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Transcript
EVOLUTION- a change in the gene frequency of a population Two important consequences of evolution for ecology (the first is important for understanding life history patterns, the second for understanding of community interactions) 1. MECHANISMS OF EVOLUTION 1. mutations 2. migration 3. genetic drift 4. natural selection 1. mutations - mistake in replicating genetic information. e.g. single nucleotide deletion. A change in genetic instructions. -RANDOM (no matter how the env changes, will not determine type of mutation), whether beneficial or harmful depends on the environment. - only way that new alleles arize, i.e. the original source of variation 2. migration - immigration of new individuals either carrying new alleles or a different frequency of alleles. How much populations move around is very difficult to determine. Can not follow every individual. Larger animals can be tagged..... 3. genetic drift - random changes in the frequency of genes due to small population size. If only 6 individual (3 AA and 3 aa) and there is an equal chance of death before reproduction and three die, then a significant chance one allele will go extinct. If 1000 AA and 1000 aa, and half die, very unlikely that all AA will die. 4. natural selection - the only mechanism that explains how organism adapt to their environment. Is a non-random process but is effected by random processes of mutation and environmental change. -predictable Proposed by C. Darwin and A. Wallace; 3 conditions: (both recognized diversity of earth and geologic record in voyages to tropics) 1) Traits are Inherited e.g..... Certainly can observe heritability of traits in nature. 2) Variation in Traits e.g..... Variation occur by mutation, recombination, (sexual reproduction), migration and hence is random. However, variation is limited (a population can only carry so many types of genes and mutations are random not produced when needed) so that the potential directions of evolution for a given population are limited 3) Differential survival and reproduction of heritable traits depending on the environment and interacting organisms, some traits increase the chance of survival and hence reproduction. Population have the potential to grow fastest enough to outstrip food. i.e. N.S. is differental survival and reproduction of genes Higher survival increase chance of reproduction (blueprint) The more copies of the blueprint passed on by one type of individual, the more prevalent those blueprints become in the population. Environmental change directs evolution. Environment may change randomly, but organism will adapt to change through natural selection (very nonrandom) i.e. explains correlation of environment with adaptation. (adaptation- an existing structural, physiological, or behaviorial of an individual that promotes survival and reproduction under prevailing conditions). No perfecting trend (e.g. parasite evol.) Is larger size adaptive? Can not say a trait (or a gene) is adaptive unless you consider the evironment it is in. e.g. a gene that causes thicker body fur will only be adaptive in a colder environment. Selection is not survival of the strongest (fittest is not the most fitness in the exercise sense). "Natural selection is not an active agent with physical properties, much less a mind. It is no more than a statisical measure of the difference in survival and reproduction among entities that differ in one or more characteristics. Selection is not caused be differential survival and reproduction; it is differential survival and reproduction, and no more" Dawkins - The Blind Wacthmaker i.e. no way that N.S. can anticipate future change. Modes of selection- Env determines direction of selection. STABILIZING SELECTION birth weights, horseshoe crabs DIRECTIONAL SELECTION peppered moths, pest resistance, antibiotics and sphyllis DISRUPTIVE SELECTION multi-phenotype mimics If same selection pressure is applied for 40 generation, number of AA and Aa become less than 1% of population If selection pressure is only a 1% difference in survival, applied for 40 generation, number of AA is 20% of population, down 5% Small differences in selection pressures can make significant evolutionary changes because selection is cumulative because living systems replicate (Other systems undergo selection, but do not reproduce. Hence selection is not cumulative in nonbiological systems. In biological systems selection is cumulative, systems can be complex because changes build on previous changes) Natural selection is the ability to pass traits on to future generations, not just the next generation. If a trait becomes more common in the next generation, but those individuals can not survive, the trait will never be selected for in the long run because selection is cumulative. If gene A causes larger body size than gene B (assuming no heterozygotes, e.g. asexual) which increases reproduction but decrease survival (e.g can not hide).. Gene A, results in 1.0 survival (100% survive to reproduce) and 1.0 offspring per individ. Gene B, results in .5 survival (50% survive to reproduce) and 3.0 offspring per individ. then 100 individual with gene A X 1.0 X 1.0 = 100 offspring 100 individual with gene B X 0.5 X 3.0 = 150 offspring How will the population change? (which gene will become more common?) Sexual selection demonstrates that the success of reproduction can outweigh cost of increased mortality. However, excessive reproduction no good if at the cost of low parent and offspring survival. Must not only be able to reproduce but survive till reproduction and produce offspring that are able to survive. Natural selection is not always survival of the strongest or most physically fit, but the ability to pass genes on to the next generation (instruction that increase survival and reproduction will become more common in the next generation) Thus, success among individuals (and thus ultimately among populations) is measured only in terms of successful offspring. When to reproduce, how often, how many at each time, how long to live is physically constrained. Natural selection should optimize these strategies depending on the environment. This optimization of traits to maximize offspring represents the LIFE HISTORY PATTERN Limit to how much energy available, so must be partitioned between repro and surv. (and further partioned within each; e.g. age of reproduction, brood size....). How it is done depends on env. and strategies of competitors, predators, etc. Selection based on individual (those genes that are more likely to be passed on increase), Lamarck- env causes change in genes, Darwin - genes determines whose genes surv/repro., random mutation changes genes. Again, variation is limited because new adaptation are not produced just because they are needed so that the potential directions of evolution for a given population are limited. Thus ind. selected causing evolution of population, but don't think populations are selected: Species are not selected, individuals are. Traits that favor the survival of the species but not the individual are less likely to be passed on (the good of species). Cheater's gene win under most conditions. Some behavoirs may appear alturistic, but usually can be shown to improve individual survival (e.g. crow calls scatter flock, confusing predators). Wynn-Edwards: populations managed themselves, overpopulation prevented because individuals limited reproduction or increased mortality (lemmings). NO MECHANISM (Ockham's razor, atfter the 14th century friar William of Ockham: it is in vain to do by more what can be done by less - the simplest explanation is the best). Explanation for decreased reproduction at higher populations density: Wynn-Edwards - individuals limit reproduction for good of species. Darwian - as population size increases those with fewer young are better able to feed their young and those survive and pass on genes for fewer offspring Alturistic behaviors can evolve under Darwian natural selection in kin selection. An individual shares a certain number of genes with siblings so more likely to take risks saving them. (does not mean selfness is best, only that we can understand biological basis. As humans we can think through the consequences of selfness). (Also, some models of group selection proposed but require very specialized conditions). molecule genes cells individuals family groups popul. comm. ecosys. Heritability and Repro. Variability -------decreasing-----------------> /--------none----------/ Another consequence decreasing heritablity with increasing levels of organization is that ecosystems (and communities) are not likely to be similar. Thus making generalizations about ecology difficult. MAIN POINTS ON NATURAL SELECTION: -natural selection is differential survival and reproduction of inheritable traits. Ecological strategies must be considered not only in light of survival, but also reproduction. (the number of genes passed onto future generations depend both on survival of parents, number of offspring, and survival of offspring). The resultant optimization of reproduction and survivorship to maximize offspring is the life history strategy. (answer questions like why is one species more abundant than another, or why has a population evovle such and such life history patterns?) -the direction of natural selection depends on the environment which results in individuals being selected causing populations to evolve. Populations are regulated by external factors; they do not regulate themselves -variation is limited so that the potential directions of evolution for a given population are limited