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Population genetics Species: a group of individuals that are more or less alike and are able to interbreed under natural conditions to produce fertile offspring. Population: is a group of individuals of the same species living together in a particular place at a particular time. Gene pool: all the genotypes of individuals capable of reproducing in a population are pooled together. This is referred to as gene pool- a collection of all their genotypes. Population genetics Allele frequency: in studying the gene pool geneticists are interested in how often each allele of a gene occurs in the population. These are called allele frequencies of that population. Allele frequencies are determined by keeping tabs on frequencies of characteristics. Half the alleles from each parent are passed on to the next generation. How the alleles are passed on in reproduction can determine the characteristics in future populations. We can now look at the process of natural selection in terms of frequencies of alleles in the gene pool of a population. Within a particular gene pool, the frequencies of the alleles have changed over time. Darwin’s theory Not all individuals in a population survive to maturity to reproduce and pass on their genetic code. The birth rate exceeds the survival rate Reason: there is a ‘struggle for existence’ (DARWIN) Factors affecting survival: Climate Availability of resources: food, water, shelter Competition with other animals Predation & disease Characteristics of the organism, itself: - some individuals have better resistance to disease, strength, speed etc. Chance Struggle of the fittest ‘Struggle of the fittest’ refers to organisms who have alleles that are adapted to a change in the environment. Fittest refers to reproductive fitness; the ability to reproduce offspring with alleles that allow them to survive. Changing environments put ‘selective pressure’ on populations, resulting in altered gene frequencies (i.e. evolution). Speciation – formation of a new species The formation of a new species results from the effects of: 1) 2) 3) variation Isolation Natural selection Speciation – formation of a new species 1. VARIATION Variation refers to the genetic differences between individuals. Causes of variation: Sexual reproduction produces new combinations of genes: Independent assortment of genes into gamete during meiosis Crossing over during meiosis Random mating Chromosomal errors Mutation produces new alleles of existing genes. Speciation – formation of a new species 2. ISOLATION Many barriers prevent breeding between distinct groups of organisms. Amongst humans barriers may be: Geographical: distance/mountains/swamps/ocean/desert Socio-cultural: social position laws language religion Speciation – formation of a new species If exchange of genetic information (gene flow) is restricted between adjacent groups, then populations will be separated into different breeding groups. Their gene pools become separated. In time the populations may become so different in behaviour, physical appearance or functioning that interbreeding becomes impossible. Speciation – formation of a new species 3. NATURAL SELECTION Natural selection acts on the variation present in a population Isolated groups will be subject to different environmental stresses. Favourable genotypes tend to be selected while unfavourable genotypes tend to be eliminated resulting in a change in the frequency of some genes in the gene pool for a population. It is said that the population has adapted to a change in the environment by changing the frequency of alleles in the gene pool. Speciation – formation of a new species This will result in different variations becoming common in different regions, depending on the environment. (e.g. cold climates tend to favour the survival of short, stocky people ENDOMORPHIC body shape (small surface area to volume- better able to retain body heat) People living in hot climates often have a tall, slender body form ECTOMORPHIC body shape (large Surface area to volumebetter able to lose heat). Speciation – formation of a new species One of the best examples of natural selection is the English Peppered Moth. This moth is whitish with black speckles and spots all over its wings. During the daytime, Peppered moths are well-camouflaged as they rest on the speckled lichens on tree trunks. Occasionally a very few moths have a genetic mutation which causes them to be all black. Black moths resting on light-coloured, speckled lichens are not very well camouflaged, and so are easy prey for any moth-eating birds that happen by. So, they never get to reproduce and pass on their genes for black colour. With time, nature would select the white to survive and reproduce. Speciation – formation of a new species However, with the Industrial Revolution, many factories and homes in British cities started burning coal. Coal creates a lot of black soot and pollution. Since lichens are extremely sensitive to air pollution, this caused all the lichens on city trees to die. Also, as the soot settled out everywhere, this turned the city tree trunks black. This enabled the occasional black moths living in the cities to be well-camouflaged so they could live long enough to reproduce, while the “normal” speckled moths were gobbled up.