Question

Evolution is a change in the frequency of heritable traits in a population over time.  Defined more narrowly, it is the change in allele frequencies over time.   The field of population genetics is concerned with alleles that exist within populations and how the proportions of different alleles change over time.

Charles Darwin’s major contribution to our understanding of the natural world was to recognize that organisms within a population vary in heritable traits that affect their ability to survive and reproduce.  This process is known as natural selection.  Although the fact that species evolve was accepted by many in Darwin’s time (the mid-1800s), natural selection was the first mechanism (or driving force) of evolution to stand up to extensive testing.  Today we know that genes are the basis of heredity, so we can rephrase natural selection in terms of genetics.  Because some alleles increase or decrease an organism’s ability to survive and reproduce relative to other alleles, some alleles are passed on at greater frequency than others.  As the population's allele frequencies change, so do it's phenotypic frequencies, making the population better adapted overall to the current environmental conditions.

We also now know that natural selection is only one of several mechanisms of evolutionary change. Today, we recognize the following four mechanisms of evolution.

1. Natural selection – the interaction between heritable, phenotypic traits and the environment that leads to differential reproduction among individuals in a population.  Although other mechanisms can cause changes in heritable traits over time, natural selection is the onlymechanism of evolution that shapes, or adapts, populations to better fit their environment.
2. Genetic drift – change in gene frequencies in a population due to chance or random events.  Genetic drift tends to eliminate genetic variation, especially in small populations where chance events can have large overall effects.
3. Gene flow – change in gene frequencies in a population due to the immigration or emigration of individuals
4. Mutation – change to the DNA of a gene.  Mutation occurs randomly throughout the genome.  It is the ultimate source of all genetic variation.  In a large population, a single mutation would constitute a very small change in allele frequency in a population, but a change nonetheless. Once a mutation occurs, its frequency may increase or decrease in the population due to the three mechanisms listed above.

Genetic variation is the raw material of evolution. Genetic variation within an individual (having more than one allele at a locus, or heterozygosity), can benefit organisms in a number of ways.  For example, if an individual is heterozygous and produces two different versions of an enzyme, these two different versions may function well under slightly different conditions (e.g. temperature).  Having both versions therefore allows a heterozygote to tolerate greater variation of that condition.  Genetic variation within populations may allow the population to adapt to future environmental conditions.  Even alleles that are mildly deleterious in today’s environment might prove beneficial if the environment changes.  It is important to remember, however, that whether an allele increases or decreases in frequency depends on whether it is beneficial or harmful in the current environment.  An allele that will prove to be beneficial in 1,000 years will decrease in frequency if it is harmful now.  Natural selection can't plan ahead.

Which of the following can cause changes in allele frequencies (evolution) in a population? Choose all that apply.

		inbreeding
		genetic drift
		mutation
		natural selection
		gene flow

Answer 1

Allelic frequency :-Alleles two contrasting forms of a gene. Allelic frequency is the frequency e of an allele on genetic locus in a population it is usually expressed in percentage & fraction. Allelic frequency indicates the the species richness and genetic diversity of the population.

Frequency of an allele :- Occurrence of the particular allele/ total number of alleles found in the population.

Factors affecting allelic frequency in a population are:-

1:-Mutation :- ​

These are sudden inheritable changes in the genetic material of the organism. They are random and occur in all the direction. The rate of genetic mutations is very low. Some of the mutations are beneficial some are neutral whereas others are having deleterious effect.

Basically based upon their origin, mutations are of two types induced mutations and spontaneous mutation. Mutation leads to variation and diversity within a population it also introduces new gene and allele in the population which can be beneficial for the adaptation of the population.

2. Genetic drift

Genetic drift is a random process that changes the frequency of an allele in the population over a period of time. It leads to large changes during short period of time.

​​​3-Gene Flow

The gene flow of a population is affected by immigration and emigration. Immigration within in a population results in addition an occurrence of new alleles into the gene pool this ultimately changes the allele frequency of the population. The degree of change depends upon how much difference in the genotypes of immigrants and original native population.

Whereas the migration of individuals from the population leads to the removal of alleles from the gene pool.

4-Natural selection

Natural selection is the phenomena in which a particular genotype is selected and favoured by the nature over others in the population. The mechanism involves selection of the individual who is best adapted to the environment and have I reproductive fitness then compared to the rest of the population so show the selected individual reproduces more oppo and contribute greater fraction of genes to the gene pool of the next generation.

Natural selection is of of three different types

• Stabilizing selection
• Directional selection
• Disruptive selection

​​​​​​​Inbreeding involves self fertilization or breeding among closely related individuals. It increases the homozygosity and reduces heterozygote in the given population. Since there is no random mating so it does not affect the allelic frequency of the population.