September 30, Lecture 10

Transcription

1 Disruptive Natural Selection in Sticklebacks field study carried out by Robinson lakes of coastal BC wherever two stickleback species occur in the same lake, they occupy different habitats and make use of different sources limnetic form feeds on zooplankton benthic form feeds on larger invertebrate prey from sediments and submersed aquatic vegetation differ morphologically - ex. gill rakers (bony structure that diverts solids from the gills) smaller in limnetic form than benthic most lakes have one species and most will be of intermediate morphology and habitat use - sometimes feeds in limnetic zone and sometimes in the benthic zone some individuals will mirror the difference in species pair: limnetic form and benthic form: different phenotypes Hypothesis: Prediction: Experiment 1 Experiment 2 disruptive selection is driving evolution because optimization for one form entails costs to adopting the other form difference between two forms are heritable - descendants of either form will remain true to it, even in neutral habitat divergence in morphology will be reflected in foraging efficiency for limnetic or benthic prey - variation in trait must lead to differences in fitness reared offspring of both forms one species under identical lab conditions and diet - do differences persist? Results: yes, traits are heritable feeding trails of foraging efficiency in artificial limnetic and benthic habitats using two food types released fish of one phenotype into either a limnetic or benthic and counted how many prey it caught intake rate: # of prey captures/min capture effort: # bites per prey caught Results: limnetic form: better at eating limnetic prey in limnetic aquarium benthic form: better at eating benthic prey in benthic aquarium Therefore, foraging efficiency is related to phenotype September 30, Lecture 10 Speciation and Hybridization Speciation: evolution of a new species when gene flow is reduced between populations, they may then diverge genetically as a result of mutations, natural selection and genetic drift genetic divergence may eventually lead to speciation - creation of a new species usually creates two or more distinct species from a single ancestral group finches some common ancestor populated all islands, eventually populations diverged (evolved in different environments) Adaptive radiation: process in which one species gives rise to multiple species that exploit different features of the environment 14

2 BIOL 150 Fall 2013 To make distinct species, we need: reproductive isolation: new species created when the diverging population can no longer reproduce sympatric speciation: occurs without geographic isolation - usually due to disruptive selection polymorphism forms, likely due to patchy habitat polymorphism: existence of more than one distinct form of individuals (phenotypes) in a population may cause speciation is differences in form affect reproductive morphology, reproductive timing or reproductive behavior to cause reproductive isolation of the two polymorphs ecological speciation: special case of sympatric morphology, behavior, timing (traits changes due to natural selection pressure - targets of selection) divergent natural selection ex. soapberry bugs use beaks to reach seeds inside fruits native plant fruits (large fruits), nonnative plant fruits (small fruit) evidence for disruptive selection on beak length found that short beaked populations growing on non-native plants and long beaked populations growing on native plants speciation is allopatric: occurs as a result of geographic separation of a population into two or more subpopulations with no movement between them ex. Diane Dodd and fruit flies - divided population into 2, fed one starch and the other maltose brought the two together, only starch flies wanted to mate with other starch, and maltose with maltose ex. lizards - one continuous population, island population begins to diverge due to drift and selection river changes course, runs through population of lizards, population begins to diverge due to drift Sympatric vs. Allopatric Speciation ex. mosquito fish inhabiting blue holes in the bahamas has evolved larger caudal region and smaller heads in the presence of predators than in their absence Sympatric Allopatric Similarities both involve the formation of a new species via reproductive isolation of the gene pool from existing species both occur when natural selection creates genetic divergence between new and ancestral populations Differences involves a reproductive or behavioral separation populations occupy same geographical areas ex. polyploidy in wheat strains involves the physical separation of populations populations occupy different geographical areas ex. adaptive radiation of Galapagos finches Hybridization: if they are so genetically and phenotypically distinct, they may not be able to interbreed - reproductively isolated - offspring have lower fitness hybrid offspring do not develop or reproduce normally called reinforcement, because the traits that isolate populations reproductively are selected for, so the speciation is reinforced sometimes the two species can mate successfully (fertile offspring that survive) may reverse speciation - parents no longer reproductively isolated 15

3 may create new species - if hybrid cannot back-cross with either parent; if it can meet with other hybrids, you have created a whole new species may animal hybrids are sterile because of an uneven number of chromosomes ex. horse: 64 chromosomes; donkey: 62; mule: 63 - odd number, cannot reproduce lots of plant hybrids (~70% of flowering plants) because it can reproduce asexually (apomixis and vegetative reproduction) even if hybrid is sterile, it can persist polyploidy: multiplication of chromosome number offspring cannot back-cross with parents - speciation through reproductive isolation how does it happen? spontaneous doubling after fertilization; union of unreduced gametes if they can: i. mate with other polyploids of the same chromosome number ii. reproduce vegetatively iii. reproduce by apomixis (asexual reproduction in which seeds are formed without meiosis or sexual recombination) then they may be a new species! no species is even perfectly adapted to its environment trade-offs - traits of benefit for one environment will have a cost - energy allocation environment is in flux - not constant natural selection acts only on available variation in the gene pool: just a filter; not creative rand chance - some degree of extinction is not related to fitness: eg. volcano, ice storm, etc. correlation among genes on chromosomes - a beneficial gene may be on the same chromosome as a deleterious one or neutral one Phenotypic plasticity: ability of the genotype to give rise to different phenotypes under different environmental conditions individuals can respond to temporal and spatial changes of environment by moving to a more suitable location and by a direct influence of the environment on gene expression - often see phenotypic plasticity in plants because they can t move norm of reaction: the set of phenotypes expressed by a single genotype across a range of environmental conditions same phenotype in different environments different phenotypes in the same environment if lines intersect - at the place where they intersect, you have the expression of the same phenotype (but still two different genotypes acting) developmental plasticity: occurs during growth - irreversible common in plants (ex. Polygonum persicaria) acclimation: reversible changes in physiology (bullhead catfish in summer vs. winter temperature ranges), morphology or behaviour ex. fish - as long as temperature changes slowly, it can move its optimal temperature range even though these changes are not heritable, natural selection can act on the capacity for plasticity essential plasticity is a way of being adapted to variability in the environment by altering phenotypic expression October 2, Lecture 11 Simbio Lab Review Frequency histogram - graphical representation of how common a particular value is distribution of variable is the arrangement of its values that indicates their frequency of occurrence bar graphs - not always bell shaped 16

4 BIOL 150 Fall 2013 Evolution by natural selection change in allele frequencies within the populations between generations requires: variation in phenotypes - selective forces can only select among the variation that already exists in the population heritability - crabs eat mostly thin shelled snails, but the surviving thick shells reproduce, so the next generation starts over differential survival of different phenotypes if shell thickness is variable and heritable, but variation in thickness does not affect thickness Key: for evolution by natural selection 1. variability within a trait (phenotypic variation) 2. trait must be heritable 3. differences in phenotype must result in differences in survival and reproduction (fitness) there is no selective force acting in any direction because there is no variation in fitness related to phenotype one could evolve, but only because of random change, not by natural selection Mutations change in the structure of genes/chromosomes allows creation of new alleles new shell thicknesses mutation does not occur in any particular direction or in response to any environmental condition mutation is random and generates on average as many thinner as thicker shells (non-directional) variation in the population phenotypes arises by a random chance through mutations despite fact that thicker and thinner shells are equally likely to result from mutation, the snails evolve towards thicker shells only surviving snails breed only thick shelled snails survive thickness of shells is passed down from the parents mutations supplies the raw variability in heritable traits on which natural selection acts to cause evolution towards increased average fitness of the population Good Experimental Design Design Component Independent variable Dependent variable Control Replication Duration Description choose one variable that will be manipulated or changes choose on or more variables that measure the experimental outcome extraneous variables must be held constant so that they don t lead you to the wrong conclusion experiments should be repeated or involve multiple groups to avoid drawing conclusions from a single unusual result run the experiment long enough to measure an effect but no so long that other factors come into play 17

5 October 4, Lecture 12 Behavioral Ecology Part 1 Behaviour: response to a stimulus - alters the relationship between an organism and its environment stimulus may be external ex. vervet monkeys - visible cur of predator - causes monkey to give a response (verbal call) - hearing the verbal call from other monkeys, modify their behavior based on the call stimulus may be internal ex. hunger pains Types of behaviour: innate: inherited or inborn inflexible, i.e. not affected by learning or environmental conditions stimulus triggers a response automatically ex. Kangaroo rat and rattle snake rattle ex. baby s cry when uncomfortable reflexive - simple ex. the withdrawal reflex - i.e. when you touch a hot object helps you avoid injury needs to be fast it bypasses the brain (you don t have to think about it - automatic response) 1) pain receptors to spinal chord 2) spinal chord to nerves controlling muscle instinctual - more complex ex. Wildebeest calves stand and walk immediately after birth - avoids predators, keeps up with the herd flexible condition dependent flexible in response to environmental conditions ex. spiny lobsters hide more when there are more predators cost benefit analysis to determine how much time they should spend hiding and how much they should spend foraging learned: changes in response to learning ex. food choices - grizzly bears teach cubs - passing techniques and fishing spot to their cubs condition dependent and learned learning is a change in behaviour that results from a specific experience in the life of an individual Learnability not inherited - must be taught/learned and is not coded in DNA but the capacity to learn could be a heritable trait (i.e. intelligence) What should I eat? ex. fruit fly larvae have a genetic predisposition to foraging (for gene) patterns rover alleles vs. sitter alleles (different phenotypes) when food is abundant, rovers trails longer, when food is less, both are similar in their food trails higher population density -roving is adaptive - higher probability of encountering unexploited food low population density - sitting is adaptive - waste energy moving around condition dependent - move to new sources - but innate ex. white-fronted bee-eaters A Highly stereotyped fixed: little variation Flexibility C Innate: no modification through learning B Highly flexible Condition dependent D Originates and modified through learning 18

6 BIOL 150 Fall 2013 nest in cliff caves - very sandy habitat - where they tend to live, not very much habitat, so they live together but where they feed, they are very territorial, so some have to travel far away for food sources - each trip by parent is for one baby (only enough food for one to carry in their beaks) optimize foraging based on distance between foraging territory and their nesting colony those that that feed far away from roost site carry more food back minimum cost of finding/ingesting food and risk of predation max usable energy taken in - may influence which food items to take, where you re going to find food When should I mate? cues for it s time to mate males and females synchronize - changes in sexual organs through the year can be really important to the success of a population widest selection of possible mates risky time - bright displays, bird songs timed to coincide with abundance of resources and favorable climate ex. barn swallows feed on insects and nest in human made structures - before humans, they nested in caves question: does tail length in barn swallows affect female choice of mates? hypothesis: females prefer to mate with the longest tailed males null: tail length in males has no influence on female choice of mates procedure: alter tails (snip and reattach) to find which males are the first to mate and which have a second nest that season results: short tails took the longest to find mates, longest tails mated sooner (therefore more could have second nests, so more offspring) conclusion: females prefer long-tailed mates why? hypothesis: long-tailed mates are more efficient in flight, therefore more successful at finding food - higher fitness reproduction synchronizes with food availability and predation risk (babies have more food, and less likely to be eaten) seasonal cues - day length triggers hormonal change in spring social cues ex. Anolis lizards - experiment hypothesis: exposure to breeding males synchronizes reproductive activity in females prediction: females in aquaria with breeding males will produce eggs earlier than females alone or with castrated males females need to exposed to spring like light likelier to produce eggs produce more quickly when exposed to breeding males results: two types of stimulation are necessary to trigger breeding in Anolis lizards Summary: Behavioral ecology - broad - helps to know about neurons, genetics, phylogeny behaviour - response to a stimulus: internal or external can be: innate vs learned flexible and condition dependent vs. reflexive or instinctual what to eat? genetics - innate cost-benefit analysis - condition dependent when to mate? synchronize to max mate options, offspring survival min predation risk who to mate with? traits that confer a fitness advantage choose attractive mates to have attractive sons 19