Transcript Evolution1
Vanessa Couldridge Richard Knight Available at http://planet.uwc.ac.za/nisl/Eco_people/Presentations/ http://en.wikipedia.org/wiki/Image:Origin_of_Species_title_page.jpg Evolution: Basic Principles What is Evolution? Evolution can be defined as: Changes in the traits of living organisms over generations Traits are genetically determined, inherited characteristics Evolution is responsible for the diversity of life that we observe today http://en.wikipedia.org/wiki/Image:Phylogenetic_tree.svg Genetic variation Genetic variation is necessary for evolution to occur Three main sources of genetic variation: Mutation Migration Recombination A mutation is a change in the base pair sequence of genetic material Usually occurs as a result of errors in DNA replication or repair Ultimate source of all genetic variation Mutations are random http://www.scq.ubc.ca/wp-content/uploads/2006/07/dna1.gif Mutation Migration Migration occurs when individuals move from one population to another Introduces new genes into a population Recombination Genetic recombination occurs as a result of sexual reproduction Has the effect of reshuffling genetic material to create new combinations of genes Changes in Trait Frequencies Traits will increase or decrease in a population as a result of either: Genetic drift (random) Natural selection (non-random) Genetic Drift Random fluctuation of the frequency of a trait in a population over time due to chance events Can lead to a trait either becoming fixed in a population, or disappearing completely Beneficial traits do not guarantee an individual’s survival, they only improve its odds Small populations are more vulnerable to drift Natural selection is the evolutionary process whereby beneficial traits become more prevalent in a population Proposed in 1859 by Charles Darwin in his book “The Origin of Species” “I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection” http://en.wikipedia.org/wiki/Image:Charles_Darwin_aged_51.jpg Natural Selection Natural Selection In order for natural selection to occur, there needs to be: Variation in a trait Differential reproduction Heritability of the trait This process results in adaptations – traits that help an organism to survive and reproduce in its environment http://en.wikipedia.org/wiki/Image:LeafInsect.jpg Peppered moth (Biston betularia) in England Two morphs – Light and Dark Majority of the population originally of the light morph Light morphs better camouflaged against lichen covered trees on which they rested Industrial Revolution caused trees to blacken from air pollution Dark morph was now better camouflaged against the black trees and increased in frequency Lichen covered tree Soot covered tree http://www.biology.iupui.edu/biocourses/N100/images/16melanism.gif Natural Selection: Example Types of adaptations Adaptations may be classified into the following types: Structural, e.g. body shape, colour Physiological, e.g. digestion Behavioural, e.g. migration Traits that are not necessarily adaptive may arise as the result of pleiotropy Pleiotropy occurs when a single gene has more than one phenotypic effect Natural selection may operate to increase the frequency of a trait, while the pleiotropic effects are carried along For example, the gene that codes for white fur in cats also causes deafness http://en.wikipedia.org/wiki/Image:WhiteCat.jpg Pleiotropy Fitness Darwinian fitness is the number of viable offspring a particular genotype contributes to the population relative to other genotypes Generation 1 Generation 2 Lower Fitness Higher Fitness Inclusive fitness encompasses not only conventional fitness (production of viable offspring), but also indirect fitness through relations An individual increases its fitness by leaving behind genes in the population and those genes can belong to either direct or indirect descendents Behaviours that benefit close relatives will be selected for The closer the relation, the more genes are shared, and the more likely an individual is to act in an altruistic way For example, Belding ground squirrels produce an alarm call to warn others of predators, and are more likely to do so when in the vicinity of a close relative http://www.msu.edu/user/mueckeem/BGS.html Inclusive Fitness Sexual selection is a type of natural selection Characteristics that help an individual to obtain mates or copulate successfully are selected for It was put forward by Darwin as an explanation for the presence of characteristics that did not appear to be adaptations to the environment http://upload.wikimedia.org/wikipedia/commons/1/1c/Pfau_imponierend.jpg Sexual Selection Result of unequal parental investment by the sexes Males are usually limited by the number of partners they can mate with, whereas females are limited by the rate at which they can produce eggs or young Females become a limited resource for which males must compete There are two types of sexual selection: Male-male competition (intrasexual selection) Female choice (intersexual selection) http://www.getreligion.org/archives/ovum-lg-thumb.jpg Sexual Selection Males compete directly for access to females Traits that help males to win contests with other males will be selected for http://upload.wikimedia.org/wikipedia/commons/b/b6/RedDeerCaithness.jpg http://www.ports.parks.ca.gov/?page_id=23700 Male-Male Competition http://en.wikipedia.org/wiki/Image:Red_Postman.JPG Male-Male Competition Can take on more subtle forms in situations where females mate with more than one male These include sperm competition, copulatory plugs, anti-aphrodisiac scents For example, male Heliconius erato butterflies leave a scent on females after copulation that acts as a deterrent to other males Females choose to mate with males having certain ornaments or behaviours Traits that serve to attract females will be selected for Females may select males on the basis of: Direct benefits Indirect benefits http://library.thinkquest.org/J002558F/birdofparadise2.jpg Female Choice Female’s choice of mate has an immediate benefit on her reproductive success Females select mates that provide them with resources Example: Male dance flies give females a nuptial gift (food) which she eats while he copulates with her http://www.elkhornslough.org/journal/journalpix/050309dance-flies.jpg Direct Benefits Used to explain the presence of male traits that are often detrimental to the survival of the male, e.g. bright colours that attract predators Females select males on the basis of genetic benefits Elaborate male traits can arise through either: Runaway selection “Good genes” Sensory bias http://www.sergiosakall.com.br/index/antilophia-bokermanni-ciro-albano.jpg Indirect Benefits Proposed by R. A. Fisher in 1930 Genes for a male trait and genes for female preference for that trait are correlated Both preference and trait become more exaggerated – leads to a runaway process Male trait is arbitrary The Irish elk is sometimes used as an example – became extinct because its antlers became too big and cumbersome http://virtuallaboratory.net/Biofundamentals/lectureNotes/AllGraphics/irish%20elk.jpg Runaway Selection Females select male traits that are a reliable indicator of a male’s superior genetic quality Example: There is a positive correlation between survival ability and the size of the spurs on the legs of male pheasants. Females selecting males on the basis of spur size are therefore selecting males that are stronger and healthier http://en.wikipedia.org/wiki/Image:Male_common_pheasant.jpg Good Genes Females have a pre-existing sensory bias for a male trait before it arises in the population When the trait arises by chance, females will preferentially mate with males that have it Example: In swordtail fish, the preference for swords appears earlier in the phylogeny than male swords do http://en.wikipedia.org/wiki/Image:Xiphophorus_helleri_02.jpg Sensory Bias Co-evolution is when two or more species influence each other’s evolution Occurs when species have close ecological relationships Three types: Mutualism Competition Predator-prey or parasite-host http://en.wikipedia.org/wiki/Image:Bombus_6867.JPG Co-evolution Mutualism Beneficial relationship between species Example: Clownfish and anemones protect each other from predators Example: Ants receive food and shelter and acacia receives protection from herbivores Species compete with each other for a limited resource They evolve ways to reduce or avoid the competition Example: Five species of warbler in the same spruce forest feed at different heights and at different areas in the same tree http://en.wikipedia.org/wiki/Image:Capemaywarbler02.jpg Competition http://en.wikipedia.org/wiki/Image:Anolemeal6127.jpg One species (predator/parasite) evolves strategies to better exploit another species (prey/host), while the species being exploited evolves strategies to avoid the other species http://en.wikipedia.org/wiki/Image:MistletoeInSilverBirch.jpg Predator-Prey / Parasite-Host Evolutionary Arms Race Species continually adapt and counter-adapt in order to gain an advantage over the other Example: Passiflora produces toxins in leaves to avoid herbivory caterpillars of certain butterflies overcome these toxins plant evolves spots on leaves that mimic eggs to deter egg laying Animals sometimes evolve bright colouration as a warning to predators that they are unpalatable Predators learn to associate the bright colours with unpalatability and so avoid eating them http://en.wikipedia.org/wiki/Image:Dendrobates_pumilio.jpg http://en.wikipedia.org/wiki/Image:Micrurus_tener.jpg Warning Colouration Mimicry Mimicry occurs when a species evolves to look similar to another species or to the environment Batesian mimicry: A palatable species mimics an unpalatable one Aggressive mimicry: An animal resembles an object that attracts prey Defensive mimicry: an animal mimics a dangerous organism Müllerian mimicry: different unpalatable species converge on a particular colour combination Drone fly mimics a bee Tongue lure in a snapping turtle Most monarch butterflies are unpalatable Viceroy butterflies resemble monarch butterflies and some are palatable (Batesian mimicry) However, some viceroy butterflies are unpalatable (Müllerian mimicry) Some monarch butterflies are palatable (automimicry) Viceroy butterfly Monarch butterfly http://en.wikipedia.org/wiki/Image:Monarch_Butterfly_Showy_Male_3000px.jpg http://en.wikipedia.org/wiki/Image:Viceroy_Butterfly.jpg Mimicry: Example