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Patterns of Evolution External 3 Credits Do Now • Define the following terms; • • • • Population Species Gene pool Natural Selection Answers • Population; is a group of individuals of the same species in an area. • Species; is a groups of individuals who normally interbreed to produce fertile offspring and who belong to the same gene pool. • Gene pool; all the alleles available to the population of a species. • Natural Selection; the process were the organsims with the best suited phenotype in a particular environment is select for (has increased survival) Lesson Objectives • Review Yr 12 Evolution A new idea? The idea that life has evolved is not new and goes back to the civilisations of the ancient Greeks. Christianity has a different perspective and is detailed in various passages in Genesis in the bible. EVOLUTION • Macro-Evolution Large changes in a gene pool over a long period of time, as in the formation of a new species, extiction and adaptive radiation • Micro-Evolution Small changes in the frequency of alleles in a gene pool over successive generations Who evolves? Write this down!!! • Individuals do not evolve – only populations evolve. • All the genes in a population are called a gene pool - the ratio of alleles and genotypes in a population can change over time. • As this changes, so evolution occurs. Sources of Variation 1. Meiosis (covered in Yr 12) 2. Mutations Processes of Evolution 1. Genetic Drift 2. Founder Effect 3. Bottleneck Effect 4. Gene migration 5. Natural Selection Meiosis • Independent Assortment • Segregation • Crossing over What is a gene pool? Remember!! All the alleles present at all gene loci in all members of a population GENE FLOW (results from Migration) Individuals migrate between populations. Immigrating individuals introduce new alleles. Emigrating individuals remove alleles. • Gene flow can change a gene pool due to the movement of genes into or out of a population Mutation changes alleles ► Natural selection leads to differential reproductive success ► Genetic Drift • A change in the gene pool of a small population due to chance Genetic Bottleneck Founder Effect There are several potential causes of microevolution • Genetic drift is a change in a gene pool due to chance • Genetic drift can cause the bottleneck effect Original population Bottlenecking event Surviving population Figure 13.11A • Population bottleneck – genetic drift due to high mortality in a population. • Unlikely that gene pool of the remaining population is representative of original population. Decreased genetic diversity among Cheetahs. Cheetahs underwent 2 population bottlenecks: 1st during last ice age 2nd during nineteenth century due to excessive hunting. Today, just two isolated populations live in South & East Africa, numbering only in a few thousand animals between them. The South African cheetahs are so genetically alike that even unrelated animals can accept skin grafts from each other. • or the founder effect Figure 13.11B, C Founder effect genetic drift • Equivalent to due to a few individuals leaving a large population to found a new group. Unlikely that gene pool of founding population is representative of original population. Founder effect Direction of movement Mainland Population Island Population 28 61% 4 44% 12 26% 5 56% 6 13% Founder effect • Thus isolated populations of a species may have very different genes from the parent population and would therefore have a different susceptibility to the effects of natural selection on them at these new localities. The Laysan Finch Story • Small population on Laysan Island and of conservation concern. • A group of 10 males and 10 females were captured and transported to a similar island 500 nm away. • They were individually marked and samples of their DNA were taken prior to release. • The introduced population thrived and samples of their DNA taken 5 years later showed a greater variation than the initial population. • How come? Laysan Island 5 genes go with the original population. 500 nm New Island 2 new genes appear, giving a new total of 7 genes in the ‘new’ population. Non-random Mating Non-random mating causes certain alleles to become more common in future generations (some individuals leave more offspring than others). Gene migration (Gene Flow) • Most populations are not closed systems. • Immigration from other populations brings in new gene combinations. • Those individuals, who leave the population (emigrate), take their genetic combinations with them. • I=in e=exit Mutation • A change in the DNA - introduces ‘new’ alleles into the population. Mutations can be beneficial, have no effect (silent) or be harmful. Increase or decrease of genetic diversity??? • Mutations and immigration increase genetic diversity. • Natural selection, emigration, non-random mating and genetic drift decrease genetic diversity. Important Slide!!!! Write this down!!!!! Natural Selection Write this Down!! The differential survival and reproductive success of organisms whose genetic traits- PHENOTYPES increase their chance to survive and reproduce in a particular environment. It is considered to be the major driving force of evolution. Natural Selection Summary Over-production of young Competition Genetic Variation (ie different phenotypes) Differential Fitness (Reproductive Success) Fittest outcompete others to pass their “fit/favourable” genes onto their offspring • These “favourable” genes will then increase in frequency • • • • • Speciation • Speciation is the formation of a new species • Remember: A species is a group of organisms that normally interbreed in nature to produce fertile offspring & belong to the same gene pool • There are 2 types of speciation: Allopatric Speciation Sympatric Speciation Allopatric speciation • Species can be allopatric – living in geographically different areas. Species B Species A Sympatric species • Species can by sympatric – living together in the same geographical area. Species A & B live in all areas in the same geographical area. The mechanism of speciation Allopatric speciation This is how most species come about. A single population occupying a uniform environment Species undergoes an expansion of range Migration into new environments on the edge of the distribution Gives rise to subspecies as a result of different selection pressures. There is gene flow between all populations still. Vegetation change Selection the same River course change Selection the same Further migration, environmental differences and the development of geographical barriers, gives rise to geographical isolation of some races and populations. This isolation halts gene flow between this and the original population. Selection the same Selection different Different alleles being selected for. Some of the isolated populations develop genetic and chromosomal differences that no longer allow inter-breeding with the parent population. The subspecies is genetically and geographically isolated from its ancestral population. Further changes in the environment remove the geographical barrier and allow the groups to live side by side. There is no interbreeding because some of the groups are now reproductively isolated, due to the different selection pressures they have been exposed to. Gene flow can occur between these populations still as they have been exposed to the same selection pressures. There is NO gene flow between these populations now, because of different selection pressures resulting in Genetic changes. These now become Sympatric and Allopatric populations Allopatric Speciation – an example The mechanism of speciation Sympatric speciation There are very few authenticated reports of speciation by this route. If it does occur, it happens within one generation. Starting with one population A very small portion of the population undergoes a random mutation, which gives them instantaneous reproductive isolation from the rest of the species. It has to occur in a male and female in the same generation and must confer an immediate evolutionary advantage and separation from the parent population. Polyploidy • This is the abrupt and almost instantaneous formation of a new species. • The main cause of this is a problem of separation of the chromosomes at Meiosis into the gametes and we will deal with it later on. • Rare in vertebrates but common in plants. Aims for Today • To be able to explain the pre-zygotic and post-zygotic isolating mechanisms that lead to speciation. Formation of species • Prevention of gene flow between populations can result in the formation of new species. • These are called isolating mechanisms. • There are pre and post-zygotic isolating mechanisms. How does it happen? • Pre-zygotic isolating mechanisms prevent the fusion of gametes to form a zygote. • Post-zygotic isolating mechanisms prevent the zygote from developing further, if fertilization occurs. Pre-zygotic mechanisms Geographical barriers • Populations are geographically isolated. • Scale is important. Pre-zygotic mechanisms Ecological barriers • Live in different areas with different temperatures, humidity, altitude tolerances etc. e.g. Arctic Fox and Fennec Fox Pre-zygotic mechanisms 1. 2. 3. 4. 5. Habitat differences. Breeding season differences. Behavioural differences; territoriality & courtship and the context of the displays. Mechanical differences. Mating takes place but no zygote formed (duck sperm does not survive reproductive tract of a hen) Post-mating mechanisms 1. 2. 3. Hybrid inviability – zygote formed but it does not develop. Hybrid sterility – hybrid forms but it is sterile – mule. Hybrid breakdown – hybrid is fertile but offspring cannot reproduce. A lion x tiger hybrid Weighs 450 kg and is 3 m From nose to tail. We’ve talked about mules What about a zeedonk? Isolation by time A species, which has gone extinct, can obviously not interbreed with a species existent today. Isolating Mechanisms – an overview Speciation • The formation of a new species - speciation and can occur in the following ways: • Reduced selection pressure – a population moves into a new area, where the selection pressures are different. • An increase in population results in the expression of alleles, which were previously selected against. More speciation • Migration into new areas, which might have different selection pressures. • Some isolated populations develop genetic and chromosomal differences that no longer allow interbreeding with the parent population. Formation of species • Prevention of gene flow between populations can result in the formation of new species. • There are two main schools of thought as to how this is achieved. Time Phyletic gradualism – genetic change takes place gradually over a long time. Amount of difference Punctuated equilibrium – rapid genetic change followed by long periods of stability. Time Extinction Amount of difference Patterns of Evolution • Sequential Evolution; species may accumulate genetic changes that, over time, result in the emergence of what can be recongised as a different species. • Coevolution; where two species reciprocally affect each other’s evolution. • Convergent Evolution; Species from different evolutionary branches may come to resemble each other if they have similar ecological roles and natural selection has shaped similar adaptations. • Divergent Evolution; the process where two species have diverged from one common ancestor. (most common form of evolution) Convergent evolution • Species from different evolutionary branches may come to resemble each other if they have similar ecological roles and natural selection has shaped similar adaptations. Convergent evolution • Homologous-descended by inheritance from a common ancestor e.g. the pentadactyl limbs shown in the diagram Convergent Evolution • Analogous structures; similar function and often the same superficial structure, but of different evolutionary origins • E.g the wing of a bird and the wing of an insect. Co-evolution Co-evolution • Co-evolution is used to describe cases where two or more species reciprocally effect each other’s evolution. • Each of the species involved exerts selective pressures on the other and over time the species develop a relationship that involves mutual dependency. • Co-evolution is likely to occur when species have close ecological interactions with one another. New Zealand’s pollinators • When New Zealand split away from Gondwana its insects did not include types of bees or any other insects that are attracted to bright colours. • So….. Insects and plant needed to evolve adaptations that enabled the insects/birds present to obtain food from the plant while at the same time carrying pollen from flower to flower. Coevolution • As a result insect pollinated flowers in New Zealand flowers become dull in colours with strong nectar scents. This attracted small beetles, butterflies, moths and small bats. • Several of the birds of the forest developed adaptations such as long, feathers tongues for feeding on nectar. At the same time some forest trees adapted to attract birds by evolving bright colour and nectar production. E.g Kowhai and tui’s Adaptive radiation • Adaptive radiation is the diversification (both structural and ecological) among descendants of a single ancestral group to occupy different niches. • In adaptive radiation is a pattern of evolution that involves an ancestral species evolving into a variety of new species, each adapted to survive in a different niche. Adaptive Radiation EXAMPLE: The radiation of the mammals occurred after the extinction of the dinosaurs, which has made niches available for exploitation. Arboreal herbivore niche Marine predator niche Terrestrial predator niche Underground herbivore niche Freshwater predator niche Flying predator/ frugivore niche Browsing/ grazing niche Megazostrodon an early mammal ancestor Divergence and Radiation of the Ratites Mesozoic Era Cenozoic Era All other living birds Birds evolved from a dinosaur ancestor about 150 million years ago Moa 1: Anomalopteryx Moa 2: Pachyornis Moa 3: Dinornis Moa 4: Megalapteryx Little Spotted Kiwi Great Spotted Kiwi Fossil evidence suggests that ratite ancestors possessed a keeled breastbone and an archaic palate (roof of mouth) Ratites diverge from the line to the rest of the birds about 100 million years ago Brown Kiwi Emu Cassowary Ostrich Elephantbird Rhea 1 Rhea 2 Tinamou (can fly) 6 important events 1. 2. 3. 4. 5. Isolation Mountains Sea level changes Climate change North and South extent Isolation • Isolation is a key reason why the endemic species of New Zealand are so different. • 300 million years ago New Zealand was part of Gondwanaland. Gondwanaland was made up of Africa, South America, Antarctica, India and Australia • Initially New Zealand was under the sea and was attached to the eastern side of Australia • 70 to 80 million years ago, New Zealand was pushed away from the Australia Isolation Isolation • When New Zealand sperated from gondwanaland only the plants and animals present at the time of sepration could become the ancestors of the present species • As a result New Zealand only had a small range of kinds of plants and animals • These species eveloved in isolation for many millions of years Mountian Building • 24 million years ago New Zealand was sitting on tectonic plate boundary. • Pressure within the earth caused uplift into large mountians • This created a new alpine habitat for things already living there • It also caused isolation which seprated different parts of the country and weather patterns such as wetland on the west coast of the south island and the dry plains in Canterbury and Otago Climate Changes • Mountian building and ice ages contributed to dramatic changes in New Zealand’s climate • 20,000 year ago most fo New Zealand was covered in snow fields, this caused • Some plant species to move north e.g nikau tree • Some plants to become extinct e.g. eucalypts • Some plants to evolve to better suit the conditions. Changing Sea levels • At times changes in the sea level isolated or joined different parts of the country • As recently as 12,000 years ago the cook strait was dry land and island such as great barrier were part of the main land of New Zealand • As the last ice age finished the sea level rose and separated the north and south island North to South Extent • New Zealand is long and narrow, with varying climates all the way along it. • This enabled tropical species to evolve in the north and alpine species to evolve in the south. New Zealand Hebe • There are more than 80 species of Hebe in New Zealand. • Most species are restricted by their adaptations to very specific areas. • The original New Zealand Hebe was probably a shrub with normal-sized leaves in an alternate pattern with pale flowers. • Today there are three main groups of hebes. Large –Leafed hebe • Large hebes are most like the ancestrial hebe. • Leaves are untoothed and either broad or narrow but never overlap. • Flowers are pale and larger than the leaves • Found in lowland shrub, on the coast, in forest margins • NOT FOUND IN EXTREME ENVIRONMENTS Medium-leafed hebes • Have features that would help the plant to withstand dry, arid conditions with wind and cold. • Toothed, fleshy leaves • Leaves are flat and concave and are short and closely set…………………… • Flowers are spikes crowded together • Found in sub-alpine regions, mainly on rocks Small-leafed hebes • Have adaptations to withstand cold and snow and are able to survive the harsh conditions of bare rock • Plants are small and spreading……….. • Leaves are very small, overlapping and tough………….. • Flowers have only a few spikes, crowed near the tips of the branches.