Week 6 notes
Week 6 notes GE 70B
Popular in Evolution of Life and the Cosmos
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General Education Cluster 80A Frontiers in Human Aging
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This 5 page Class Notes was uploaded by Sarah Doberneck on Tuesday February 16, 2016. The Class Notes belongs to GE 70B at University of California - Los Angeles taught by Dr. Friscia in Winter 2016. Since its upload, it has received 40 views. For similar materials see Evolution of Life and the Cosmos in General at University of California - Los Angeles.
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Date Created: 02/16/16
Asexual reproduction Many species can reproduce without sex Advantages to asexual reproduction Makes adaptation difficult o Sexual reproduction can combine beneficial mutations that Aries in different lineages o Asexual reproduction Doesn't combine beneficial mutations so adaptation may take longer Send maintains genetic variation Asexual reproduction takes inbreeding to the extreme Millers ratchet: deleterious alleles accumulate within lineages o All alleles passed in, instead of half as in sexual reproduction o Un more difficult or impossible to purge bad alleles o Sends asexual lineage on a path to extinction Sex shuffles allele combinations (generates new genotypes) each generation Red queen hypothesis Sex may also provide a way to evade parasites o A host gets a mutation that exists infection, selection will increase the frequency of this allele o The parasite counters with its own mutation which then also increases in frequency o Host mutation is no longer resistant so frequency decreases o Decrease in parasite mutation follows, cycle continues What does sexual reproduction have to do with this? o Parasite typically have much shorter generation time the hosts Allows their populations to evolve more quickly o Sex allows the host to keep up with the parasites evolution and resistance o Evidence: snail population s in New Zealand Sexual selection Darwin was puzzled by certain male traits that seemed to hammer survival (e.g. Large, colorful peacock feathers) Sexual selection is the selection for a trait that increases mating opportunities. Occurs when three is competition for mates o Direct competition between individuals of the same sex o Indirect competition to n the most attractive to opposite sex Females pit Moore energy into reproduction o Ovulation and pregnancy take a lot of energy and time Batemans principle o Females are a limited mating resource because egg as are costly. Also the parental obligations take time. So females are very choosy and then makes have to be impressive Natural selection vs. sexual selection Bright colors and conspicuous ornaments would seem to attract predators, how do they evolve and persist? Runaway selection (example): o Female peacock liked a big bright tail o Sons inherit dads. Big bright tail o Daughters inherit moms preference for it o Runaway selection ensues, tails get bigger and brighter o Stops only when dangerous threshold is met Reproductive benefit to being conspicuously sexy must be deed the cost of being more visible and vulnerable to predators What might drive female preference in the first place? o Could be arbitrary and runaway selection would still work As long as daughters inherit the preference, makes with preferred traits have fitness advantage o Or, attractive male traits may be a sign of vitality Could initiate or reinforce male preference o What about liars? The trait must have a cost. Energy is expended to attract the female using that trait and shows that the individual is healthy Monogamy vs polygamy Monogamy: when male and female mate only with each other during a breeding season o Rae among animals o Typically arises only when care from both parents gives strong selective advantage to offspring Females are often polyandrous to hedge their bets and make sure they are investing in good sperm o Eggs limit their production so they are more choosy o Creates direct competition between sperm o Make strategies to ensure competitive sperm Increase quantity of sperm Increase quality of sperm Remove sperm of previous males Aggregation of sperm allows it to swim faster. The sperm combines with other sperm from the same male o Makes need to get around female defenses against fertilization o One eggs. Many sperm (polyspermy) is usually bad To prevent this, females evolve ways to avoid fertilization E.g. Gamete Recognition proteins evolve very rapidly Proteins on the surface of sperm must recognize those on egg surface Egg surface proteins evolves rapidly to avoid being recognized Sperm evolution has to keep up, millions of sperm to one egg helps them do that Role reversals What conditions cause females to compete for mates? o Syngnathids (sea horses and pipefishes) o Males carry developing embryos o Females compete for mates Family sacrifice Offspring quality can be just as important as quantity This may explain why male sand gobies eat their developing babies o Paternal care (guarding eggs) is common in gobies o Male sand gobies sometimes eat eggs they're guarding but why? o When density of eggs is high in the nest, makes eat mor eggs Low oxygen outs all eggs at risk, eliminating some of those potential offspring can help the rest survive rather than them all dying Sex ratios 1:1 ratio of male to females is a typical expectation for Mendelian inheritance of sex chromosomes o It, it's also easy to image that mutations could lead either x or Y sperm to be more successful, skewing the sex ratio o But why are they still 1:1 R.a. Fishers adaptive explanation o Let's say mutation lead to more female births o Makes now have reproductive advantage (easier to find mates o Any mutation that leads to more sons now has the advantage o Presence of more males eventually return selective advantage to mutations for more daughters o Frequency dependent selection/: traits are only favored if they are rare Survival vs. Reproduction Guppies in Trinidad o Tradeoff between investing in early growth and early reproduction o When small guppies are preferred prey, they prioritize growth before reproduction o When large guppies are preferred prey, they reach sexual maturity sooner o Evolution of flayed reproduction observed in guppies transplanted from high to low predation environment Semelparity: semelparous species reproduce only one in their life time o Many of these mothers die immediately or soon after giving birth o Natural selection doesn’t favor genes for post reproductive survival o Most species of octopus are examples of this. They live for just a few years so they invest most of their energy into one reproductive event Iteroparity: iteroparous species have multiple reproductive cycles o Mothers have multiple birth events in a single lifespan o Natural selection favors longer life spans because it allows for more reproduction Menopause in Humans Life span of human mothers goes far beyond their reproductive age. How might this be adaptive? Mother hypothesis: prevents women from having their resources divided between older children and babies. Counters increased risk of death during childbirth at old age Grandmother hypothesis: genetic constraint while increased life span favored by selection. Older women can help raise their grandchildren who have 1/4 of their genes Or maybe it’s not adaptive at all o Guppies also experience a form of menopause but no grandmother care and presence of predation makes no difference o Maybe just a tradeoff of aging The Origin of Species We refer to this process of origination of new species as speciation Species seem to be very easily to identify but there are a couple things that make it more difficult to distinguish different species o Individuals of the same species look very different when there is a lot of INTRAspecific variation Sexual dimorphism is when different sexes have different traits Or, there is just a lot of variation o Interspecific variation can cause different species to go unnoticed Referred to as cryptic species Defining species o Species concept: a set of criteria used to determine how many species are present in a group of individuals o In the absence of obvious visual clues, other types of evidence like reproductive biology or genetics Biological species concept o Species consist of populations and organism that can reproduce with one another and do not interbreed with other such populations o Problems with this are asexual reproduction and how some populations are able to interbreed physically but are geologically separated Phylogenetic Species Concept: species are defined by genetic distinctiveness, specifically by monophyletic groups separated by relatively long branches o Problems: how long is a relatively long branch? What happens when different genes give you different answers/branch lengths? o Usually more evidence than just phylogenetic trees are needed to justify formal description of a new species Speciation o No matter how you define a sexually reproducing series, reproductive isolation is critical in the process that generates more of them o Reproductive isolation is when two populations either cannot or do not interbreed o This violates the random mating assumption of the hardy Weinberg equilibrium Allopatric speciation o A population diverges into two species after being physically isolated by some sort of barrier o These barriers come from plate tectonics, island formation, climate, and migration o Let’s say a new river begins to run through the middle of a species' habitat. Genetic drift or natural selection cause different sets of alleles to become fixed on either side of the barrier. Over time the combinations of alleles in each group become so different from each other that they are eventually considered two distinct species. At this point their genes are so incompatible that even if the barrier disappears, they can’t interbreed Isthmus of panama o Connected north and south America about 3 million years ago o Terrestrial organisms separated for millions of years migrated between the two continents o Also separated the pacific and Atlantic oceans creating geminate species pairs Many of these species that were isolated became distinct from one another Secondary contact o When isolated populations come back onto contact, reproductive isolation may not be complete o However, hybridization may half or reverse the speciation process o Also reinforcement may facilitate competition Pre-zygotic isolation o Individuals either avoid mating or aren’t given the opportunity o Examples include sea animals that release their sperm and eggs at different times, so there is no way for the two to interbreed o Also many birds use song as a tool for mate recognition and only breed with birds who sing the same song Post-zygotic isolation o Individuals can and do mate, but they produce offspring with decreased fitness or no offspring at all o Even a slight hybrid disadvantage can reinforce assort ice mating, which drives the species even farther apart o Incipient speciation: species in the process of diverging but could potentially hybridize back into one species Ring species o Long term migratory route wraps around a barrier o More distant populations share fewer alleles due to drift and selection o Adjacent populations can interbreed o But where the ends meet on the other side of the barrier they cannot Allele fixation o When two populations of the same species stop interbreeding with each other, what happens next? o Drift and/or selection o Allele frequencies change between the two populations o Some alleles become fixed or lost (evolution) o As time passes, populations continue to diverge until each one is a new species Allopathy: geographic barrier separates two populations Sympatry: two populations overlap in space o Happens through host shift, divergent selection, and sexual selection o Once reproductive isolation is established, different alleles become fixed in each mating population o Examples: apple maggot flies, Darwin's finches, African cichlids Instant speciation o Allopolyploidy o Two species with different numbers of chromosomes hybridize o If the chromosome number of hybrid is odd, it can reproduce asexually but not sexually o If it then duplicates its genome, its back to even number of chromosomes and can now sexually reproduce, but probably not with either parent species o Just like that, it has become its own new species
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