factors Producing Change
Competition between Species
Reaction to Deficiencies in the Environment
b.
Descent from a Common Ancestor
(3)
The Search for Evidence
(4)
Two Limitations of Evolution
Rapid Environmental Change and Spandrels
|
.png){kind=link}
