Evolution for beginners

Biological evolution is still not well understood by general public, and when we speak of it in our language abound expressions that confuse even more how mechanisms that lead to species diversity work. Through questions you may have ever asked yourself, in this article we will have a first look at the basic principles of evolution and debunk misconceptions about it.

IS EVOLUTION REAL? IT IS NOT JUST A THEORY OR AN IDEA WITHOUT EVIDENCES?

Outside the scientific field, the word “theory” is used to refer to events that have not been tested or assumptions. But a scientific theory is the explanation of a phenomenon supported by evidence resulting from the application of the scientific method.

The scientific method. Image by Margreet de Heer.

Theories can be modified, improved or revised if new data don’t continue to support the theory, but they are always based on some data, repeatable and verifiable experiments by any researcher to be considered valid.

So few people (sic) doubts about the heliocentric theory (the Earth rotates around the Sun), or the gravitational theory of Newton, but in the popular imagination some people believe that the theory of evolution made by Charles Darwin (and Alfred Russell Wallace) is simply a hypothesis and has no evidence to support it. With new scientific advances, his theory has been improved and detailed, but more than 150 years later, nobody has been able to prove it wrong, just the contrary.

WHAT EVIDENCE WE HAVE THAT EVOLUTION IS TRUE?

We have many evidences and in this post we will not delve into them. Some of the evidence available to us are:

• Paleontological record: the study of fossils tell us about the similarities and differences of existing species with others thousands or millions old, and to establish relationships respect each other.
• Comparative anatomy: comparison of certain structures that are very similar between different organisms, can establish whether they have a common ancestor (homologous structures, for example, five fingers in some vertebrates) if they have developed similar adaptations (analogous structures, for example, the wings of birds and insects), or if they have lost their function (vestigial organs, such as the appendix).

Homologous organs in humans, cats, whales and bats

• Embryology: the study of embryos of related groups shows a strong resemblance in the earliest stages of development.
• Biogeography: The study of the geographical distribution of living beings reveals that species generally inhabit the same regions as their ancestors, although there are other regions with similar climates.
• Biochemistry and genetics: chemical similarities and differences allow to establish relationships among different species. For example, species closely related to each other have a structure of their DNA more similar than others more distant. All living beings share a portion of DNA that is part of your “instructions”, so there are also found in a fly, a plant or a bacterium, proof that all living things have a common ancestor.

IS IT TRUE THAT ORGANISMS ADAPT TO THE ENVIRONMENT AND ARE DESIGNED FOR LIVING IN THEIR HABITAT?

Both expressions, frequently used, mean that living beings have an active role to adapt to the environment or “someone” has designed them to live exactly where they are. It is a typical example of Lamarck and giraffes: as a result of stretching the neck to reach the higher leaves of the trees,  currently giraffes have this neck for giving it this use. They have a necessity, they change their bodies to success. It is precisely upside down: it is the habitat that selects the fittest, nature “selects” those that are most effective to survive, and therefore reproduce. It is what is known as natural selection, one of the main mechanisms of evolution. It needs three requirements to act:

• Phenotypic variability: there must be differences between individuals. Some giraffes necks were slightly longer than others, just as there are taller people than others, with blue or brown eyes.
• Biological fitness: this difference has to suppose an advantage. For example, giraffes with a slightly longer neck could survive and reproduce, while the others don’t.
  
• Heredity: these characters must be transmitted to the next generation, the offspring will be slightly different to that feature, while “short neck” feature transmits less and less.

The variability in the population causes individuals with favorable characteristics to reproduce more and pass on their genes to the next generation, increasing the proportion of those genes. Image taken from Understanding evolution

Over the years these changes are accumulated until the genetic differences are so big that some populations may not mate with others: a new species has appeared.

If you thought that this is similar to artificial selection that we do with the different breeds of dogs, cows who give more milk, trees bearing more fruit and larger, congratulations, you think like Darwin as it was inspired by some of these facts. Therefore, living beings are mere spectators of the evolutionary process, depending of changes in their habitat and their genetic material.

WHY ORGANISMS ARE SO DIVERSE?

Genetic variability allows natural selection act. Changes in the genetic material (usually DNA) are caused by:

• Mutations: changes in the genome that may be adverse or lethal for survival, indifferent or beneficial to survival and reproduction. If they have benefits, they will pass to the next generations.
• Gene flow: is the motion of genes between populations (migration of individuals allows this exchange when mate with others in a different population).
• Sexual reproduction: allows recombination of genetic material of different individuals, giving rise to new combinations of DNA.

Populations that have more genetic variability are more likely to survive if happen any changes in their habitat. Populations with less variability (eg, being geographically isolated) are more sensitive to any changes in their habitat, which may cause their extinction.

Evolution can be observed in beings with a very high reproduction rate, for example bacteria, since mutations accumulate more quickly. Have you ever heard that bacteria become resistant to our antibiotics or some insects to pesticides? They evolve so quickly that within a few years were selected the fittest to survive our antibiotics.

ARE WE THE MOST EVOLVED ANIMALS?

Theory of Evolution has various consequences, such as the existence of a common ancestor and that therefore, that we are animals. Even today, and even among the young ones, there is the idea that we are something different between living beings and we are in a special podium in the collective imagination. This anthropocentric thinking caused Darwin mockery and confrontations over 150 years ago.

Caricature of Darwin as an orangutan. Public domain image first published in 1871

We use our language to be “more evolved” as a synonym for more complex, and we consider ourselves one species that has reached a high level of understanding of their environment, so many people believe that evolution has come to an end with us.

The question has a mistake of formulation: actually evolving pursues no end, it just happens, and the fact that millions of years allows the emergence of complex structures, it does not mean that simpler lifeforms are not perfectly matched in the habitat where they are. Bacteria, algae, sharks, crocodiles, etc., have remained very similar over millions of years. Evolution is a process that started acting when life first appeared and continues to act in all organisms, including us, although we have changed the way in which natural selection works  (medical and technological breakthroughs, etc.).

SO IF WE COME FROM MONKEYS, WHY DO STILL MONKEYS EXIST?

The truth is that we don’t come from monkeys, we are monkeys, or to be more rigorous, apes. We have not evolved from any existing primate. As we saw in a previous post, humans and other primates share a common ancestor and natural selection has been acting differently in each of us. That is, evolution has to be viewed as a tree, and not as a straight line, where each branch would be a species .

First scheme of the evolutionary tree of Darwin in his notebook (1837). Public domain image.

Some branches stop growing (species become extinct), while others continue to diversify. The same applies to other species, in case you have asked yourself, “if amphibians come from fish, why are there still fish?”. Currently, genetic analyzes have contributed so much data that they make so difficult to redesign the classical Dariwn’s tree.

Classification of live organisms based on the three domains Archaea, Bacteria and Eukarya, data of Carl R. Woese (1990). Included in Eukarya there are the Protista, Fungi, Plantae and Animalia kingdoms. Image by Rita Daniela Fernández.

Evolution is a very broad topic that still generates doubts and controversies. In this article we have tried to bring to uninitiated people some basics, where we can delve into the future. Do you have any questions about evolution? Are you interested into a subject that we have not talked about? You can leave your comments below.

REFERENCES

• Darwin’s Tree of Life is a Tangled Bramble Bush
• Algunas reflexiones sobre la clasificación de los seres vivos
• Museos vivos. Evidencias de la evolución
• Las ideas en la ciencia: Teoría, hipótesis y leyes
• Frequently asked questions about evolution
• Brock et. al. 1999. Biología de los microorganismos. Ed. Prentice Hall.
• Massa, Renato. El origen de la vida. La evolución de las especies. Ed. Susaeta.
• Cover image