Seed Magazine recently published an article on recent human evolution that was prompted by a scholarly article by John Hawks, Gregory Cochran, Eric Wang, Henry Harpending, and Robert Moyzis. Recent Acceleration of Human Adaptive Evolution was published in December of 2007 in the Proceedings of the National Academy of Science vol. 104 no. 52. Seed raises some interesting points in the course of their article but before I discuss those I’d like to talk a little bit about the basics of evolution. The American Heritage Dictionary defines evolution in biology as, “a gradual change in the characteristics of a population of animals or plants over successive generations.” What does that mean though? It means that evolution is more than believing we share a common ancestor with chimpanzees. The theory of evolution is far more complex and subtle than that. I will focus on evolution as observable changes in populations over time. The fact of evolution is evidence that we can see; it is the change in trait ratios over observable periods of time. It is the observable evidence for evolution that leads scientists to speculate about long-term changes. It might be helpful to think of evolution as biological change since the term evolution has come to be so associated with “progress,” “improvement,” and “advancement.” This change is not inherently good or bad in the long run. Instead it is good or bad for certain circumstances or environments that exist in a particular place or time. Whether or not a change is good or bad can change over time as the environment changes. Typically the processes of evolution (mutation, migration, genetic drift, and natural selection) will produce changes that benefit an organism’s ability to reproduce and pass on its genes to future generations.
Scientists have discovered that genes mutate randomly at a predictable rate over time Mutations can be caused by gene copying errors, UV or radiation exposure, chemicals, or viruses. When several of these mutations occur to affect the same gene, a new variation of the gene is created. If a mutation becomes common enough in a population then the likelihood of the mutation occurring in the majority of the population over time increases. An example of this is the development of blue eyes. Scientists believe that blue eyes originated in a single individual in the Black sea region between 6,000 – 10,000 years ago. The trait was passed onto the individual’s children and eventually became common enough that 20-40% of all Europeans today have blue eyes.
Genetic drift is one of the most difficult concepts within evolution to understand. Genetic drift is a random change in the frequency of alleles within a population. That is to say, at any given point in time an allele that has a high frequency within a population may not actually confer any fitness benefits to individuals who possess the allele; rather the allele’s high frequency may be due to chance. The University of California – Berkley has a great example of how genetic drift can change allele frequency in a population. Say you have a group of beetles that is made up of 3 green beetles and 5 brown beetles. Here there are nearly equal numbers of green and brown beetles. However, if a person walked through the middle of the group of beetles and happened to step on two of the green beetles then the frequency of the brown allele will go up in relation to the whole population. The next generation of beetles in this case would be much more likely to be mostly brown because there is only one green beetle left, not because being brown helps a beetle’s fitness. Fitness here is an organism’s ability to reproduce and pass on its genes to future generations.
Natural selection is far less random than genetic drift. For natural selection to occur there must be diversity within a population. Diversity is a byproduct of gene mutation because it produces many different variations of alleles. An allele is a variation of a gene that serves the same basic function as another variation for the same gene. For example, eye color in humans is determined by allele variations. In a simplified version, each human has two alleles for eye color encoded in their DNA – one from each parent. If a baby’s mom and dad have green and brown eyes respectively then the baby will get one “brown” allele and one “green” allele. Thus, the baby could have either brown or green eyes depending on which allele is dominant. When the baby grows up he/she could pass on either the brown or green allele no matter which color the baby’s eyes actually are. Natural selection comes into play if one allele has a fitness benefit that the other alleles for the same gene don’t have.
To explain this I’ll go back to the beetle example. Say we have a population of 3 green beetles and 3 brown beetles. If the beetles live on trees that have brown bark, the brown beetles aren’t as visible to predators that eat beetles. If birds eat all the green beetles, then the next generation will get mostly brown alleles because being brown kept some beetles alive by camouflaging them against predators. On the flip side, if the beetles live in the grass then the brown ones might be more likely targets for hungry birds. Thus we can see that whether or not a certain allele confers a benefit to an organism is highly dependant on the organism’s environment. Another example of natural selection can be found in the bright colors of a male peacock’s feathers. Female peacocks are attracted to brighter color displays on a male’s feathers. This means that a male with very colorful feathers will get more mates than a male with less colorful feathers – which means his genes are more likely to be passed on to the next generation. In this sense natural selection can also be determined by sex appeal – whether or not an individual looks healthy or attractive to the opposite sex.
Migration is the last major mechanism for evolutionary change. Allele frequency can be changed in a population if enough members of the population that have one gene variation leave the population. The same is true of incoming members to a population. For example, if we start with a group of 4 green and 4 brown beetles and 3 of the brown beetles leave to go live with another group then the brown allele frequency of the original group has dropped significantly. If the 3 brown beetles that left the first group join another group that has 2 brown beetles and 3 green beetles to start with then the frequency of brown beetles in the second group will go up significantly.
These mechanisms of evolutionary change are always at work and can have drastic impacts on a population over time. This change is not necessarily good or bad overall; it just tends to represent a successful solution to the present environmental circumstances that exist at any given time. Of course evolution and the mechanisms of evolution tend to be much more complicated than how I’ve represented them here I hope that this post helps clear up some of the basic confusions of evolution so that we can go more in depth next time. In my next post I plan to discuss some of the issues raised in the Seeds article, “How We Evolve.”
As usual, here are some extra links to sites that you might find interesting or helpful for further clarification on the topic of Evolution:
University of California – Berkley, Evolution 101:
How are human eye colors inherited?:
PNAS: Darwin’s greatest discovery – Design without designer: