Unit 3: Evolution
Lesson 5: Types of Natural Selection
Evolution occurs when natural selection acts on the genetic variability within populations. Genetic
variation arises by chance through genetic mutations and recombination. The process of natural
selection, however, does not occur by chance. The environment favours certain individuals over others.
Just as human breeders have artificially selected for specific characteristics in domesticated plants
and animals, the environment selects individuals that are better suited to their environment.
Question: Why is there a higher incidence of sickle-cell anemia in Africa over anywhere else in the
world?
Ex: Sickle-cell anemia and malaria.
In humans, the sickle-cell allele resulted from a single base mutation in the DNA coding for haemoglobin.
Heterozygous individuals for sickle-cell anemia are resistant to the disease malaria. Therefore, those who carry the allele
for sickle-cell anemia have a better chance of survival against malaria than those who do not carry the allele.
Let S represent the normal phenotype and let s represent a sickle-cell anemia. Therefore SS – normal for sickle cell by
susceptible to malaria. Ss does not express the sickle cell disease, but is also resistant to malaria. ss may be resistant to
malaria, but also have sickle-cell anemia and thus have health problems and shorter life span. So, people who are SS and ss
(homozygous) will have less chances of surviving than those who are heterozygous. The homozygous individuals are selected
against while the heterozygous individuals are selected for.
Types of Selection
Selective pressures may result from any number of abiotic or biotic factors. Selective pressures can
result in different patters of natural selection. List some examples of selective pressures.
Ex: diseases, climatic conditions, food availability, predators, choice of mate etc.

Directional Selection: selection that
favours an increase or decrease in the
value of a trait from the current
population average

Stabilizing Selection: selection
against individuals exhibiting traits
that deviate from the current
population average

Disruptive Selection: selection that
favours two or more variations of a
trait that differ from the current
population average
The figure shows examples of selection in a
population of hummingbirds. (a) In a new
environment with longer flowers, directional
selection will favour individuals with longer
bills. (b) In stabilizing selection, individuals with an average bill length are favoured. (c). In disruptive
selection, the environment favours individuals with long and short bills over individuals with average
bill lengths.
Sexual Selection: differential reproductive success caused by variation in the ability to obtain mates;
result in sexual dimorphism, and mating and courtship behaviours.
Sexual selection is the favouring of any trait that specifically enhances the mating success of an
individual. Sexual selection often leads to the males and females of a species evolving appearances and
behaviours that are quite different from each other.
The most common forms of sexual selection are female
mate choice and male-versus-male competition. In many
species, females choose mates based on physical traits,
such as bright colouration, size or behaviour.
Sometimes, males must engage in mating calls or mating
rituals to attract a mate. In other species, males have
evolved larger body size and other physical attributes
such as antlers that often used in direct competition.
The males often fight each other to establish control
over a territory that is home to females with which
they mate. The difference between success and failure
can be dramtic. For example, a very successful male elephant seal may mate with dozens of
females each year and hundreds of femles in his lifetime, while a weak male may live a longer life
but produce no offspring. In this case the genes of the short-lived but dominant male are destined
to beome more common in succeeding generations.
While traits such as bright colouration and large antlers can be favoured by sexual selection, they
are often a disadvantage when it comes to longevity. Avoiding predators is not made easier by
brilliant plumage or a distinctive song.
Sexual selection is not limited to animal populations. Colourful flowers and scents are the most
obvious sexual features of plants. Rather than attracting mates, these features attract
pollinators. By maximizing their chances of being pollinated, plants have a greater likelihood of
contributing more alleles to the next generation’s gene pool.
Homework: Research at least 3 different examples of evolution in the plant, animal, and one other
kingdom (protists, archae, eubacteria) world. Be prepared to give your example tomorrow.
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Examples of Evolution:
Birds with pumping of feathers and humming
Birds that collect treasures
Crabs with the large claw
Tungara frogs – mating calls
Fringe-lipped bats – mating calls
Silver-back gorilla
Males aren’t always bigger – spiders, prey mantis – sacrifice their bodies after mating; ensures
the mom is fed
Peacocks and peahens
Antlers
Manes on male lions
What about the weak? How do we explain altruism?
- Killing of offspring but that sucks for females
Examples:
1. Mexican Minnow fish: In the small pools of water along a mountain, you will find sexual and
asexually reproducing minnows. The asexual minnows bear a higher parasite load meaning that
asexual minnows will have more parasites then the sexual reproducing minnows. The thought
here is that asexual minnows lack the genetic variability to combat the evolving parasites and
therefore are more susceptible.
BUT, and interesting thing happened. There was a large drought that killed off many of these
fish except some of the higher pools. When the water returned and you looked at the fish at
the top mountain pools, you found that the sexually reproducing fish actually carried the higher
parasitic load compared to the asexual ones. This was perplexing because the idea was that
sexual reproduction provides the species with the genetic variability to combat and evolving
parasitic group. The only way to explain it was to look closely at this sexually reproducing
minnow. It turns out that after the drought, when there were so few minnows remaining, the
fish had to inbreed thereby reducing the genetic variability. When the scientists took fish
from other ponds and introduced them to the higher ponds, then that increased the genetic
variability and then the sexual minnows lost the higher parasitic load and the asexual ones bore
that load.
Though very close in genetic relationship and virtually next-door neighbors, chimpanzees and a less-wellknownspecies called bonobos in Zaire are socially poles apart. Only identified as a species separate from chimps in
1929, bonobos intrigue biologists with their easygoing ways, sexual equality, female bonding, and zeal for
recreational sex.
How did bonobos, which live in humid forests south of the Zaire River, evolve such a different social structure from
chimpanzees since the two species split about 2 million years ago? Male dominance plays a big role in chimp society.
Disputes are often resolved by threatening displays or by fighting. Female chimps lead a life much more solitary than
that their bonobo cousins, and are sometimes harassed by the much larger males. Sex is strictly about reproduction,
and reproductive tactics can include infanticide -- the killing of offspring unrelated to a male chimp. Infanticidal
individuals remove potential competitors to their own offspring, and the mother, without an infant to care for, will
become available for mating again much sooner.
In contrast, bonobo society is marked by the strong bonds that develop between unrelated females and by almost
constant sexual activity amongst all members of a group. Bonobos apparently use sex to reinforce bonds within the
group and to resolve conflict. What evolutionary advantages do these behaviors offer?
Seeking the answer to that question, researchers noted that infanticide is almost unknown among bonobos. Their
constant sexual activity obscures paternity, removing the incentive for infanticide, and the pervasive bonding of
female bonobos, who form coalitions for mutual support and protection, removes the opportunity. Preventing
infanticide is a huge evolutionary advantage for bonobo females, because more of their offspring will survive.
Why, then, have chimps not evolved this social structure? The answer may lie in the history of the habitats they
occupy. Both species of primates live in tropical forests along the Zaire River -- chimps north of the river, bonobos to
the south. Their environments seem to be quite similar today. But about 2.5 million years ago, there seems to have
been a lengthy drought in southern Zaire that wiped out the preferred food plants of gorillas and sent the primates
packing. After the drought ended, the forests returned, but the gorillas did not.
Chimpanzees in this environment south of the river had the forest to themselves, and could exploit the fiber foods
that had previously been eaten by gorillas -- foods that are still eaten by gorillas to the north. With this additional
food to tide them over between fruit trees, they could travel in larger, more stable parties, and form strong social
bonds. They became bonobos.
On the north side of the river, the chimps had to share their niche with gorillas, which eat the fiber foods. The
chimps have to compete for fruit, and occasionally meat, food resources that tend to be widely scattered. Female
chimps disperse into the forest with their infants to find enough to eat, and cannot spend time together to forge
strong bonds. The changes in social behavior that occurred in response to this environmental factor may be what led
chimps down a different evolutionary path, toward a society more prone to violence.
A subtle difference in environment, it appears, had profound implications for their evolution.
OTHER RESEARCH TOPICS FOR HOMEWORK:
- Camouflage mantids
- Galapagos finches
- Edith’s checkerspots
- Blister beetles
- Gartner snakes
- Cichlid fish
- Deer antlers
- Asian jungle fowl
- Pygmy elephants, rhinocerous
- Blue-footed boobies
- Deer mouse
- Antelope squirrels in the grand canyon
- Sea hourses
- Cactus
- Lizards
- Bees
- Pacific salmon
- Gorillas
- Apes
- Beetles
- Butterflies
- Birds – lots of types
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Bring the specific name of the plant or animal
Explain its evolutionary example
Try to identify what type of selection plays a role in that species.