biology4alevel600

1 Genetic variation within a population is the raw material on which natural selection can act.  2 Meiosis, random mating and the random fusion of gametes produce genetic variation within populations of sexually reproducing organisms. Variation is also caused by the interaction of the environment with genetic factors, but such environmentally induced variation is not passed on to an organism’s off spring. The only source of new alleles is mutation.   3 All species of organisms have the reproductive potential to increase the sizes of their populations, but, in the long term, this rarely happens. This is because environmental factors come into play to limit population growth. Such factors decrease the rate of reproduction or increase the rate of mortality so that many individuals die before reaching reproductive age.   4 Within a population, certain alleles may increase the chance that an individual will survive long enough to be able to reproduce successfully. Th...

Isolating mechanisms can lead to the accumulation of different genetic information in populations, potentially leading to new species . Species and speciation species : a group of organisms with similar morphological, physiological, biochemical and behavioural features can interbreed to produce fertile offspring reproductively isolated from other species speciation : the production of new species 1. Allopatric speciation - geographical isolation - population of species split and move to different areas - each new population experiences different selective pressures --> features change over time, mutations occur - when the different populations are reintroduced, they can no longer interbreed --> new species have evolved 2. Sympatric speciation - ecological and behavioural separation - sympatric speciation usually occurs through polyploidy polyploidy organism: has more than 2 complete sets of chromosomes happens when meiosis goes wrong when forming gametes tetraploidy : 2+2 = 4; ...

General theory of evolution : organisms have changed over time. Molecular comparison between species 1. Comparing amino acid sequences of proteins Number of difference in the nucleotide sequences measure how closely related the species are. 2. Comparing nucleotide sequences of mitochondrial DNA Human mtDNA : inherited through the female line zygote contains the mitochondria of the ovum mtDNA is circular so can't undergo any form of crossing over, changes in nucleotide sequence can only arise by mutation Different human populations show differences in mitochondrial DNA sequences. This provides evidence for the origin of different populations --> ' molecular clock ' hypothesis: there is a constant rate of mutation over time the greater the number of differences in the sequence of nucleotides, the longer ago those individuals shared a common ancestor 'clock' can be estimated from fossil evidence Extinction Species can become extinct through a variety of mechani...

Humans use selective breeding (artificial selection) to improve features in ornamental plants, crop plants, domesticated animals and livestock. - selective pressure: humans - individuals with desirable features are chosen to interbreed = selective breeding - some of these desirable alleles are passed onto offspring - offspring with the most desirable features are chosen to interbreed - this is repeated over many generations Over many generations, alleles deemed desirable by the breeder increase in frequency, while the 'disadvantageous' ones may completely disappear over time. Dairy cattle - desirable features: docility, fast growth rates, high milk yield - cows with desirable features are chosen to interbreed, and so are their offspring. This is repeated over many generations. Bulls cannot be assessed for milk production as this is a sex-limited trait. Therefore, progeny testing is used: the performance of the bull's femail offpsirng is looked at to see whether or not t...

Populations of organisms have the potential to produce large numbers of offspring, yet their numbers remain fairly constant year after year. Natural selection occurs as populations have the capacity to produce many offsprings --> compete for resources --> individuals best adapted to survive breed and pass on their alleles . Variation means some individuals in a population will have features which give them an advantage in the 'struggle for existence'. Environmental factors - biotic : caused by other organisms     e.g.: predation, food competition, infection by pathogens - abiotic : caused by non-living components of the environment     e.g.: water supply, nutrient level of soil Selection pressures control the chances of some alleles being passed on to the next generation.     e.g.: predators - individuals that can better camouflage themselves survive more, pass on alleles The effects of such selection pressures on the frequency of alleles in a p...

The variation that exists within a species is categorised as continuous and discontinuous . The environment has considerable influence on the expression of features that show continuous (or quantitative) variation. Discontinuous variation - qualitative differences - genetic basis: different alleles at a single gene locus have large effects on the phenotype different genes have quite different effects on the phenotype - e.g.: eye colour Continuous variation - quantitative differences - genetic basis: different alleles at a single gene locus have small effects on the phenotype different genes have the same/additive effect on the phenotype polygenes - large number of genes have a combined effect on a particular phenotypic trait - e.g.: height; weight Environmental effects on phenotype e.g.: hair colour of Himalayan rabbits, Siamese and Burmese cats - development of dark extremeties: tips to ears, nose, paws and tail - caused by an allele that allows formation of dark pigments onl...

17.1 Variation 17.2 Natural and artificial selection 17.3 Evolution Charles Darwin and Alfred Russel Wallace proposed a theory of natural selection to account for the evolution of species in 1858. A year later, Darwin published On the Origin of Species providing evidence for the way in which aspects of the environment act as agents of selection and determine which variants survive and which do not. The individuals best adapted to the prevailing conditions succeed in the ‘struggle for existence’.  Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and unfamiliar contexts. Learning outcomes  Candidates should be able to: 17.1 Variation The variation that exists within a species is categorised as continuous and discontinuous. The environment has considerable influence on the expression of features that show continuous (or quantitative) variation. a) describe the differences between continuous and discontinuous variation and expla...