A population geneticist's defintion of evolution

Are humans evolving?

In 2005, Stefasson et al. reported the fascinating discovery of an allele in humans whose presence is associated with increased fertility in Icelandic and European populations. Females with at least one copy of the allele have approximately 3.5%, and males 2.9%, more children on average than non-carriers. The exact mechanism by which the allele affects fertility is unknown.

1. Predict: Consider what you know about evolution. Do you think these populations are likely to be evolving with respect to this allele? Why or why not? Please make sure your response illustrates your understanding of evolution.

2. Test: Describe one way you could test your prediction quantitatively . What data would enable you to conclude that these human populations are evolving? What results would support the contention they are not?

Differential survival and reproduction underpin evolution

The fascinating discovery above should have called to mind the key causes, a.k.a. 'agents', of evolution

• natural selection
• sexual selection
• genetic drift (including bottle necks and founder effects)
• immigration/emigration
• mutation

and their consequences. All cause a population to evolve by altering the frequency with which particular phenotypes, their underlying genotypes and most importantly the responsible alleles, occur. This quantitative description of the genetic consequences of these evolutionary mechanisms is encapsulated by the population geneticists definition of evolution:

• Evolution is a change in the allele frequencies observed in a population over time (i.e. over generations).

Agents of evolution cause allele frequencies to change because they result in differential survival and reproduction. That is, not every individual has an equal chance of surviving, reproducing and contributing surviving offspring to the next generation. (The survival of offspring is key; if you reproduce but your kids don't survive to reproduce then you are evolutionarily inconsequential in terms of your allelic contribution to future generations.) Instead, for reasons that vary with the agent, some phenotypes, and their responsible genotypes, are more likely to survive or to reproduce and thus, to leave behind offspring than other phenotypes (genotypes). As a result, alleles of reproductively successful individuals become more common, and those associated with relatively unsuccessful individuals become less common, in subsequent generations. This change in allele frequencies is, of course, evolution.

Conversely, a population will not evolve if every phenotype (genotype) has an equal probability of surviving and producing surviving offspring. To imagine this, conjure a population in which all of the following conditions are simultaneously met:

• all phenotypes are equally likely to survive and to reproduce surviving offspring; there is no natural selection.
• all phenotypes are equally attractive or have equal access to potential mates; there is no sexual selection.
• no phenotypes leave behind more offspring than others just by chance; the population must be very large as there is no genetic drift.
• breeding individuals (and their genotypes) are not leaving or entering the population; there is no emigration or immigration.

The genetic consequence of all this equality is that the same allele frequencies are maintained from one generation to the next so the population does not evolve.

Of course, not all organisms reproduce sexually but the point is that a population will not evolve if all genotypes are equally likely to leave behind offspring with their alleles, even if reproduction occurs asexually.

3. Consider these definitions as you reflect your answers to questions 1 and 2 in the "Are Humans Evolving?" scenario above. Have you learned anything that would encourage you to modify your answers? If so, please do. If not, explain why your responses are appropriate.

Definitions

• frequency - the number of times an event or observation, for example a particular measurement or condition like blue eyes, is observed in a collection of events or observations like those comprising a sample, population or study. In this statistical sense, a frequency is equivalent to a proportion. For example, the frequency of a particular allele is equal to the number of times that allele is observed in a population over the total number of alleles for that locus in the population. Can be expressed as a fraction, a percentage, a decimal, or a probability.

Works cited

• Stefansson, H., Helgason, A., Thorleifsson, G. et al. 2005. A common inversion under selection in Europeans. Nature Genetics . 37:129-137.