|
Go to site main page, student resources page. |
File last modified:
Go to Essay 3, Introduction, Essay 5, |
This text was drafted by award-winning MMW TAs Tara Carter and Beth Peterson, (Anthropology) in the summer of 2008, supported by the MMW-based research funds left in his account on the death of Prof. Donald F. Tuzin, long an instructor and supporter of MMW. The text was substantially modified by Professor David K. Jordan in summer, 2011 to fit it to the needs of MMW-11.
This text may be freely reproduced for non-commercial educational purposes.
Most high school biology classes include an introduction to genetics, including cell division, meiosis, mitosis, and the way genes are moved around in the course of sexual reproduction. A review of this material is available as Appendix 1 to this set of essays. (Open in new window.)
However, genetic recombination in sexual reproduction is not the only source of variation within a population. We also have to consider mutation, gene flow, and genetic drift.
Sometimes in the process of DNA replication, which is necessary for cell division, mistakes in copying the DNA occur, called mutations. Often these mistakes are fatal. Most mutations that take place during meiosis result in genes that do not function the way they are supposed to. These mistakes may lead to a gene producing the wrong proteins, becoming activated at the wrong time, or not functioning at all. Most of the time if a gamete (sperm or egg cell) carrying a mutation becomes a part of a zygote (fertilized egg) spontaneous abortion or miscarriage takes place.
However, not all mutations are fatal, and some may merely lead to survivable developmental or physical disabilities: the geranium that drops its flowers buds before they open, the dog with no sense of smell, the fish missing a fin, and that sort of thing. And some mutations can have only very trivial effects. A very few mutations may prove to be advantageous and confer a benefit on the organism.
Non-fatal mutations produce more variation in a population, variation upon which natural selection can act. If a mutation is ultimately beneficial for an organism, it will lead to increased reproductive success, producing more descendant organisms with the beneficial mutation that are also more likely to reproduce and pass on that mutation. This can be part of the process of species change. (Obviously, some mutations may be beneficial in some environments but detrimental in others. A fat bottom pads you in a desk job, but it is a detriment if you are a long-distance runner.)
Another source of variation is gene flow. Gene flow refers to the movement of genes between populations. A population , in this context, is the community of other individuals of the same species in which an individual typically reproduces. Gene flow happens when individuals migrate from one population to another. When an individual from one population migrates to another population and reproduces there, he or she introduces new genetic material into that population.
Human history has been characterized by a great deal of migration by individuals and, therefore, a great deal of gene flow. Depending on the reproductive success of these migrants in their new environments, they can produce significant evolutionary changes. Similarly, when gene flow stops between populations, significant evolutionary changes can occur as they continue to respond to their separate environments.
For most plants and animals, geographical and territorial boundaries typically define populations, but for humans it is primarily social rules that determine mating patterns. Cultural beliefs and values shape people’s ideas about potential mates. These cultural factors have a significant influence on the biological make-up of human populations. In other words, we need to think of human evolution as biocultural evolution .
A good example of the influence of gene flow on human populations and the importance of cultural values is the distribution of genes in African-Americans across the United States. Most African-Americans trace their African ancestry to West Africa but there has been significant admixture over time with European-derived Americans. Studies of the frequencies of different alleles in African-Americans in different U.S. cities can illuminate mating patterns which also reflect cultural values. In northern and western states, about one-quarter of alleles in African-American communities are of non-African origin. In the south, about 10% of alleles are of non-African origin illustrating that there has been less gene flow between African- and European-Americans in the south than in the north. This difference in mating patterns is influenced by social traditions and the historical tendency for greater segregation in the south.
Random factors can also play a role in evolution. The effect of random factors on allele frequencies (evolution) in populations is called genetic drift. (Definition) Since genetic drift is caused by random factors, its effects are most prominent in small populations, where each individual is a larger proportion of the whole gene pool. If a particular allele is already rare in a population that is also small, there is a pretty good chance that the trait will not be passed on, reducing the genetic variation of the population.
Suppose, for example, that a small population of hunter-gatherers has only three individuals who are left-handed. Being left-handed in this population has no real fitness consequences. Left-handed people are just as likely to be successful hunters and accomplished gatherers as right-handed people, but there just are not many of them in the population. Now, suppose that, by chance, all of the left-handed people are standing under a rock cliff to stay out of the rain and the cliff collapses, killing all three of them. The likelihood of that gene being passed on in the population just got significantly reduced, having a long-term consequence on the genetic variation of the population.
One particularly important type of genetic drift is called the founder effect. The founder effect occurs when a small group of people from within a larger population migrates or is otherwise cut-off from reproductive access to the parent population and becomes reproductively isolated. Over time, if this population remains reproductively isolated from the parent population and does not mate within a new population, then all of the members will be descended from the original founders.
For example, imagine that a plane crashed or a ship capsized leaving a handful of survivors on an isolated and uninhabited island. Over generations, all of the members of the population would be descended from the original survivors, of course, and would exhibit a genetic makeup that showed as much. The founder effect is seen in some endogamous (in-marrying) communities around the world. Often, social and cultural traditions discourage mating with individuals outside of the original culture. For example, the Amish of the northeastern United States are a relatively reproductively isolated population that is descended from the original settlers from Germany and the Netherlands in the 1800’s.
How does all this —almost none of which was known to Darwin and his colleagues— help us understand natural selection and evolution? When Darwin developed his theory of evolution by natural selection, he knew nothing about genes or DNA, and nearly all of the words set in boldface in this essay would have made no sense at all to him. Darwin still came up an elegant and largely accurate picture of how natural selection works, of course. But now that we know about genes, we can refine Darwin’s theory of natural selection by combining it with knowledge of genes and DNA.
In a non-gene-aware world, Darwin and his colleagues could think of evolution as changes in a population over time. Evolution rested on the fact that individuals within populations varied and that this variation led to differential reproductive success. If enough changes accumulated in a population over a very long period of time, new species might emerge.
Now we can add our knowledge of genetics and define evolution as a change in the frequency of alleles in a population from one generation to the next. Allele frequencies tell us about the genetic makeup of a population, about what percentage of the population has a particular allele. The problem that Darwin could not solve —where does the variation come from?— has now largely been worked out.
An important implication is that this new understanding can dodge the difficult issue of where one species ceases to exist and has “evolved into” a successor species. Although species tags are useful, evolutionary biologists no longer worry very much about when Beast One has somehow become Beast Two. Evolution is viewed not as a series of “stages” or “species,” but as a continuing process. What label is put upon a particular manifestation of a particular branch of the family tree of all life becomes a matter of linguistic convenience, not an exigency of nature itself.
A review quiz is available for this essay.