Microevolution pdfMicroevolution
powerpoint to be used in class
How to explain
allele frequencyThere are a number of factors that affect the rate of
gene flow between
different populations. One of the most significant factors is mobility,
as greater mobility of an individual tends to give it greater migratory
potential. Animals tend to be more mobile than plants, although pollen
and seeds may be carried great distances by animals or wind.
As an analogy, imagine representing organisms in a population with a
large number of marbles, half of them red and half blue. These two
colors correspond to the two different gene alleles present in the
population. Put 10 red and 10 blue marbles in a jar; this represents a
small population of these organisms. Each generation the organisms in
this population will reproduce at random and the old generation will
die. To see the effects of this, imagine randomly picking a marble from
the jar and putting a new marble of the same color as the one you
picked into a second jar. After your selected marble has "reproduced",
put it back, mix the marbles, and pick another. After you have done
this 20 times, the second jar will contain 20 "offspring" marbles of
various colors. This represents the next generation of organisms. Now
throw away the marbles remaining in the first jar - since the older
generation of organisms eventually die - and repeat this process over
several generations.
Since the numbers of red and blue marbles you pick out will
fluctuate by chance, the more common color in the population of marbles
will change over time, sometimes more red: sometimes more blue. It is
even possible that you may, purely by chance, lose all of one color and
be left with a jar containing only blue or red offspring. When the jar
only contains one color (allele), say red, the other allele, in this
case the blue, has been removed or "lost" and the remaining allele
(red) becomes fixed. Given enough time, especially in a small
population, this outcome is nearly inevitable. This is genetic drift -
random variations in which organisms manage to reproduce, leading to
changes over time in the allele frequencies of a population.
An example of a bottleneck: Northern elephant seals have reduced genetic variation probably because
of a population bottleneck humans inflicted on them in the 1890s. Hunting
reduced their population size to as few as 20 individuals at the end of the 19th
century. Their population has since rebounded to over 30,000—but
their
genes still
carry the marks of this bottleneck: they have much less genetic variation
than a population of southern elephant seals that was not so intensely
hunted.
The
cheetah has unusually low
genetic variability and a very low
sperm count, which also suffers from low
motility and deformed
flagellae[6].
Skin grafts between non-related cheetahs illustrate this point in that
there is no rejection of the donor skin. It is thought that it went
through a prolonged period of
inbreeding following a
genetic bottleneck during the
last ice age.
Founder effects
A founder
effect occurs when a new colony is started by a few members of the original
population. This small population size means that the colony may have:
- reduced genetic variation from the original population.
- a non-random sample of the genes in the original population.
For example, the Afrikaner population of Dutch settlers in South Africa
is descended mainly from a few colonists. Today, the Afrikaner population
has an unusually high frequency of the gene that causes Huntington’s
disease, because those original Dutch colonists just happened to carry
that gene with unusually high
frequency. This effect is easy to recognize
in genetic diseases, but of course, the frequencies of all sorts of genes
are affected by founder events.
PROS AND CONS OF INBREEDING
Copyright 1996, Sarah Hartwell
Adapted, with permission, from Cat
Recourse Archive and
edited by Dog Breed Info.
Inbreeding is the mating together of closely related dogs, for
example mother/son, father/daughter and sibling/sibling matings. For
breeders, it is a useful way of fixing traits in a breed - the pedigrees
of some exhibition dogs show that many of their forebears are closely
related. For example, there is a famous cat by the name of Fan Tee Cee (shown in the 1960s and
1970s) appeared in more and more Siamese pedigrees, sometimes several
times in a single pedigree, as breeders were anxious to make their lines
more typey. Superb specimens are always much sought after for stud
services or offspring (unless they have already been neutered!) having
won the approval of show judges.
However, inbreeding holds potential problems. The limited genepool
caused by continued inbreeding means that deleterious genes become
widespread and the breed loses vigor. Laboratory animal suppliers
depend on this to create uniform strains of animal which are immuno-depressed
or breed true for a particular disorder e.g. epilepsy. Such animals are
so inbred as to be genetically identical (clones!), a situation normally
only seen in identical twins. Similarly, a controlled amount of
inbreeding can be used to fix desirable traits in farm livestock e.g.
milk yield, lean/fat ratios, rate of growth etc.
To some extent humans
even have done this themselves. A Viking who wanted to have strong sons
might marry a strong woman. A woman who wanted to make sure her family
was well provided for might select a loyal and reliable man who had good
hunting skills - or is a good farmer . A tribal leader who wanted
to make sure that he produced many children might only marry a woman who
had already borne a child, proving her fertility.Inbreeding
and GeneticsStabilizing Selection. A good classic example of this is
human birth weight. Babies of low
weight lose heat more quickly and get ill from infectious disease more
easily, whereas babies of large body weight are more difficult to
deliver through the pelvis. However, the recent improvements in human
nutrition in
developed countries has led to rising rates of caesarean sections, since babies are routinely out-growing the female reproductive tract.
[1]Directional Selection. A useful example can be found in the breeding
of the greyhound dog. Early breeders were interested in dog with the greatest
speed. They carefully selected from a group of hounds those who ran the
fastest. From their offspring, the greyhound breeders again selected those
dogs who ran the fastest. By continuing this selection for those dogs
who ran faster than most of the hound dog population, they gradually produced
a dog who could run up to 64km/h (40mph).
Disruptive Selection. Disruptive selection, like directional selection, favors the extremes
traits in a population. Disruptive selection differs in that sudden changes
in the environment creates a sudden forces favoring that extreme. Think
about the changes in the environment when that meteor crashed into Earth
65mya. A sudden decrease in light levels as the dust rose over large portions
of the Earth. Extremely large tidal waves washing miles over the land.
Increased seismic activity. The sudden lost of food along the coast, possible
plague due to the high initial death rate, dust filling the lungs of animals
would have been the most stressful on larger animals. Large animals need
a large oxygen supply to supply energy to their muscles. They also need
a large, constant supply of food. The the sudden drop of oxygen due the
the dust, and the drop in fresh food, large animals would be stressed.
If a plague started by the high death rate also hit these stressed animals,
they would have been sorely pushed to survive. Evidence shows that they
did not. So disruptive selection occurs quickly, selecting for those extreme
traits that help organisms survive in the new environmental conditions.
A population may find itself in circumstances where individuals
occupying one extreme in the range of phenotypes are favored over the
others.
Since 1973, Peter and Rosemary Grant — aided by a succession of
colleagues — have studied Darwin's finches in the Galapagos Islands.
When rainfall, and thus food, are plentiful, the ground finches tend to
- have a varied diet, e.g., eat seeds of a range of sizes;
- show considerable variation in body and beak size (large beaks
are better for large seeds but can handle small seeds as well as small
beaks).
From 1976 through 1977, a severe drought struck the islands,
with virtually no rainfall for over a year. This caused a precipitous
decline in the production of the seeds that are the dietary mainstay of
Geospiza fortis, the medium ground finch.
The graph (from P. T. Boag and P. R. Grant in
Science 214:82,
1981) shows how the population declined from 1400 to 200 on the island
of Daphne Major, a tiny (10-acre) member of the Galapagos Islands.
One of the plants to make it through the drought produces seeds in
large, tough fruits that are virtually impossible for birds with a beak
smaller than 10.5 mm to eat.
Sampling the birds that died as well as those that survived showed that
- the larger birds were favored over the smaller ones;
- those with larger beaks were favored over those with smaller ones.
