Sex and genetic recombination

Organisms that are asexual, has the characteristic of they genes are inherited together, or linked, as they cannot mix with genes in other organisms during reproduction. However, the descendants of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent as sortment. In a related process knowing as homologous recombination, sexual organisms exchange DNA between two matching chromosomes.

Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing descendants with new combinations of alleles. Sex usually increases genetic variation and may increase the rate of evolution. However, asexuality is advantageous in some environments as it can evolve in previously-sexual animals. Here, asexuality might allow the two sets of alleles in their genome to diverge and gain different functions.

Recombination allows even alleles that are close together in a strand of DNA to be inherited independently. However, the rate of recombination is low (approximately two events per chromosome per generation). As a result, genes close together on a chromosome may not always be shuffled away from each other, and genes that are close together tend to be inherited
together, a phenomenon known as linkage. This tendency is measured by finding how often two alleles occur together on a single chromosome, which is called their linkage disequilibrium. A set of alleles that is usually inherited in a group is called a haplotype.

When alleles cannot be separated by recombination – such as in mammalian Y chromosomes, which pass intact from fathers to sons – harmful mutations accumulate. By breaking up allele combinations, sexual reproduction allows the removal of harmful mutations and the retention of beneficial mutations. In addition, recombination and reassortment can produce individuals with new and advantageous gene combinations.

These positive effects are balanced by the fact that sex reduces an organism's reproductive rate, can cause mutations and may separate beneficial combinations of genes.The reasons for the evolution of sexual reproduction are therefore unclear and this question is still an active area of research in evolutionary biology, that has prompted ideas such as the Red Queen hypothesis.

Vriation in Genetics

An individual organism's phenotype is teh result of both its genotype and the effect of the interaction with environment it has lived in. An important part of the variation in phenotypes in a population is caused by the differences between their genotypes. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will fluctuate, becoming more or less prevalent relative to other forms of that gene.

Evolutionary forces act by driving these changes in allele frequency in one direction or another. Variation disappears when an allele reaches the point of fixation — when it either disappears from the population or replaces the ancestral allele entirely.

(Image: Duplication of part of Chomosome)

Variations are the result of mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes through sexual reproduction. Variation also comes from exchanges of genes between different species; for example, through horizontal gene transfer in bacteria, and hybridization in plants. Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species. However, even relatively small changes in genotype can lead to dramatic changes in phenotype: chimpanzees and humans differ in only about 5% of their genomes.

Heredity

The evolution in all living things has its origin in heritable traits (particular characteristics of an organism). In humans, for example, eye color is an inherited characteristic, which individuals can inherit from one of their parents. Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.

The complete set of observable traits that make up the structure and behavior of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment. Thus we have that not every aspect of an organism's phenotype is inherited. Suntanned skin results from the interaction between a person's genotype and sunlight exposure; thus, suntans are not passed on to people's children. However, people have different responses to sunlight, arising from differences in their genotype.

Hhereditary characteristics are transmitted among generations via DNA, a molecule that contains genetic information. DNA is a polymer composed of four types of bases. The sequence of bases along a particular DNA molecule specify the genetic information, in a manner similar to a sequence of letters specifying a sentence. Portions of a DNA molecule that specify a single functional unit are called genes; different genes have different sequences of bases.

Within cells, the long strands of DNA form condensed structures called chromosomes. A specific location within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.

However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes.

Source: Wikipedia

Evolution

Eevolution is a change in the genetic of a population of organisms from one generation to the next. Though changes produced in any one generation are generaly small, cumulative changes with each generation can cause important changes in the population, a process that can culminate in the emergence of new species. Characteristics in common among species imply that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.

The basis of evolution is the genes that are passed on among generations; these produce an organism's inherited traits. These results cause organisms showing heritable differences (variation) in their traits. Evolution is the product of two opposing forces: processes that constantly introduce variation, and processes that make variants either become more common or rare. New variation arises in two main ways: either from mutations in genes, or from the transfer of genes between populations and between species. In species that reproduce sexually, new combinations of genes are also produced by genetic recombination, which can increase variation between organisms.

Two major mechanisms determine which variants will become more common or rare in a population. One is natural selection, a process that causes helpful traits (those that increase the chance of survival and reproduction) to become more common in a population and causes harmful traits to become more rare. This occurs because individuals with advantageous traits are more likely to reproduce, meaning that more individuals in the next generation will inherit these traits.

Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of the variants best-suited for their environment. The other major mechanism driving evolution is genetic drift, an independent process that produces random changes in the frequency of traits in a population. Genetic drift results from the role that chance plays in whether a given trait will be passed on as individuals survive and reproduce.

Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time. However, the mechanism driving these changes remained unclear until the theories of natural selection were independently discovered by Charles Darwin and Alfred Wallace. Darwin's landmark 1859 work On the Origin of Species brought the new theories of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the diversity of life on Earth.