Lecture 4 Flashcards
Sexual selection was initially described by who
Sexual selection was initially described by Darwin
what does sexual selection come from
Arises from the observation that many animals develop features whose function is not to improve survival but to maximize their reproductive success
does sexual selection always increase survival
in some cases, the feature may actually decrease survival, while still improving fitness because it increases reproduction
what is Intrasexual selection (within a sex)
individuals intimidate, deter or defeat same-sex rivals:
territory defence
fighting or other direct competition
what is Intersexual selection (between the sexes)
individuals make themselves more attractive to the opposite sex
mate choice/mate preferences
In general, selection can act not just on morphology, but also on_____
In general, selection can act not just on morphology, but also on behaviour
give an example of sexual selection
Females preferentially mate with large clawed males
Over time (generations) this will skew the population toward larger claws
Any mutation that allows for even larger claws will thrive
Female preferences for larger claws drives an increase in claw size
Natural selection may limit increase in claw size
the crab with the claw too large will be eliminated because now they cant escape from predators, get food, dig a borrow, etc… even though the females prefer the larger claws… so they will end up eliminated
natural selection puts a limit on the distribution
how does sexual selection stay in line
sexual selection pushes against natural selection and vice versa
so this explains how we might end up with such elaborate traits, but it doesn’t explain how it would get started
what are some questions remaining
Why are there preferences?
Why does one sex tend to be choosy?
who is the flashy sex
The flashy sex: whichever sex has greater VARIANCE (or SKEW) in reproduction will evolve more elaborate traits
where is the mean on a graph
the mean is were half of the points are above and half are below (dotted line)
what does the variance tell us on a graph
the variance tells us about the width of the distribution (the width)
what does the skew tell us on a graph
the skew tells us about the shape of the distribution (the shape wont always have the normal distribution (bell curve))
The greater the variance, the _____ apart the extremes are
The greater the variance, the farther apart the extremes are
The greater the skew, the ______ asymmetric the distribution
The greater the skew, the more asymmetric the distribution(if it is sort of mushed to one side or the other… blue curve)
if the average reproduction success is equal (every offspring has a mother and father)
what is the reproduction skew like
average reproduction success is equal (every offspring has a mother and father)
reproductive skew is high for males because some males mate, some do not
if the average reproduction success is equal (every offspring has a mother and father)
what is the variance like
reproductive variance is similar because males and females form pairs, work together
what are the 3 situations/possibilities that sexual selection can result in
everyone gets to mate and variance is very small
some mate and some don’t (males) but all female mate (with only a select few males) so this is skewed so we have more variance in males (more flashy)
some mate and some don’t (females) but all males mate (with only a select few females) so this is skewed and we have more variance in females (more flashy)
who has more investment, typically
By virtue of the size and content of eggs vs. sperm, it is often thought that females have a greater investment in reproduction
There are definitely more convincing examples of females having greater investment, in particular in species where there is greater MATERNAL than PATERNAL care
are mammals more flashy
Here, I should point out that in mammals males might not be more “flashy” but rather be bigger and more aggressive and territorial
what did Bateman hypothesize
that there was a relationship between mating success and reproductive success
And that this relationship could differ between males and females
what does Bateman’s Gradient say
In species where investment might limit the reproductive output of females but not males, mating more frequently doesn’t improve reproductive success for females, but it would for males
Predicts that sexual selection will be stronger on males
In species where investment might limit the reproductive output of males
Predicts that sexual selection will be stronger on females
The flashy sex: whichever sex has ______-
Greater VARIANCE or REPRODUCTIVE SKEW (some males mate, some do not) Or, for biparental species for example, a steeper BATEMAN’S GRADIENT
Females that are not able to increase reproductive success with additional mating may be able to improve the quality of offspring (and therefore their reproductive success) by choosing doing what
Females that are not able to increase reproductive success with additional mating may be able to improve the quality of offspring (and therefore their reproductive success) by choosing a better mate
is Reproductive success measured only y the number of offspring
Reproductive success is influenced not only by the number of children a female has but also the number of grandchildren, great grandchildren, etc is influenced not only by the number of children a female has but also the number of grandchildren, great grandchildren, etc
so what makes a better mate
theories include;
Sexy Sons (also known as Fisher’s runaway selection model):
Good Genes/Immunocompetence hypothesis
Pre-existing sensory Bias and Sensory exploitation
what is Sexy Sons (also known as Fisher’s runaway selection model):
In a population, females show a preference for a particular character (and mate with males displaying that character)
The sons of those females will display that character
The daughters of those females will display the preference
Over time, the preference and the trait will become genetically coupled
And may continue to co-evolve (males produce more elaborate versions of the trait, females prefer more elaborate versions of the trait)
This hypothesis is thought to produce extreme traits that are contrary to (and eventually limited by) natural selection
what is Good Genes/Immunocompetence hypothesis
William Hamilton and Marlene Zuk, 1982:
Some males are genetically more resistant to parasites or infection
Those males would survive longer and have more offspring
AND they would be less likely to infect females
AND their offspring would also survive longer, reproduce more
Males produce a trait that signals their health or vigor
Females prefer males which display that trait
There are many steps to demonstrate this:
Parasites are costly
Heritable variation in immunity
Ornaments depend on the parasite
Females choose males with fewer parasites
But there is some support of this hypothesis in a handful of species
what is Pre-existing sensory Bias and Sensory exploitation
Michael Ryan, 1990
Females have a bias for particular types of sensory stimuli (e.g. particular colors or sounds). These biases result from the way that their brain is set up and has evolved for reasons outside of sexual selection (for example, for foraging)
Males produce a signal that stimulates that part of the female sensory system and females preferentially mate with males producing the signal
how is Pre-existing sensory Bias and Sensory exploitation entirely different from that of the good gene and runaway hypotheses
The preference and trait do not evolve in concert (no co-evolution)
The signal doesn’t indicate anything about the “quality” of the male
what is Phylogeny
a history of organismal lineages as they change through time
how is phylogeny different from a pedigree
Similar to a pedigree or family tree, except that instead of marking each generation, we indicate speciation events when big changes occur
what is the difference between a derived and ancestral trait
DERIVED trait is one that is more recent
what is PARSIMONY
how to get to the pattern of trait expression in the descendants with the fewest changes
give an example of parismony
Let’s say our green arrow indicates the appearance of a preference for chucks
No preference —> Preference for Chucks
this is the LEAST Parsimonious possibility
One Last Question, why would ancestral females prefer chucks?
It has to do with the way their ears are set up
females were programmed to be biased towards the chucks
Thus, with regard to the sensory bias/sensory exploitation hypothesis
Male Tungara frogs have “exploited” an ancestral sensory bias for low frequency sounds that is present in the female auditory system
we’ve been talking about “traits” as unitary things i.e. that females prefer a particular trait
why is this
This is mostly for simplicity, as clearly there are often multiple traits that appear to be selected
Determining which traits are selected can be challenging
give an example of this
is all of the elaborate plumage in a peacock under sexual selection, or is there pleiotropy?
what is Pleiotropy:
when a single gene influences many phenotypic traits
what is an example of pleiotrophy
Frizzle trait in chickens Chickens with feathers that turn out (look “frizzled”) also have high metabolic rates and blood flow, greater digestive capacities, and abnormal body temperatures
Pigmentation and deafness in cats: Cats with white fur and blue eyes are also often deaf
Regardless, it is important to remember that Natural Selection is not the only mechanism of evolution, what is another mechanism
Genetic Drift
what did Stephen J. Gould and Richard Lewontin argue
They argued that too often scientists assumed that EVERY trait must be adaptations
This meant that when trying to understand the evolution of a trait, scientists would come up with one “just-so-story” after another in order to explain how that trait was adaptive
While not denying the existence of adaptations, Gould and Lewontin argued that not everything had to be an adaptation
They used as a metaphor the spandrels in the San Marco Basilica in Venice
spandrel, as they described it, is the extra space that is made when you create an arch in a rectangular space (for example the red areas below)
The San Marco Basilica has lots of arches as well as domed ceilings so there are lots of spandrels, including some 3D ones
The spandrels all have paintings on them
So if you knew nothing about the basilica, you might imagine that spandrels were created in order to provide a place for the paintings
But Gould and Lewontin argue that would be incorrect as the spandrels actually exist just as a consequence of the architecture
Similarly, the short arms on a Tyrannosaurus Rex
Had they arisen de novo (that is, if T. Rex was the first dinosaur with arms) they might be
an adaptation
But, because they are just smaller than other arms, maybe they became small because of some other change (or adaptation) but not because selection was acting directly on them
This idea has been hotly debated ever since
In fact, there has even been considerable discussion by architects as to whether the metaphor is true architecturally
what is a NULL hypothesis
our population when nothing is happening
why would we need to know the NULL hypothesis
if we know what SHOULD be happening, we can see if evolution is occurring
what is the Hardy-Weinberg Equilibrium
We start with a tiny, imaginary population with:
8 individuals, 2 alleles each = 16 alleles
Frequency of G = 8 / 16 = 0.5
Frequency of g = 8 / 16 = 0.5
If these individuals mate completely at random, and there is no evolution happening (no drift, no selection, no mutation, no gene flow, nothing)
We can predict the frequency of each genotype in the next generation (hint: it will be exactly the same as it is in this generation!)
Why would you want to calculate this? (Hardy-Weinberg Equilibrium)
Compare the predicted frequency and the actual frequency to determine whether the population is evolving
Can also use the equation to estimate the frequency of “carriers” (heterozygotes) of rare recessive diseases
what equation is used to calculate the Hardy-Weinberg Equilibrium
p^2+ 2pq + q^2= 1
explain genetic drift: evolution by chance
Now, our tiny, imaginary population has
6 individuals, 2 alleles each = 12 alleles
Frequency of G = 8 / 12 = 0.66 Frequency of g = 4 / 12 = 0.33
We can calculate how it will change using: p^2+ 2pq + q^2= 1
Now, in the next generation, the number of each genotype will change
Now, our tiny, imaginary population has
5 individuals, 2 alleles each = 10 alleles
Frequency of G = 8 / 10 = 0.80 Frequency of g = 2 / 10 = 0.20
Again, we can calculate how it will change using:
p^2+ 2pq + q^2= 1
Let’s say something happens, just by chance, to our population.
Maybe the pink ones get caught in a flood
Now, in the next generation, the number of each genotype has changed And then, maybe there’s an earthquake
Current allele frequencies:
Frequency of G = 0.80
Frequency of g = 0.20
Starting allele frequencies:
Frequency of G = 0.50
Frequency of g = 0.50
Just by chance:
The flood and earthquake didn’t select against phenotypes
Those individuals were just unlucky
This is the basic idea behind genetic drift
Now, in the NEXT generation, the number of each genotype will REALLY change
a couple key points
Because effects are random, completely by chance, would be just as likely for things to go the other way. i.e. for the gray lizards to disappear.
Also, the effects of drift are stronger in small populations. That’s why we could change our tiny population so quickly.
what are the Two Ways that genetic drift can result in big changes
Genetic bottleneck
Founder effect
what is Genetic bottleneck
An extreme example of drift where an event results in a big decrease in:
population size
genetic diversity
what is the founder effect
Similar to a genetic bottleneck except here a subset of the population “founds” a new population somewhere else, like on an island
Still results in a decrease in:
population size
genetic diversity
what is the definition of the founder effect
founder effect— a founder population contains only a subset of the alleles present in the original population. I.e. there is less genetic diversity in the founder population
There is less variation for selection to act upon – influencing adaptability of the population
Allele frequencies are more susceptible to chance effects – bigger effects of genetic drift
so, just by moving to the island, there are already some changes in the allele frequencies because of the founder effect
Let’s say a mutation leads to longer claws
These individuals can climb trees
Once in the trees, there is less competition for food
Perhaps they eventually establish a tree-dwelling population
Once on the island our island population can begin to adapt to their new environment
In addition to long claws, these iguanas also evolve to be smaller
Meanwhile, the beach iguanas might also adapt to be less visible to predators
Or there could be sexual selection for increased size or ornaments
And suddenly (over millions of years) we have very different iguana populations on the island versus on the mainland
what can separate species
Natural barriers can serve to separate species
how do Natural barriers serve to separate species
Natural barriers can serve to separate species
they prevent gene flow between populations
what is speciation
Speciation occurs when gene pools are separated, gene flow is restricted and populations diverge genetically over time
Can result from a non-adaptive evolution (genetic drift)
e.g. founder effect plus additional mutation over time
Or from natural or sexual selection
Or all of the above!
what is a species
a population whose members can interbreed to produce viable (can live after birth) and fertile (can reproduce) offspring
what is Reproductive Isolation
mechanisms that prevent gene flow between members of different species
what are the 2 types of reporductive isolation
pre-zygotic or post-zygotic
what is a Zygote:
cell formed by the joining of two gametes (i.e. sperm and egg)
what is Pre-zygotic mechanisms
prevent formation of a zygote
they do so by preventing interactions, mating, or the actual union of gametes
There are a lot of pre-zygotic isolating mechanisms
wat is Post-zygotic mechanisms
occur after the formation of a zygote
the zygote either doesn’t survive or cannot reproduce
Why are there more pre-zygotic than post-zygotic mechanisms
how is mating costly
Finding and attracting a mate takes time and energy
Producing eggs and offspring can require even more time and energy
who should selection favour
Selection should favour individuals that are better at identifying and mating with their own species
what happens to Individuals who mate outside their species
Individuals who mate outside their species will produce fewer offspring, their alleles will be less represented in the next generation
what is allopatric speciation
Reproductive isolation is often set up by some sort of physical isolation (e.g. on an island)
This is called allopatric speciation
what is sympatric speciation
this is when species arise in the absence of some sort of physical isolation or barrier (long debate wether this even exists)
i.e. species arise even though individuals live in the same place
difficult to prevent gene flow when there aren’t physical barriers between individuals
THIS RARELY OCCURS
what is parapatric speciation
parapatric speciation is when species live immediately adjacent to each other and only share a small contact zone
You can think of it as being somewhere between allopatric (no overlap) and sympatric (complete overlap)
what does parapatric speciation result in
It results in really interesting patterns of species distribution that we call ring species
what is a ring species
In biology, a ring species is a connected series of neighboring populations that can interbreed with relatively closely related populations, but for which there exist at least two “end” populations in the series that are too distantly related to interbreed
what is the definition of Allopatric speciation–
new species result when populations are physically separated –most common
what is the definition of Sympatric speciation
—new species arise when populations are in overlapping ranges –less common, difficult to prevent gene flow when there aren’t physical barriers between individuals
what is the definition of Parapatric speciation
Parapatric speciation– new species arise sequentially when populations only overlap in a small contact zone.
The species at the start and end of the sequence are very different –also less common but creates really interesting patterns
Before Darwin and Wallace, one theory of evolution was proposed by who
Jean-Baptiste Lamarck
what did Jean-Baptiste Lamarck propose
Over a single lifetime, a giraffe would stretch to reach higher leaves, making its neck longer
These use-dependent changes would be passed down to the offspring
what are epigenetics
Heritable changes in gene expression and function without alterations in the DNA sequence
For example: DNA is usually wrapped around histone proteins
keeps DNA compact
regulates gene expression
what is Histone modification
Changes in the attachment of methyl groups to histone proteins or DNA influence which genes are expressed and which are not
give examples of epigenetics
Daphnia that have been exposed to a predator grow spines
spine expression persists over many generations, even in the absence of predators
Agouti mice overeat and become obese (and have a number of obesity related diseases)
And they have kids that overeat and become obese (and have a number of obesity related diseases)
Agouti moms fed food rich in methyl donors have slimmer, healthier offspring
Baby rats with attentive moms tend to be brave and calm (for rats, anyway)
High nuturing mothers raise high-nuturing offspring
Offspring of high-nuturing mothers have different methylation patterns in the brain than baby rats whose moms were less attentive
