Hooded heritage
Question: I have a problem which has been perplexing me for some time. I was
idly wondering how many of my ancestors were alive at the time of Robin Hood. If
a generation can be said to be about 25 years, and the aforementioned Mr Hood
allegedly lived in the 12th century, using this approximate information there
must have been 232 (4 294 967 296) of my forebears extant at the time.
The available breeding population of the areas in England and Ireland where
my ancestors originated, and where transport was limited, can only have been a
tiny fraction of this number, and I do not think it is reasonable to say that I
am related as closely as this fraction implies to everyone from that part of the
world.
Can I be amazingly inbred? Am I doing my sums wrong? Are everyone鈥檚 family
trees so closely entwined in a great global hedge? Please advise.
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Answer: It is true that the number of ancestors is terribly overestimated by
simply raising two to the power of generations past. But that does not mean that
your children will be compulsive banjo players with malformed dental
structure.
If you follow your parental lineage several generations back, chances are you
start seeing the same names popping up again and again at different places in
your family tree, meaning the same individual is your ancestor in more than one
way. To avoid drastic overestimation of the number of ancestors, you must
combine different lineages of your family tree wherever you see the same
ancestor. So instead of a family tree, you get something more like an interwoven
family web with tangles stretching out here and there.
So indeed, everyone鈥檚 family trees are entwined. And, in fact, you are in a
way closely related to everyone else. So why aren鈥檛 we all inbred? The answer
lies in the definition. A part of your genome is said to be inbred if both of
its copies are derived from the same ancestor in two different
ways鈥攖hrough both your parents. The chance of that happening halves, due
to meiosis where cell chromosomes divide rather than replicate, for each step
you take over the loop that lies through your parents and their common
ancestor.
My girlfriend and I are fourth and fifth-generation cousins. That is, my
great-grandfather and her great-great-grandmother were siblings. The route
through our common ancestor is 10 steps long, so the chances of our children
being inbred (called the Inbreeding Coefficient in the jargon of population
genetics) are approximately one in a thousand. That鈥檚 not a high figure, even
though we鈥檙e quite closely related. Family relations at deeper levels in the
tree become negligible fast, so don鈥檛 worry about being inbred from a family in
Nottinghamshire in the 12th century.
Hjorva Petursson
Department of Biology, University of Iceland
Answer: Yes indeed, as you go back in time, the number of actually different
ancestors dwindles rapidly compared to the number of places to be filled. In
German, the word Ahnenschwund has been used for the fact. Most people
know little about this because even devoted ancestry researchers rarely know
more than a couple of single family lines going back many generations.
Manfred Mahnig
Meckenheim, Germany
Bee clever
Question: A few years ago, I observed a bumble bee bypassing the normal
pollination process. The flower was columbine, clearly not intended to be
pollinated by British bees because the long tail of the flower puts the nectar
well out of reach of a bee鈥檚 proboscis.
Instead, the bumble bee attacked the plant from the outside, carefully
puncturing the long tail of the flower to get at the nectar, without going
anywhere near the pollen. Most of the columbine flowers nearby had been
punctured in the same place.
Presumably the flowers provide no external guide marks, because the
pollination process is being short-circuited, so how does the bee know where to
get the nectar? And is this action learnt behaviour on behalf of the bees in my
locality or is it instead genetically programmed?
Answer: Some bumble bee species have tongues long enough to reach nectar in
the long spurs of columbine, and others don鈥檛. Short-tongued species like
Bombus lucorum are notorious for acting as primary robbers (biting holes in
the corolla) or secondary robbers (reusing holes bitten by others) on deep
flowers such as red clover and field bean. If you crouch in a bean field at dawn
you can hear the snapping of the mandibles as primary robbers perforate the
day鈥檚 new flowers.
Robbing behaviour is learnt. Individuals often develop a habit of biting
consistently on the left or right-hand side of a flower.
In his paper 鈥淔rom naivet茅 to experience: bumblebee queens foraging on
Corydalis cava鈥� (Journal for the Kansas Entomological Society
vol 69 (special supplement 4), p 274) Jens Olesen showed that as the flowering
season progressed, the bees got better at positioning their holes in the best
place for access to the nectar.
Sarah Corbet
Department of Zoology, Cambridge University
Answer: Bumble bees frequently use their mandibles to perforate flowers that
have inaccessible nectar. This process is termed nectar robbery and has been
shown in Corydalis, a spurred flower. When flowering begins, bumblebees
perforate various parts of the spurs at random, but after a few days they become
more precise, biting the hole near the surface of the nectar. Other insects
whose mandibles are too weak to perforate the flowers, such as hoverflies,
indulge in secondary robbing, using the holes made by the bees.
Nectar robbery is normally expected to have an adverse effect on plant
reproductive success because the removal of nectar without effecting pollination
forces the plant to produce more nectar. This uses up energy, weakening the
plant and reducing its chances of producing viable fruit. Furthermore, robbers
compete with pollinating insects, so the plant has less chance of being
pollinated.
However, in experiments with Mertensia ciliata (Ecology,
vol 77, p 1451), researchers did not find significant differences in
reproductive success between plants which had been robbed and those which had
not. It is not clear how this can be explained.
Maria Bennett
Aberdeen
This week鈥檚 questions
Live links: Pine forests in Europe are frequented by a caterpillar known in
Spanish as processionales because of its habit of migrating by
attaching itself to other individuals to form a snake-like procession. I have
observed as many as 50 in a procession a metre or more in length. This makes
them quite conspicuous. What is the advantage of this behaviour?
Quintin Davis
Leatherhead, Surrey