Evolution

Scientific background information

What is Evolution?

Evolution refers to the processes, more or less gradual, that have transformed life on Earth from its earliest forms to the vast diversity that characterizes it today. The first primitive living organisms are dated as far back as 3800 million years ago. In fact, everything in the Universe evolves, nothing stays still. We can talk about the evolution of a star, the evolution of the lithosphere… and the evolution of living organisms. It seems obvious that living beings have appeared and disappeared throughout geologic time, and that many species, nowadays with different characteristics, have common ancestors. It is known that there are species that existed in past times and are now extinct, and others which did not exist in older geologic times but that exist today. Generally speaking, evolutionary change is based on the interactions between populations of organisms and their environment. As there have been changes in the environment, different characteristics have been needed in order to adapt to such changes.

Evolutionary theories

Lamarck’s Theory of Evolution

Evolution was driven by an innate tendency towards higher and greater complexity, towards perfection. (vital force).
This innate tendency to perfection is influenced by environmental changes, therefore becoming better adapted to their environments. These environmental changes would create ‘new needs’ in organisms, needing to use certain organs or characteristics more than others.
This use and disuse of certain organs causes those organs that are used more often to cope better with the environment, making them become larger and stronger, while those that are not used deteriorate. In other words: the function makes the organ. So, organisms could acquire certain characteristics throughout its lifetime, and lose others.
Lamarck believed that the modifications an organism acquires during its lifetime are passed to its offspring (inheritance of acquired characteristics).
One species cannot transform into any other species, but only into a similar one where the species can evolve gradually. Lamarck established the fact of evolution, but his theory had several flaws:
- There is no evidence to prove that innate tendency (vital impulse) of organisms toward complexity.
- Acquired characteristics are not inherited. We all know that physical exercise will make our muscles develop, but such acquired physical characteristics (which are not genetic) are not passed down to offspring. Such acquired characteristics are produced by environmental factors and development, but not by genes. Only those characters which are regulated by genes are hereditary, and only if such genes are found in the reproductive cells. Whatever happens to somatic cells due to their use or disuse will not affect the genes in the gametes, therefore it will not be hereditary. F.L. August Weissmann (1834-1914) carried out the following experiment to refute Lamarck’s theory: He cut off the tails of fifteen hundred mice, repeatedly over 20 generations, and reporting that no mouse was ever born in consequence without a tail. According to Lamarck and his theory, the disuse of the tail should have given offspring without a tail or with an undeveloped tail, however that was never the case.

The Darwin-Wallace theory of evolution

Charles Darwin was 22 years old when he sailed from England with the Beagle in December 1831; a voyage around the world that would last five years. During the expedition Darwin collected thousands of specimens of the exotic and exceedingly diverse faunas and floras of different places. He was also able to observe the various adaptations of plants and animals that inhabited such diverse environments.

The species that inhabit the Earth today are not the same species that existed in the past, although they do resemble them. This aspect of evolution was very apparent to Darwin from the fossil record.
Each species possesses a number of characters that adapt individuals within that species to their way of life and their particular environment. Much of The Origin of Species is devoted to detailed descriptions of the adaptations of individual species, for example the various beak shapes of finches on the Galapagos Islands.
Selective breeding of domestic species can produce characters in a diversity of forms. For example, dog breeders have produced numerous breeds that differ in characters such as ear length, stature and behaviour: different breeds have different forms of a character.

Another naturalist, Alfred Wallace who was working in the East Indies, sent a manuscript to Darwin in which he developed a theory of natural selection essentially identical to Darwin’s.

In 1858 they published together their theory of Evolution, which replaced Lamarck’s evolutionary theory. Darwin explained this theory in his book The Origin of Species, where he presented his theory of natural selection as the mechanism of evolution.

Within a given species, more individuals are produced by reproduction than can survive within the constraints (e.g. food supply) imposed by the species' environment.
Consequently, there is a struggle for existence, because of the disparity between the number of individuals produced by reproduction and the number that can survive.
Individuals within a species show variation; no two individuals are exactly alike. Those with advantageous characters have a greater probability of survival, and therefore of reproducing, in the struggle for existence.
Individuals produce offspring that tend to resemble their parents (the principle of inheritance). Provided that the advantageous characters that promote survival are inherited by offspring, individuals possessing those characters will become more common in the population over successive generations because they are more likely than individuals not possessing those characters to survive and produce offspring in the next generation.

The essence of Darwin's theory is that natural selection will occur if three conditions are met. These conditions are a struggle for existence, variation and inheritance.

Some clarifications about this theory:

According to Darwin, evolution is gradual, slow and continuous: there are no sudden or brusque changes. It is a gradual accumulation of minute changes.
Environmental factors are what drive natural selection. With time, those individuals with characteristics less fit to their environment would disappear, and those with favourable variations would be perpetuated. (Survival of the fittest).
Over vast spans of time, and always due to environmental factors, a group of individuals could accumulate a considerable amount of favourable characteristics giving rise to a new species from the original population.
Still nowadays, many people mistakenly believe that it is Darwinism that explains evolution. There are many objections to the Darwinian view, (many of those objections were presented whilst he was still alive).
When Darwin had to explain what caused individual variations among organisms of a population he had to borrow Lamarck’s acquired characteristics theory (Weissman will not carry out his experiments until 1880). Mendel enunciated the laws of heredity six years after Darwin published his book, however Mendel’s work was not recognized or appreciated until 30 years later, which considerably slowed down the understanding of evolutionary mechanisms.

The modern evolutionary synthesis or neo-Darwinism

A comprehensive theory of evolution that became known as the modern synthesis or neo-Darwinism was forged in the early 1940s. This theory is not the work of one but many scientists. It is called the synthesis because it integrated discoveries and ideas from many different fields, including population genetics, paleontology, taxonomy, etc. It brings together Charles Darwin's theory of the evolution of species by natural selection with Gregor Mendel's theory of genetics as the basis for biological inheritance.

We can define evolution as the natural selection (which is seen through differential reproduction), that acts upon genetic variations (which are the outcome of mutations or sexual recombination) which appear among the members of a population.

A population is a localised group of individuals belonging to the same species, where the individuals interbreed amongst themselves, and occasionally, with members of other populations.
As a result of this closed sexual interaction in a population, a gene flow is created (that is to say that the genes of the different individuals circulate within the population, and could be transmitted to the entire population).
The genetic variations among individuals could be due to:
- mutations (this variations can take place in individuals with both types of reproduction, asexual and sexual); and
- meiotic or sexual recombination (this takes place during meiosis I, therefore, it can only happen in individuals with sexual reproduction).
Because of mutations and meiotic recombinations, individuals with new characteristics are constantly born. If those individuals survive and have offspring, their new characteristics will stay within the gene pool or group of genes of the population. In successive generations those genetic variations could be passed down to many (or all) members of the population.
Whether that would happen or not depends on natural selection (some individuals have more offspring than others). Those organisms best adapted to a given environment will be most likely to survive to reproductive age and have offspring of their own. Organisms that are successful in their environments will be more likely to be successful in reproduction, and therefore the better-adapted organisms will reproduce at a greater rate than the less well-adapted organisms. Therefore, the genes of the ones that reproduce more will predominate in the gene pool of the population. This is known as differential reproduction.
It is important to understand that it is not just how well adapted they are, but how well they are able to reproduce, in order to be able to pass down the new characters. For example, it could be that an individual is very well adapted to its environment, however, it has a mutation that makes it sterile. Would this individual have any impact in the evolution of its population?
The process of evolution has therefore two stages:
- genetic variation must occur due to mutations and/or meiotic recombinations;
- those variations must spread throughout the population by differential reproduction (natural selection) in successive generations.
Individuals do not evolve, populations do. The genetic characteristics that an individual has usually don’t vary throughout its life and once it dies they disappear. However, if the individual reproduces sexually, it could introduce genetic variations to its offspring.
The process of evolution does not follow a fixed pattern, as genetic variations happen randomly. This does not mean that evolution lacks a driving force: as the individuals that are most successful in reproduction are those that are best adapted, the driving force of evolution is natural selection. - Therefore, natural selections acts by means of differential reproduction, and not by the struggle for survival.

Examples

Evolution of the giraffe according to Lamarck:

The ancestors of the giraffe (with a short neck and short legs), also ate leaves. As the lower leaves were eaten, it had to stretch its neck to new lengths in pursuit to eat the higher leaves. Due to that ‘innate impulse’, its neck and legs began to grow. The long neck and legs of the giraffe Lamarck reasoned, evolved gradually as the cumulative product of a great many generations of ancestors stretching higher and higher, which would be inherited by offspring.

Evolution of the giraffe according to Darwin:

The variability which appeared in giraffe populations in each generation (some would stretch their neck or legs more than others) gave rise to the appearance of some individuals within the population with longer legs and a longer neck. These characters were transferred to their offspring. At first, this was not of any advantage to the individual, as there were enough low leaves on trees. However, as the lower leaves became scarse, only the individuals with the longer neck and longer legs could reach the higher leaves, which allowed them to survive through generations and have more offspring. With time it became the only type of giraffe that existed.

Evolution of the giraffe according to the modern evolutionary synthesis:

The ancestors of today’s giraffes did not have a long neck nor long front legs. By mutation and/or genetic recombination new individuals presenting a long neck and/or long legs appeared in a population. Those new individuals were better adapted to the environment. They ate more, found mating partners more easily and therefore reproduced more often. With time they became the only existing giraffes. Obviously, the individuals that presented the short neck and short legs (both negative characteristics), as they were less well adapted to the environment, they did not reproduce as successfully as the well adapted giraffes. And eventually the less favorable genes disappeared from the population.

Evolution Myths

Myth 1: Evolution produces perfectly adapted creatures

Natural selection is not all-powerful. There are many reasons that natural selection cannot produce “perfectly-engineered” traits. For example, living things are made up of traits resulting from a complicated set of trade-offs — changing one feature for the better may mean changing another for the worse (e.g., a bird with the “perfect” tail plumage to attract mates maybe be particularly vulnerable to predators because of its long tail). And of course, because organisms have arisen through complex evolutionary histories (not a design process), their future evolution is often constrained by traits they have already evolved. For example, even if it were advantageous for an insect to grow in some way other than molting, this switch simply could not happen because molting is embedded in the genetic makeup of insects at many levels. After all you don’t have to be perfectly adapted to survive, you just have to be as well adapted as your competitors.

There are many reasons why natural selection may not produce a “perfectly-engineered” trait. For example, you might imagine that cheetahs could catch more prey and produce more offspring if they could run just a little faster. For example, here are a few reasons why natural selection might not produce perfection or faster cheetahs:

Lack of necessary genetic variation
Selection can only operate on the available genetic variation. A cheetah might run faster if it had “faster” alleles — but if faster alleles are not in the population from mutation or gene flow, evolution in this direction will not happen.
Constraints due to history
Perhaps a different arrangement of leg muscles and bones would produce cheetahs that run faster — however, the basic body form of mammals is already laid out in their genes and development in such a mutually constrained way, that it is unlikely to be altered. There really may be “no way to get there from here.”
Trade-offs
Changing one feature for the better might change another for the worse. Perhaps faster alleles exist in the cheetah population — but there is a trade-off associated with them: the alleles produce cheetahs with longer legs (and hence more speed), but these long legs are hazardously delicate. Although longer limb bones increase stride, their chances of failing due to bending loads increases as well. In this case, perhaps no net increase in fitness would result from the faster alleles.

Myth 2: Evolution is just a theory

In science, a theory is an explanation for a phenomenon that can be tested and verified by the scientific method. It includes multiple sources of evidence that build up into a framework of understanding that can change over time to account for new discoveries and new evidence.

Theories are the explanations that science seeks to find. In everyday use the word is rather less concrete.

The misconception “Evolution is just a theory” stems from a mix-up between casual and scientific use of the word theory. In everyday language, theory is often used to mean a hunch with little evidential support. “Scientific theory” on the other hand, does not imply uncertainty. It is a coherent group of general propositions used as principles of explanation for a class of phenomena. In order to be accepted by the scientific community, a theory must be strongly supported by many different lines of evidence. In the case of the theory of evolution, the following are some of the evidence:

Life appeared on earth more than two billion years ago;
Life forms have changed and diversified over life's history;
Species are related via common descent from one or a few common ancestors;
Natural selection is a significant factor affecting how species change.

The theory of evolution has proved itself in practice. It has useful applications in epidemiology, pest control, drug discovery, and other areas.

Besides the theory, there is the fact of evolution, the observation that life has changed greatly over time. The fact of evolution was recognized even before Darwin's theory. The theory of evolution explains the fact.

If "only a theory" someone should also reject the theory of gravity, atomic theory, the germ theory of disease, and the theory of limits (on which calculus is based). The theory of evolution is no less valid than any of these. Even the theory of gravity still receives serious challenges. Yet the phenomenon of gravity, like evolution, is still a fact.

So, evolution is a well-supported and broadly accepted scientific theory; it is not ‘just’ a hunch.

Myth 3: Evolution follows a preordained, straight path (or if human evolved from apes, why are there still apes?)

Another misconception is that evolution is a strictly linear process — that is, it occurs in a straight line from primitive to advanced.

**Figure 1.** Cladogram: Bird branch of the Amniota clade

Originating with Plato and Aristotle the traditional view of how the world was organized was through a “progression in perfection.” This concept is explicit in the idea of the “scala naturae”: All beings on earth, animate and inanimate, could be organized according to an increasing scale of perfection from, say, mushrooms at the bottom up through lobsters and rabbits, all the way to human beings at the top. This view gets three main things wrong:

It holds that nature is organized hierarchically. It is not a random assortment of beings.
It envisions two organizing criteria: things progress from simple to perfect and from primitive to modern.
It supposes there are no intermediary stages between levels in this hierarchy. Each level is a watertight compartment of similar complexity. No one is halfway between two steps.

The truth is that we didn't evolve from any of the any animals that are alive today. Humans didn’t evolve from the gorillas or the chimpanzees we see at the zoo and is a common misconception that apes are a step away from becoming human. According to Darwin, all current organisms are equally evolved and are all still affected by natural selection. So, a starfish and a person, for example, are both at the forefront of the evolution of their particular building plans. And they happen to share a common ancestor that lived about 580 million years ago. That means humans descended from common (and now extinct) ape ancestors that lived millions of years ago. The bottom line is that all humans are apes and, as such, all humans are related to other apes.

Darwin’s theory doesn’t presuppose any special direction in evolution. It assumes gradual change and diversification. And, as evolution is still operating today, all present organisms are the most evolved of their kind.

Evolution is not a linear process that starts with more “primitive” looking organisms we can observe today, and ends in mankind (as shown in Figure above). A cladogram captures the most important aspect of the evolutionary process: “branching,” or what biologists refer to as cladogenesis. Cladogenetic events are the moments in time during which one species “splits” into two species – these events are also known as speciation events. The branching nature of cladogenesis has an important consequence: Because two or more new species always originate from an ancestor species (and this process has been occurring since the origin of life), any two species we observe in the present are related.

Myth 4: Evolution is unscientific because it is not testable or falsifiable. It makes claims about events that were not observed and can never be re-created.

The Science of Evolution is divided into at least two areas: microevolution and macroevolution.

Microevolution looks at changes within species over time that may be preludes to speciation, the origin of new species.
Macroevolution studies how taxonomic groups above the level of species change. Its evidence draws frequently from the fossil record and DNA comparisons to reconstruct how various organisms may be related.

These days even most creationists acknowledge that microevolution has been upheld by tests in the laboratory (as in studies of cells, plants and fruit flies) and in the field (as in the Grants' studies of evolving beak shapes among Galpagos finches). Natural selection and other mechanisms (chromosomal changes, symbiosis and hybridization) can drive profound changes in populations over time.

Macroevolutionary study involves inference from fossils and DNA rather than direct observation. Yet in the historical sciences (which include astronomy, geology and archaeology, as well as evolutionary biology), hypotheses can still be tested by checking whether they accord with physical evidence and whether they lead to verifiable predictions about future discoveries. For instance, evolution implies that between the earliest known ancestors of humans (roughly five million years old) and the appearance of anatomically modern humans (about 200,000 years ago), one should find a succession of hominin creatures with features progressively less apelike and more modern, which is indeed what the fossil record shows. But one should not (and does not) find modern human fossils embedded in strata from the Jurassic period (65 million years ago). Evolutionary biology routinely makes predictions far more refined and precise than this, and researchers test them constantly.

Myth 5: Species are immutable.

The notion that living species of animals and plants are immutable is probably as old as humankind. A casual observation of the natural world does not readily suggest that species evolve. This is because human life spans are too short to witness these events directly. Speciation might take centuries. Furthermore, recognizing a new species during a formative stage can be difficult because biologists sometimes disagree about how best to define a species.

Nevertheless, the scientific literature does contain reports of apparent speciation events in plants, insects and worms. In most of these experiments, researchers subjected organisms to various types of selection—for anatomical differences, mating behaviours, habitat preferences and other traits—and found that they had created populations of organisms that did not breed with outsiders.

Though we can’t run an experiment that will tell us how the dinosaur lineage radiated, we can study many aspects of evolution with controlled experiments in a laboratory setting. In organisms with short generation times (e.g., bacteria or fruit flies), we can actually observe evolution in action over the course of an experiment. The Evolution of Bacteria on a The Evolution of Bacteria on a “Mega-Plate” Petri Dish. In another example, William R. Rice of the University of New Mexico and George W. Salt of the University of California, Davis, demonstrated that if they sorted a group of fruit flies by their preference for certain environments and bred those flies separately over 35 generations, the resulting flies would refuse to breed with those from a very different environment. In some cases, biologists have observed evolution occurring in the wild (e.g. mosquito populations that evolve rapid resistance to DDT, antibiotic-resistant Bacteria and drug-resistant HIV can evolve resistance to drugs very rapidly).

Myth 6: The use or disuse of a structure during one’s lifetime, could be inherited by its offspring and lead to a change in a species.

Lamarck proposed a theory of evolution based on the principle that physical changes in organisms during their lifetime—such as greater development of an organ or a part through increased use—could be transmitted to their offspring.

Biologists define an acquired characteristic as one that has developed in the course of the life of an individual in the somatic or body cells, usually as a direct response to some external change in the environment or through the use or disuse of a part. The inheritance of such a characteristic means its reappearance in one or more individuals in the next or in succeeding generations. but there is no evidence supporting this case.

As well as growing, individual organisms may develop particular skills or physical characters during the course of their lives as a result of differences in the way they live. For example, the human practices of ear-piercing, circumcision and decorative body scars. These characters, which are acquired deliberately during the course of an individual's life, are not inherited by that individual's offspring even though the practice may have been carried out for hundreds of generations. Likewise, a plant that has grown particularly large in a patch of good ground, or a toad that has grown very big because it lives in a garden full of food, will not pass their large size on to their progeny. So, inheritance of acquired characters does not occur.

Another example would be found in the supposed inheritance of a change brought about by the use and disuse of a special organ. The blacksmith’s arm (or any other set of muscles) enlarges when used continually against an external resistance, such as the weight of the hammer. If the effect were inherited, the smith’s children at birth would have unusually large arms—if not at birth, then when they became adults, even though they had not used their arms excessively. There is no evidence supporting this case.

A more subtle illustration is found in the supposed inheritance of an increased dexterity of the hands of a musician through practice. The skill acquired, although causing no visible increase in the size of the fingers, might be imagined to be passed along to the musician’s children, and they might then be expected to play skilfully with minimal practice. Just how the intricate interplay of cerebral sequences that has given the dexterity to the musician’s fingers could ever be transferred to the musician’s sex cells (spermatozoa or ova), and through them to any potential children, has never been brought within the range of biological possibilities.

Evolution misconceptions

On occasion, teachers find themselves on the receiving end of questions about the inclusion of evolution in the curriculum. Students may bring up challenges in class. Parents may object to their child learning about evolution. School administrators or other teachers may fail to support teachers in their efforts to teach evolution. Community members may present intentional challenges to individual teachers, to school districts, or even to statewide entities by attempting to influence science education standards or by changing legislation.

All teachers, even those in communities thoroughly supportive of teaching evolution, should keep in mind that some students perceive evolution to be incompatible with religious faith. Although many religious views are compatible with evolutionary theory and although many religious organizations support the teaching of evolution, students may be unaware of these facts.

The perception of a clash between science and students’ beliefs can cause discomfort in class. To make these students more comfortable, teachers can help them understand that evolution, like all of science, seeks to explain natural things through natural causes. It need not be considered incompatible with their faith because science does not rely on, and cannot evaluate or test, supernatural explanations. At the same time, your teaching should reflect the fact that evolution is the only scientifically valid and accepted theory that accounts for our observations of the biological world. Alternative “theories” that have been proposed for insertion into the science curriculum have not been supported by valid science.

Below are presented some of the common misconceptions about evolution and how teachers can address them.

Misconception 1: Evolution is a theory about the origin of life

Evolutionary theory does encompass ideas and evidence regarding life’s origins (e.g., whether or not it happened near a deep-sea vent, which organic molecules came first, etc.), but this is not the central focus of evolutionary theory. Most of evolutionary biology deals with how life changed after its origin. Regardless of how life started, afterwards it branched and diversified, and most studies of evolution are focused on those processes.

Misconception 2: Evolutionary theory implies that life evolved (and continues to evolve) randomly, or by chance.

Some important mechanisms of evolution are non-random and these make the overall process non-random.

For example, natural selection, results in adaptations (e.g., the ability of bats to echolocate). Such amazing adaptations clearly did not come about “by chance.” They evolved via a combination of random and non-random processes.

The process of mutation, which generates genetic variation, is random, but selection is non-random. Selection favoured variants that were better able to survive and reproduce (e.g., to navigate in the dark). Over many generations of random mutation and non-random selection, complex adaptations evolved.

Misconception 3: Individual organisms can evolve during a single lifespan.

Evolutionary change is based on changes in the genetic makeup of populations over time. Populations, not individual organisms, evolve. New gene variants (i.e., alleles) are produced by random mutation, and over the course of many generations, natural selection may favour advantageous variants, causing them to become more common in the population.

Misconception 4: Because evolution is slow, humans cannot influence it.

Since humans often cause major changes in the environment, we are frequently the instigators of evolution in other organisms. Some examples of human-caused evolution:

Insects like bedbugs and crop pests have evolved resistance to pesticides.
Bacteria, HIV, malaria, and cancer have evolved resistance to drugs.

Misconception 5: Humans are not currently evolving.

Humans are now able to modify our environments with technology. We have invented medical treatments, agricultural practices, and economic structures that significantly alter the challenges to reproduction and survival faced by modern humans. So, for example, because we can now treat diabetes with insulin, the gene versions that contribute to juvenile diabetes are no longer strongly selected against in developed countries. Some have argued that such technological advances mean that we have stopped evolving.

However, this is not the case. Humans still face challenges to survival and reproduction, just not the same ones that we did 20,000 years ago. The direction, but not the fact of our evolution has changed. For example, modern humans living in densely populated areas face greater risks of epidemic diseases than did our hunter-gatherer ancestors (who did not come into close contact with so many people on a daily basis) — and this situation favours the spread of gene versions that protect against these diseases. Scientists have uncovered many such cases of recent human evolution. Explore these links to learn about:

Misconception 6: Natural selection involves organisms trying to adapt.

Natural selection leads to the adaptation of species over time, but the process does not involve effort, trying, or wanting. Natural selection naturally results from genetic variation in a population and the fact that some of those variants may be able to leave more offspring in the next generation than other variants. That genetic variation is generated by random mutation — a process that is unaffected by what organisms in the population want or what they are “trying” to do. Either an individual has genes that are good enough to survive and reproduce, or it does not; it can’t get the right genes by “trying.”

For example, bacteria do not evolve resistance to our antibiotics because they “try” so hard. Instead, resistance evolves because random mutation happens to generate some individuals that are better able to survive the antibiotic, and these individuals can reproduce more than other, leaving behind more resistant bacteria.

Misconception 7: Humans can’t negatively impact ecosystems, because species will just evolve what they need to survive.

Natural selection does not automatically provide organisms with the traits they “need” to survive. Of course, some species may possess traits that allow them to thrive under conditions of environmental change caused by humans and so may be selected for, but others may not and so may go extinct.

If a population or species doesn’t happen to have the right kinds of genetic variation, it will not evolve in response to the environmental changes wrought by humans, whether those changes are caused by pollutants, climate change, habitat encroachment, or other factors.

For example, as climate change causes the Arctic Sea ice to thin and break up earlier and earlier, polar bears are finding it more difficult to obtain food. If polar bear populations don’t have the genetic variation that would allow some individuals to take advantage of hunting opportunities that are not dependent on sea ice, they could go extinct in the wild.

Misconception 8: The fittest organisms in a population are those that are strongest, healthiest, fastest, and/or largest.

In evolutionary terms, fitness has a very different meaning than the everyday meaning of the word. An organism’s evolutionary fitness does not indicate its health, but rather its ability to get its genes into the next generation. The more fertile offspring an organism leaves in the next generation, the fitter it is. This doesn’t always correlate with strength, speed, or size.

For example, a puny male bird with bright tail feathers might leave behind more offspring than a stronger, duller male, and a spindly plant with big seed pods may leave behind more offspring than a larger specimen — meaning that the puny bird and the spindly plant have higher evolutionary fitness than their stronger, larger counterparts.

Misconception 9: Evolution is not science because it is not observable or testable.

that all science depends on controlled laboratory experiments, and
that evolution cannot be studied with such experiments.

First, many scientific investigations do not involve experiments or direct observation. Astronomers cannot hold stars in their hands and geologists cannot go back in time, but both scientists can learn a great deal about the universe through observation and comparison. In the same way, evolutionary biologists can test their ideas about the history of life on Earth by making observations in the real world.

Second, though we can’t run an experiment that will tell us how the dinosaur lineage radiated, we can study many aspects of evolution with controlled experiments in a laboratory setting. In organisms with short generation times (e.g., bacteria or fruit flies), we can actually observe evolution in action over the course of an experiment. The Evolution of Bacteria on a “Mega-Plate” Petri Dish. In some cases, biologists have observed evolution occurring in the wild.

Misconception 10: Evolutionary theory is invalid because it is incomplete and cannot give a total explanation for the biodiversity we see around us.

This misconception stems from a misunderstanding of the nature of scientific theories. All scientific theories (from evolutionary theory to atomic theory) are works in progress. As new evidence is discovered and new ideas are developed, our understanding of how the world works changes and so too do scientific theories. While we don’t know everything there is to know about evolution (or any other scientific discipline, for that matter), we do know a great deal about the history of life, the pattern of lineage-splitting through time, and the mechanisms that have caused these changes. And more will be learned in the future.

Evolutionary theory, like any scientific theory, does not yet explain everything we observe in the natural world. However, evolutionary theory does help us understand a wide range of observations (from the rise of antibiotic-resistant bacteria to the physical match between pollinators and their preferred flowers), does make accurate predictions in new situations (e.g., that treating AIDS patients with a cocktail of medications should slow the evolution of the virus), and has proven itself time and time again in thousands of experiments and observational studies. To date, evolution is the only well-supported explanation for life’s diversity.

Misconception 11: Gaps in the fossil record disprove evolution.

While it’s true that there are gaps in the fossil record, this does not constitute evidence against evolutionary theory. Scientists evaluate hypotheses and theories by figuring out what we would expect to observe if a particular idea were true and then seeing if those expectations are borne out. If evolutionary theory were true, then we’d expect there to have been transitional forms connecting ancient species with their ancestors and descendents.

This expectation has been borne out. Paleontologists have found many fossils with transitional features, and new fossils are discovered all the time. However, if evolutionary theory were true, we would not expect all of these forms to be preserved in the fossil record. Many organisms don’t have any body parts that fossilize well, the environmental conditions for forming good fossils are rare, and of course, we’ve only discovered a small percentage of the fossils that might be preserved somewhere on Earth. So, scientists expect that for many evolutionary transitions, there will be gaps in the fossil record.

Misconception 12: Evolution leads to immoral behaviour.

Evolution does not make ethical statements about right and wrong. Some people misinterpret the fact that evolution has shaped animal behaviour (including human behaviour) as supporting the idea that whatever behaviours are “natural” are the “right” ones. This is not the case. It is up to us, as societies and individuals, to decide what constitutes ethical and moral behaviour. Evolution simply helps us understand how life has changed and continues to change over time — and does not tell us whether these processes or the results of them are “right” or “wrong”. Furthermore, some people erroneously believe that evolution and religious faith are incompatible and so assume that accepting evolutionary theory encourages immoral behaviour. Neither are correct.

Misconception 13: Evolution and religion are incompatible.

Because of some individuals and groups stridently declaring their beliefs, it’s easy to get the impression that science (which includes evolution) and religion are at war; however, the idea that one always must choose between science and religion is incorrect. People of many different faiths and levels of scientific expertise see no contradiction at all between science and religion. For many of these people, science and religion simply deal with different realms. Science deals with natural causes for natural phenomena, while religion deals with beliefs that are beyond the natural world. Of course, some religious beliefs explicitly contradict science (e.g., the belief that the world and all life on it was created in six literal days does conflict with evolutionary theory); however, most religious groups have no conflict with the theory of evolution or other scientific findings. In fact, many religious people, including theologians, feel that a deeper understanding of nature actually enriches their faith. Moreover, in the scientific community there are thousands of scientists who are devoutly religious and also accept evolution.

Misconception 14: Teachers should teach “both sides” of the evolution issue and let students decide. Or teachers should give equal time to evolution and creationism.

Equal time does not make sense when the two “sides” are not equal. Religion and science are very different endeavours, and religious views do not belong in a science classroom at all. In science class, students should have opportunities to discuss the merits of arguments and evidence within the scope of science. For example, students might investigate and discuss exactly where birds branched off of the phylogenetic tree: before dinosaurs or from within the dinosaur clade. In contrast, a debate pitting a scientific concept against a religious belief has no place in a science class and misleadingly suggests that a “choice” between the two must be made. The “fairness” argument has been used by groups attempting to insinuate their religious beliefs into science curricula.