How Does Crossing Over Ensure That Variation in a Species Continues

Summary

• Variation describes diversity in the genetic make-up of a species
• Variation arises from mutations
• Variations usually have a small effect on phenotype (continuous variation), but there are examples where a variant has a large effect on a phenotype, and generates distinct categories (discontinuous variation)
• Variation in a population can be influenced by mutation rate, genetic drift, sexual reproduction, meiosis and gene flow
• Variation is important for adaptation and evolution.

Evolution begins with the inheritance of new genetic variation. Variation describes differences in the genetic make-up between individuals within a species. Variation can exist in all cells of the body but is only passed on to the next generation when it is present in the gametes. In this way, genetic variation can influence future generations and alter the genetic pool of the population.

Having a large gene pool with lots of variation within it improves the biological fitness of the species, and allows them to adapt and survive intense periods of selection. This is because diversity in genetic information allows the population to remain flexible to changes in their environment. Lots of different combinations of genes variants confer different advantages under different conditions such as climate changes and disease resistance).

Variation is crucial in the process of evolution of a species, as genetic diversity is a factor enabling natural selection to occur. That is, certain variants are selected for and against based on the environment. Different versions of a gene (or variants) that are present within the population are called alleles.

How does variation arise?

There are several causes for variation being present within a population:

• Mutations

Mutations are the original source of DNA mutations. Mutations occur when the sequence of the DNA is changed and can arise spontaneously by errors in DNA replication. Mutations can include the deletion or substitution of individual nucleotides or larger sections of DNA.

Many mutations are harmful to the organism, but, occasionally, a new allele could be created that benefits the organism. This could lead to the organism being more likely to reach sexual maturity and reproduce. Therefore, the mutation will be selected for, and be increased in frequency in the gene pool. This advantageous allele is inherited by members of the next generation. One single mutation can have a large effect on a phenotype, but usually evolution of new phenotypes is based on the accumulation of lots of mutations over time.

• Gene flow

Gene flow is the movement of genes from one population to another. This occurs in plants when they send their pollen by wind (or it is carried by insects) to pollinate other populations some distance away, or in animals when individuals travel from one geographic location to another. This flow of individuals in and out of the group changes the gene pool within the previous population and introduces new genetic information to other populations. Conversely, continuous gene flow between two populations can lead to a combination of the two gene pools, reducing the genetic variation between both groups.

• Genetic drift

Genetic drift describes variation in the frequency of different genotypes in a small population, because of random fluctuations in allele frequencies. This can cause the chance disappearance of certain alleles (usually the rarer alleles), biasing the variation present in a population. This is common after an event such as a natural disaster causes a genetic bottleneck, where the population is drastically reduced. Genetic drift can also cause a newly isolated population to be genetically distinct from the original population, which has led many to believe that genetic drift can be involved in the evolution of new species.

• Sexual reproduction and random fertilisation

Sexual reproduction occurs through the joining of a male and female gamete, a process called fertilisation. Gametes have only half (one copy) of the genetic information stored within other cells of the body, and can therefore combine with another gamete to produce offspring with a complete set of genetic information. Sexual reproduction, therefore, produces offspring carrying genetic information from both parents, producing variety through the random combining of new genetic material. Sexual reproduction can introduce new gene combinations into a population. This genetic shuffling is another important source of genetic variation.

• Meiosis

Meiosis is the special kind of cell division that produces haploid gametes from diploid cells and is covered in detail in another section. A crucial part of meiosis is when homologous chromosomes pair up during prophase I. When the pair, they also exchange pieces of DNA with each other in a process called crossing-over or recombination. This generates a new opportunity for genetic variation, as it results in the shuffling of alleles on each chromosome to create new combinations of genes.

Types of variation

Variation can either be continuous or discontinuous.

Discontinuous variation describes variation in which different alleles of one gene have a large effect on the phenotype of the organism. With this type of variation, individuals can be grouped into distinct classes. One example is in the case of blood types. Your blood group (A/B/AB or O) is an either/or scenario, you cannot be an intermediate blood group. Another example is tongue rolling, you either can or cannot roll your tongue.

Continuous variation describes variation in which many alleles have small effects on the phenotype. There is no clear separation into distinctive categories, as measurements are on a continuous scale. An example of continuous traits as height and IQ.

Read more about Genetic Inheritance

The effect of variation

With changing allele frequencies come changing genotype and phenotype frequencies. Different genotypes can be associated (subtly or dramatically) with positive or negative effects on fitness in that environment. Therefore, some of these phenotypes/genotypes are favoured, and these individuals can reach reproductive age and pass on their genetic information, biasing the population towards a specific phenotype, and altering allele frequencies in the gene pool.

Further reading and references:

[1]. https://www.sciencelearn.org.nz/resources/208-meiosis-inheritance-and-variation Image meiosis

[2]. https://thenaturalhistorian.com/2016/03/30/the-great-genetic-bottleneck-that-contradicts-ken-hams-radical-accelerated-diversification-ie-post-flood-hyper-evolution/ Image bottleneck

[3]. https://www.nature.com/scitable/topicpage/the-genetic-variation-in-a-population-is-6526354

[4]. https://evolution.berkeley.edu/evolibrary/article/evo_17

[5]. https://www.yourgenome.org/facts/what-is-genetic-variation

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Source: https://alevelbiology.co.uk/notes/variation/

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