Natural Selection Flashcards
General principle of Thomas Malthus
populations could grow exponentially but in practice they dont ; therefore they must be limited by incomplete survival or reproduction
-> competition bw members of same population to be in the part of population that survives and reproduces
Mutation and Environment
Mutation is unrelated to demands of environment
Fitness of an allele
-> fitness is defined as the number of offspring that organisms with a particular genotype or phenotype leave behind, on average, as compared to others in the population
absolute / direct fitness
contribution of an individual to the gene pool of the next generation ( How many childrens a person has)
relative / indirect fitness
the contribution an individual makes to the gene pool of next generation relative to contributions of other individuals ( eg more efficient food collection -> more power -> more eggs )
How does natural selection change allele frequency?
natural selection increases the frequency of those alleles with high fitness and eliminates alternative alleles with low fitness
-> causes evolution by selecting favorable phenotypes
adaptive evolution
evolution resulting in better match between organisms and their environment
Linnean hierarchy
history of life with organisms at avrious levels related through descent from common ancestors
genetic drift
random chance events cause allele frequencies to fluctuate unpredictably from one generation to the next
-> more likely to occur in small populations
-> GENES IN CURRENT ARE ONLY SAMPLE OF GENES CARRIED BY PREVIOUS GENERATION -> things get lost
consequences of genetic drift
can lead to loss of genetic variation -> influences how effectively population is
Founder effect
few individuals become isolated from larger population and establish a new gene pool different from source population
Bottleneck effect
sudden change in environment drastically reduces size of population ( major disaster) ->one small part of population survives
-> by chance certain alleles are over/under represented or absent
gene flow
- reduce genetic differences
traveling -> mating -> gene flow -> populations become more similar
Mutation-selection-balance
mutation introduces genetic variation into population while selection reduces it
-> level of genetic diversity depends on the relative strength of the two
Important point on natural selection and allele frequency
1 not necessarily organism but alleles with highest fitness will preserve
2 selection acts more directly on phenotype than on genotype
Purifying selection
- the selective removal of alleles that are deleterious
- makes sure that deleterious mutations cannot take over a population and that any improved structures—once fixed in a population—are maintained as long as they are needed.
- > getting rid of the extremes
Phenotypic plasticity
ability of one genotype to alter phenotype when exposed to different environments ( without altering its genome)
Stabilizing Selection
intermediate phenotypes are more fit than extreme ones
- > individuals higher or lower the average having worse reproductive fitness
- > reduces variation and maintains status quo
Directional Selection
one extreme phenotype is more fit than all the other phenotypes -> shifts overall makeup -> adaptation
Disruptive Selection
occurs when condition favours variants at both extremes of a phenotyic range over intermediate phenotypes
Heterozygote advantage
situation where individuals with one copy of a particular allele have higher fitness than individuals with either no or two copies
example sickle cell trait, which protects against malaria in heterozygotes, but causes a deadly disease in homozygotes
negative-frequency-dependent selection
where a phenotype is associated with relative high fitness when it is rare but low fitness when it is common
positive-frequency-dependent association
altruism, the more people it have, the better
force of mutation
genetic mutation will persist when force of mutation is strengthened or that of selection weakened
inconsistent ( unbeständig) selection
reason why variation may persist is if selection is inconsistent
-> decreases particular allele in one year and increases it in another
sexually antagonistic selection
optimal phenotype may not be same in male and female
adaptationist stance / hypothesis
if some feature or behavior is commonly found in a type of organism then it is probably an efficient design solution to some problem the organism has faced
-> multiple perspective model shows some evidences that can be met
ultimate explanation
How a particular design increased ancestral fitness
-> more phenotype
proximate explanation
explanation of the genetic mechanism that led to the formation of a new characteristic
-> more genotype
-> natural selection favours whatever proximate mechanisms produces the optimal phenotype
example ultimate and proximate explanation :
Black tailed Prarie Dogs Alarm Calling
ultimate : call when kin is in danger - increases fitness
proximate: all kinds of proximate machanisms could produce the required effect
Why can’t organisms become perfect?
1 Time lags ( adaptation is constantly behind)
2 Selection can act only on existing variation -> fittest phenotype not always ideal phenotype
3 works on already existing traits by adaptation rather than building new structures from scratch
4 adaptations are often compromises ( eg penguins live on land and sea)
5 Chance events ( eg genetic drift) can ruin process
6 need of anear continous gradient of improvement
hitch hiking trait
allele changes frequency not because it itself is under natural selection, but because it is near another gene that is undergoing a selective sweep and that is on the same DNA chain
example : humans made foxes tame -> many other characteristic changed too
3 ways to test an adaptionist hypothesis
1 Reverse Engineering and optimality models
2 Experimental MAnipulation and Experiments of Nature
3 Comparitive Evidence
Reverse Engineering and optimality models
Reverse engineering : examine how it works , then come up with assumptions about what function would have led it to be deigned this way
Optimality models:»_space;»> not complete yet
Experimental manipulation and experiments of nature
cmparing individuals with more and less of the characteristic
Comparitive evidence
comparative method : skin colour should be associated to average amount of sunshine that population is exposed to
sexual dimorphism
difference between male and female forms of same species
sexual selection
natural selection on the ability to gain mates
Bateman’s principle
variability in reproductive success is greater in males than in females
- > females have an upper limit when it comes to reproduction and males dont have it
- > males maximize quantity while females spend long time choosing quality
Intrasexual competition
example : fight between males
must weigh cost-benefit : even though fighting is risky -> benefit more access to more mates
female choice and ornamentation
- seccond way to compete for extra females is by attractiveness
the sexy son hypothesis
there is a genetic correlation between the preferene for the traits and the trait itself
-> mating with a male who has a favorable traits means higher chance that own son has trait too
the good genes hypothesis
females choose males with best traits , because these traits provide that they do well in general in current environment
-> high quailty traitsshow higher quality in general ; not just to be more attractive but to survive better
Intersexual competion
females choose a male
trade offs : cost vs benefits
Reformulation of Batemans principle due to sex role revearsal
whichever sex invests less per episode of reproduction will have the larger variance in reproductive success and will therrefore compete for mates
Extra-pair mating
female mates with males other than the social partner -> usually the extra mate is the ‘‘good gene’’ mate whereas their actual partner is of lesser quality but needed to raise the offspring
alternative male reproductive strategies
1 wait in one colony and have extra pair matings, wairing for territory to become free
2roaming from colony to colony picking up occasional extra pair matings and no need for further investment
Batemans principle in humans
males reproductive success tends to be higher than females but depends in environment and socital circumstances ( having several wives and money)
Males parental investment compared to females
men : women should be fertile and produce fertile offsprings -> high value on physical appereance
women: material resources of men are of great importance
male short term mating
mating can be expensive for male but not necessarily -> choosy for long term relationships
‘just so’ explanation
the opposite of adaptation -> logical story is not always the correct story!
Two types of adaptation
1 Being adopted of an individual -> proper functioning in environment
2 Becoming adapted as an population -> increasing frequency of adapted individuals
NS in terms of fitness
any consistent difference among phenotypycally different classes of organisms
adaptation
better functioning = more offspring = higher fitness
- an adaptation is a character that raises relative fitness of an individual
- adapation has no evultionary effect unless phenotypes differ in genotypes
What are mistaken views on adaptations?
1 all differences between populations are adaptive
2 all characters of of organisms are adaptations
adaptations are trade offs
benefits always come with costs
Why is HW still relevant?
- > no need to look at adaptations etc
- > used to detect evoution
