Examples of Natural selection in action
You should recall that Darwin proposed evolution was the inevitable outcome of 4 postulates:
1. There is variation in populations. Individuals within populations differ.
2. Variation is heritable.
3. In every generation some organisms are more successful at surviving and reproducing than
other. Differential reproductive success.
4. Survival and reproduction are not random, but are related to variation among individuals.
Organisms with best characteristics are ‘naturally selected.’
If all four of these postulates are true then the population will change from one generation to the next,
i.e. evolution will occur.
The term Darwinian fitness describes the ability of an organism to survive and reproduce in its
environment. Fitness is ultimately measured in numbers of surviving offspring. It’s also important to
remember that an individual’s fitness is measured relative to other individuals of its species.
Consequently, individuals that produce proportionally fewer offspring than others will contribute
proportionally fewer of their genes to the next generation and so relatively speaking their alleles will
decline in frequency in the next generation.
An Adaptation is a characteristic or trait of an organism that increases its fitness relative to individuals
that do not possess it. For them to be selected for, adaptations must have a heritable basis.
Natural Selection and coat color in the oldfield mouse
Is there variation?
The oldfield mouse is widely distributed in the southeastern U.S. It is preyed upon by a variety of
visually hunting predators such as hawks and owls. The mouse displays considerable variation in coat
color both within and between populations across its range. Most populations of the mouse are dark
colored, but populations on beaches and barrier islands have lighter colored coats.
Hoekstra et al. carried out a series of experiments to evaluate the hypothesis that natural selection
favors a match between coat color and background color. There is considerable phenotypic variation in
coat color in oldfield mice.
For natural selection to occur the variation must be heritable. Hoekstra et al. have shown that several
genes affect coat color in these mice. The first gene is the melanocortin-1 receptor gene (Mc1R). This
gene switches between producing a dark pigment (Eumelanin) and a light pigment (Phaeomelanin)
depending on how it is affected by signals from other genes.
If a protein called alpha-MSH binds to the McR1 gene then the dark pigment eumelanin is produced.
If alpha-MSH cannot bind to the Mc1R gene either because of the presence of a different protein called
ASP (agouti-signaling protein) or because a mutation in the Mc1R gene so it cannot bind alpha-MSH then
the light-colored pigment phaeomelanin is produced.
Hoekstra et al. have shown in populations with large numbers of light-colored mice that two mutations
are common: one that prevents alpha-MSH binding to Mc1R and the second a mutant agouti allele that
produces much more ASP than usual.
Both mutant alleles result in light-colored mice. Thus there is a clear genetic basis for the observed
variation in coat color.
Does variation affect fitness?
Does coat color affect the survival and ultimately reproduction (i.e. fitness) of oldfield mice? Two
experiments suggest it does.
Kaufman (1974) carried out an experiment in which pairs of mice (one dark-coated, one light coated)
along with an owl were placed in large cages located in habitats with different backgrounds (light or
dark and with different vegetation densities). In all cases mice that better matched the background
survived better than mice that matched less well.
Kaufman et al. carried out a follow-up experiment in which they made silicone mouse models painted
light or dark to mimic either the dark or light background. They placed the models in different habitats
and measured from beak and claw marks how often the models were attacked. They found clear
differences in attack rates. Models that matched their background were attacked much less.
Thus for oldfield mice all 4 postulates are satisfied. There is (i) variation in coat color and it is (ii)
heritable. There is (iii) differential reproductive success (or in this case differential survival which is a
necessary precursor to reproduction). That differential reproductive success is (iv) related to the
variation (different coat colors survive better in different habitats).
Another example of natural selection: Darwin’s finches
Evolution of beak shape in Darwin’s Finches. Peter and Rosemary Grant’s (and colleagues) have worked
for many years on Medium Ground Finches Geospiza fortis that breed on Daphne Major, one of the
islands in the Galapagos
Postulate 1. Is the population variable?
Finches vary in beak length, beak depth, beak width, wing length and tail length.
Postulate 2: Is variation among individuals heritable?
Remember it is possible that variation can due to both environmental effects and genetic effects. We
can figure out how important genes are by measuring the heritability of traits. Heritability is defined as
the portion of the total variation in a trait in a population that is due to variation in genes. Peter Boag
compared average beak depth of parents with that of their adult offspring and he found a strong
relationship between offspring and parent beak depths. (the slope of the regression line of that
relationship is a measure of heritability [we’ll deal with this in more detail later in the semester]).
Postulate 3: Do individuals differ in their success at survival and reproduction?
In 1977 there was a major drought on the island and 84% of G. fortis individuals died, most from
starvation. In two other droughts 19% and 25% of the population died.
During the drought seed densities declined rapidly and the small soft seeds were consumed first.
Consequently, the average size and hardness of remaining seeds increased over the course of the
drought as the softer seeds were eaten.
Postulate 4: Are survival and reproduction nonrandom?
Do those who survive and reproduce have different characteristics than those that don’t? As the
drought progressed small soft seeds disappeared and large, hard Tribulus seeds became a key food item.
Only birds that had deep, narrow beaks could open these seeds.
At end of the 1977 drought the average survivor had a deeper beak than the average non-survivor and
also a larger body size. Chicks that hatched in 1978 when the survivors of the 19977 drought
reproduced had deeper beaks on average than those hatched in 1976. Clearly, the population evolved.
Variation in weather from year to year on Daphne Major over 30 years has led to variation in the traits
that are favored by selection and as a result the population has evolved over time. Between 1970 and
2000 the population beak size evolved rapidly. It rose sharply during drought then in later years
declined to pre-drought dimensions. In non-drought conditions smaller beaks appear to be more
advantageous as birds with smaller beaks appear to be more nimble in handling food.
Milk drinking: evidence for natural selection in humans
Milk contains the sugar lactose and young mammals produce an enzyme, lactase, to break it down.
Most humans (about 70%) stop producing lactase after weaning, but many western Europeans retain
the ability to digest lactose into adulthood.
Humans began to domesticate cattle in NW Europe about 10,000 years ago and this new food source
favored individuals able to digest milk into adulthood. The frequency of alleles for lactose tolerance are
highest in NW European populations and lowest in SE Europe the in populations furthest from the origin
of cattle domestication.
A similar pattern is found when comparing animal herding societies with nearby non-herding
populations. The herders have much higher tolerance for lactose than their non-herding neighbors.
Humans as agents of selection
Humans act as strong agents of selection. This has occurred through deliberate choice (artificial
selection for desired traits in crops and domesticated animals) and inadvertently through environmental
change.
Artificial Selection. Humans have selectively bred for desirable traits in domestic animals and plants for
millennia. Process has produced our crop plants, garden plants, pets, and domestic animals.
Recall: Darwin closely studied pigeon breeding as a process analogous to natural selection.
Cauliflower, broccoli, kale, brussels sprouts all descended from wild cabbage. All these crops can be
crossed and produce fertile offspring.
Cauliflower: edible bit is the inflorescence or flower stalk. Cauliflower has large dense infloresence. This
results from mutant ‘loss of function’ alleles of two genes that affect flower structure and infloresence
density. Early farmers choosing among their crops selected those with largest infloresences. Process has
resulted in cauliflowers that are homozygous for both loss of function alleles.
Resistance to pesticides
Insects and plants treated with chemicals designed to kill them have rapidly developed resistance.
Heavy spraying creates an environment in which any mutations that offer resistance are strongly
selected for and spread rapidly.
Farmers are now using evolutionary biology to reduce rate of evolution of resistance.Resistance
frequently comes with a cost and in pesticide-free environments non-resistant pests may have an
advantage and outcompete resistant forms.
To maintain non-resistant genes in pest populations farmers are now setting aside pesticide free refuges
that are not sprayed. For example farmers using BT-corn (corn containing a gene that produces a
natural pesticide) must set aside 20% of their plantings as non-BT corn. States in which large areas of
refuges were used have shown much slower rates of BT-resistance in pests than states where smaller
areas of refuges were set aside.
Hunting and fishing as agents of selection
Humans have intensively fished all the world’s oceans and that fishing pressure has resulted in fish
populations evolving in response. For example, because under fishing pressure few individuals survive
to breed late in life, fish such as cod today mature much younger and at smaller sizes than they did 20
years ago.
In a similar fashion selective shooting by trophy hunters of males with larger horns has led to the
evolution of smaller horns in hunted populations.