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NATURAL SELECTION Content Statements: D4.1.1 Natural selec/on as the mechanism driving evolu/onary change D4.1.2 Roles of muta/on and sexual reproduc/on in genera/ng the varia/on on which selec/on acts D4.1.3 Overproduc/on of offspring and compe//on for resources as factors that promote selec/on D4.1.4 Abio/c factors as selec/on pressures D4.1.5 Differences between individuals in adapta/on, survival and reproduc/on as the basis for natural selec/on D4.1.6 Requirement that traits are heritable for evolu/onary change to occur D4.1.7 Sexual selec/on as a selec/on pressure in animal species D4.1.8 Modelling of sexual and natural selec/on based on experimental control of selec/on pressures NATURAL SELECTION Natural selec/on describes the changes that occur in the composi/on of a gene pool as a consequence of environmental selec>on pressures. Natural selec/on requires a number of key condi/ons in order to occur (ICE AGE): • Inherited varia>on must exist within a popula/on (via muta/ons, etc.) • Compe>>on for survival results from an overproduc/on of offspring • Environmental selec>on pressures lead to differen/al reproduc/on • Adapta>ons which benefit survival will be passed on to the offspring • Gene pool composi>on changes (there is a change in allele frequency) • Evolu>on occurs within the popula/on (a cumula/ve change in traits) GENETIC VARIATION Gene/c varia/on can be created within a popula/on via muta/on and sexual reproduc/on. Random point muta/ons can generate new alleles within a popula/on, while sexual reproduc/on promotes varia/on by crea/ng new gene combina>ons in offspring. This recombina/on of traits occurs via meiosis and involves two dis/nct processes: crossing over (in prophase I) and random assortment (in metaphase I). Addi/onally, the subsequent fusion of male and female gametes is random, further increasing the varia/on of offspring.COMPETITION Species have a natural tendency to produce more offspring than the environment can support, as this will improve the chances of survival and maintain con/nuity. When resources are abundant, popula/ons grow exponen/ally (bio>c poten>al). But as a popula/on outgrows its resource base, environmental resistance sets in, causing growth to plateau as mortality increases – leading to a struggle for survival (compe//on). SELECTION PRESSURES Selec/on pressures describe the external agents that affect an organism’s ability to survive in a given environment. These pressures can func/on to either reduce the frequency of a trait (by reducing survival prospects) or increase the frequency of a phenotype. Selec/on pressures include both abio>c factors (non-living condi/ons such as light intensity, temperature and pH) or bio>c factors (living agents such as predators and pathogens). Selec/on pressures can be density-dependent or independent according to whether they are affected by popula/on size. Some examples include: Predators, Available shelter, Nutrients, Disease, Abio/c factors (PANDA) ADAPTATIONS An adapta>on is a feature that aids in the survival of an organism by sui/ng them to a given environment. • Structural adapta>ons – Any physical difference in biological structure (e.g. the neck length of a giraffe) • Behavioural adapta>ons – Differences in pa^erns of observable ac/vity (e.g. opossums feigning death) • Physiological adapta>ons – Varia/ons in the ac/vity of internal organs (e.g. homeothermy, night vision) When exposed to a specific environmental selec/on pressure, organisms with beneficial adapta/ons will be more likely to survive long enough to reproduce and pass on the alleles. This will lead to a change in allele frequency within the popula/on’s gene pool (i.e. evolu/on). If environmental condi/ons change, then what cons/tutes a beneficial adapta/on will also similarly change – meaning popula/ons are constantly evolving. SEXUAL SELECTION Sexual selec/on is a bio/c pressure that results from the compe//on that exists in popula/ons for ma/ng. Intrasexual selec>on occurs when members of the same sex compete for mates (e.g. via combat or ma/ng calls), whereas intersexual selec>on occurs when members of one sex choose individuals of the other sex for ma/ng (based on factors like elaborate plumage or courtship rituals). Sexual selec/on may result in the development of characteris/cs that do not provide any survival benefit but maximise reproduc/ve success. John Endler modelled natural and sexual selec/on by controlling the exposure of guppies to a predator. In the absence of the predator, guppies evolved bright spots (sexual selec/on) – but when predators were present, drab coloura/on became the predominant phenotype (natural selec/on). When guppies with drab colours were moved to regions without predators, the popula/on evolved to favour brightly coloured fish. CLASSIFICATION Content Statements: A3.1.1 Varia*on between organisms as a defining feature of life A3.1.2 Species as groups of organisms with shared traits A3.1.3 Binomial system for naming organisms A3.1.4 Biological species concept A3.1.5 Difficul*es dis*nguishing popula*ons and species due to divergence of non-interbreeding popula*ons during specia*on A3.1.6 Diversity of chromosome number in plant and animal species A3.1.7 Karyotyping and karyograms A3.1.8 Unity and diversity of genomes within species A3.1.9 Diversity of eukaryo*c genomes A3.1.10 Comparison of genome sizes A3.1.11 Current and poten*al future uses of whole genome sequencing CLASSIFICATION SCHEMES Varia*on is a defining feature of life in that no two individuals are iden*cal in all their traits. Classifica*on schemes are employed by scien*sts to collect, sort and group informa*on about organisms. They allow for organisms to be iden*fied and compared based on recognised characteris*cs, predict evolu*onary paSerns and show relatedness, as well as ensure that all organisms can be named according to global conven*ons. TAXONOMIC RANKS Organisms with shared characteris*cs are grouped according to a series of hierarchical units (called taxa). The more similar two organisms are, the more taxonomic ranks they will share. The taxa used to classify all living organisms are domain, kingdom, phylum, class, order, family, genus, species (hint: dumb kids put chili on food – get smashed). Every organism is given a scien*fic name that consists of both the genus and species. In this binomial system, genus is capitalised and the name is underlined or is in italics. Example: Homo sapiens (humans), Bos taurus (cows), Staphylococcus albus (bacteria). SPECIES CONCEPT A species represents the most precise unit of classifica*on and is composed of organisms that will share the greatest number of morphological and gene*c features. However, species are not typically defined by their structural characteris*cs. Non-interbreeding popula*ons may look very different due to gene*c divergence but s*ll belong to the same species. Conversely, different species may appear phenotypically similar due to the presence of analogous structures (arising via convergent evolu*on). According to the biological species concept, a species represents a group of organisms that can interbreed and produce fer8le, viable offspring. Excep*ons to this defini*on include organisms that reproduce asexually (such as bacteria) and ring species. Addi*onally, it is difficult to establish breeding poten*al in geographically isolated popula*ons or in fossils. CHROMOSOME NUMBER In order for members of a species to interbreed, they must have a compaCble geneCc profile. The gene*c informa*on of each parent must be organised onto the same number of chromosomes, with comparable sizes and gene loci posi*ons. Organisms with different diploid numbers will be unable to interbreed as the chromosomes cannot form the homologous pairs required for meio*c division. Hence, chromosome number is a characteris*c feature of members of a species and can be used to deduce evolu*onary rela*onships. For example, humans have 46 chromosomes and chimpanzees have 48 chromosomes. It is hypothesised that chromosome 2 in humans arose from fusion of chromosomes 12 and 13 in a shared ancestor. KARYOGRAMS Karyotyping is the process of pairing and ordering the complete set of chromosomes within a cell to provide a snapshot of an individual gene*c profile. Cells will be arrested mid-division as chromosomes are condensed, before chromosomes are arranged into pairs according to size and centromere posi*on. Chromosome sets are then photographed to produce a karyogram, which can be used to iden*fy species, sex and any abnormali*es. A karyogram from a human male is shown on the right. GENOMIC DIVERSITY A genome is all of the gene*c informa*on within an organism (including both nuclear and organelle DNA). Organisms in the same species will share most of their genome, but varia*ons such as single-nucleoCde polymorphisms will give some diversity. Genomes vary in overall size as well as base sequence. Varia*ons between species is much larger than varia*on within a species. While viruses and bacteria tend to possess smaller genomes, there is no clear correla8on between organism complexity and genome size in species. WHOLE GENOME SEQUENCING Whole genome sequencing is the process of determining the DNA sequence of an en*re genome at a single me. Technological advancements have allowed this process to occur with increasing speed and decreasing cost. For research purposes, this sequencing can allow for the determinaon of evoluConary relaConships. For more personal uses, genome sequencing can be used to map individual pedigrees and also facilitate the development of individualised medicines (via pharmacogenomics) for the treatment of disease condi*ons. BIODIVERSITY Biodiversity describes the varia9on seen in living organisms at all levels of biological organisa9on. A higher level of diversity is vital for the integrity of ecosystems – making them more stable and resistant to change. Biological diversity can involve varia9on within a species (gene9c), between species or within ecosystems. Gene:c Diversity Variety of genes and traits available within a species Species Diversity Variety of different species found within an ecosystem Ecosystem Diversity Variety of habitats / niches in an area of land or water MEASURING BIODIVERSITY When assessing the level of biodiversity in a par9cular region, two key measures are typically inves9gated: • Species richness describes the number of different species present in an area (more = greater richness) • The rela)ve abundance of different species describes species evenness (similar abundance = evenness) Species richness and evenness can be quan9tated by es9ma9ons such as percentage land coverage (plants) or by popula9on sampling techniques such as the capture-mark-release-recapture method (lincoln index). BIODIVERSITY CRISIS The level of biodiversity is constantly changing. Random muta9ons, specia9on and ex9nc9on all change the levels of biodiversity. Evidence from fossils suggest that there may be more species on Earth today than at any 9me in the remote past. However, there has also been a significant loss of biodiversity in recent 9mes. Assessments of biodiversity are made by mul9na9onal agencies and are based on contribu9ons from both expert scien9sts and civilians. The accuracy of diversity es9mates is influenced by methods of classifica9on: • Lumpers will classify species according to shared similari)es – resul9ng in lower biodiversity es9mates • SpliDers focus more on the differences between organisms – resul9ng in higher biodiversity es9mates. ANTHROPOGENIC CAUSES The current biodiversity crisis is widely recognised to be anthropogenic (due to human ac9vity). The overarching cause is the global growth of the human popula9on, which is placing various strains on ecosystems: • Overexploita:on (overfishing and excessive / unnecessary hun9ng) • Climate Change (greenhouse gases impact environment condi9ons) • Urbanisa:on (leading to the direct loss or fragmenta9on of habitats) • Pollu:on (damage to environment from microplas9cs and fer9lisers) • Invasive Species (spread of pests and diseases via global transport) • Deforesta:on (the clearing of land for either industry or agriculture) ECOSYSTEM LOSS Ecosystems can become threatened by direct human ac9vity or ecosystem loss can involve indirect ac9vity. An example of ecosystem loss due to direct ac9vity is the extensive removal of mixed dipterocarp forests in South East Asia. Dipterocarps are a family of trees that func9on as a keystone species with rainforests – the trees provide an important habitat for na9ve species and provide nutri9onal support (via fruits and nectar). These forests are being cleared for 9mber and to provide space for agriculture (palm oil planta9ons). This is endangering local species (like the orangutan) as well as threatening water security and food sovereignty. An example of ecosystem loss due to indirect ac9vity is the mass bleaching of the Great Barrier Reef off the coast of Australia. Coral polyps receive nutri9on from photosynthe9c algae (zooxanthellae) that live within the polyp’s endodermis. Anthropogenic greenhouse gas emissions are increasing oceanic temperatures and decreasing pH (ocean acidifica9on). This has caused the zooxanthellae to leave the coral polyp, resul9ng in bleaching. As a reef provides shelter and spawning grounds for marine life, this loss threatens biodiversity. EXTINCTION Ex9nc9on is the total cessa9on of a species or higher taxon level, reducing the overall level of biodiversity. It can result gradually (phyle)c ex)nc)on) as one popula9on progressively evolves into something else, or it can occur rapidly (abrupt ex)nc)on) when one species does not leave any descendants and ceases to exist. Mass ex:nc:on events are categorised by an unusually high number of species dying out in a short period. There have been five mass ex9nc9on events in the past history of the Earth, while human ac9vity is causing a current sixth mass ex9nc9on (which is anthropogenic). It is es9mated that Earth has experienced roughly a 73% decline in the average size of all monitored wildlife popula9ons over the last 50 years (source: WWF). SPECIES EXTINCTIONS Species that have been declared ex9nct as a consequence of anthropogenic ac9vity (overhun:ng) include North Island giant moas (terrestrial megafauna), Caribbean monk seals and the Tasmanian 9ger (thylacine). The giant moa was a large flightless bird that occupied New Zealand’s North Island. Prior to human colonisa9on, giant moas had few natural predators – making them extremely vulnerable to ecosystem disturbances. Following the arrival of humans in the 1200s, the giant moa was driven ex9nct by overhun9ng and the progressive destruc9on of habitat. The Caribbean monk seal was a marine species that lived in the oceans around the Gulf of Mexico. Their docile nature made them easy prey to humans, who hunted them for their oil and blubber. Addi9onally, constant overfishing led to starva9on. The species was officially declared ex9nct in late 2008 – with the last recorded sigh9ng occurring in 1952. Tasmanian :gers are ex9nct carnivorous marsupials that were once na9ve to Australia. They died out on the mainland roughly 3,500 years ago (possible due to compe99on with dingoes), but con9nued to exist on Tasmania. Upon the arrival of European sehlers, the Tasmanian 9gers were rapidly hunted to ex9nc9on (the last one died in 1936 in cap9vity). CONSERVATION Conserva9on involves protec9on and maintenance of natural resources in order to preserve biodiversity. In situ conserva9on involves the preserva9on of plant and animal species within a natural habitat (on-site), while ex situ conserva9on is the preserva9on of plant and animal species outside natural habitats (off-site). In situ conserva9on allows wildlife to maintain normal behaviour and occupy their natural posi9on in food webs, while ex situ conserva9on allows for greater control of condi9ons to improve the chances of survival. In situ Conserva:on Techniques: • Using legisla9on and funding to designate protected areas of land as na:onal parks or nature reserves • Ac9vely or passively restoring a damaged ecosystem un9l it can become fully sustainable (rewilding) • Repurposing areas used for human ac9vity (reclama:on) to restore previous ecosystems (e.g. quarries) Ex situ Conserva:on Techniques: • Cap9ve breeding programs (zoos) involving animals raised and bred in containment to ensure survival • Botanical gardens are areas devoted to collec9ng, cul9va9ng and displaying a variety of plant species • Biological samples are stored and catalogued at secure sites to preserve diversity (seed or :ssue banks) EDGE OF EXTINCTION PROGRAMME The EDGE of Existence programme selects species for conserva9on priori9sa9on based on two condi9ons: • Species have few close rela9ves and represent unique phylogene9c branches (evolu:onarily dis:nct) • Species are classed ‘at risk’ according to the IUCN red list for threatened species (globally endangered) The EDGE of Existence programme is used to inform stakeholders of priori9es – it does not make decisions regarding interven9ons (keystone species and culturally significant species may require greater resources)