Natural Selection
Adaptations
The process of natural selection favors individuals within a population that
have genetic variations that adapt them to their environment. Since these
individuals are more likely to survive and reproduce, the future population
will include more of these offspring with the selected-for traits, or
adaptations. Adaptations may result from cumulative small changes. They
may be simple changes such as beak depth or complex adaptations such as
the eye. Adaptations may be structural or behavioral, but each must have a
genetic basis. Acquired structures, such as muscles built up by strength
training, or behaviors, such as the use of tools, are not heritable.
Mimicry: The Orchid and the Bee:
Some people -- and animals -- will go to any lengths to attract members
of the opposite sex, including the use of aliases and lies. As many as
10,000 species of dainty orchids in the floral world also utilize deception
in order to be get pollinated: Over time, they have evolved elaborate
ruses to lure insects.
Generally we think of flowers as offering food to the insects that
pollinate them. Evolution has produced many flower species that pretend
to have food that insects want, emitting scents of coconut or even rotting
meat. Some orchids, however, appear to offer the promise of sex: They
have evolved to resemble female versions of certain insects.
The pseudo-copulation strategy of the orchid enables it to spread its
genes widely. The mimicry is near perfect. For example, the Australian
hammer orchid has taken advantage of a mating ritual of the Thynnid
wasp, which involves a female wasp waiting on top of a branch or plant
for a male to spot her. The hammer orchid's flower mimics the female
wasp looking upward for a male flying by, complete with a fake shiny
head and furry body. The orchid even releases an enticing female wasp
pheromone.
When the male wasp tries to mate with the dummy female, he fails, but
the orchid succeeds in getting pollen on the wasp. He flies away, only to
be fooled again by another orchid pulling the same trick. In the process,
the wasp transfers pollen from flower to flower.
Place a real female wasp next to the orchid mimic, and a male wasp will
spot the real deal. For this reason, natural selection has favored flowers
that happen to bloom in the period when male wasps are flying but
females are not out yet.
Why orchids have evolved sexual mimicry for pollination is open for
debate. Plants that are farther away from each other are more likely to be
distant relatives, so mimicry may reduce inbreeding. Posing as a sexual
suitor may be a strategy that allows the geographic spread of plants over
a wide area -- generally, insects will travel further to find a mate than to
find a meal. To test these hypotheses, scientists are studying both the
mating behavior of the insect pollinators and the growth and
reproduction of the deceptive orchids.
Introduction of Genetic Variation
Genetic variation can be introduced either through selective or nonselective means.
Selective Mechanisms
1. Genetic Variation By Mutation
Natural selection acts upon two major sources of genetic variation:
mutations and recombination of genes through sexual reproduction.
Most mutations do not affect the reproductive fitness of individuals -some may be beneficial, some may be harmful, and many may be neutral.
Mutation rates per gene are generally low. However, because there are so
many genes in organisms (current estimates are 30,000-60,000 genes in
humans), about 10 percent may be phenotypically expressed (i.e., they
affect the anatomy and physiology of the organism) and acted upon by
selection.
2. Genetic Variation By Recombination
Recombination is usually the most immediate cause of variability.
Recombination can occur in at least three ways:
1. Crossing over of chromosomes during meiosis
2. Random assortment of maternal and paternal chromosomes during the
production of egg and sperm
3. Random combination of egg and sperm at fertilization.
Why sex? The strange case of the minnows
For over 25 years Robert Vrijenhoek has studied unique populations of
minnows in the small hillside pools of the Sonoran desert in Mexico. Here
he found two different species of minnows living side-by-side, one an
asexual reproducer and the other a sexual reproducer. Vrijenhoek has been
trying to understand which conditions might favor the sexual minnows and
which favor the asexual ones. The variability in the sexual minnows is
primarily caused by recombination of chromosomes during sexual
reproduction:
 the random assortment of maternal and paternal chromosomes during
the production of sperm and eggs,
 the random joining of gametes at fertilization, and
 the crossing over of chromosomes during meiosis.
Variability can also be caused by mutations, but in this example sexual
reproduction is the most immediate cause. It is this variability in individuals
that allows those best adapted to their environment to survive and reproduce
to create future populations. Thus, the sexual minnow population, with its
variability, was better adapted to resisting the parasite than the asexual
population. This is a clear demonstration of the process of natural selection
at work—the primary mechanism of evolution in populations. An interesting
change in Vrijenhoek’s pools demonstrated another mechanism for
evolution. A bad drought dried up the pools and killed most of the minnows.
Eventually, the water returned and so did the minnows. They had hopped up
stream like trout. But, when Vrijenhoek checked the top pool, he made a
surprising discovery. Now the parasites were decimating the sexual
minnows and the clones were doing quite well. Vrijenhoek was stymied. He
collected the fish and examined them carefully. He found that the sexual
minnow population had lost its genetic variation; it had become inbred and
lost its advantage. The sexual fish were clone-like in their variability and
since they outnumbered the true clones, they were the biggest targets of the
parasites. To test his idea that reduced variability in the sexual minnow
population had caused the turn of events, Vrijenhoek tried an experiment. He
brought sexual minnows from a lower pool, where the fish still had genetic
variability, up to one of the higher pools. A year later he came back to see
what had happened. To his delight, the situation had reversed itself to the
normal pattern. Now in fact, the asexual minnows again were more parasiteprone and the genetic variability of the sexual minnows had returned. The
recolonization of the upper pools demonstrated another mechanism for
evolution—the founder effect. This happens when a small population with
limited diversity founds a new population in a new location. Because of this,
the limited population of sexual minnows became inbred. This mechanism,
unlike natural selection, is random. It is by chance that this particular group
of individuals recolonized the upper pools. Vrijenhoek’s work demonstrates
key mechanisms for evolution: the genetic variability created by sexual
reproduction and the effect of natural selection on individuals within a
population. It also shows how a non-selective mechanism, the founder
effect, can cause evolution within a small population.
Nonselective Mechanisms
Although natural selection is the major mechanism for evolution and
speciation, there are several other mechanisms of evolution that are
nonselective. These nonselective evolutionary mechanisms cause changes
in populations because of chance fluctuations in the frequencies of genes.
These chance fluctuations occur in large and small populations, but tend to
be significant to evolution only in small populations.
Besides mutations, changes in the gene pool of populations can be caused
by:
• gene flow: changes in the gene pool of a population because of the
introduction of genes from another population by migration. Example:
the gene pool in Southeast Asia was changed when U.S. soldiers had
children with Vietnamese women during the Vietnam War in the
1960s and early 1970s.
• genetic drift: changes that take place because of random fluctuations
of gene frequencies due to small population size (e.g., sampling size).
Example: The frequency of Ellis-van Creveld syndrome -- a rare form
of dwarfism that includes extra digits -- in the Amish population of
eastern Pennsylvania, which has intermarried over many generations.
• founder effect: changes in a population when a small population
moves to a new location bringing only a small fraction of the genetic
variation of the parent population. The population then will contain
only those genes the initial individuals brought with them. Example:
The finches of the Galapagos Islands have the gene pool the original
founding population of finches brought with them.
Genetic Drift and the founder Effect
Eastern Pennsylvania is home to beautiful farmlands and countryside,
but it's also a gold mine of information for geneticists, who have
studied the region's Amish culture for decades. Because of their
closed population stemming from a small number of German
immigrants -- about 200 individuals -- the Amish carry unusual
concentrations of gene mutations that cause a number of otherwise
rare inherited disorders, including forms of dwarfism. One form
of dwarfism, Ellis-van Creveld syndrome, involves not only short
stature but polydactyly (extra fingers or toes), abnormalities of the
nails and teeth, and, in about half of individuals, a hole between the
two upper chambers of the heart. The syndrome is common in the
Amish because of the "founder effect." When a small part of a
population moves to a new locale, or when the population is reduced
to a small size because of some environmental change, the genes of
the "founders" of the new society are disproportionately frequent in
the resulting population. If individuals in the group tend to marry
within it, there's a greater likelihood that the recessive genes of the
founders will come together in the cells that produce offspring. Thus
diseases of recessive genes, which require two copies of the gene to
cause the disease, will show up more frequently than they would if the
population married outside the group. In the Amish, in fact, Ellisvan Creveld syndrome has been traced back to one couple, Samuel
King and his wife, who came to the area in 1744. The mutated gene
that causes the syndrome was passed along from the Kings and their
offspring, and today it is many times more common in the Amish
population than in the American population at large. The founder
effect is an extreme example of "genetic drift." Genes occurring at a
certain frequency in the larger population will occur at a different
frequency -- more or less often -- in a smaller subset of that
population. As in the example of human diseases, genetically
determined traits that would ordinarily be uncommon in the overall
gene pool might crop up with distressing frequency in a small subset
of that pool.
Sexual Selection
Sexual selection is a "special case" of natural selection. Sexual
selection acts on an organism's ability to obtain (often by any means
necessary!) or successfully copulate with a mate.
Selection makes many organisms go to extreme lengths for sex:
peacocks maintain elaborate tails, elephant seals fight over territories,
fruit flies perform dances, and some species deliver persuasive gifts.
After all, what female Mormon cricket could resist the gift of a juicy
sperm-packet? Going to even more extreme lengths, the male redback
spider literally flings itself into the jaws of death in order to mate
successfully.
Sexual selection is often powerful enough to produce features that are
harmful to the individual's survival. For example, extravagant and
colorful tail feathers like those found in peacocks. Darwin simply did
not understand how a peacock’s brightly colored, long tail enabled it
to survive quite well. He expected that the males' long tails would
make them unwieldy for flight, be heavy to drag around, and would
put them at a disadvantage when escaping from predators. Eventually,
he recognized the value of color and ornamentation: it attracted
females and improved the males' chances of reproducing. Recent
research has demonstrated that long and brightly colored tail-feathers
indicate a very strong, healthy male and will therefore be more
attractive to peahens. In an experiment where a peacock had his tail
feathers cut it proved to be a long a lonely mating season!
Sexual selection usually works in two ways, although in some cases
we do see sex role reversals:
 Male competition
Males compete for access to females, the amount of time spent
mating with females, and even whose sperm gets to fertilize her
eggs. For example, male damselflies scrub rival sperm out of the
female reproductive tract when mating.
 Female choice
Females choose which males to mate with, how long to mate, and
even whose sperm will fertilize her eggs. Some females can eject
sperm from an undesirable mate.
Based on the reading and your own research, answer the following
questions:
1. In your own words, and with reference to natural selection,
describe what constitutes an adaptation.
2. Provide an example of:
a) an adaptive trait that has a genetic basis, and
b) an acquired trait.
3. How is variation introduced into genes?
4. Citing an example (you may use the example given in the
reading), explain how sex is important in increasing the fitness
of a population.
5. After reading about the non-selective mechanisms at work in
natural selection, provide two more examples of gene flow and
genetic drift that have been documented.
6. Provide two additional examples of male competition and
female choice, giving the names of the organisms and
describing how each is accomplished.
.