Evidence and Mechanisms of Biological Evolution
Evidence of Evolution
Biological evolution is possibly the most important process that affects all living creatures on Earth. This process does not shine directly on certain living beings, since it is a process that lasts a long time and takes thousands or millions of years to manifest. Nevertheless, it is an unstoppable process that began with the appearance of life and, since then, has lost none of its vigor. We can be more certain of the existence of this process in the past because, as we just saw, evolution cannot be proved at present because of its extreme slowness. This certainty, however, can be obtained from a series of events that are going to prove its existence.
Biogeographical Evidence
Biogeographical evidence is found scattered all over the world. It consists of the existence of groups of species, more or less similar and related, living in places linked together by their proximity, location, or characteristics. For example, a set of islands where each species group has adapted to specific conditions. Evolutionary testing occurs because all these species come from a nearby single ancestor species that gave rise to all others as small groups of individuals adapted to the conditions of a particular place, which were different from other places. Typical examples of this are the Galapagos finches studied by Darwin, Drepano birds of the Hawaiian Islands, or the large flightless birds distributed throughout the southern hemisphere: South American rheas, ostriches in Africa, the elephant bird of Madagascar (extinct), the cassowary and emu of Australia, or the giant moa of New Zealand (also extinct).
Paleontological Evidence
The study of paleontological evidence from fossils gives us a very direct idea of the changes species were undergoing to be transformed into each other. There are many series of fossils of plants and animals that allow us to reconstruct how they were adapting to changing environmental conditions, such as the series of English sea urchins, the reptiles to birds passing through the Archaeopteryx, or the evolution of horses to adapt to the Great Plains that opened up by running.
Anatomical Evidence
Anatomical evidence is perhaps what provides us with the most information because it is a direct reflection of environmental adaptations. In many living beings, there are atrophied, non-functional bodies that appear in ancient ancestors as perfectly functional, but that over the generations were no longer useful. These bodies are called vestigial organs. For example, in humans, the appendix, wisdom teeth, body hair, or the coccyx; in other animals, the vestigial hind ankle of whales and pythons, vestigial leg bones of horses, and vestigial wings of ostriches and penguins. On the other hand, the study of the anatomy of different species teaches us that there are many that are very similar, and species that are evolutionarily close, separated by a different adaptation to different media. In other words, organs and organ structures have very similar anatomy since they have the same evolutionary origin but have different functions. They are what is called homologous organs, such as the fin of a dolphin and a bat’s wing. These are organs with the same internal structure, but one is for swimming and one for flying. At the same time, there are evolutionarily widely separated species that must adapt to the same environment, and therefore develop similar structures, but no evolutionary relationship. These are called similar bodies, which are anatomical patterns that have been successful in a concrete way and so it mimics several species, like the wing of an insect and a bird’s wing. Similar bodies represent a phenomenon called adaptive convergence, by which living things repeat formulas and designs that have been successful. They represent the counterparts of adaptive divergence, by which living beings shape their bodies according to their way of life, the environment they’re in, etc.
Embryological Evidence
Related to anatomical evidence, the study of vertebrate embryos gives us an interesting insight into the evolutionary development of groups of animals, since the early stages of this development are the same for all vertebrates. It is impossible to differentiate between them. Only by moving forward in the process does each group of vertebrates have an embryo different from the rest, being much more similar as they are more closely related species. This is what Haeckel summed up by saying that “ontogeny recapitulates phylogeny.”
Biochemical Evidence
Finally, the latest and most likely evidence consists of comparing certain molecules that occur in all living things so that these molecules are much more alike the smaller the developmental differences between their owners and vice versa. This has been done mostly with proteins (e.g., blood proteins) and DNA.
Operation of Evolution
Living beings are what we are thanks to the genetic information we have stored in our cells. This information has been more or less shaped by the environment in which we live, which can naturally modify the genetic information over the life of a living being, but the changes it produces will never be passed on to our descendants. The only thing we pass on to our children is our genes.
- Genetic information and the environment are the basis of evolution.
At first, living things of the same species and the same population should have identical genetic information, the same genes, and the same alleles (see the discussion of Genetics). All individuals would, in principle, be equally adapted to their environment, unless there are individual environmental differences (for example, those who feed more will be stronger). The question is, why do different individuals come in time within populations? In a population of bears, at first, they would all have short hair; no long-haired bears exist. How did the long hair come about? The answer to these questions is in the genetic mutations that make a gene change enough to remain the same gene, but result in a somewhat different character, thus becoming what is called an allele. For example, bears only had information for short hair, but by a mutation, an allele arises that carries information to get the hair a little longer.
- When a human being is born, they develop a series of characters for which they have genetic information, and these characters are shaped by the environment in which they live.
Any being lives better or worse in the place where it lives according to the characters it has developed. So, for example, if you have a thick covering of hair, you will stand the cold well; if you have agility to climb trees, you will escape predators; and if you can swim, you will not drown when crossing a river. This ability to live better or worse is what we call adaptation to the environment. Those that are better adapted live better, eat well, escape from predators, live longer, and, therefore, will have more offspring and leave more offspring to the next generation who will carry their genes. This is the survival of the fittest.
- Beings better adapted to their environment leave more offspring to the next generation.
In a negative sense, individuals who are less well adapted live less and leave fewer offspring, so that after several generations their genes tend to disappear, leaving only the genes that are a better fit. In other words, nature selects the best genes for a particular environment; this is what we call natural selection. In the example of the bears, in a hot medium, short-haired bears live better than those with long hair, and it will be somewhat hot, which will affect their daily lives (run less, be more tired, etc.). Short-haired bears live longer and better and leave more offspring to subsequent generations. Eventually, fewer bears will be born with long hair. If at any given time there is a sustained change in the environment in which a population lives, everything will change, and the best-adapted individuals may cease to be, and, conversely, those who previously lived worse and left few descendants may now be the best adapted. In this case, natural selection will act now, favoring those who it harmed before. If the place where our bears live is getting colder, long-haired bears who used to live worse are now going to become the fittest, and short-haired bears who used to live better now do not bear the cold, live worse, and leave fewer offspring, changing the evolutionary trend. After many generations, the population will have disappeared from the alleles of short hair; all bears are long-haired, and the bear species have changed slightly. Now maybe we have a new subspecies characterized by having long, thick hair to protect it from the cold.
Evolutionary Forces
As we have seen, the main evolutionary force is genetic mutations, which are responsible for most of the genetic variability of populations, but they are not the only evolutionary force that acts, as there are others that are also very important:
- Sexual reproduction, which is responsible for the mixing of genes and alleles in individuals.
- The number of individuals in the population, because if the population is very small, genetic changes occur more quickly (genetic drift).
- Movements of individuals, migration, altering the set of genes and alleles of the population.
- And, of course, natural selection, which will select those most favorable genetic combinations in that environment, making those individuals best adapted to produce more individuals and their biological effectiveness is greater.
Microevolution and Macroevolution
Sometimes natural selection acts by favoring alleles that result in major changes in populations, so that over time new species can arise similar to the above, what we call microevolution. Or groups of living things may become completely different species, even earlier extinct, what is called macroevolution. It all depends on mutations arising alleles or new genes, suggesting the existence of characters very different from those existing, and that these different characters are selected by mean better adaptation to the environment.
Speciation
When a population changes its genetic information by mutations, the combination of sexual reproduction and natural selection favors new genetic combinations. Eventually, the population will belong to the species and will become a new species; this is what we call speciation. This process is slow and gradual, as the Darwinists and neo-Darwinism say, or is quick and sharp, as the “saltationists” or punctuated equilibrium say.