Founder effect
One of the most important manifestations of genetic drift is in the founding of new populations by a small number of colonists. For example, the Afrikaner population of the country of South Africa today descends from Dutch colonists who arrived during the seventeenth century. Some of the earliest colonists to arrive had a large genetic contribution to the later Afrikaner population, because they had a chance to have lots of offspring who intermarried with later arrivals. The first Dutch colonists landed in 1652, and one of these colonists was a man who carried an allele causing Huntington's disease, a rare genetic disorder of the nervous system. Huntington's is a dominant genetic disorder, affecting all individuals who carry the allele, but it exerts most of its effect late in life --- after people generally reproduce. Although this harmful allele was carried by only one individual, it was a relatively large proportion of the new founder population --- much higher in frequency than it had been in Holland. After strong population growth, today's Afrikaners have a high frequency of the Huntington's allele, mainly from this single founder (Ridley 2002). This phenomenon of genetic drift is often called the \term{founder effect}.
\subsection{Population structure and genetic drift}
A simple reason for variability in reproduction is the different reproductive efforts of males and females. Female mammals face a high cost of reproduction. Mothers provide space and nutrients to their developing young while they still in the womb, and mothers provide high-energy milk and protection to their young after they are born. Although female fish and frogs may lay hundreds --- or even thousands --- of eggs, female mammals are limited to many fewer offspring over the course of their lifetimes. Males, on the other hand, do not face the same reproductive costs. If a male can mate with many females, he can potentially have many times the number of offspring of any single female. But males face a different cost: if they want to mate at all, they must first face competition from other males. In many species, a lucky few males may mate with many females, while most males do not mate at all. Thus, males are often much more variable in their reproductive success than females. Each generation of offspring in such a population includes the genes of many different females but only a few males. Only a few genes may be responsible for the and all the genes of these few males are boosted by genetic drift.
Human history appears to have included some cases where single male lineages had exceptionally high mating success. Geneticists can trace male reproduction through the Y chromosome, which is passed from only from father to son. Because of this unique pattern of inheritance, the Y chromosome marks \term{patrilines}, lineages of males. In many human societies, social status or power may also be passed along patrilines, as kings and chiefs pass power to their sons. This cultural pattern of inheritance generally lasts only for a few generations, as some member of the male lineage ultimately fails to have a son as an heir, or the patriline simply loses power. But the history of some cultures gave a few patrilines exceptional mating opportunities, as kings and other high-ranking men sometimes kept harems of dozens or more women for their own exclusive mating.
\begin{figure}
\includegraphics[width=\textwidth]{genghis.png}
\caption[Frequency of ``Genghis Khan'' Y chromosome haplotype in Asia]{Frequency of the ``Genghis Khan'' Y chromosome haplotype in samples of Asian populations. The ``star cluster'' refers to the rapid expansion in numbers of the haplotype in different populations since its origin around 1000 years ago. Reprinted from Zerjal \emph{et al.} (2003).}
\label{fig:genghis}
\end{figure}
Two Y chromosome haplotypes in Asia are shared by many millions of men, even though they emerged within the past thousand years. One of these, carried by 8 percent of men in Central and Northeast Asia, appears to have originated in Mongolia around a thousand years ago [1]. At this frequency, the haplotype would occur in as many as 16 million men, all descendants of a single man within the past 1000 years. The large current population implies that these men descend from an exceptionally widespread and productive patriline. During the past 1000 years in Asia, the best candidate for such a patriline is that of the Mongol emperor Genghis Khan, who lived from around A.D. 1162--1227. After conquering history's largest land empire, Genghis and his descendants installed their male relatives as rulers of much of Asia. These descendants themselves must often have had extraordinary reproductive opportunities, so that their Y chromosomes became more and more common in Asian populations. A second Y chromosome haplotype is carried by around 3 percent of people in China and Mongolia, and may derive from the Manchu dynasty, which dates to the year 1644 [2]. Together, these haplotypes illustrate the chance for some rare alleles to increase greatly in frequency due to genetic drift in human history.
References
- The Genetic Legacy of the {Mongols}. American Journal of Human Genetics. 2003;72:717–721.
- Recent Spread of a Y-Chromosomal Lineage in {Northern China} and {Mongolia}. American Journal of Human Genetics. 2005;77:1112–1116.
- Can you think of other populations in human history that might have undergone a founder effect?
- What evidence can we use to test whether a founder effect can explain the high frequency of an allele?
