BACK TO MAIN A 2 BIOLOGY MENU Variation

BACK TO MAIN A 2 BIOLOGY MENU Variation, overproduction, natural selection Evolution The Darwin-Wallace theory Species and speciation Artificial selection Classification 5 kingdom classification Round up

Variation – genetic and environmental Genetic variation arises through: Independent assortment of chromosomes during meiosis Crossing over of homologous chromosomes during meiosis prophase I Allele reshuffling Random mating between organisms within a species Random fertilisation of gametes Mutation New allele production These are all changes in the genes that can be inherited by the offspring Environmental variation e. g. body mass due to food availability, do not involve genetic changes and are thus not passed on to the offspring.

Overproduction numbers Populations have the potential for exponential growth but are normally checked by environmental pressures or ‘factors’. Biotic factors: e. g. predation, food supply, infection Abiotic factors: e. g. water supply, soil nutrient levels. If the pressure of environmental factors is sufficiently great the population may fall before later possibly rising again. Thus populations may oscillate about a mean over a period of time. myxomatosis Denser populations increase the probability of disease transmission. Why? Time (months)

Overpopulation Lemmings The chart below outlines the lemming population changes over a decade. The populations of most species show a far less spectacular oscillation, but the pattern is similar – more young are produced than can survive into adulthood.

Natural selection e. g. rabbits What determines which rabbits will survive the environmental or ‘selection’ pressures? Successful allele Alleles Selection pressure Brown (agouti) / white colour Fox predation brown Blends better with background Long / short ears Fox predation long Better sound detection Thick / thin tooth enamel Limiting food thick Teeth last longer Advantage Natural selection increases the frequency of advantageous alleles.

Evolution Stabilising and directional selection Stabilising selection The environment is not changing and only extremes of the population are selected against New Zealand tuatara – no change in 200 million years Stabilizing selection Selective pressures select against the two extremes of a trait. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height.

Evolution Stabilising and directional selection Directional selection The environment changes shifting the range of variation in a particular direction. The population shift is an adaptation to the new environment In directional selection, one extreme of the trait distribution experiences selection against it. Thus the population's trait distribution shifts toward the other extreme and the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favour individuals with long necks.

Evolution New alleles are produced by mutation. Nearly all mutations are harmful, conferring a disadvantage on the individual. Very occasionally advantageous mutations arise, e. g. here, white coat colour Hares possessing the allele are more difficult to see and are less likely to be predated. They survive to pass the allele on and over many generations almost all hairs will inherit the allele. Such changes in allele frequency are the basis of evolution Mountain hare

Evolution Antibiotic resistance C Amp P Strep Tet Petri dish with lawn of Staphylococcus bacteria and antibiotic discs Which is the most effective antibiotic against this sample of Staphylococcus? Some Staphylococcus bacteria produce an enzyme called penicillinase which renders them immune to penicillin. 1 resistant bacterium can produce 100000 descendants in 24 hours under favourable conditions. Similar antibiotic resistant strains of bacteria are constantly emerging. The selection pressure causing this is the presence of the antibiotic. The more antibiotics are used, the greater the selection pressure to evolve resistance.

Evolution Industrial melanism Peppered moths During the day the moth rests underneath the branches of trees and relies on camouflage to protect it from insect-eating birds Until 1849 only the speckled variety was observed. From 1849 (industrial pollution getting worse) until the late 1960’s the black variety was reported in ever-increasing numbers Speckled form A single gene controls appearance. The alleles are - C: black, c: speckled Trees in unpolluted regions often had growths of speckled lichen. This lichen was killed off in polluted areas, causing bark to look much blacker Black form Selection pressure: Bird predation. Successful allele?

Evolution Industrial melanism The advent of clean air acts from the 1960 s began to reduce the level of industrial pollution Edinburgh Newcastle Distribution of the pale and dark forms of the peppered moth, Biston betularia in the early ’ 60 s Manchester Birmingham Cardiff London

Evolution Sickle cell anaemia HS HS Most people die from anaemia before adulthood HN HS Most people show no symptoms. Also, reduces the chance of death during malaria by about 20 x 2 selection pressures on the sickle cell allele: Selection against those who are homozygous Selection for heterozygous individuals – malaria resistant The frequency of the HS allele in an area depends on a balance between these 2 pressures The malarial parasite, Plasmodium, cannot live in sickle cells. Thus sickle cell confers immunity to the carrier.

Evolution Sickle cell anaemia The frequency of the HS allele in an area depends on a balance between : Selection FOR (malaria rersistance in heterozygotes) Selection AGAINST (death due to anaemia in homozygotes) There is selection pressure for both alleles to remain in malaria-affected areas

Evolution Artificial selection The application by humans of selection pressures to populations For cattle desired features may include: docility, fast growth rates and high milk yields. Chillingham White Cattle breed. Similar to the original wild cattle which once roamed the forests of Britain from earliest times. Guernseys are bred for the production of large quantities of fat-rich milk

Evolution Fossil Evidence Richard Dawkins – explains the evidence for evolution http: //www. youtube. com/watch? v=IQWv_0 Mjq 0&feature=player_embedded Fossils show the development of legs from fins over 20 million years

Evolution The Darwin-Wallace theory of evolution by natural selection Charles Darwin Observation 1 Organisms produce more offspring than are needed to replace the parents Observation 2 Natural populations tend to remain stable in size over long periods Deduction 1 Observation 3 Deduction 2 Alfred Russel Wallace There is a competition for survival – a struggle for existence There is variation amongst individuals of a given species The best adapted variants are the fittest and survive to reproduce

Speciation Introduction The Darwin Wallace theory did not attempt to explain how new species could arise. The mechanisms discussed in this unit explain how new genes / alleles arise by mutation, and how selection pressures determine the spread of these through the population An important question is ‘how did different species arise? ’ A species is: A group of organisms with similar morphological, biochemical, physiological and behavioural features which can interbreed to produce fertile offspring, and which are reproductively isolated from other species. The feature that really decides whether 2 organisms belong to the same species is their ability to interbreed successfully (A problem is that the 2 organisms may be dead, fossils, the same sex etc!) How does a new species arise (speciation)? Here are 2 models:

Classification The study of the classification of organisms is called taxonomy. A key element in classification is the identification of shared or homologous features In vertebrates one such feature is the pentadactyl limb. frog reptile horse human bat Each has the same basic design, suggesting a shared or common ancestor.

Classification Species which share many homologous features are grouped into the same genus. e. g. horse, donkeys and zebras all belong to the genus Equus, probably evolving from what was a single species along time ago. Every species is given a 2 -word Latin name called a binomial Genus Species e. g. zebra: Equus burchelli. Note the italics (if hand written, underlined) Kingdom Animalia Phylum Chordata Class Mammalia Order Perissodactyla Family Equidae Genus Equus Species burchelli This classification reflects the evolutionary history of the zebra and its relationship with other organisms. The evolutionary history of living organisms is called phylogeny.

Classification - the five kingdoms The system was proposed in 1988 by Margulis and Schwartz. The kingdoms are: Prokaryotae, Proctista, Fungi, Plantae and Animalia Prokaryotae Proctista Fungi Plantae Animalia

Classification - the five kingdoms Prokaryotae (Prokaryotes) Escherichia coli. A rodshapes gram negative bacterium. Scanning electron microscope Prokaryotes Light micrograph of filamentous cyanobacterium Cylindrospermum Eukaryotes Generalised bacterial cell

Classification - the five kingdoms Protoctista Amoeba. A unicellular heterotrophic proctist. Some species live in the sea, others in fresh water. Some are parasites, e. g. causing the disease amoebic dysentery Spirogyra. A multicellular photosynthetic protoctist found in ponds and streams Fucus vesiculosis. A multicellular photosynthetic protoctist. A seaweed! Protoctists are simple eukaryotes. Some are single cells, others multicellular. Some are photosynthetic, others heterotrophic.

Classification - the five kingdoms Fungi Saccharomyces yeast cells Fungal hyphae with inset drawing based on EM studies. Light microscopy photo of yeast cells, Saccharomyces Some fungi are unicellular, e. g. yeasts. Others are multicellular containing mycelium composed of hyphae. They feed heterotrophically, some are saprophytes, others parasites (athlete’s foot). They don’t have chlorophyll and although they have cell walls they are not made from cellulose (usually chitin) Boletus. Growing on leaf litter Different species of Aspergillus growing on a lemon Spores of of Aspergillus niger on decaying fruit

Classification - the five kingdoms Plantae A mixture of moss species e. g. Eurhychium crassinervium A fern Polystichium setiferum A flowering plant Ranunculus Plants are multicellular eukaryotes that feed using photosynthesis. The kingdom includes mosses, liverworts, ferns, conifers and flowering plants. Plants have cell walls containing cellulose (bacteria have cell walls but not made of cellulose). Many plant cells have chloroplasts and often large vacuoles.

Classification - the five kingdoms Animalia Orang-utan. Pongo pygmaeus. A mammal. The common house spider Tegenaria domestica. An arthropod. A jellyfish Animals relatively simple organisms such as worms as well as more complex ones such as birds. Their cells do not have cell walls and they don’t contain chloroplasts. They feed heterotrophically

AT THE END OF THIS UNIT YOU SHOULD BE ABLE TO: describe one example of artificial selection Explain how natural selection may bring about evolution explain why variation is important in selection explain how all organisms can potentially overproduce; describe the processes that affect allele frequencies in populations with reference to the global distribution of malaria and sickle cell anaemia explain the role of isolating mechanisms in the evolution of new species use the knowledge gained in this section in new situations or to solve related problems.

PROCTISTA Amoeba

Fungi Penicillium spores and hyphae Aspergillus niger spores and hyphae

Fungi Fungal hypha Yeast – drawing from EM

Migrations In Scandinavia, lemmings become restless in years when their populations are high. In the mountainous terrain of Norway, for example, when lemmings begin to move they tend to go downhill and get funneled into valleys. The result is that large numbers eventually reach the sea or a large lake. They may proceed onto sea or lake ice or jump into the water, which has given rise to the popular conception that they are committing mass suicide to relieve a problem of overpopulation. The Inuit have no legends about migrating lemmings and it is difficult to believe that they would have overlooked such an event, especially if it occurred repeatedly. Spring is a time of social upheaval caused by the environmental changes associated with snow melt, and the physiological changes associated with onset of the breeding season.