Transcript ppt
Coevolution Coevolution I. Types of Interactions A. Overview: Effect on Species 2 Positive Effect on species 1 Positive Neutral Negative Neutral Negative Coevolution I. Types of Interactions A. Overview: Effect on Species 2 Positive Neutral Negative Positive mutualism commensal consumer Neutral commensal - amensal Negative consumer amensal competition Effect on species 1 Co-evolution requires reciprocal evolutionary feedbacks… such that response by one species affects the selective pressures on another that encourage further responses. Coevolution I. Types of Interactions A. Overview: B. Competition: - the nature of the interaction: scramble (for resource) or contest (access) Coevolution I. Types of Interactions A. Overview: B. Competition: - intraspecific competition: - for food, mates, territories Coevolution I. Types of Interactions A. Overview: B. Competition: - interspecific competition: - for food (nutrients), territories Coevolution I. Types of Interactions A. Overview: B. Competition: - outcomes: 1. reduction in growth, reproduction of individuals or populations. 2. local extinction (competitive exclusion) 3. Resource Partitioning and Character Displacement Coevolution I. Types of Interactions A. Overview: B. Competition: - outcomes: - not coevolution. Once one species reduces the intensity of the interaction, selection pressures on the second species are RELAXED, stimulating no further change. Competition Coevolution I. Types of Interactions A. Overview: B. Competition: C. Predation, Herbivory, and Parasitism: Toxicity Coevolution I. Mimicry Types of Interactions A. Overview: B. Competition: C. Predation, Herbivory, and Parasitism: - selects for defense in prey Resistance (encapsulate wasp egg) Avoidance crypsis Coevolution I. Types of Interactions A. Overview: B. Competition: C. Predation, Herbivory, and Parasitism: - selects for defense in prey - Response exerts a pressure on predator: ‘coevolution’ (arms race) “you must keep running to stay in the same place” The ‘Red Queen Hypothesis’ Predation Coevolution I. Detection, and capture Types of Interactions A. Overview: B. Competition: C. Predation, Herbivory, and Parasitism: - selects for defense in prey - Response exerts a pressure on predator: ‘coevolution’ (arms race) - selects for: ‘resistance to ‘resistance’ Detoxification C. Predation, Herbivory, and Parasitism: - Results: Geographic variation in Relationships ‘Mosaic theory of coevolution' Coevolution I. Types of Interactions A. Overview: B. Competition: C. Predation, Herbivory, and Parasitism Batesian Mimicry is an ‘arms race’ Toxic species gains protection from predators ‘aposematic coloration’ Selection favors harmless mimics that look like toxic species. This weakens the correlation between toxicity and coloration, selecting for changes in toxic species so it can be recognized as toxic. Three toxic species Papilio dardanus C. Predation, Herbivory, and Parasitism: - Results: Geographic variation in Relationships C. Predation, Herbivory, and Parasitism: - Results: Geographic variation in Relationships C. Predation, Herbivory, and Parasitism: - Results: Coevolutionary alternation Cuckoo – parasitizes 4 bird species in England. Lays eggs in their nests. Reed Warbler Cuckoo chick pushes other eggs and nestlings out of the nest Hard work feedin the cuckoo chick… C. Predation, Herbivory, and Parasitism: - Results: Coevolutionary alternation Reed Warbler (discriminates) Pied Wagtail (discriminates) Cuckoo – parasitizes 4 bird species in England. Lays eggs in their nests. Dunnock – no discrimination C. Predation, Herbivory, and Parasitism: - Results: Coevolutionary alternation Reed Warbler (discriminates) Pied Wagtail (discriminates) Sp. 1 responds Cuckoo – parasitizes 4 bird species in England. Lays eggs in their nests. Dunnock – no discrimination C. Predation, Herbivory, and Parasitism: - Results: Coevolutionary alternation Reed Warbler (discriminates) Pied Wagtail (discriminates) Sp. 1 responds Cuckoo – parasitizes 4 bird species in England. Lays eggs in their nests. Sp. 1 responds Dunnock – no discrimination C. Predation, Herbivory, and Parasitism: - Costs: Defenses are expensive Selected lines exposed to parasites encapsulate eggs at a greater rate than naïve flies When placed in competition with populations unexposed to wasps, they lose when resources become limiting (competition intense). C. Predation, Herbivory, and Parasitism: - Costs: Defenses are expensive Relaxation of selection causes reversion to asexual reproduction. Wildtype outcrossing rates over time. Outcrossing rates in wildtype populations were not manipulated and free to evolve during the experiment. The wildtype populations were exposed to three different treatments: control (no S. marcescens; dotted line), evolution (fixed strain of S. marcescens; dashed line), and coevolution (coevolving S. marcescens; solid line) for thirty generations. Error bars represent two standard errors of the mean (SE). C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Predators want to kill prey ASAP – to reduce period of interaction Parasites that kill their host before dispersal are selected against. Why are pathogens virulent? C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? - coincidental evolution hypothesis: Some are deadly by chance. They don't mean to kill you, it just happens. The soil bacterium Clostridium tetani produce a potent neurontoxin as a secondary metabolite (just as a consequence of their metabolism). This metabolite is a strong neurotoxin that kills human hosts. But, C. tetani does not normally live in humans. The evolution of this toxin was probably NOT a response to the dynamic relationship between humans and the bacterium. Perhaps it is to kill soil nematodes - a major competitor in the soil microfauna. C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? - short-sighted evolution hypothesis: the pathogen may go through several generations inside the host, and selection will favor within-host fitness over traits which might increase the probability of host survival. So, a strain might cripple it's host as it is selected for within that host (over other strains that are nor as damaging). Organismal Selection overriding Group Selection. C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? - trade-off hypothesis: virulence and transmission are intimately related. Virulence can increase if transmission increases or stays high. But, if population density of hosts decline, then transmission will probably decline and natural selection will favor strains that do not kill their host. Living hosts are more likely to encounter new hosts and transmit the pathogen than dead hosts... (except that cultural practices become important.... handling and burying the dead, or eating tissue from dead bodies, can increase transmission regardless of density and maintain virulence, as well.) Deadly pathogens (Ebola) have to be highly contagious to survive. Myxoma and rabbits in Australia C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? How do they overwhelm host immunity? – MUTATION RATE - HIV - The reverse transcriptases that make the DNA from RNA are error-prone HIV has the highest mutation rate of any virus or organism measured - over 1/2 of the reverse transcriptases made have a new mutation. - AZT - reverse transcription inhibitor - it is a base analog that, when incorporated, stops transcription because it has a N-group instead of a 3' OH group. Other reverse transcriptase inhibitors lock up the active site. Thus, there is rapid selection for rt'ases that fail to bind AZT - rendering this treatment ineffective in 6 months. But, these new variants do not replicate as quickly; so when AZT treatment is stopped, there is reverse selection for original variant that reproduces more rapidly, and AZT is again effective. Trade-offs C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? How do they overwhelm host immunity? – MUTATION RATE - HIV - The reverse transcriptases that make the DNA from RNA are errorprone - HIV has the highest mutation rate of any virus or organism measured over 1/2 of the reverse transcriptases made have a new mutation. Thus, there is rapid selection for rt'ases that fail to bind AZT - rendering this treatment ineffective in 6 months. But, these new variants do not replicate as quickly; so when AZT treatment is stopped, there is reverse selection for original variant that reproduces more rapidly, and AZT is again effective. • HIV variants exist that are not as virulent. However, neither are they as transmissible. So, they are rare in the population. Trade-offs C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? How do they overwhelm host immunity? – MUTATION RATE - HIV - There are also resistant people, who have a mutant coreceptor protein with a 32 base deletion. THis is called 'delta-32". Viruses attacking these people can't bind the coreceptor, and thus can't infect the cells. The frequency of this variant varies across human populations, up to 9% in some northern European countries. (Non-existent in Africa or Asia). It may be high because it conferred advantages to previous plagues like smallpox or even bubonic plague (bacterial). C. Predation, Herbivory, and Parasitism: - Parasitism is a bit different Why are pathogens virulent? How do they overwhelm host immunity? – VIRAL TRANSDUCTION INCREASING VARIANCE FLU •Hemagluttinin is a surface protein. New viral strains evolve that have new AA sequences in these surface proteins that are not bound by existing antibodies; requiring a novel immune response. • Some sites are antigenic sites that the antibodies respond to. • Over 20 years, the flu has evolved and the surviving strain is the one with the greatest frequency of mutations in the antigenic sites. Selection by the human immune system has favored amino acid changes in the antigenic sites. • Over the last eleven outbreaks, nine occurred with the strain from the previous year that had the greatest number of changes in the 18 codons of the antigenic sites.... gives some predictive ability • If a radical new sequence evolved, it could crate a pandemic. This is possible, because infection with multiple virions allow for recombination of the eight chromosomes in the new viruses. Sometimes, cross infection of non humans by human influenza and pig or avian flu creates new genetic combinations that can reinfect humans and have entirely different antigenic sites. (Bird flu, swine flu, etc...)