Unit 5 Forces Of Evolution PDF

Unit 5 Forces of Evolution Mutation, Geneflow, Selection, Genetic Drift, Co-adaptation and co- evolution, anthropogenic activities, extinction (in – brief) – periodic and mass scale – causes and events. Forces of Evolution There are four known forces of evolution...

Unit 5 Forces of Evolution Mutation, Geneflow, Selection, Genetic Drift, Co-adaptation and co- evolution, anthropogenic activities, extinction (in – brief) – periodic and mass scale – causes and events. Forces of Evolution There are four known forces of evolution, they are mutation, gene flow, natural selection and genetic drift. Mutation introduces new alleles into the population due to the occurrence of copying errors in DNA replication and transcription. Natural selection (due to the individual differences on the survival and reproductive success) and genetic drift (due to random sampling in small populations) differentially transmit alleles into the next generation. Gene flow, through migration, contributes alleles from one population to other (Vasulu, 2012). Forces of Evolution The interaction of the evolutionary forces is studied using genetic models to understand the distribution, turnover and diversity of alleles at the particular level. Evolutionary forces disturb the Hardy-Weinberg equilibrium and are responsible for the process of evolution. Interaction of evolutionary forces contributes to variation and spreading alleles within or between populations Mutation It serves as a raw material of evolution. If evolution has to happen mutation should occur. Mutation occurs in every generation. Mutation is mostly random change in phenotype (colour, size and shape) or genotype forms. Genotype changes involve alteration in the sequence of DNA. Mutation introduces new alleles in the population and changes the allele frequencies. The survival of introduced allele depends on how it can affect the survival fitness of the population. If the introduced new allele is advantageous to the population it is supported by natural selection. Mutation Vernon Ingram was the first to identify change in the amino acid of valine in place of glutamic acid in the mutant haemoglobin of patients with sickle cell anaemia. The frequency of mutation in humans is one in 500 million nucleotides (Sudi and Ali– Dunkrah, 2005). Mutation includes all type of heritable changes. The process of introduction of mutation in a gene is called mutagenesis and resulting product is called mutant. The agent used to induce mutation is called a mutagen. Mutation frequency Depending on the frequency of mutant different terms are used. If frequency of genetic mutant is less than 1% it is called variant and if ≥1 it is termed as polymorphism. Mutations are known by different names depending on the number of bases, mechanism and region of localisation of variants as, single nucleotide polymorphisms, indels (insertion and deletion), short tandem and variable number of tandem repeats and copy number variations. Mutation rate is low and varies with the sites of the genome.  In some sites its frequency is higher than others and these are known as hot-spots of mutation. Higher rate of mutations occurs in intronic, repetitive sequences and non- coding regions of mitochondria (hypervariable regions 1 and 2, HV1 and HV2 respectively). Mutations in the HV1 and HV2 are useful to study the maternal history of population using haplo-group or population approaches. The rate of spontaneous mutations ranges from one in 104-108 gene per generation. Alu transposition element (short interspersed elements or repeats) events occur in every new birth. In born errors of Metabolism Harmful effects of Mutation Mutations are responsible for genetic disorders and cancer. The genetic disorders involve single genes both autosomal (Sickle cell anaemia, Thalassemia, Cystic fibrosis, Huntington disease and Marfan syndrome) and sex linked genes (Haemophilia A and Vitamin-D resistant rickets) and also copy number variations or chromosomal abnormalities (Down syndrome, Jacob syndrome, Klinefelter syndrome, Turner syndrome, Super females, Patau syndrome and Edward syndrome). Mutations in BRCA1 and BRCA2 (tumour suppressor genes) cause breast, ovarian, pancreatic and prostate cancers. Beneficial effects of Mutation Carriers of sickle cell trait (HbAS) are known to be protected against Plasmodium falciparum malaria. Carriers of sickle cell anaemia with alpha thalassemia showed lower incidence of cardiac complications, jaundice, gall stone, pallor, splenectomy, acute splenic sequestration crises, stroke and avascular than those without alpha thalassemia (Ali Al-Barazanchi et al., 2021). Neutral Effects Mutations such as silent mutations do not any have effect either positive or negative effect on the organisms and do not change the amino acids they encode. Silent mutations have been shown to interfere in the splicing function of exon splicing enhancers in the desired place in nucleotide sequence (Dickson and Hyman, 2013). Gene flow Diffusion of alleles across populations can be called gene flow. Human beings move from one population and join another population due to various socio- economic reasons, and choose mate and involve in interbreeding. This causes the gene flow or genetic admixture. As a result of gene flow, allele frequency changes in the population left and also the population in which they have joined. Examples of genetic admixture or gene flow (Figure 2) are the Anglo Indians, the American Blacks and the Siddis (Indo-African population). Gene flow In Latin American countries, admixed populations are seen due to random mating among indigenous populations, Europeans and Africans. Another example is gradient distribution of the B blood group allele. B allele believed to have originated in Asia spread to western countries due to genetic admixture forced by invasions. The barriers to gene flow are geographical, cultural, linguistic, and political factors. For gene flow to occur, not only the large scale migrations and choosing of mates in the settled areas but also the mate selection in particular direction over a long span of time may be responsible. Gene flow – Models To explain the change in allele frequency contributed by gene flow, Sewall Wright proposed ‘island’ and ‘isolation by distance’ models whereas Kimura and Weiss contributed ‘stepping stone model’. Stepping stone model assumes that a line or ring of populations exchange migrants with their immediate neighbours only whereas island model allows exchange of migrants between populations. In contrast, ‘isolation by Genetic Structure of Human Populations distance model’ proposes that gene flow decreases as the geographical distance increases. Natural Selection Natural selection is defined as differential survival and reproductive success. Mutation adds new alleles into the population. Natural selection determines the fate of newly added allele in the population. If it is not advantageous or not suitable to the environment, natural selection eliminates it by negative or purifying selection. The survival of allele in particular environment depends on its fitness/ adaptive value/ selective value. Various factors such as mate selection, female fecundity and survival in the reproductive age contribute to the fitness. Natural Selection Those who are fit to survive, have reproductive success and contribute more offspring than less fit people. This is also called adaptation. Natural selection facilitates the adaptation of the organism to the environment and the allelic composition carried by the fit individual is promoted that result in the change in allele frequency of the population due to the replacement of allele composition carried by the less fit individuals. In another words this is called positive selection or diversifying selection. Natural Selection When people of opposite sex carrying different genotypes marry, they pass on unequal alleles to their offspring as a result there may be a difference in the allele frequency of parents or progeny. An individual is the unit of natural selection. Examples of natural selection are skin colour, physiological and genetic adaptation to high altitude, lactose persistence, sickle cell trait and Duffy blood group. Natural Selection Skin Color: temperature and color of the skin; The light skin allowed ultraviolet rays to enter and produce vitamin D to absorb calcium levels. For those staying near equator natural selection favoured dark skin to prevent the loss of folic acid (vitamin B9) required for the development of healthy foetuses (Humanorigins.si.edu). High altitudes: Human beings living in plains when enter into the high altitudes, undergo hypoxic stress and develop adaptive responses like increase in the levels of red blood cells, greater lung volume and increased chest dimensions and respiration. Natural Selection Lactose persistence: Lactase is an enzyme that hydrolyses lactose, a carbohydrate present in milk, into glucose and galactose. The lactase activity decreases after weaning period in most mammals but some humans continue to produce this enzyme and digest the lactose in the milk. This trait is known as lactase persistence (LP) and has been observed in pastoralist populations. Resistance to Malaria: Duffy is one of the blood groups identified in humans. RBCs lacking Duffy antigens are resistant to Plasmodium vivax. Types of Selection 1. Selection for heterozygotes 2. Selection against heterozygotes 3. Selection with the co-dominant allele 4. Selection against dominant allele 5. Selection against recessive homozygote Selection types Selection for the heterozygotes (heterozygotes have higher fitness than homozygote) Selection against heterozygotes or under dominance (lower fitness assigned to heterozygotes against two homozygotes). Homozygous individuals containing alternate forms of a chromosomal translocation marry, heterozygotes are fit because they have single copy of all genes from both parents and when these heterozygotes marry each other they give birth children among them a fraction of them lack important trans locational segment and not viable. If we observe this fitness phenomenon over generations this can be called as underdominance or selection against heterozygotes. Selection types Selection with the co-dominant allele (alleles are co-dominant and one allele (heterozygotes) is favoured). Example for this type of selection is ABO blood group. Co-dominance means no allele can mask the expression of other allele. If an individual inherits B allele from mother and A allele from father both are expressed, he/she has AB blood group. In AB blood group both are co-dominant alleles and in heterozygous state which favoured 4) Selection against dominant allele (expressed as heterozygote). Achondroplasia is an example for this type of selection. This is an abnormality of bone growth occur in 1 in 20-30,000 live births. T Selection types Selection against recessive homozygotes (harmful allele is favoured). Example for this type of selection is Tay Sachs disease. Its incidence is 1 in 250-350 people. Most commonly observed in Ashkenazi Jews, South eastern Quebec and among Cajun of Louisiana. The bottleneck effect: The seal population on the island is reduced due to a volcanic eruption. The surviving individuals form the new population with different allele frequencies than the original population. The founder effect: A few lizards are isolated from the main population of lizards from a continent by chance and drift to a distant island. Their numbers gradually increase on the new island, but the allele frequencies of the new population are different compared to the original population. Bottleneck effect occurs when a population suddenly gets much smaller. This might happen because of a natural disaster such as a forest fire or a volcanic eruption (see the Figure). By chance, allele frequencies of the survivors may be different from those of the original population. Founder effect occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left.