Download Evolution, 2e

Some misconceptions promulgated Discovery series: “Life” (3/21-10)
• 1. female poison frog: “has to get tadpoles to safety.”
• 2. female octopus: “blows water over her eggs to help them
breath.”
• 3. “amphibians have no scales, no claws, so they have had to
evolve different (survival) strategies.
• 4. female chameleon in Namib Desert: moving across the
dunes “looking for love.”
• 5. male African bullfrog: “tadpoles will begin to suffocate
within one hour unless he can do something about it.”
• 6. Sea krait: “needs to make sure eggs are safe.” “found a
solution.”
Life histories and reproductive mode
• Two principles:
• 1. Small genetic component in life history variation.
– Flexibility is important
• 2. Life history characteristics have not evolved in
order to perpetuate a species.
– Shaped by natural selection increasing fitness of
individuals.
• Life histories and reproductive maturity
• E.g., insects
Reproductive effort—Index of the proportion of biomass allocated to
inflorescences—in annual (semelparous) and perennial (iteroparous)
species of British grasses
Semalparity in males of
Antechinus sps.
Trade-off between number of seeds and their size among Goldenrods
(Solidago)
Trade-off between age of reproductive maturity and adult
mortality rate
Guppies: selection for age at reproductive maturity
♂
♀
Selection for size of female (first birth) and size of offspring
♀
Sex and Reproductive Success
• Sex as an Adaptation
• Since most species are bisexual (gonochoristic), there must be
some advantage to sexual reproduction.
• Resistance against parasites?
• Sexual reproduction promotes genetic recombination
• Trade-off: separating favorable gene combinations.
• If A1 is always associated with B1 and A2 with B2, the genes
are in linkage disequilibrium.
• Linkage equilibrium is achieved by crossing over.
• Eventual result = association of 2 or more alleles at the
frequency predicted by their individual frequencies.
Bringing together
A1 B1/ A1 B1 and
A2 B2 / A2 B2 in
the same population
A1 B1/ A2 B2
Decay of linkage
disequilibrium
Continues until alleles at
The 2 loci are randomly
associated with each
other
A and B are in linkage
equilibrium if
(gAB*gab) – (gAb*gaB) = 0
Countering linkage equilibrium: Primulus vulgaris
1. Keep genes
together that
promote outcrossing
Heterostylous
G and A closely
linked
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•
•
•
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2. Reproduce asexually
e.g., unisexual vertebrates
Shared features
1. Originate by hybridization
2. Exclusively female
3. There is an aberrant gametogenic mechanism
– mitotic or meiotic
• 4. Genetic recombination is usually absent so that
inheritance is clonal.
• 5. Polyploidy can occur if synapsis between nonhomologous chromosomes does not have to occur
• or if homologous chromosomes can be produced.
Four Reproductive Modes
• An hypothesis (Maynard Smith): Asexual reproduction
(all females) should be twice as advantageous as sexual
reproduction--where half of the population are males.
• Theoretical basis – Female fitness is maximized if her
genes are not “wasted” in the production of males.
• However, when actual asexual populations are studied e.g.,
(parthenogenetic vertebrates) there are some obvious
disadvantages.
• 1. An asexual mechanism in vertebrates is difficult to
produce.
• 2. The two sexual species that hybridize to produce
successful asexual entities:
– A. must be sufficiently divergent genetically to modify
normal meiosis but
– B. can’t be too divergent genetically or development
will be abnormal.
• a proportion of parthenogens hatched in the lab have
developmental deformities