Lectures 6-10

From Biol301

• Natural Selection will work most efficiently when an organism is near the carrying capacity of the environment and thus struggle for survival and the more fit phenotypes dominate.

• The wild-type and melanic forms of Biston betularia are important because they exhibit how the fittest phenotype will predominate. Initially the wild-type was the white peppered moth but as the environment changed due to industry the white ones became less fit. H.B.D. Kettlewell and E.B. Ford released a controlled number of these moths into a soot covered forest and collected them a few days later to discover more of the white moths were eaten.

• Steps of the scientic method are: Observation, Prediction, Hypothesis, Test

• The selection coefficient is the distance between the advantage and disadvantage associated with a particular phenotype. This allows us to make mathematical calculations about evolution.

• Three basic conditions must be demonstrated for a trait if a researcher proposes it has evolved under natural selection. Which are variance, heritability and fitness due to the variability.

• The comparative method is statistically establishing a non-random pattern of phenotype within independent lineages that are faced with similar (environmental) selection pressures. This shows that the phenotype is due to convergence not homology because the species are unrelated. The method shows that the environment plays a large role in the phenotype.

• Optimality Studies show how close a biological phenotype is to an optimal (fittest) state calculated from the first principles. The leaf insect resembles the shape and color of the plant it lives on. Studies can show the colors of various foods that birds will eat against the leafy background and compare the color least fed on with the color of the bug. Also the bug’s behavior is selected for because if the bug was the right shape and color he would still get eaten if he moved around a lot.

• Ethology is the study of animal behavior.

• Optimal foraging is the investment of energy for getting food versus the reward. E.g. there is an optimal size of shellfish for a crab to eat that can be calculated and is actually the size most selected for by crabs.

• Carrying capacity (of an environment) is the largest size a population can be based on available resources

• Facultatively sexual means the creature can choose to reproduce sexually or asexually; obligate sexual reproduction is (like humans) when you cannot choose which way to reproduce.

• Sexual Reproduction is a derived state and has developed in a rather small portion of the tree of life.

• The direct (two-fold) cost of sex is that it halves an organisms reproductive success; offspring contain only half of the parents genes.

• The indirect cost of sex is that resources are spent on ornamentation, fitness is reduced due to ornamentation and lots of time, effort, and expense go into finding a mate. Also it requires complex machinery that can malfunction.

• Sexual reproduction involves the mixing of genetic material and reproduction. Sex without reproduction is conjugation (E. coli) and reproduction without sex is asexual.

• Fisher’s hypothesis that sexual reproduction speeds up adaptive evolution was that sex shuffles genes which allows mutations to spread faster in a population. With asexually reproduction all beneficial mutations would have to arise together so that they can all spread in the population otherwise they would be in competition.

• G.C. Williams proposed the balance argument which states that there must be a short term advantage to sexual reproduction because there are facultative sexual species. (A facultative species would simply see one more fit organism change to .)He also produced the theory of sib competition which states that the parents producing the most varied (hence sexual reproduction) seeds are statistically more likely to survive

• Hamilton’s Red Queen theory suggests that larger organisms have to reproduce sexually to keep up with the parasites otherwise they would be out evolved. "You must run as fast as you can just to stay where you are."

• Anisogamy is unequal size of gametes and isogamy is equal sized gametes.

• Zahavi’s handicap principle states the organisms that have enough resources to waste them on sexual ornamentation are the healthiest. Thus by handicapping themselves (by being more susceptable to predators) they show off their health and parasite freeness.

• The defining characteristic between males and females is gamete size: males have small gametes, females have larger gametes.

• Trivers said that if a heritable mutation arises that leads to slightly different sized gametes the parent with the larger gametes would have more to lose and the difference would be intensified through disruptive selection.

• The emperor penguin after the female lays the egg will balance it on its feet for about 65 days and stands out in the artic cold. Usually found in large crowded moving masses that cycle so that everyone gets a turn in the warm middle.

• Adaptive landscapes help illustrate the associated fitness based on many phenotypic traits. The environment presents challenges which have hypothetical ideal solutions at the "top" of the landscape. Natural selection selects organisms such that the population approaches the peak. Once at the peak, natural selection can be seen as a stabilizing or purifying selection --selecting against organisms venturing down the slope via random mutation.

• Fisher’s microscope theory is an example of gradualism. It demonstrates that the best way to approach a point is to make small changes and determine after each change if you are closer to the ideal answer --if so, change again, if not, undo the change. Thus evolution occurs in small steps much like focusing a microscope.

• The adaptive landscape was first proposed by Sewall Wright in 1932.

• Jacob’s statement "Evolution is a tinkerer", refers to the observation that evolution will occur wherever the organism starts. It will climb the nearest peak.

• Purifying selection is the process by which an organism at its adaptive peak stays at its adaptive peak --mutations placing organisms slightly off the peak will be selected against. This is very often the kind of selection that is occuring for present populations because after 4.6 billion years of evolution most organisms are at their peaks and any mutations would be harmful.

• Adaptive peaks shift if there is a change in the environment. Most likely due to humans but really any other organisms could change the environment.

• For natural selection to work three conditions must be met:

1. self-replicating units that vary in properties
2. variation affects reproductive success
3. variation is heritable

• • This most likely only occurs in the individuals not the species or populations.

• Natural selection and sexual selection amount to the same thing so long as you look at what traits leave more offspring and are therefore more fit. While ornamentation may make an organism more vulnerable it also lets the females know he is parasite free and can afford to waste resources. Thus the females choose him over a less ornamented male because he is in fact more fit.

• The fluke is a parasite that manipulates the behavior of its host. Flukes live in sheep’s livers. The fluke eggs leave the sheep and must be eaten by a snail. The snail gets rid of the fluke in a slime ball and the slime is eaten by an ant. The fluke needs to be in a sheep’s liver to grow so the parasite alters the ant’s behavior and causes it to hang from a blade of grass until a sheep comes around and eats it.

• Also there is Toxoplasma gondii that is a protozoan parasite that is found in 35% of rats but prefers to live in cats. The infected rats lose their natural fear of cats because of the parasite and therefore have a greater chance of being eaten by a cat.

• Wynne- Edwards appeared to observe adaptations that only made sense at the group level (Group-Selection). Wynne observed that a bird laying only 2 or 3 eggs and incorrectly proposed that it was doing so to keep the population at a reasonable level. An optimality study would show that only 2 or 3 eggs are laid because that is what is optimal (i.e. the most a parent can efficiently and effectively take care of) not because it is good for the species.

• Satisficing theory states that individuals perform tasks of living and reproducing at the threshold of efficiency (just enough to survive and reproduce). For example: prey would run just a little bit faster than their predator. This is not correct an individual needs to run faster than the one next to him. Once the slowest dies, the standard is raised hence we still see "progress" along a spectrum. This is called intraspecific competition.

• Richard Dawkins’ distinction between ‘vehicles’ (organisms) and ‘replicators’ (genes) is that changes of the vehicle are not inherited whereas changes of the replicator are inherited.

• A gene for altruistic behavior will spread if the total of benefit of other copies of the gene multiplied by the relatedness is greater than the cost to a particular copy of the gene. This was Hamilton’s (1964) distinction of "Inclusive fitness" this ties in with "Kin selection" because closely related organisms have a greater relatedness (obviously) and are therefore more likely to have altruistic behaviors around family. From the genes point of view everything is going great if the other copies are doing well.

• Aposematism – warning colors

• The haplo-diploidy of hymenopteran insects (ants, bees and wasps) predisposes them to eusocial living (that is, helping to raise others' young instead of having it's own young, having sterile castes in the population) because an individual would be more like a sibling than its own child. For example female bees are diploid and males are haploid; by definition half the genes in the diploid female are from the mother and half from the father. The chance a sister has the gene from the mother is ½ and the chance a sister has the gene from the father is 1 with the average of the two being ¾. So sisters share ¾ of their genetic material and if the female had a child it would only be ½ related so it makes more sense (in terms of a gene perpetuating itself) that she would raise her sisters over having her own child. Also, note males are identical to their brothers.

• Cytoplasmic genes are predominately found in the female’s larger gamete and to that cytoplasmic gene it makes more sense to have more female offspring than male because males are lacking in this gene (or more precisely their gametes don’t pass on cytoplasmic genes).
  • Feminizers are cytoplasmic genes that override the nuclear gene so that regardless the individual is female. An examples is the Wolbacteria in the common woodlouse.
  • Parthenogens are cytoplasmic factors that make females reproduce asexually and only produce females. An example is Wolbachia trichogramma.

• Intragenomic conflict is when different genes within a different organism have antagonistic fitness landscapes. An example of this is in isogamy --the cytoplasmic genes compete for dominance and only one can "win". This competition can weaken or even kill the zygote.
  • Anisogamy resolves the intragenomic conflict of isogamy by making it so that the female’s cytoplasm will dominate in the zygote with little to no struggle.

• Selfish genes arise because they are "concerned" with replicating themselves. In asexual reroduction there is no competition --all the genes in the parent will be transferred to the children, hence, we do not see selfish genes in asexually reproducing animals.

• Meiotic Drive (=‘segregation distorter’) gene is the name given to an allele that selfishly promotes its own spread through the population by acting immediately after meiosis to kill all gametes that do not contain it. Can be found in mice, plants and Drosophila melanogaster.

• The toxin / anti-toxin plasmid system is, as the name suggests, a plasmid (smaller ring of DNA separate from chromosomal DNA) that contains both a toxin and antitoxin. There is always toxin in the cytoplasmid but as long as the organism has the anti-toxin gene it will survive. If a daughter cell does not get the anti-toxin gene it will still get a cytoplasm full of toxin! An example is the pSM19035 plasmid from Streptococcus pyogenes.

• Memes proposed by Richard Dawkins (1976) are self replicating thought units. E.g. a catchy song or belief.

• Bateson (1915) suggested that even under the strictest application of natural selection it is unnecessary to suppose that every part of an animal is useful.

• Hardy Weinberg Equilibrium is a constant allele frequency over time and the conditions that must be met for the equilibrium are:

1. Infinite population size
2. No migration
3. No mutations
4. Random mating
5. No natural selection

• • This does not occur in a population of finite size because it tells us the probabilities of allele frequency but with a small sample there is no way to achieve the fraction of a allele after meiosis. You either have it or you don’t so after one round of mating the allele frequencies change and you are not in equilibrium.

• Genetic drift is the random change in allele frequencies. The amount of genetic drift is inversely proportional to population size. It is most apparent in small populations. Drift has nothing to do with fitness and allows even deleterious alleles to increase in frequency. Also, populations are freed from purifying selection and can undergo radical changes even become new species. Two examples of drift are:
  • Founder Effect: when a small group is detached from the main population and founds a new population (for example the Amish).
  • Population Bottleneck: when an environmental disaster reduces the population to a small fraction of its normal size.

• Selection coefficients are an indication of the strength of drift or selection. High coefficients mean natural selection dictates change in allele frequency and small coefficients mean genetic drift dictates.