Question

Consider a comparison of two cladistic analyses aimed at reconstructing the phylogeny of a particular set of mammalian species. One of these studies used Y-chromosome sequences and the other used mtDNA sequences, and both studies successfully avoided the problems associated with homoplasy. Explain why the species trees estimated by these studies would likely be very similar, despite the fact that these data sets were generated by lineage histories that do not contain ANY individuals in common (Y-chromosomes evolve thru males only, while mtDNA evolves thru females only)?

Answer 1

DNA contains the genetic information of a person.Our autosomal DNA is inherited from both parents, but Y-chromosomal DNA (Y-DNA) is inherited only from father to son, and mitochondrial DNA (mtDNA) is inherited only from our mother. Shortly after the process of fertilization, the sperms mitochondria die away, and the embryo is only left with maternal mitochondria. As these two contains genetic information one can trace back its common ancestral origin.

Even though everyone on Earth living today has inherited his or her mtDNA from one person who lived long ago, our mtDNA is not exactly alike. Random mutations have altered the genetic code over the millennia.For example, let's say that 10,000 years after the most recent common ancestor, one of the mtDNA branches experienced a mutation. From that point on, that line of mtDNA would include that alteration. Another branch might experience a mutation in a different location. This alteration would also be passed on. What we would eventually end up with are some descendants who have mtDNA that is exactly or very much like that of some people's, somewhat like that of others, and less like that of yet others. By looking at the similarities and differences of the mtDNA of all of these individuals, researchers could try to reconstruct where the branching took place. The same can be said for the Y chromosome inheritance in males.

The phlogenetic trees constructed by these two studies will be quite similar as the will have less number of mutations than that of the nuclear DNA and both can trace back the point from which divergence occured in the set of mammalian species.

For the original 1987 Nature article, the three authors (Rebecca Cann, Mark Stoneking, and Allan Wilson) looked at the mtDNA of 147 people from continents around the world (though for Africans, they relied on African Americans^[2]). Later, with the help of a computer program, they put together a sort of family tree, grouping those with the most similar DNA together, then grouping the groups, and then grouping the groups of groups. The tree they ended up with showed that one of the two primary branches consisted only of African mtDNA and that the other branch consisted of mtDNA from all over the world, including Africa. From this, they inferred that the most recent common mtDNA ancestor was an African woman.

References

"Mitochondrial DNA and Human Evolution." Rebecca L. Cann, Mark Stoneking, Allan C. Wilson. Nature, January 1, 1987.