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BIOL 359 Evolution 1 > Final Exam (Unit 1 and 3) > Flashcards

Final Exam (Unit 1 and 3) Flashcards

1
Q

Evolution

A

Derived from the Latin word “evolutio” meaning “unravelling” or “unfolding”

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2
Q

what is evolution

A

Decent with modification

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3
Q

what isn’t evolution

A
  • Does not include the developmental change in the life of an organism.
  • It does not proceed along some predictable course.
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4
Q

who proposed that populations of organisms evolve over time

A
  • Anaximander in Ancient Greece (6th Century B.C.) believed that species were formed from water, and that humans and other animals descended from fish.
  • Empedocles (5th century B.C.) proposed that heads, limbs and other organs were joined at random and only some combinations were fit for survival.
  • Plato (4th century BC) believed that each natural type of object in the observed world is an imperfect manifestation of the ideal
  • Aristotle: Student of Plato and elaborated on his ideas.
  • Envisioned a static world in which a Deity had created fixed essences, which are templates for all species
  • Gradation from inanimate to barely animate through plants and invertebrates up to “higher” forms in a Scala Naturae or Scale of Nature, moving from least perfect to most perfect
  • Christianity expanded on this: God’s creation must follow a plan- the ladder of nature, or Great Chain of Being
  • Permanent and unchanging since change would imply imperfection in the original creation
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5
Q

where did species originate

A
  • Theory of Special Creation
  • Created by a deity.
  • Species were created independently of one another.
  • Species remain unchanged since their creation.
  • Variability within a species is minor/limited.
  • Predicted that the creation event occurred 26 October 4004 B.C. at 9:00
  • Theory of Evolution-descent with modification
  • Idea was proposed by several people in the late 1700s and early 1800s.
    Erasmus Darwin (1731-1802).
  • Laws of Organic Life (1794-1796).
  • “Integrative” approach.
  • “The final course of this contest
    among males seems to be, that the
    strongest and most active animal
    should propogate the species which
    should thus be improved.”

-Lamarckism
- Proposed: All species are derived by gradual change from other species.
- Mechanism: Inheritance of acquired characteristics.
- Organs become improved if used a lot, reduced to vestiges from disuse.
- This mechanism required that bodily changes have a way to be registered in the germ cells
- “The giraffe lives in places where the ground is almost invariably parched and without grass. Obliged to browse on trees it is continually forced to stretch upwards. This habit sustained over long periods of time by every individual of the race has resulted in the forelimbs becoming longer than the hind ones, and the neck so elongated that a giraffe can lift his head to a height of six meters without taking his forelimbs off the ground” (1809)

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6
Q

Carl Linnaeus

A
  • Swedish naturalist.
  • Classified plants based on morphology into Classes, Orders, Genus and Species.
  • Introduced a system of binomial nomenclature for species identification (Systema Naturae 1735).
  • Individual species names consist of a genus name and a species name.
  • Remains the standard even today
  • Established the framework for modern classification (the binomial system e.g., Homo sapiens) in Systema Naturae in the hope of discovering the pattern of God’s creation
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7
Q

Catastrophism vs uniformitarianism

A
  • Catastrophism- Biblical interpretation of the history of the earth.
  • Earth was created by supernatural means approx 6000 years b.p.
  • Had been affected by a series of catastrophic events such as the biblical flood
  • Uniformitarianism – More modern view of the earths history.
  • Earth is unimaginably old.
  • Landforms present today are the result of geological processes that are still occurring.
  • Ideas originated from the work of James Hutton-late 1700s.
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8
Q

Charles Lyell

A
  • Published “Principles of Geology” (1830-1833)
  • Challenged the prevailing belief regarding the earth’s history and age.
  • Argued the earth is hundreds of millions of years old.
  • And…earth’s geology was created by natural forces including earthquakes, volcanoes, winds, rain, etc.
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9
Q

Charles Darwin

A
  • Observations from South America:
  • Fossils of animals in S America that were no longer extant in the region (e.g. Thoatherium).
  • So…species can/do go extinct
  • Fossils of animals in a region were different from modern day equivalents (e.g., armadillos and sloths).
  • So.. species have ancestors, and change through time.
  • In traveling from N to S, one species was replaced by a similar, but slightly different species (e.g., Rhea).
  • So.. species differ across space/geographical distance
  • Vestigial Organ- “functionless or rudimentary version of a body part that has an important function in other, closely allied species”
  • Artificial selection- over time humans have modified many species to suit their needs through careful breeding programs.
  • e.g., Shetland pony, Arabian horse, many plants
  • So..humans have ‘caused’ the evolution of several species from a single ancestral species.
  • e.g. many sp. of domestic fowl derived from red jungle fowl
  • In 1838 he read “An Essay on the Principle of Population” by Thomas Robert Malthus (1766-1834).
  • Discussed how the reproductive
    capacity of the human population
    exceeds the food supply available
    nourish the expanding population.
  • Therefore, humans compete
    among themselves
  • This struggle would also exist in nature and would constitute a ‘natural’ selection process.
  • This process would occur without a plan or conscious.
  • Prepared an essay of his ideas in 1844.
  • Showed essay to Lyell who urged him to write a book.
  • Darwin did not publish his ideas until much later…
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10
Q

Fossil records

A
  • Species go extinct
  • Law of succession
  • Transitional forms
  • Environmental change
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11
Q

what is homology

A
  • The study of likeness.
  • “The same organ in different animals under every variety of form and function”
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12
Q

Wallace

A
  • From a working class family.
  • Father died when he was young.
  • He worked with his brother as a surveyor following schooling for 4 years.
  • Met Henry Walter Bates who introduced him to botany.
  • The 2 travelled to S. America and collected for 4 years.
  • Lost all their collections in fire during return voyage in 1852
  • Also read essay by Malthus
  • During a fit of Malarial fever he also came to the idea of natural selection.
  • Wrote an essay “On the Tendencies of Varieties to Depart Indefinitely from the Original Type” and sent it to Darwin to review
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13
Q

Darwins observations of natural selection

A

Observation 1:
- potential exponential increase of populations
Observation 2:
- steady state (stability of populations)
Observation 3:
- heritable variation in organisms

Deduction 1:
- struggle for existence among individuals
Deduction 2:
- differential survival (natural selection)

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14
Q

Mendel

A
  • Austrian monk and mathematics teacher.
  • Conducted extensive experiments in plant hybridization 1856-1863 using common peas (Pisum sativum).
  • Credited for discovering the laws of heredity (basis for all modern genetics).
  • Significance of his work not really recognized until 1900s
  • Traits are passed from parent to offspring through the gametes in specific discrete units, or factors.
  • These factors can be assorted differently in different individuals.
  • These factors are now called genes.
  • Individuals have 2 copies of each gene.
  • One received from each parent.
  • Alternate forms of the gene are called alleles

First Principle - Principle of segregation
- Each gamete (egg or sperm) can carry only one allele from a parent at a given gene

Second Principle – Independent Assortment
- The segregation of the members of one pair of alleles occurs independently of of the segregation of the members of another pair of alleles during gamete formation

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15
Q

Evidence for decent with modification

A
  1. microevolution
  2. speciation
  3. macroevolution
  4. common ancestry
  5. age of earth and life
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16
Q

what is microevolution

A
  • Selective breeding
  • vestigial structures
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17
Q

what is speciation

A
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18
Q

what is macroevolution

A

-extinction and succession
- transitional forms

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19
Q

what is common ancestry

A
  • Descent with modifications contends that all organisms have a single common ancestor.
  • Homology is a crucial line of evidence
  • Traits are homologous if they are derived, evolutionarily and developmentally, from the same source structure
    -structural and molecular homology
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20
Q

what is age of earth and life

A
  • Uniformitarianism (Hutton/ Lyell) vs. Catastrophism
  • Geologists recognized that considerable amounts of time were required to build mountains and create the geological strata observed in nature.
  • Based on geological evidence, it was recognized that a 6000 year old Earth (Arch Bishop Ussher) and catastrophism were not consistent with accumulating evidence
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21
Q

theory of evolution by natural selection postulates

A
  1. Individuals in a population are different from one another.
  2. The differences are, at least in part, passed from parents to offspring.
  3. Some individuals are more successful at surviving and reproducing than others.
  4. The successful individuals are not merely lucky; instead they succeed because of the variant traits they have inherited and will pass to their offspring.

simplified
- Living things produce more offspring than can be supported
- There is a constant struggle for existence.
- Individuals in a population vary in their phenotypes.
- Some of this variation is heritable (genotypes).
- Those individuals best adapted to current conditions are most likely to survive and reproduce themselves
- If these adaptations are heritable, they will be passed on to their offspring.
- Natural selection acts on phenotypes, but only if there is a change in allele frequencies in a population has evolution occurred.
- Population genetics is the study of allele and genotype frequencies and their changes in entire populations

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22
Q

what is fitness

A
  • ability of an individual to survive and reproduce in its environment.
  • a relative term, i.e., relative to the fitness of other members of the population.
  • May be objectively measured in nature by means including:
    1. counting offspring of individuals over their lifetime;
    2. observing individuals ability to survive some natural event (e.g., drought)
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23
Q

what is an adaptation

A

A trait that increases the ability of an individual to survive and reproduce compared with individuals without that trait

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24
Q

what is a trait

A

any characteristic of an individual ranging from outward appearance (e.g., height) to molecular characteristics (e.g., shape of a protein)

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25
Q

evolution by natural selection

A
  • Requires statistical thinking.
  • Natural selection acts on individuals, but evolutionary consequences occur in populations.
  • Natural selection acts on phenotypes, but evolution consists of changes in allele frequencies.
  • Natural Selection is not forward looking
  • Natural Selection acts on existing traits only, but new traits may evolve
  • Natural Selection does not lead to perfection.
  • Natural Selection is nonrandom, but it is not progressive.
  • Natural Selection acts on individuals, not for the ‘good’ of the species
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26
Q

the modern synthesis

A
  • Reformulation of the theory of evolution based on integration of genetics with Darwins 4 postulates.
  • Derived from works published 1932-1953
  • Grounded on 2 propositions:
    1. Gradual evolution results from small genetic changes that are acted upon by natural selection.
    2. The origin of species and higher taxa, or macroevolution, can be explained in terms of natural selection acting on individuals, or microevolution
27
Q

restatement of Darwins postulates

A

Darwin’s first
Individuals in a population are different from one another.
Restatement:
As a result of mutation creating new alleles, and segregation and independent assortment shuffling alleles into new combinations, individuals within populations are variable for many traits

Darwin’s second
The differences are, at least in part, passed from parents to offspring.
Restatement:
Individuals pass their alleles on to their offspring intact

Darwin’s third
Some individuals are more successful at surviving and reproducing than others.
Restatement:
In every generation, some individuals are more successful at surviving and reproducing than others

Darwin’s fourth
The successful individuals are not merely lucky; instead they succeed because of the variant traits they have inherited and will pass to their offspring
Restatement:
The individuals that survive and reproduce, or who reproduce the most, are those with the alleles and allelic combinations that best adapt them to their environment

Summary
Alleles associated with higher fitness increase in frequency from one generation to the next.

28
Q

human induced selection

A
  • Human harvesting practices can often impose selection regimes on natural populations with unintended (and often undesirable) evolutionary consequences
  • Consider the evolutionary implications of hunting and fishing regulations, which often stipulate that only animals over a certain size can be taken
29
Q

what is environmental variation

A

Individuals can possess different phenotypes as a result of exposure to different environments, despite identical genotypes.

30
Q

what is genetic variation

A

Individuals can possess different phenotypes as a result of genetic differences (different genotypes), transmitted from parents to offspring

31
Q

what is genotype by environment interaction

A

Individuals can possess different phenotypes as a result of the interaction of their genotypes with the environment

32
Q

what is phenotypic plasticity

A

refers to the fact that genetically identical individuals can have different phenotypes in different environmental conditions

33
Q

what is reaction norm

A

refers to the pattern or range of phenotypes that the same genotype can possess as a result of different environments

34
Q

mutation

A
  • Ultimate source of genetic variability
  • A change in the base sequence of DNA.
  • Raw material for evolution.
  • Discovered in the early 1900s by T.H. - Morgan and colleagues in fruit flies
  • Any heritable change in the genetic material (DNA)
  • Mutations can generally be classed as single gene mutations, which affect a single gene only, or chromosomal mutations which affect many genes.
  • Initially, any new mutation at a diploid nuclear locus will have a low frequency in a population (1/2N, where N is the population size).
  • As result, on its own, mutations have limited impacts on population allele frequencies
  • However, when combined with other forces such as selection, they can be very important
35
Q

DNA mutations

A
  1. mutation due to DNA alteration
  2. copying error
  3. point mutation
    -transitions (between same type of base)
    -transversions (between different types of bases)
36
Q

impacts of point mutations

A
  1. Synonymous mutations- no change in amino acid
    - Due to redundancy in the genetic code
  2. Non-synonymous mutations- change in amino acid
  3. Non sense mutation-premature STOP
    Often produces non-functional proteins
37
Q

results of mutations

A
  • Neutral- no impact whatsoever
  • Deleterious- reduces fitness of the individual
  • Beneficial- increases fitness of the individual
  • Lethal- results in organisms death prior to reproduction
38
Q

where do new genes come from

A
  1. gene duplication vis 2 methods:
    a. unequal crossing over
    b. retroposition
    - New gene may reintegrate into a chromosome and represent a new copy of existing gene.
    - May be a pseudogene-non functional because it does not have the regulatory sequences for transcription to RNA
    - However, by chance may insert near a regulatory sequence, or acquire such a sequence so it may become functional
  • Genes formed from unequal crossing over have the same introns as original genes and typically occur in tandem with original genes on the same chromosome
  • Genes formed from retroposition typically lack introns and may be found far from the original gene
  1. derived from scratch
    -derived from non-coding region of DNA
    May affect only gene order and organization and /or produce duplication or deletions that affect a large amount of genetic material.
    Two types include :
    a. Inversions
    b. Genome duplication
39
Q

mutation and natural selection

A
  • Is the rate at which mutations occur subject to natural selection?
  • Does heritable variation exist in the accuracy of enzymes that synthesize and repair DNA? Yes
  • Is there variation in fitness among variants in particular environments? Yes
  • Individuals with higher mutation rates seem to have higher fitness only when exposed to novel environments.
40
Q

population genetics

A
  • Integrates theory of evolution with mendelian genetics
  • Changes in the relative abundance of traits in a population are tied to changes in the relative abundance of alleles that influence them.
  • Provides the theoretical foundation for much of our modern understanding of evolution
  • Evolution is the change in allele frequencies in a population from generation to generation
41
Q

genetic variation

A
  • How can we measure genetic variation?
  • Requires determination of genotypes at particular genetic loci for individuals
  • Heterozygosity- measure of genetic variation
  • Number of heterozygotes/number of sampled individuals
  • AA=25, AB=50, BB=25; Heterozygosity (H) = 50/100=0.5

How do we determine genotypes?
1. Infer genotype based on phenotype.
- e.g. intestinal schistosomiasis
- Susceptibility influenced by 1 gene
- AA-susceptible
- AB-moderate susceptibility
- BB-resistant
- By determining susceptibility one can infer genotype

  1. Examine the proteins encoded by the alleles
42
Q

measure variation with electrophoresis

A
  • Protein electrophoresis was the first method to determine an individual’s genotypes and measure the diversity of populations at a locus
  • Protein electrophoresis in the 1960s and 70s revealed substantial genetic variation
  • How can we measure genetic variation?
  • Requires determination of genotypes at particular genetic loci for individuals
  • How do we determine genotypes.
  • Infer genotype based on phenotype.
  • e.g. intestinal schistosomiasis
  • Examine the proteins encoded by the alleles.
  • Examine the DNA itself
43
Q

H-W model

A
  • Applies to all populations of diploid organisms
  • Population-level model
  • Null model
  • Assumes an idealized/panmictic population
  • Simple assumptions:
    1. No selection
    2. No mutation
    3. No gene flow
    4. No genetic drift (large population size)
    5. Individuals choose their mates at random
  • Under those stated simple assumptions, the H-W principle says:
    a. Genotype frequencies can be predicted from allele frequencies.
    b. Allele frequencies will remain stable in a population over time
44
Q

panmictic mating

A
  • A panmictic population can be thought of as a pool of gametes (eggs and sperm), or a pool of alleles
  • Gametes carrying a specific allele are produced in the same proportion as the frequency of the allele in the population
45
Q

questions about increasing fitness

A
  1. Should it reproduce sexually or asexually?
  2. At what age should it start to reproduce?
  3. How many times should it attempt to reproduce?
  4. Should its offspring be few in number but large and high quality or large in number and small in size and low quality?
  5. Should it reproduce early in life, neglect its own maintenance and have a short life?
  6. How should individuals of the limiting sex choose among its suitors?
46
Q

sexual vs. asexual reproduction

A

Asexual
1. No partner required
2. No gametes
3. Genetically similar to parent
4. Cell divisions are mitotic
5. Simple
- Parthenogenesis- asexual reproduction where offspring develops from unfertilized eggs
Sexual
1. Requires partner
2. Gametes
3. Offspring have genetic variation
4. Gametes produced by meiosis and zygote develops by mitosis
5. Takes longer and is more complicated

47
Q

sexual reproduction advantages

A
  1. Where fathers provide support for the care of
    offspring, females can produce more offspring
  2. Meiosis with crossing over
    - Linkage disequilibrium
  3. Mating between unrelated individuals
  • Hermaphrodites- have both males and female sex organs and are capable of self fertilization
48
Q

what is fitness

A
  • An individuals ability to survive and reproduce
  • Those with high fitness are those whose offspring make up a higher proportion of the next generation
  • Much of the variation between the sexes is explained by natural selection
49
Q

why is there sexual dimorphism

A
  • Fitness is about survival and reproduction.
  • Just because an individual survives to sexual maturity does not mean they will be able to convince a member of the opposite sex to mate with them.
  • Sexual selection: Differential reproductive success due to variation among individuals in success at getting mates.
  • If there is heritable variation in a trait that affects the ability to obtain a mate, then these successful variants will increase in frequency
50
Q

sexual selection vs. natural selection

A

Similarities
- for both, there must be heritable variation in a trait or traits within a population.
- selection operates when individuals with certain variants have more surviving offspring than others.
- selection operates among individuals of the same species

Difference
- For sexual selection individuals compete against other members of the same sex, instead of against all other individuals in the population, as it is for natural selection

51
Q

parental investment/ asymmetric limits on fitness

A
  • For it to occur it must act on the sexes differently.
  • Driven by the difference in reproduction allocation
  • Males and females differ in reproductive allocation
  • In addition to the cost per gamete, females invest more in game production:
  • Typically, females invests 3X the energy required for basal metabolism, whereas males invest 4/1000s the energy required for basal metabolism
  • If the goal is to have as many offspring as possible, then:
  • females may do this by mating once per breeding season.
  • males can have more offspring with each mating
52
Q

Sexual selection- newts

A
  • Rough-skinned Newts.
  • Males wait at a pond site for females.
  • Females arrive, select a mate.
  • After mating she lays up to 300 eggs.
  • Neither parent provides parental care.
  • Thus, costs of reproduction is solely the costs of making eggs and sperm.
  • Therefore, females make larger investment than males
  • Jones et al. captured all the males and females from a pond after mating occurred.
  • Induced egg laying by the females in separate containers.
  • Genotyped all adults and a sample of the eggs from each female
  • Determined the number of mates and offspring for each adult in the population
  • So, if males are limited by access to mates and females are limited by ability to make and rear young then:
  • Males should be competitive.
  • Females should be choosy.
  • Note: in some species access to mates is limited for females, and males invest most of the energy in each offspring. In these cases females should compete with other females and males should be choosy
53
Q

sexual selection summary

A
  • Males compete for mates
  • For males, the more mating, the more fitness
  • Typically there are more males than receptive females
  • Males compete against each other for access to females
  • Females select among males
  • A female doesn’t need many matings to fertilize her eggs
  • She can produce better offspring if she selects a “good” mate
  • Therefore females choose among the available males for the preferable one
54
Q

male-male competition

A
  • Intrasexual competition for mates.
  • How do males compete?
    1. Combat
    2. Alternative mating strategies
    3. Sperm competition
    4. Infanticide
55
Q

combat

A
  • Males that dominate other males during combat obtain access to more mates.
  • Leads to the evolution of traits that are important for combat –e.g., weapons such as antlers
56
Q

alternative mating strategies

A
  • A ‘sneaky’ male strategy in coho salmon
  • Large and armoured hooknoses vs small jacks
57
Q

sperm competition

A
  • Just because a male copulates does not mean he will sire the females offspring (if the female has multiple mates).
  • Therefore, for species in which the females mate with multiple males selection should favor individuals which produce more sperm
58
Q

infanticide

A
  • Competition beyond conception.
  • Male lions that invade a pride will often kill the offspring of the previous males so that the female will enter breeding condition sooner
  • In terms of fitness, male infanticide creates a conflict between new males who take over a lion pride, and it’s resident females.
  • The females will sometimes defend their cubs against new males, and sometimes be killed in the process
  • Gelada Baboons live in a social structure where one or a few males control up to12 females and their offspring. - Males often fight to take over groups of females, and if successful, infanticide of the previous male’s offspring typically follows.
  • Pregnant females will often abort pregnancy, rather than continue to invest in ‘doomed’ offspring.
  • The Bruce Effect: Pregnancy termination in the presence of unfamiliar males
59
Q

female choice

A

Why might females be choosy?
1. Acquiring genes for her sons that will make him attractive
2. Acquiring genes for her offspring that confer general fitness benefits
3. Acquire resources
4. Perhaps she may prefer males that exploit pre-existing sensory biases of her nervous system

60
Q

types of social interactions

A

actor +, recipient + –> mutual benefit
actor +, recipient - –> selfishness
actor -, recipient + –> altruism
actor -, recipient - –> spite

61
Q

altruism

A
  • Why would a behavior that benefits others at the actors own expense persist?
  • Hamilton’s rule (1964)
    1. An individual is rarely the sole repository of his or her alleles.
    2. Individuals that are closely related are likely to carry alleles that are identical by descent.
    3. Therefore, it is possible that the behavior of individuals towards others has the potential to influence the probability of passing copies of their alleles to the next generation.
    4. Key parameter is relatedness of actor and recipient
  • Br – C >0
  • Where:
  • B is benefit to the recipient
  • C is cost to the actor
  • r is relatedness of actor and recipient. A measure of genetic similarity.
  • Relatedness- often measured as the probability that a gene copy is ‘identical by descent’
  • The probability that a gene copy sampled in one individual is identical by descent to a gene copy present in another individual (i.e., arose by replication from the same ancestral copy of an allele)
62
Q

direct vs. indirect fitness

A
  • Direct fitness- an individuals direct contribution to the next generation by reproduction (number of offspring contributed).
  • Without the help of relatives
  • Indirect fitness- additional reproduction by relatives that results from an actor’s actions (e.g., assistance).
  • It is additional reproduction that would NOT have been achieved without the assistance provided by the actor.
  • Inclusive fitness = direct fitness + indirect fitness
  • Natural selection leading to the spread of alleles that increase the indirect fitness is called kin selection
63
Q

evolution of social behaviour

A
  • Social interactions generate opportunities for cooperation and conflict
    1. Parent-Offspring Conflict
  • Weaning conflict- Towards the end of nursing mothers decrease nursing but offspring demand care
  • E.g., Recruitment as helpers
  • E.g., Siblicide in 2 species of booby
  • siblings kill one another, often while the parent looks on passively. Why would parents not defend vulnerable offspring?
  • Masked and blue-footed each lay 2 eggs 2-10 days apart.
  • Older sibling may kill the younger one, while parent often does not intervene
  • How could siblicide increase the fitness of the parent?
  • Food shortages may lead to the death of the second chick anyway.
  • Siblicide is more likely to occur if food it limited.
  • Blue-footed boobies will intervene to prevent siblicide sometimes
  • Siblicide should occur if the individual killing the other gains enough direct fitness benefits by removing competition for food to outweigh the indirect fitness costs of killing a sibling
  • Social interactions generate opportunities for cooperation and conflict
  • Thus far the interactions we have considered have all been between kin. What about non-kin cooperation?
    2. Cooperation among non-kin
  • Reciprocity- exchange of favors among non kin
  • May be costly in the short term to the actor, but if the favor is returned then it might be of greater value in the long term
  • Baboons
  • Favors include: grooming and support during aggressive encounters.
  • Individuals that had recently been groomed by an individual were more likely to move towards their threat calls, presumably to their support
64
Q

reciprocal altruism

A
  • e.g., vampire bats
  • Feed on blood
  • Females form colonies consisting of 8-12 adults and their offspring.
  • Wilkinson (1998) was able to determine the degree of relatedness and association among individuals of a population
  • Meals are often hard to get and individuals may regurgitate blood meals to unsuccessful individuals.
  • If they do not feed for 3 days they will likely starve to death.
  • Reciprocity is more likely to evolve under the conditions when the same individuals:
    1. repeatedly interact with one another
    2. have many opportunities to return favors
    3. sometimes need a favor and sometimes can offer a favor
    4. Have good memories

BIOL 359 Evolution 1 (2 decks)

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