Lec 2 Flashcards
What are the key components of Darwin’s theory of natural selection?
More born than can survive; struggle for existence; variation via natural selection
What are three broad classes of evidence used to support evolution and natural selection?
Artificial selection
Island radiation
Fossil record
Behavioral data
Morphological data
Embryological data
Molecular data
What are two examples of features Darwin had trouble explaining by natural selection?
Evolution of complex traits
Traits with little importance (i.e. male nipples)
Elaborate traits with no fitness advantage
Source of variation
Natural Selection and Adaptation
Individuals with alleles best suited to their environment survive and reproduce
Components of Natural Selection: 4 main observations led to Darwin;s discovery of Natural Selection
10 Individuals within species are variable
2) Some of these variations are passed on to their offspring - they are HERITABLE
3) In every generation, more offspring are produced than can survive to reproduce
4) Survival and reproduction are non random - individuals carrying specific variations best suited to their environment survive and reproduce the most (have HIGHER FITNESS)
Case Study: California Drought
California declared a state of emergency every year from 2000-2004
Plants were particularly hard hit - drought SHORTENED THE GROWING SEASON of Brassica rapa, a species of mustard in Souther California
In particular, later in the summer became too hot and dry for mustard to grow
What pattern is shown in this figure?
Descendant (post-drought) plants flower earlier than ancestral plants
Based on your understanding of evolutionary theory, what might you hypothesize the response to intense drought to be in mustard populations?
Flower earlier
Is this the pattern you would hypothesize under natural selections?
Yes - a shorter growing season due to drought should select for plants that flower earlier
Drought selects for earlier flowering time
Plants derived from the seeds of the 2004 parents flowered earlier, on average, than plants derived from the seeds of the 1997 parents
What is variation?
Much of variation is caused by differences in DNA sequences; Darwin did NOT know what the cause of variation was
What does variation look like at the genetic level?
All of the alleles (or genetic variants) from all of the individuals in this population make up the GENE POOL
AAACTGTTT
AAACTTTTT
AAACTATAT
Key vocabulary: Phenotype
The outward appearance of an organism
Key vocabulary: Genotype
The genetic constitution of an individual organism
Genetic variation underlying the phenotype
Phenotype vs Genotype
Natural selection does not directly act on genotypes, but rather it acts on phenotypic differences among the individuals in a population
Phenotypes are sets of observable characteristics
They represent the interaction of genes and environment
To understand natural selection, we have to understand how the interaction between genotype and environment determines the phenotype
Key vocabulary: Heritability
The mount of variation in a phenotype due to genotype
How much is CAUSED by the genotype; some variation NOT heritable because it is due to the environment
Not all variation is heritable: Phenotypic Plasticity
The same genotype expresses different phenotypes in different environments
i.e. a house plant put in a dark corner with no water won’t grow very tall; take the same plant and put it outside in the sun with lots of water, it grows tall; plant has the SAME genotype but is provided different nutrients to express different phenotypes
Phenotypic plasticity: Reaction Norms
Norms of reaction - each line shows a different genotype. The x-axis is DIFFERENT ENVIRONMENTS, and the y-axis is EXPRESSION of phenotype. The same genotype looks different in different environments
How we show reactions in particular environments; basically a graph
Each line is a _______
Reaction norm
Do most genotypes show a similar relationship between leaf area and light intensity? (Based off graph)
Yes, most genotypes have the largest leaf area at intermediate light intensity
Resemblance among relative: Genes vs. environment
Second of Darwin’s observations that led to natural selection is that offspring resemble parents
Language APPEARS to be heritable: offspring resemble parents, and there are clear differences among populations
Language is NOT heritable - if you raise the child of English-speaking parents in a Spanish-speaking household, the children will speak Spanish. This trait is 100% environmentally determined
Only traits that have a genetic basis can evolve over time
Traits that are 100% environmentally determined CANNOT be acted on by natural selection
Is language heritable?
NO
100% environmentally determined
Why can’t 100% environmentally determined traits evolve by selection?
a) They are too variable and selection would be ineffective
b) Evolution by selection is change in allele frequencies over time and there are no alleles in these traits
c) Offspring don’t resemble parents in environmental traits
b) Evolution by selection is change in allele frequencies over time and there are no alleles in these traits
Only traits that are GENETICALLY determined can be selected
What is fitness?
Fitness: RELATIVE genetic contribution of individuals to future generations
2 components of fitness:
1) Survival
2) Reproduction
For your genes to make it to the next generation, you need to SURVIVE and PRODUCE OFFSPRING
Relative Fitness
Fitness is the RELATIVE genetic contribution of individuals to future generations
If individual fitness > mean population fitness, relative fitness HIGH; if individual fitness < mean population fitness, relative fitness LOW
Relative Fitness = Individual Fitness/Mean Population Fitness
Mean Population Fitness = Sum of each population/number of populations
This means that how fit you are doesn’t just depend on who many offspring YOU have, it depends on the ratio you have to everyone else
In this course, we consider how selection affects relative fitness
Who has the higher relative fitness within its population, the WHITE MOUSE in population 1 or population 2?
Pop 1: W = 10,000 offspring
B = 1000 offspring
Pop 2: W = 100 offsrping
B = 10 offspring
Rel. fitness for pop. 1:
Mean = (10,000+1,000)/2 = 5,500
Rel. fitness = 10,000/5,500 = 1.82
Rel. fitness for pop. 2:
Mean = (100+10)/2 = 55
Rel. fitness = 100/55 = 1.82
They have the SAME FITNESS
Fitness means the “strong” survive T or F
FALSE
Darwinian Fitness
The ability of an individual to survive and reproduce in its environment
NOT the same as reproductive fitness
Fitness measures:
The variable success in reproduction of the individuals in a population, comparing them to each other
It is NOT how many offspring YOU have, it is how many you have COMPARED to everyone else
Darwinian fitness is ___________ - your fitness only matters in relation to other individuals in your population
Relative, not absolute
Fitness (is, is not) fixed
is NOT
If your environment changes, your fitness may also change
Why do we care about fitness?
Fitness is the currency of natural selection/evolution
Examples of natural selection: Peromyscus polionotus
Shows all 4 of Darwin’s observations:
-Variation in the phenotype (gradient of light and dark colors)
- Variation is heritable
- -Phenotype variation can arise through genetic variation along, variation in environment alone, or through a combination of both
- -ONLY heritable variation can be acted on by natural selection
- Genetic variation and phenotype
- -Beach mice typically carry a recently evolved form of the Agouti allele and a mutation at the Mc1R gene that contributes to their lighter coat color
- -Underlying genetic basis of coat color is the Mc1R gene (receptor pathway)
- -Mutation is genes to results in this lighter color
Are there fitness consequences to color variation (is there differential survival in different environments)?
YES, population will evolve to have coat to suit environment
Put mice down on varying colors of soil: shown percentage of mice captures by predators
Light mice captured more in dark environments; vice versa is true as well
Fitness consequences: Experimental test using mouse models
Natural selection leads to populations of pale mice in light habitats and dark mice in dark habitats
Dark mice attacked in light habitats, same for light in dark habitats
Consequence of natural selection:
-Light mice in light environments will survive and be more fit; proportions of light mice will go up in light habitats
What does relative fitness mean?
Who has the higher relative fitness within its population, the WHITE MOUSE in population 1 or population 2?
Pop. 1: W = 10,000 offspring
B = 15,000 offspring
Pop. 2: W = 10 offspring
B = 5 offspring
Rel. fitness 1:
Mean = (10,000+15,000)/2 = 12,500
Rel. fit = 10,000/12,500 = 0.8
Rel. fitness 2:
Mean = (10+5)/2 = 7.5
Rel. fit = 10/7.5 = 1.33
Population 2 has the higher relative fitness
Adaptation
Adaptive means “reproductively advantageous for individuals” or “beneficial for the genes underlying development of the trait”
An adaptation is a trait associated with the highest relative fitness in a given environment
-You will have more offspring survive if they have the particular ADAPTATION that helps with survival
Natural selection is the only mechanism that consistently causes adaptive evolution
-Evolution can occur via processes other than natural selection; however, natural selection is the ONLY type of evolution that leads to adaptive evolution since it isolates the ones that work and those that don’t
Adaptations ___________ to be substantial modifications from the surrounding population’s average for a trait
Do NOT need
______________ in size, shape, color, strength, speed, intelligence, or cooperative behavior over time can drive a population to a considerable evolutionary change
Very small alterations
Adaptations to living in dry environments
Sharp spines to deter herbivores
Waxy cuticle prevents water loss
Accordion-like structure allows the plant to expand and contract with water availability
Flower entice pollinators
Low surface area-to-volume ratio minimizes evaporative water loss
Shallow root system collects water after infrequent rains
Adaptations vary among species
Traits that are adaptive in some species may not be adaptive in others
There is no single “perfect” phenotype that everyone is working toward
Brain size is linked to intelligence and evolutionary success in humans, but in fruit flies large brains reduce fitness as they require more energy to maintain and the flies do not benefit from the extra brain power
Determining whether traits are under natural selection and are adaptive can be challenging
We can easily make up “stories” about why specific traits might be adaptive in a particular environment
It is important to test falsifiable hypotheses in a rigorous, scientific way, ideally with experiments or other forms of manipulation
Adaptations are:
a) Any aspect of the phenotype
b) Traits that always increase fitness in all populations
c) Traits organisms acquire to enhance their fitness
d) A trait associated with the highest relative fitness in a particular environment
d) A trait associated with the highest fitness in a particular environment
Selection on life history strategies
David Reznick, Anne Magurran, and John Endler did guppy experiment
Trinidadian guppy system: Has been studied for many years now; same guppies found at fish store
Guppies live in streams in Trinidad; little pools separated by waterfalls
Below waterfalls, there are predators
Higher up on stream, predation pressure is far LESS
Trinidad guppy populations face different environmental pressures
Natural selection should favor different traits in different populations - different traits should be adaptive in different environments
“Life history” traits are things like timing of breeding, number of eggs, size of offspring, age of reproduction
Downstream (high-predation_ fish mature faster, and females produce more broods with smaller offspring
What is a testable hypothesis for why females produce more clutches and smaller offspring at the downstream site?
1) Females have evolved to produce many small offspring downstream, because predation risk is high. Having many offspring increases the likelihood that some of them survive.
2) Females upstream do not like to have many offspring, and so they choose to have fewer, larger offspring
3) Females at the downstream site have brighter colors than females upstream because there is more food available
1) Females have evolved to produce many small offspring downstream, because predation risk is high. Having many offspring increases the likelihood that some of them survive.
Hypothesis:Females have evolved to produce many broods with smaller offspring when predation risk is high because it increases the chance that some offspring will survive
Females at low predation sites can invest more resources in fewer offspring, and therefore have smaller broods of larger babies
Experimental Manipulation
Moved downstream (high predation) fish upstream to low predation pools
Studied these fish for 12 years
Measured change in number of offspring and offspring size over time
Found that over time in these new populations, downstream-derived females produced fewer, bigger offspring as predicted
The environment changed, and a different life history strategy was under selection
IMPORTANT NOTE: Individual females did NOT start producing fewer, larger offspring. The population produced fewer, larger offspring over time
What that means is that the females that naturally had fewer, larger babies had higher relative fitness than the females with more, smaller babies
Clutch size and offspring size were HERITABLE, so those variants were passed on
Over time, the population average offspring size and clutch size change
Predictions
When you generate a hypothesis, you want to clearly identify what evidence would falsify your hypothesis, and what might support it
Here our hypothesis would be falsified if, when the high-predation fish are moved to a low-predation site, there is no change in their clutch size or offspring size over time - then there would be no evolutionary response to these different predation environments
Our hypothesis would be supported if we saw that over time, the transplanted population evolved to have fewer, larger offsrping
This would NOT prove our hypothesis - we might be able to think of other explanations for the pattern. But it would allow us to rule out some possibilities and open the door for further study
What happened to school size
School size is the number of fish in a group. The plot at right shows school size for high, medium, and low predation sites. At which site is school size largest?
1) High predation
2) Medium predation
3) Low predation
4) No difference
1) High predation
Behavior also evolves
in addition to offspring size, guppies increase thier group size at high predation
-Guppies live in much larger groups when predation is high
Guppies transplanted from a high predation stream to a no-predation stream evolved behavior similar to what was observed at low-predation sites - they had smaller school sizes
-Over time, the guppies that were transplanted from downstream (high predation) to upstream (low predation) gathered SMALL groups
Experiments showed these behaviors are heritable
Natural selection in the lab
Evolution in E. coli has been studied in the lab for over 60,000 generations (more generations than there have been of humans in all of history)
Step I: To initiate experiment, take a single bacterial clone and create 12 genetically identical lines
Step II: Carry out daily protocol for 3000+ days
Step III: Evolved strains and frozen ancestors are now available for wide range of evolutionary studies
What kind of questions can you ask with these experiments?
Is evolution repeatable?
-If we start out with 12 flasks that are identical, will we see the same result in each flask?
Do the same phenotypes evolve over and over?
If so are the same genetic mechanisms involved?
What happens to cell volume and relative fitness over time? (Graph)
a) Cell volume and relative fitness both increase
b) Cell volume and relative fitness both decrease
c) Cell volume increases but fitness decreases
d) Cell volume decreases but fitness increases
a) Cell volume and relative fitness both increase
Cell size and fitness
Both cell size and fitness increased dramatically over 10,000 generations
But was this change in a cell line random?
Can it be repeated?
The same result was observed across all cell lines
But the increase in cell size is greater in some lines than in others, even though all original cells were identical
Later generations were more fit than earlier generations
Some components of natural selection were PREDICTABLE and REPEATABLE, but they are not identical - there is still some randomness in the process
Key point: Selection acts on individuals, but POPULATIONS evolve
Darwin argued that selection acted primarily on individuals
However, it is not the individuals who evolve, but the populations of naturally selected individuals
Favorable heritable variations will gradually spread within a population over time due to natural selection on individuals
Population
A group of actually or potentially interbreeding organisms (typically in geographic proximity)
Which of these is NOT true:
a) Evolution can be predictable and repeatable over time
b) Similar phenotypic changes can arise independently in similar environments
c) Evolution always produces identical results in similar environments
d) Experiments allow us to test hypotheses about evolution
c) Evolution always produces identical results in similar environments
Evolution of Complex and Novel Traits
Darwin’s critics have argued that his theory could not explain the origin of complex traits, such as the human eye or molecular “machines”
Natural selection CAN result in complex traits
TWO explanations for complex traits:
- Intermediate, adaptive steps
- Evolutionary co-opting (exaptations)
intermediate steps
There is a common misconception that evolutionary “steps” involve huge, novel mutations that create big phenotypic changes
Complex structures evolve via a series of intermediate steps
Intermediate, beneficial stages and gradual steps of focusing eyes can be seen across animals today
Simulations indicate eyes could have evolved in half a million years - and they appear in the fossil record 500Mya
It is rare for new, totally novel phenotypes to emerge fully formed from mutations of large effect
It is much more common for complex structures to evolve via a series of individually adaptive intermediate steps
Complex eye evolved through various simple steps; lens evolved several times convergently
Novel structures and exaptations
The evolution of an essentially new phenotypic trait, derived from variation existing in an ancestral trait
Most “new” structures derive from existing structures
ALL of these derived from a wing precursor (had an extra small set of wings that spouted out of proximal end of thorax; not critical for survival of species, vestigial, become co-opted to do other things)
Exaptation
A trait evolves for one function but is co-opted for different function
Exaptations - Cranial sutures
Cranial sutures allow mammals to deform their brain cases during live birth, but probably evolved to allow brain growth in birds
-Evidence: Sutures seen in bird and reptile skulls
In other words, live birth was possible in part because brain sutures already existed
Exaptations - The evolution of feathers
Feathers probably for body insulation and temperature regulation
Clear from fossils that feathers evolved prior to flight
Because some dinosaurs had feathers they were able to evolve the ability to fly
-Birds evolved from dinosaurs well-accepted among scientists
Molecular exaptations
Gene sharing: proteins that serve one function are recruited to new or different functions elsewhere
Suggests that proteins have been co-opted over time to serve different functions
Lock-and-key systems
How could these evolve? GENE DUPLICATIONS duplicate the receptor
The receptor was able to bind several different types of hormones
Millions of years later, mutations led to the rise of a new hormone that was able to bind to the existing duplicated receptor
Origin of “lock-and-key” mechanism evolved by accident by the random duplication of the receptor and eventual mutation in the hormone
Key point: Natural selection favors gene copies that maximize the fitness of the individual
There is NO goal to natural selection
Evolution is just a generation-by-generation response to prevailing ecological conditions
“Tree of life” shows very simple organisms at bottom, humans at top, moving “up”; this is INACCURATE, moves generation by generation
Which is NOT true about how complex and novel traits typically evolve:
a) New, complex structures typically appear full formed via mutations of large effect
b) Complex structures can evolve via many intermediate stages
c) Structures that serve one function are frequently co-opted for a new function
d) Gene duplication and gene sharing enable evolution of novel traits at the molecular level
a) New, complex structures typically appear full formed via mutations of large effect
Adaptations are constrained by genetic variation and physical laws
Pigs do not fly, not because it would not necessarily be advantageous for them, but because they lack genetic variation for the growth of a wing from the shoulder
Organisms are never perfectly adapted to their environments due to constraints and trade-offs
Natural Selection is tinkering, not engineering
Natural selection does not improve organisms to perfection or to the maximum efficiency
Natural selection does not “give organisms what they need”
-Very Lamarckian idea
There is no “goal” phenotype that is being optimized
As long as the phenotype of an individual works “well enough” relative to others in the population it will be selected
–As long as you work just a little better than everyone else
The phenotype of living organisms is often the result of trade-offs and physical and genetic constraints
If masks were necessary for our survival as a species we would have evolved one by now.
This idea is:
a) Darwinian
b) Lamarckian
c) Very reasonable
d) Not sure
b) Lamarckian
Trade-offs and limits on variation
Natural selection is limited by physical characteristics of an organisms and other traits under selection
For example, a two-eyed organism can EITHER have binocular vision (good for depth perception) or 360 degree vision (good for vigilance) - not both
From fitness perspective, is it better to have depth perception or vigilance? Depends on ethology…
Constrains on phenotypes
Over time, owls and spider evolved different solutions to the limited field of view
Owls - rotating head
Spiders - eight eyes
Impossible Phenotypes: Why aren’t there wolf-sized insects? Why no stilted elephants?
Wolf-sized insects would be scary and their systems aren’t meant for being that large
Long and thin legs cannot support an elephant size animal made of bones, flesh, and blood
Mammals conform to tightly defined relationship between body mass and leg diameter
Is evolution random?
Yes and no
Natural selection is NON-random because it favors adaptation of organisms to their environment in different ways depending on the environmental conditions
-Natural selection selects for strongest traits for a particular environment, NOT randomly selecting traits
However, other evolutionary processes such as mutation and genetic drift are random
Evolution is only partially random
Myths and Misconceptions about Evolution
Controversial amongst lay people
MYTH: Evolution is organisms adapting to their environment
-Individuals do NOT evolve populations do
MYTH: Survival of the fittest; makes it sound like evolution favors biggest, strongest, and fastest
-Organisms evolve to be fittest for a particular environment
MYTH: If an organism dies without reproducing, it does not effect evolution
-It has siblings and relatives carrying similar traits
Metaphors: “Natural selections”, “genes want to be selected for”
Genes are randomly selected
Key point: natural selection favors gene copies that maximize the fitness of the individual
There is NO GOAL to natural selection
Evolution is just a generation-by-generation response to prevailing ecological conditions
Can evolution produce ANY phenotype?
a) Yes, given enough time, any phenotype is possible
b) No - adaptation is constrained by physical laws
c) No - adaptation is constrained by available genetic variation
d) B&C
d) B&C
What is heritability and why can natural selection only operate on heritable?
heritability: The amount of variation in a phenotype due to a genotype; natural selection is the process of altering alleles over time, nothing else is heritable
What is phenotypic plasticity?
The same genotype expresses phenotype differently in varying environments
What is relative fitness?
The amount of offspring an individual has COMPARED to the population; individual fitness/mean population fitness
What is an adaptation?
Adaptation: Reproductively advantageous for individuals or beneficial for the genes underlying development of the trait; a trait associated with the highest relative fitness in a particular environment
What are two ways complex structures can evolve?
Intermediate steps or through co-opting/exaptation
Is evolution repeatable and predictable?
Yes, to an extent, there is still randomness involved
Is evolution engineering or tinkering?
Evolution is TINKERING
Which is NOT true about how complex and novel traits typically evolve?
a) Structures that serve one function are frequently co-opted for a new function
b) Gene duplications and gene sharing enable evolution of novel traits at the molecular level
c) Complex structures can evolve via many intermediate stages
d) New, complex structures typically appear full formed via mutations of large effect
d) New, complex structures typically appear full formed via mutations of large effect
Which of these is NOT true:
a) Similar phenotypic changes can arise independently in similar environments
b) Evolution can be predictable and repeatable over time
c) Evolution always produces identical results in similar environments
d) Experiments allow us to test hypotheses about evolution
c) Evolution always produces identical results in similar environments
What is relative fitness?
a) The acquisition of beneficial characteristics that improve survival
b) The ability of individual to survive and reproduce in its environment
c) Only the strong survive
d) The absolute number of offspring an individual produces
b) The ability of individual to survive and reproduce in its environment
Can evolution produce any phenotype?
a) No, adaptation is constrained by available genetic variation
b) No, natural selection is only effective in certain environments
c) Yes, any physical constraint can be overcome by mutations
d) Yes, given enough time, any phenotype is possible
a) No, adaptation is constrained by available genetic variation
This figure shows reaction norms for the number of leaves on plants grown at different light intensities. What is the correct interpretation of this figure?
a) Each line is a different light intensity. The number of leaves a plant produces is unpredictable based on light intensity
b) Each line is a genotype. The same genotype always produces the same number leaves, regardless of light intensity
c) Each line is a different light intensity. The same light intensity always causes the plants to grow the same number of leaves
d) Each line is a GENOTYPE. the same genotype can produce different numbers of leaves in different light intensities
d) Each line is a GENOTYPE. the same genotype can produce different numbers of leaves in different light intensities
Superhero movies like the X-Men series often refer to the next “step” or “stage” of human evolution. Why is this type of thinking incorrect?
a) Evolution is not a directed process, and there are therefore no planned evolutionary “steps”
b) Evolutionary steps are unconstrained by physical laws, and therefore unpredictable
c) Evolution is a completely random process, and it is therefore impossible to predict the next evolutionary step of human evolution
d) Evolutionary steps can only happen via mutations of large effect, which are comparatively rare
a) Evolution is not a directed process, and there are therefore no planned evolutionary “steps”
Who has the higher relative fitness within its population, the BROWN MOUSE in population 1 or population 2?
Pop. 1: W = 100,000 offspring
B = 1000 offspring
Pop. 2: W = 100 offspring
B = 10 offspring
Rel. fitness pop. 1:
Mean = (100,000+1000)/2 = 50,500
Rel. fit. 1 = 1000/50,500 = 0.0198
Rel. fitness pop. 2:
Mean = (100+10)/2 = 55
Rel. fit. 2 = 10/55 = 0.18
Rel. fit 2 =0.18 > rel. fit 1 = 0.0198
POPULATION 2 has the higher fitness
Why can’t 100% environmentally determined traits evolve by selection?
a) They are too variable and selection would be ineffective
b) Environmental traits don’t vary
c) Offspring don’t resemble their parents in environmental traits
d) Evolution by selection is change in allele frequencies over time, and there are no alleles underlying these traits
d) Evolution by selection is change in allele frequencies over time, and there are no alleles underlying these traits
Why is language not heritable?
a) It has no genetic basis, and therefore cannot be passed from parents to offspring
b) It does not vary enough among individuals
c) It is not affected by the environment
d) Offspring do not resemble their parents
a) It has no genetic basis, and therefore cannot be passed from parents to offspring
What are ecological analogues?
a) Unrelated species that exhibit similar adaptations to similar environments
b) Unrelated species that exhibit different adaptations to different environments
c) Related species that exhibit different adaptations to similar environments
d) Closely related species that exhibit similar adaptations to similar environments
a) Unrelated species that exhibit similar adaptations to similar environments
