BIOLOGY 280 Study Guide - Final Guide: Facial Symmetry, Mate Choice, Symmetry In Biology

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16 Nov 2020
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1. Can sexual selection account for human sexual dimorphism? Describe some hypothesized
evolved psychological traits that might have arisen through sexual selection and describe
the kind of evidence that is typically used to test for the hypothesized traits.
Sexual selection can account for human sexual dimorphism. Since we see sexual dimorphism
in ourselves (ex: males are usually bigger than females), then it should also be possible in
psychological traits. In the world of evolutionary psychology, psychologists usually ask: do we
have any evolved biases in our mate choice and is there a biological component in our mate
choice preference? To test these questions, we can digitally manipulate faces of females and
males to give them specific features and test to see which style attracts more of the opposite
sex. For example, when a male’s face was digitally masculinized (by adding more facial hair,
wider jaw, etc.), heterosexual females, on average, preferred intermediate males (a fac between
masculine and feminine features). However, the heterosexual females who were on their
menstrual cycle fertile phase preferred more masculine faces. More of these tests were conducted
and the results showed that in general, heterosexual males prefer more feminized female faces
and heterosexual females prefer more masculine male faces. Evolutionary psychologists say that
maybe there is something evolved about this. Maybe there are male and female mate preferences
are evolved because they ensure that preference for mates that would increase the fitness of the
chooser. Similar tests were conducted by using body types and level of facial hair, etc. However,
this specific test was on college students. In order to solidify the hypothesis of evolved
psychological traits from sexual selection, one may need to conduct tests within difference
cultures and ages.
2. Describe some evidence that male humans possess evolved mating preferences. How and
why might such preferences have evolved?
Based off of a survey data, there is a universal tendency that males look at attractiveness as
more important in the partner than females. In a different survey on age preference, males prefer
a partner that is younger, and females prefer older males. When males were asked about their
preference of female waist-to-hip ratio based on attractiveness, health, desirable as a wife, the
average ratio was calculated to be 0.7 (data based on subjects from US). However, this number is
not universal because when the same test was on a group of people of Hazda (living a
hunting/gathering lifestyle in Africa), the peak of the ratio was around 0.9. Another similar test
was conducted on a group called the Tsimane that live in the Bolivian Amazon as
hunter/gatherers. It’s preferred to look at these people because our evolved psychological traits
may have occurred in the past when we were still living in the wild as hunter/gatherers. So, the
best way to understand these evolved traits is to look at people that are in similar environments
as our evolutionary past. Another test on male mate preference is on facial symmetry. Female
faces were digitally manipulated. and results showed that heterosexual males showed a high
preference for females with very symmetrical faces.
All of the research summary conclude that males prefer females that are younger than the
male, have bright eyes, fuller lips, glossy hair, clear skin, symmetrical facial features, slim waist,
large breasts, and a waist-hip ratio of 0.7. The theory here is that these things male prefer in
mates are reliable indicators of reproductive potential. If, in our evolutionary past, preferred
mates had these features, it would increase their chances of bearing healthy and many offspring
that would be selected.
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3. Why might heterosexual women possess an evolved preference for more symmetrical
males? How do you interpret the finding that a sample of women preferred the scent of
symmetrical males?
In the Thornhill and Gangestad’s experiment on female mating preference, they took a group
of men and measured their ear length, ear width, elbow width, length of all fingers, etc. (their
body symmetry). The theory is that those who were healthy in the womb come out symmetrical.
So, those who are symmetrical have good and healthy genes and that may the reason why others
prefer people that are more symmetrical. After the males were measured, they were given new
shirts to wear for two days without taking a shower, taking it off, or eating anything that would
alter the “natural” odor. After two days, the females were given the shirts to smell and evaluate
to see if smell and symmetry are correlated. The data seems to suggest that females’ preference
for symmetry is positively correlated with probability of conception, which is based on stage of
menstrual cycle. So, the more fertile the female is, the more she will prefer a symmetrical male.
4. How were the methods of systematics (the science of phylogeny) used to infer the origin
of HIV and the timing of this origin?
It turns out that evolutionary biology played a big role in understanding the origin of AIDS,
caused by HIV-1. Shortly after the discovery of HIV, people began to make phylogenetic trees of
HIV. It turns out they are closely related to SIV, which is found in monkeys and apes. The tree
shows that two strains of HIV (1 and 2), are not closely related to one another. All of the viruses
are probably descended from a monkey virus. HIV1 seems to be mixed in with a bunch of
monkey viruses and the same goes for HIV2. The only possible explanation for this pattern is
that the virus jumped from monkeys to humans and from chimpanzees to humans. By using the
methods of systematics, we were able to see that humans got HIV from chimpanzees and
monkeys, probably when humans hunted monkeys (HIV transferred by bodily fluids).
An unrooted evolutionary tree for 159 groups of M HIVs was made to show the genetic
difference between each branch tip from one another in different patients. You can plot the
genetic differences with the year that the samples were collected and the divergence time from
the common ancestor. We can see that the virus has been diverging from its common ancestor
steadily. Since we have this steady molecular clock of increasing divergence from its common
ancestor, we can extrapolate that line back in time to calculate the origin of when the virus
jumped from chimpanzees to humans. Results showed it around sometime between 1920 and
1940.
5. How have the methods of systematics been used to discover how pandemic flu strains
arise and to discover which flu strain is most likely to give rise to the most successful
(commonly transmitted) future flu strains.
Although the flu is less fatal than HIV, there are many more flu infections, so the death poll
is pretty high as well. Influenza A virus is an RNA virus encapsulated by a protein shell with two
types of surface proteins (hemagglutinin and neuraminidase). Hemagglutinin has five antigenic
sites, which are the sites recognized by human immune systems and triggers it to respond. If a flu
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virus that has hemagglutinin with antigenic sites not recognized by our immune system will
make us really sick because our body does not know how to respond to it. The virus is always
evolving so it has antigenic sites that’s avoiding the human immune system response. Every year
when the flu breaks out, epidemiologists take a sample of all the different flus from that year to
see the evolution of the flu from the origin. If you look at nucleotide substitutions in the genome
of the flu virus, you see that they accumulate at a steady rate overtime. This is important to know
because flu shots are the weapons against the flu, so they have to be specific to the genetic
structure of the flu virus that is present in the sample that year. However, vaccines take months
to make and you have to be able to predict, in advance, what the genetic makeup of the flu virus
is going to be in the next year in order to make an effective vaccine.
Based on a phylogenetic tree of flu viruses, it shows that some are able to leave descendants.
In the extinct lineages, the mutations on antigenic sites and non-antigenic sites are about the
same. In the surviving lineages, the ratio of mutations on antigenic sites is much higher than non-
antigenic sites. This shows us that the surviving viruses tend to have substitutions on their
antigenic sites than non-antigenic sites of the neuraminidase protein. They are able to survive
because they evolve to be harder for their antigenic sites to be recognized by human immune
systems. If we look at this year’s viruses and we try to predict where the mutations will be, we
should look closer at antigenic sites. Methods of systematics are important here because they
look at past patterns of mutations in the flu virus to predict mutations of the future flu virus in
order to create a vaccination ahead of time to target that new flu virus. When a new flu arises and
are able to kill anyone, those are called pandemic strains.
By using a phylogeny of the nucleoprotein gene, we are able to see the specific strains based
on their host that group together. Looking at specific branches of human viruses, the phylogeny
shows that the transition from H1N1 (hemagglutinin 1 and neuraminidase 1 strain) to H3 made it
very deadly (around 1968). The question is how H3 got into the human strain that made it
pandemic? Another phylogeny was made based on the hemagglutinin gene and it shows that the
H3 strain in humans came from birds. When specific cells are infected with different virus strains
from different animals, the cells will reform later on with combinations of the different DNA
from the different animals. One important strain is H1N1 that can infect humans, which contains
a combination from multiple pig host viruses. Unlike HIV, these viruses can be transmitted via
air, so it easier to transmit to another host. Unless we understand the evolution of specific virus
strains, it is hard to predict the future strains to make a vaccine to prevent another pandemic. You
must know where and when it began and how its evolving to be more resistant.
6. Describe and explain some explanations, based on evolutionary thinking, for the
continued existence of human diseases and other health problems.
The evolutionary approach can help us understand why we get sick. One reason for
vulnerability to sickness is that idea that selection is slow, and the speed left some consequences.
Most of our evolutionary history and our body evolution was in a different environment. Because
of this, there is a mismatch with the modern environment in the sense that our bodies have yet to
evolve with the modern environment. Also, the pathogens that we encounter will coevolve with
us, so our bodies are never in perfect immunity. At a given time, our bodies might be behind the
pathogens. Another reason for vulnerability is the limits placed on selection. There are
constraints with selection. For example, there is a developmental constraint of the lungs where
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