Bisc 102 final Flashcards
Evolution by natural selection in three steps
- Variation among individuals
- Heritable variation
- Variation linked to fitness
Asexual reproduction
Reproduction without fertilization or conjugation; Parent passes on 100% of its genes to all its offspring (produces clones); Can be parthenogenesis, fragmentation, budding, vegetative propagation
Sexual reproduction
Occurs through fertilization to produce a genetically novel individual; Each parent passes ~50% of its genes to each offspring; Offspring tend to resemble parents; Internal or external fertilization
Variation in sexual function
Separate sexes; hermaphrodites (simultaneous ex. flowers or sequential ex. some fish that start as female then become male)
Karyotype
A display of the chromosome pairs of a cell arranged by size and shape- homologous pairs have the same staining pattern and centromere position
46 chromosomes in humans (44 autosomes, 2 sex chromosomes) 2n = 46; 18 in carrots; 32 in cats;
1262 in adder’s fern (why so many: a lot would not be useful!)
3 events unique to meiosis
- Synapsis and crossing over during prophase 1 (formation of the synaptonemal complex and genetic rearrangement between nonsister chromosomes) 2. Homologous pairs at the metaphase plate, rather than individual chromosomes 3. Separation of homologs instead of separation of chromatids.
Sources of genetic variation
Crossing-over (= recombination), Independent assortment, Random fertilization
Crossing-over
Exchange of corresponding segments of DNA by non-sister chromatids in a tetrad during Prophase I, producing new combinations of maternal and paternal alleles
Independent assortment
Random alignment of each pair of homologous chromosomes at Metaphase I plate ie. each pair of homologs may orient with either its maternal or paternal homolog closer to a given pole, independently of any other pair of homologs (2^23 possibilities= 8.4 million)
Random fertilization
Which egg of 2^23 possibilities will combine with which sperm of 2^23 possibilities? Any 2 parents will produce a zygote with ~70 trillion (2^23x 2^23) diploid combinations
Advantages of sex
Introduces new combinations of heritable traits in offspring; Introduces new combinations of heritable traits in offspring; Potentially useful in a variable, dynamic environment
Mendelian genetics
The scientific study of how traits are passed on from parent to offspring; The study of heredity
Mendel’s 1st law of heredity
Law of segregation: The two alleles for a heritable trait separate during gamete formation and end up in different gametes
Mendel’s 2nd law of heredity
Law of independent assortment: Each pair of alleles segregates independently of each other pair of alleles during gamete formation
Genotype
The genetic makeup or set of alleles of an organism
Phenotype
The observable physical and physiological traits of an organism which are determined by it’s genetic makeup
Incomplete dominance
The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele ex. Rr gives pink flower, when RR is red and rr is white
Co-dominance
The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways ex. blood types IAIB contains both A and B proteins
Sickle cell disease is an example of what kind of phenotypic expression of genotype?
Both codominant and incomplete dominance:
RB cells are codominant (A1A1 normal, A1A2 50% sickcle, A2A2 all sickle) while the individual shows nomal or mild or full diseases symptoms
Heterozygote advantage
Approximately 1/10 African Americans is heterozygous for the sickle-cell allele. This is higher than expected, because sickle cell anemia gives partial protection from malaria
Polygenic inheritance
Multiple genes determine trait; Polygenic characters have near-normal frequency distribution; ex. Human skin colour
Multiplication law
Probability of independent events A and B = (Probability of event A) x (Probability of event B)
Autosomal recessive pedigree
If a condition is recessive (i.e. afflicted individuals have 2 recessive alleles): Homozygous recessive individuals ARE afflicted; ‘Normal’ parents can have afflicted offspring; The condition can appear suddenly or skip generations
Autosomal dominant pedigree
If a condition is dominant (i.e., afflicted individuals have 1 or 2 dominant alleles): Homozygous recessive individuals ARE NOT afflicted; Afflicted offspring have at least one afflicted parent; The condition cannot skip a generation
Autosomal vs sex-linked
In humans:
1 pair of sex chromosomes XX = female XY = male
X is larger and has more genes
If a gene is X-linked, A female has 2 alleles, A male has 1 allele (on X)
Natural selection
Process by which the individuals that have the characteristics best suited to the environment survive and reproduce better than other individuals, The main mechanism of evolution
Evolution
‘Evolution is a change in allele frequencies of a population over time’
Two scales of evolution
Microevolution : Changes within species = change in allele frequencies
Macroevolution : The evolution of new species
Allele frequency
The fraction of each allele in a gene pool, ex. 10 A and 10 B, so the frequency of A=0.5, the frequency of B=0.5, and the frequency of A and B= 1
How do new alleles originate?
Mutation;
Horizontal gene transfer
Mutation
Random, heritable changes in DNA that introduce new alleles into a gene pool
Point mutation;Gene duplication; Genome duplication
Mutation rates are generally low, can be deleterious, neutral, or beneficial. Only mutations that occur in germ line (generate gametes) are inherited
Why would a duplicated gene accumulate mutations more quickly than a non-duplicated gene?
because the mutations can accumulate without an effect, because the unreplicated gene on the homologous chromosome, and can therefore be passed on to the next generation (no fitness consequences)
Horizontal gene transfer
Genes passed from one organism to another
Common among bacteria
Likely origin of eukaryotic chloroplasts and mitochondria
What affects allele frequency
Genetic drift; Gene flow; Selection
Genetic drift
Random (=chance) fluctuations in allele frequencies from one generation to the next; examples are bottleneck effect or founder effect
Gene flow
Transfer of alleles from one population to another; Immigration & emigration / accidental movement; Important in mobile organisms
Directional selection
favours variants at one extreme of distribution ex. mice, from all colours to favouring dark coloured
Disruptive selection
favours variants at both extremes of distribution ex. favouring light or dark mice over intermediate brown coloured mice
Stabilising selection
favours intermediate variants and removes extremes of distribution
Natural selection in the wild: Coloration of Trinidadian guppies is an example of
directional selection- Above waterfalls, bright coloured for females, Below waterfalls, dull coloured in the presence of Pike-cichlid
Natural selection in the wild: Bill size in Darwin’s finches is an example of
Disruptive selection- During a drought, birds with large bills could eat seeds or by bark stripping, birds with smaller bills could eat rotting parts, birds with medium sized bills who ate fruit died
Natural selection in the wild: Birth weight in humans is an example of
Stabilising selection- think of the graph: infant mortality is at its lowest when the birth weight is between 6-8, goes up on either side
Types of questions: proximate vs. ultimate
PROXIMATE = explanations based on immediate cause (stimuli, genetics, hormones, experience)- Ethology / psychology ULTIMATE = explanations based on survival value or function (evolution)- Behavioural ecology
Is behaviour heritable?
It is much more complicated than physical traits, but it is partly genetic! There is a lot of evidence for this: artificial selection for mating speed in fruit flies and errors in a maze for mice.
Evolution optimises every trait, including behaviour, through natural selection
Blackcap migratory behaviour
Experiment where birds were breeded for migratory behaviour- it took 3 generations for 100% of birds in one population to show migratory restlessness, and 6 generations for all birds to show non-migratory behaviour
What is behavioural ecology?
The study of the survival and reproductive value (i.e. the adaptive significance) of behaviour; The ‘ecology’ part is due to the fact that the way in which behaviour contributes to survival and reproduction depends on ecology
Behavioural ecologists ask ultimate questions
Why do these behaviours occur? Why have they evolved?
Does behaviour influence survival and reproduction?
Feeding behaviour: - How & where to search for food? - What type of food to eat? - Forage alone or in a group? • Sexual behaviour: - How to search for a mate? - Which mate to choose? - Which mating strategy to use? • Territorial behaviour: - Defend a territory or not? - How large and where?
A general theme of BE: Natural selection affects
gene survival and individuals (vehicles for genes) should behave to maximise inclusive fitness
What is fitness?
How well an individual (= a gene) does relative to others in the population
Fitness = 0 no representation of allele x in next generation
Fitness = 1 100% allele x in next generation
‘to ensure the survival of the species’ is incorrect
Individual advantage is what counts! think of infanticide in lions
Evolution of altruistic behaviour
Acting to increase another individual’s lifetime
number of offspring at a cost to one’s own survival and reproduction
Helping in bee eaters is an example of
Inclusive fitness: DNA analysis shows that helpers are related to breeding pair! Altruistic behaviour is therefore genetically selfish!
Inclusive fitness
Fitness gained:
directly, through personal reproduction, and
indirectly, by contributing to the survival and reproductive success of relatives
Relatedness
r = coefficient of relatedness between two individual
= the probability that a particular allele, present in one individual, is also present in another individual because of their descent from a common ancestor
= the proportion of genotype in 2 individuals that are identical because of their common descent
r = Σ 0.5^L
L is the number of generation links between the 2 individuals concerned
ex. parent- offspring = 0.5, siblings = 0.5, grandparent-grandchild= 0.25, cousins= 0.125
Kin selection
Process by which characteristics (or alleles) are favoured due to their beneficial effect on the survival or reproduction of relatives
Kin selection should occur if Hamilton’s rule is met
Hamilton’s rule
rB – C > 0 (** note the GREATER THAN not greater than or equal to)
where B = benefit to the recipient of the altruistic act
C = cost to the donor
r = coefficient of relatedness between donor
and recipient
reciprocal altruism
What about altruism towards non-kin? Whenever Brecipient > Cdonor AND help is reciprocated later then both participants benefit
Vampire bats regurgitating is an example of
Reciprocal altruism- Vampire bats favour kin, but they also help non-related roost mates
THE fundamental difference between males and females
Females produce few, large, energetically expensive gametes; Males produce numerous, small, cheap gametes
Bateman’s principle
Male reproductive success is determined by number of mates
Female reproductive success is not
Comparing the reproductive success of each sex
Selection for males to maximise quantity so males compete with each other
Selection for females to maximise quality so females choose their mates carefully
