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Mechanisms of Evolution The Gene Pool The gene pool can be though of as a genetic sum of a population. It is the sum total of all of the genes possessed by the individuals in a population. Remember: a population may contain many different alleles An individual can only contain two alleles evolution is the change in the genetic makeup of a population Therefore, a change in the gene frequency Therefore, a change in the gene pool The Hardy-Weinberg Principle (12.2) – a mathematical basis for evolution under certain conditions of stability, both gene frequencies and genotype ratio’s remain constant from generation to generation in sexually reproducing populations Conditions of Genetic Equilibrium: 1. the population must be large enough to make it highly likely that chance alone could significantly alter gene frequencies 2. mutation does not occur 3. no immigration or emigration 4. reproduction must be totally random 1. and 3. are sometimes met but 2. and 4. are NEVER met! if a population is not in equilibrium it will undergo natural selection…. microevoloution The Hardy-Weinburg Theorem Allows us to predict what to expect for a non-evolving population p = frequency of dominant allele q = frequency of recessive allele p + q=1 based on % adding up to 100% or 1 (p + q)2 = 12 p2 + 2pq + q2 = 1 AA Aa aa Ex. if p = 0.9 (A) and q = 0.1 (a), what are the genotype frequencies? p2 + 2pq + q2 = 1 (0.9)2+2(0.9x0.1) + (0.1)2 = 1 AA Aa aa 81% 18% 1% pg. 549 1&2 But for all this to work, the population must remain in equilibrium so… These calculations are not always what we see in nature in fact, populations are rarely in perfect equilibrium as frequencies change from generation to generation Therefore, HW is not maintained evolution is occurring!!!!! (mutation, migration, genetic drift, non-random mating,natural selection) Sample of Allele Frequencies in a Population of Moths: Random Change (12.3) – chance happenings that lead to BIG changes over time HW gives us insight into what causes evolution: small population chance fluctuations can cause changes in allele frequencies Non-random mating preferred mates pass on their genes more frequently Mutation new allele or change in allele will change frequencies of new and original alleles Migration removal of alleles Natural Selection certain alleles will out survive others therefor increasing their frequencies When a population is not in equlibrium many things can happen: 1. Genetic Drift: changes in the gene pool of a small population due to chance. (Pg. 550) a) Bottleneck Effect Decrease in population because of some disaster. The population is no longer ‘representative’ as certain alleles are lost – elephant seal (population was down to 20 in 1890s) b) Founder Effect Genetic drift in a new colony. Usually occurs because of the small number of pioneers in an isolated area. 2. Gene Flow: A population may gain or lose alleles by immigration or emigration. 3. Mutation: changes to the gene pool of a population these are always occurring (most genes undergo mutation once every 50 000 to 100 000 duplications) this is the original source of all variation!!! can be neutral, harmful or beneficial to the organism it is those beneficial ones that usually lead to increased reproductive success = adaptation = evolution! 4. Non-Random Mating: an organism’s genotype influences its selection of a mate The physical efficiency and frequency of its mating, its fertility, total # of zygotes, % of successful births etc. Also: 1. Mating with nearby individuals promotes inbreeding 2. Tendency to mate with partners like oneself in some ways: Assortative mating 3. Sexual Selection The brutal truth eliminates all but the most favorable gene combinations. This is why Darwin referred to nature as: “Red in Tooth and Claw” (A bit chilling …) Patterns of Selection (12.4) – the ‘not-so-chance’ changes in populations … there are selection pressures mutations provide a continuous supply of new genetic variation natural selection leads to a variety of outcomes when this genetic variation occurs within competitive populations Ex. Sickle-Cell Anemia 3 selection pressures: homozygous s-cv: strong negative against (severe affliction) heterozygous: postive/negative dependent on area (anemic but resistant to malaria) homozygous (normal Hb): positive/negative dependent on area (no malaria resistance, but all functional Hb) these pressures lead to differential success in reproduction (heterozygous is most favored in areas where malaria is common) So… Although genes provide the source of variation, natural selection acts on individuals and their phenotypes. Types of Selection 1. Stabilizing Selection pg. 557 once a species has become well adapted to their environment, selection pressures will tend to prevent them from changing (= unseen evolution) the most common phenotypes are most favoured by the environment natural selection eliminates extreme variations of a particular trait most common selection pressure 2. Directional Selection environment favours individuals with more extreme variations of a trait can result in observable change in a population especially in large population with short generation times 3. Disruptive Selection favours individuals with variations at opposite extremes of a trait over individuals with intermediate variations environment may favour more than one phenotype this is a significant evolutionary mechanism for the formation of distinctive forms within a populations can lead to isolated breeding populations which leads to speciation (creation of new species) 4. Sexual Selection favours the selection of any trait that influences the mating success of the individual ex.: sexual dimorphism physical appearance of males and females is very different and behavior is different between the sexes female selection of males based on: appearance behavior male vs. male competition not environmental or else both sexes would have the trait can result in detrimental traits – ex. body parts that are obvious to predators The bottom line of all of this is that natural selection is the mechanism that drives the evolution of a species. Cumulative Selection (12.5) Did the eye just appear suddenly in one generation? OR Did it evolve piece by piece over millions of generations? (um … probably #2) the complete eye we know today evolved gradually through a series of accumulated beneficial mutations Picture here the environment would have favoured the individual capable of sensing light get out of the sun or away from predators evolution of a complex structure is a cumulative process Evolution of Insect Pollination 1. Wind pollination = 2. gain slight lots of pollen stickiness = less pollen attract insects = even has to be produced 3. make structures to less pollen to produce Altruism one organism benefits much more from the behaviour of another organism than that organism itself Darwin’s “Fatal” part of his theory ex. wasp colonies of females helping only 1 female to reproduce (sisters) May not pass on their genetics but at least passing on the genetics of the family Speciation: The Formation of a New Species (12.6) the formation of an entirely new species evolution at the species level is called microevolution Species: members of a population that have the ability to breed with each other under natural conditions often we can differentiate between species solely on physical or morphological differences but sometimes behavior or biological methods are needed Ex. reproductive isolating mechanisms: any behavioural, structural or biochemical traits that prevent individuals of different species from reproducing successfully with each other Reproductive Isolating Mechanisms Prezygotic Mechanisms prevent fertilization 1. Ecological isolation – occupy different habitats prevent mating 2. Temporal isolation – diff. plants bloom at diff. times of day 3. Behavioural isolation – wrong signals won’t attract a mate prevent fertilization 4. Mechanical isolation – parts won’t fit 5. Gametic isolation – ex. biological markers, releasing gametes into water Some reproduction between species produces hybrids which are often sterile ex. mule – mare for a mommy and an ass for a daddy and all mules are female Postzygotic Mechanisms 1. zygotic mortality – the zygote dies 2. hybrid inviability – it doesn’t live long 3. hybrid infertility – ie. mule Modes of Speciation within isolated gene pools, any mutations and subsequent selection processes that occur in one population can no longer be shared with others – most commonly result from geographical isolation Allopatric Speciation the evolution of populations into separate species as a result of a geographical isolation ex. a canyon, or a river Sympatric Speciation the evolution of populations within the same geographical area into separate species – weird but does happen (fluke – but isn’t all evolution this???…) (pg. 575)