Chapter 20: Selection and evolution

 Genetic variation provides the raw material on which natural selection can act  Variation within a population means that some individuals heave features that given them an advantage over other members of that population Genetic variation

 Genetic variation is caused by:  Independent assortment of chromosomes (and therefore alleles) during meiosis  Cross over between chromatids of homologous chromosomes during meiosis  Random mating between organisms within a species  Random fertilization of gametes  Mutation Genetic variation

 These processes (minus mutation) reshuffle existing alleles in the population so that offspring have combination of alleles which differ from those of their parents  This produces phenotypic variation Genetic variation

 Mutation can produce completely new alleles  Ex: sickle cell allele of the gene that codes for β- globin  Such a change in a gene is called a gene mutation  The new allele is often recessive, so it frequently does not show up in the population until some generations after the mutation originally ocurred Genetic variation

 Mutations that occur in body cells, or somatic cells, often have no effect at all on the organisms, and such mutations cannot be passed on to offspring by sexual reproduction Genetic variation

 Mutations in cells of ovaries or testes may be inherited by offspring if mutation divides to form gametes  Gametes will have mutated gene  If mutated gamete is involved in fertilization the new organism will express the mutated gene in all somatic cells Genetic variation

 Genetic variation, whether caused by the reshuffling of alleles during meiosis and sexual reproduction or by mutation, can be passed on by parents to offspring  Variation in phenotype is also caused by the environment in which organisms live  Ex: some organisms may be larger because they has better access to food while they were growing. This type of variation is NOT passed on by parents to their offspring Genetic variation

 All organisms have the reproductive potential to increase their population numbers  As population increases, various environmental factors come into play to keep numbers in check  These factors may be:  Biotic : caused by other living organisms (predation, competition, pathogenic infection)  Abiotic : caused by nonliving components (ex: water supply or nutrient levels in the soil) Natural Selection

 If the pressure of the environmental factors is sufficiently great, then the population size will decreases  Over a period of time, population numbers will oscillate around a mean level Natural Selection

 The number of young produced is far greater than the number which will survive to reproduce  What determines if a newborn will survive to reproductive age? It may be just luck, but some individuals are born with a better chance of survival than others  Variation within a population means that some individuals will have features which give them an advantage in ‘the struggle for existence’ Natural Selection

 Ex: coat color in rabbits  Most rabbits have alleles that give them brown color. However, a few will be homozygous for the recessive allele that gives a white color. These white rabbits will stand out to predators (foxes) more from other and are less likely to survive than brown rabbits. The chances of a white rabbit reproducing and passing of its alleles for white coat to its offspring are very small. Natural Selection

 Predation by foxes is an example of a selection pressure  Selection pressure increases the chances of some alleles being passed on to the next generation, and decrease the chances of others Natural Selection

 In the rabbit fur coat color example, the alleles for brown color have a selective advantage over the alleles for white  The alleles for brown coat will remain the more common alleles in the population Natural selection

 The effects of such selection pressures on the frequency of alleles in a population is called natural selection  Natural selection raises the frequency of alleles conferring an advantage, and reduces the frequency of alleles conferring a disadvantage Natural Selection

 Usually, natural selection keeps things they way they are. This is stabilizing selection  Keeps allele numbers around a central mean  However, is a new environmental factor or new allele appears, the allele frequencies may also change  This is called directional selection Evolution

A new environmental factor  Imagine we are plunged into a new Ice Age. The climate becomes much colder and snow covers the ground almost all year. Now, white rabbits have a selective advantage in the new environment. Rabbits with white fur are more likely to survive and pass on their alleles.  The frequency of the white allele will increase Evolution

 Because they are random, most mutations produce features that are harmful  Other mutations may be neutral, meaning their neither benefit nor harm the organism  Mutations produce advantageous features only occasionally Evolution

 Imagine that mutation occurs in the coat color gene of a rabbit, producing a new allele which gives a better camouflaged coat than brown  Rabbits possessing this allele will have as selective advantage over brown rabbits, making them more likely to survive to reproduce  Over many generations, the new allele will become more common Evolution

 Such a change in allele frequency in a population are the basis of evolution  Evolution occurs because natural selection gives some alleles a better chance of survival than others.  Over many generations, populations may gradually change, becoming better adapted to their environments Evolution