The first of the issues on evolution that will be addressed in this lesson plan is the timescale of species change. Human evolution is commonly understood as beginning 2.5 millions of years ago with the appearance of the Homo Habilis, the first beings considered as belonging to the genus homo, within which we (Homo Sapiens Sapiens) are a subspecies of the species Homo Sapiens. For example, Homo Neandethalensis (Neanderthal Man) is a different species of Homo Sapiens. The distribution of a trait throughout a population of a particular species has usually also required extremely long timescales, but there are several interesting instances of distinct evolution having been observed in the time lapse of short-term scientific observations.
One of the quickest evolutions on record is the case of the Tropical Blue Moon Butterfly. (Click here for a picture to show to the class.) The butterflies carry a parasitic bacteria that was killing most male embryos before they hatched, reducing the male population from 50% to a mere 1% of the total. Due to a mutation, the genetic information of the butterfly started to include a gene that allowed the butterflies to produce an anti-bacterial chemical or somehow keep the bacteria in check. This led to a larger proportion of males carrying this gene and a rapid spread of the gene in successive generations of butterflies. The male population is now back to almost half of all specimens. Butterfly shows evolution at work from the BBC News, July 12, 2007) and But Madame Butterfly, Where are all the Males? by Sourish Basu, Scientific American, July 13, 2007.
There are also observations indicating that there has been a noticeable change in the size of adult fish due to man-induced selection. Regulations limit the size of the fish that may be caught. Smaller fish must be left in the sea or thrown back, which allows smaller adults an opportunity to breed. It is considered a possibility that the catching of the larger fish of highly consumed species is inducing an evolutionary pattern towards a smaller adult size. See Human-induced evolution caused by unnatural selection through harvest of wild animals Fred W. Allendorf and Jeffrey J. Hard, Proceedings of the National Academy of Sciences of the United States). Some call this “unnatural”, human-induced selection. But hasn’t it always been true that a dominant species could affect the environment, and therefore the natural selection, of other species?
Most mutations do not result in changes that are beneficial to the individual and the individual dies before birth, leading to a miscarriage, or if it lives, it will not have offspring and the mutation will die out. However, if the change caused by the mutation is one of the few that gives the individual an advantage in relation to the environment in which it lives, the individual may be more successful in leaving offspring – now with the mutated gene, which in turn produce a new group of organisms that are more apt to survive. If the advantage is great enough, over what is usually a long period of time, most members of the species will come to have that mutation. Strictly speaking it cannot be said that a particular species has “developed” a new feature. Rather, the competition for survival has selected those individuals that are more fit through a process called natural selection.
Since the environment is always changing, what is beneficial is always changing as well. An interesting example of the interplay between environment and natural selection has been the recent acquisition by human beings of the ability of adults to digest milk. In most animals, and in all human beings who lived more than 9,000 years ago, the ability to digest milk was for the young. It switched off in adulthood. In early hunter, gatherer societies, the only milk an individual would need would be the breast milk provided by the mother. But then, about 9,000 years ago people domesticated cattle. Once cattle were domesticated and formed an important source of food, being able to keep digesting milk increased the available food supply for those adults who carried the trait. Those adults obtained an advantage in natural selection.
The evolutionary trait of the ability by an adult to digest milk is especially interesting because it is a significant example of convergent evolution which occurs when different populations independently acquire the same new feature through evolution; over the last 6,000 years, the ability to digest milk by adults has been found to have appeared at least once in Europe and an additional three separate times in Africa.
At the time these changes were taking place another recent evolutionary development occurred. Between 10,000 and 6,000 years ago some human beings started to have blue eyes. Before that time everyone had brown colored eyes. This change has two interesting features worth noting: it is traceable to a single ancestor of all blue-eyed humans and scientists cannot figure out how the trait provides an evolutionary advantage to people who have it. There are other hereditary traits for which scientists have not found an evolutionary advantage. These include hair color, baldness, freckles and beauty spots. Even if there was no advantage conferred by these traits, natural selection played a role in their spread through the population because they are not disadvantageous. Professor Eiberg, one of the scientists who discovered that the mutation for blue eyes probably came from a single ancestor said, “it simply shows that nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so.”
It has been suggested by some scientists that human evolution has come to a halt. In modern civilized society many individuals who would have quickly perished if we had been living in the wild survive and have children. No genes are lost or gained due to natural selection as we fight to cure genetically induced diseases or defects. This issue is under debate within the scientific community and many articles and opinions can be found on either side. Some of the major points in this debate are as follows.
The main argument in favor of human evolution having come to a halt is that the pressure by natural selection has all but vanished, allowing anyone, not only the most fit, to survive and have children. No “disadvantageous” genes are lost. More subtly, it is argued that there is no room for further improvements in the human species, after medical and biological research have made it relatively easy to live into the child bearing years.
Also the increased mixing of genes due to the fact that in modern societies people meet and have children with partners from all over the world is said to produce a gene blending that blocks evolutionary changes. It is argued that this blending will end up making all humans of the planet more uniform, beginning with skin color, which is expected to become equally brown for all humans in a few generations.
Monogamy has also been considered an obstacle for evolution. In an animal herd, the fittest male would be father to a significantly large fraction of the offspring of all females. In addition, weaker females will die whereas stronger females will live longer and thus have much more offspring from those fittest males. In cultures that practice monogamy, no matter how genetically superior a male or female might be, the number of children will be limited.
Others see human evolution limited to the mind – perhaps better-prepared humans to live in a profit and technology-driven society are more likely to have children (because of their extra wealth), and the underlying genes – if this is a matter of genes at all – will spread faster. Yet others affirm that the wealthier and the professional class actually have fewer children than families in poverty and that the society will get less intelligent as a result.
A final argument that natural selection is no longer a force in developed societies is the is that it will move into areas of genetic preference. There are sperm banks that describe the donor’s occupation and personality traits. See e.g. International Cryogenics – Donor List. In the future, scientists may be able to manipulate genes so that certain traits are enhanced or suppressed. But if people are selecting for the traits that they feel will benefit their children in society, isn’t this a form of natural selection? After all, haven’t people been choosing mates for similar reasons for thousands of years?
Against the argument that human evolution has stopped it is argued that natural selection has not necessarily stopped for humans: environmental issues like increased UV radiation on the surface of the Earth due to the hole in the ozone layer could still select against those less prepared to endure the resulting altered conditions. Each individual might be looked after, but as a whole, communities with higher death rates due to, for example, melanoma (skin cancer) are less likely to have descendants. In third world areas there is still a high degree of mortality and therefore an evolutionary pressure that favors those more resistant to malnutrition, AIDS, malaria or other diseases that prevail due to poverty or geographic location.
A further argument points towards natural selection acting all the time but mostly going unnoticed because the effects on one individual are tiny. These effects would only become evident over generations. It is estimated that every human carries about two mutations with respect to the parent’s genetic code. Only very few are patently visible as diseases or dramatic changes in appearance: most of them might affect an individual’s capability to reproduce by a small percent. On the long run, however, it is argued, such genes do disappear (if they affect reproduction negatively) of spread (if they are positive for reproduction).
Finally, there is a middle ground position, acknowledging the validity of all the arguments as describing forces which affect natural selection in the modern world. In this view, natural selection continues but is being affected by the vast changes that technology and advanced forms of social organization have made in modern society.