microevolution

microevolution

1. Factors initiating elementary evolutionary changes(Microevolution)

2. Microevolution Microevolution is the change in the genome, or gene pool, for a given species in a relatively short period of geologic time by the alterations of successfully reproducing individuals within a population.

3. Some environmental conditions are more harsh than others, and organisms may have to adapt more to survive in that conditions. Areas where the environmental pressures are stable, or the organisms have adapted to it, exhibit non-evolving populations. In a non-evolving population, the allele frequency, genotype frequency, and phenotype frequency remain in genetic equilibrium.

4. This phenomenon was illustratedd by a German physician, Weinberg, and a British mathematician, Hardy, both working independently in 1908. Their combined efforts are now known as the Hardy-Weinberg equilibrium model.

5. Hardy-Weinberg Equilibrium To understand the Hardy-Weinberg equilibrium, assume G and g are the dominant and recessive alleles for a trait where GG = green, gg = yellow, and Gg = orange . In our imaginary population of 1,000 individuals, assume that 600 have the GG genotype, 300 are Gg , and 100 are gg . The allele and genotype frequency for each allele is calculated by dividing the total population into the number for each genotype:

6. GG = 600/1,000 = .6 Gg = 300/1,000 = .3 gg = 100/1,000 = .1 The frequency of the allele in the first generation of offspring .

7. First, determine the total number of alleles possible in the first generation. In this imaginary case, because each organism has 2 alleles and there are 1,000 organisms, the number of possible alleles in the first generation of offspring is: 2 × 1,000 = 2,000

8. For the G allele, both GG and Gg individuals must be considered. Taken separately, GG = 2 × 600 = 1,200 + Gg = 300 1,500

9. The letter  p  is used to identify the allele frequency for the dominant allele (.75) and q for the recessive allele (.25 ). Note that  p  +  q  = 1. The frequency for the G allele is therefore: 1,500/2,000 = .75

10. For the g allele, the calculation is similar: Gg = 300 + gg = 2 × 100 = 200 500 The frequency for the g allele is therefore: 500/2,000 = .25

11. Hardy-Weinberg can also predict second-generation genotype frequencies. From the previous example, the allele frequencies for the only possible alleles are  p = .75 (G) and  q = .25 (g) after meiosis. Therefore, the probability of a GG offspring is  p  ×  p  =  p 2  or (.75) × (.75) = 55 percent. For the gg possibility, the allele frequencies are q × q or (.25) × (.25) = 6 percent. For the heterozygous genotype, the dominant allele can come from either parent, so there are two possibilities: Gg = 2 pq  = 2(.75)(.25) = 39 percent.

12. Note that the percentages equal 100, and the allele frequencies ( p  and  q ) are identical to the genotype frequency in the first generation! Because there is no variation in this hypothetical situation, it is in Hardy-Weinberg equilibrium, and both the gene and allele frequencies will remain unchanged until acted upon by an outside force(s). Therefore, the population is in a stable equilibrium with no change in phenotypic characteristics.

13. The Hardy-Weinberg equation highlights the fact that sexual reproduction does not alter the allele frequencies in a gene pool. Five factors impact the Hardy-Weinberg equilibrium and create their own method for microevolution . 1.Mutation pressure 2.Immigration 3.Genetic drift 4.Cross breeding 5.Selection pressure

14. 1.Mutation pressure A  mutation  is an inheritable change of a gene by one of several different mechanisms that alter the DNA sequencing of an existing allele to create a new allele for that gene A primary mechanism for microevolution is the formation of new alleles by  mutation . Spontaneous errors in the replication of DNA create new alleles instantly while physical and chemical mutagens, such as ultraviolet light, create mutations constantly at a lower rate.

15. Mutations affect the genetic equilibrium by altering the DNA, thus creating new alleles that may then become part of the reproductive gene pool for a population. When a new allele creates an advantage for the offspring, the number of individuals with the new allele may increase dramatically through successive generations. This phenomenon is not caused by the mutation somehow overmanufacturing the allele, but by the successful reproduction of individuals who possess the new allele. Because mutations are the only process that creates new alleles, it is the only mechanism that ultimately increases genetic variation

16. 2.Immigration Gene migration is the movement of alleles into or out of a population either by the immigration or emigration by individuals or groups. When genes flow from one population to another, that flow may increase the genetic variation for the individual populations.

17. 3.Genetic Drift Genetic drift is the phenomenon whereby chance or random events change the allele frequencies in a population. Genetic drift has a tremendous effect on small populations where the gene pool is so small that minor chance events greatly influence the Hardy-Weinberg arithmetic. The failure of a single organism or small groups of organisms to reproduce creates a large genetic drift in a small population because of the loss of genes that were not conveyed to the next generation

18. Conversely, large populations, statistically defined as greater than 100 reproducing individuals, are proportionally less affected by isolated random events and retain more stable allele frequency with low genetic drift.

19. 4.Cross breeding The Hardy-Weinberg equation assumes that all males have an equal chance to fertilize all females. However, in nature, this seldom is true . In fact, the ultimate nonrandom mating is the act of self-fertilization that is common in some plants. In other cases, as the reproductive season approaches, the number of desirable mates is limited by their presence (or absence) as well as by their competitive premating rituals. Finally, botanists and zoologists practice nonrandom mating as they attempt to breed more and better organisms for economic benefit.

20. 5.Selection pressure The process by which comparatively better adapted individuals out of a heterogeneous population are favoured by the Nature over the less adapted individuals is called natural selection. The process of natural selection operates through differential reproduction.

21. It means that those individuals, which are best adapted to the environment, survive longer and reproduce at a higher rate and produce more offsprings than those which are less adapted. So the formers contribute proportionately greater percentage of genes to the gene pool of next generation while less adapted individuals produce fewer offsprings . If differential reproduction continues for a number of generations, then the genes of those individuals which produce more offsprings will become predominant in the gene pool of the population.