Inherited Variation Evolution and Natural Selection

Key Concepts

1. Genes,Genome

2. Genetic variation

3. Natural selection

4. Mutation

5. Crossing over

6. Mitosis

Introduction

The transmission of characters from parents to offspring’s is called heredity. Variations are the difference in DNA sequence or genes between individuals. The offspring of the same parent resemble each other, but they may vary in size, color, and several other characters.  

Genetic variation is the term used to describe variation in the DNA sequence in each of the genomes. Genetic variation makes us unique in terms of hair color, skin color or even the shape of our faces. 

Explanation

Natural Selection – It is an important mechanism of evolution. In this process organisms that are able to adapt to the changing environmental conditions survive and those that are unable to are naturally eliminated. The genes which are responsible for the survival of the organisms are passed from one generation to another, i.e., from parent to offspring.  

For example – The giraffes had shorter necks. Their feeding habits led to the elongation of their necks, and the genes of such animals were transferred to the future generations. 

Theory of Natural Selection  

Darwin and Wallace worked on natural selection. The adjustment of genes through the generations based on factors which help organisms survive. It is known as survival of the fittest or the adaptation of organisms. 

Process of Natural Selection  

• Inherited Variation  
• Competition  
• Selection  
• Adaptations  
• Evolution 

Main Source of Genetic Variation   

• Mutations – Change in the genetic composition of gametes. 
•  Meiosis – Crossing over (prophase I) or independent assortment (metaphase I). 

• Sexual reproduction –Combination of genetic material by two distinct sources creates a new gene.  

Mutation  

Mutation is the change in the DNA base pair which is caused by various environmental factors like UV light, or mistakes during DNA replication.”  

A gene mutation is the change in nucleotide sequence of a DNA coding for a trait. New alleles are formed by mutation.  

Crossing over (prophase I) 

During prophase I of meiosis, homologous chromosomes align in a process called tetrad. The parts of non-sister chromatids form a chiasma. Here the parts of homologous chromosomes can be exchanged. 

There is an equal chance of crossing-over at any point along the chromosome. If two genes are very far on a chromosome, it is likely that crossing-over will occur somewhere between them. 

After the crossing-over occurs, the homologous chromosomes separate to form two daughter cells. The cells go through meiosis II, during which sister chromatids separate. At the end there are four gametes. Two of them are called parental as they contain the same alleles as one of the parents. These two are called recombinants because they contain unique allele combinations which are not found in either of the parental chromosomes.

Crossing-over is an important contributor to genetic diversity as it leads to the creation of new combinations of DNA sequences. 

Random Assortment of Chromosomes (metaphase I) 

During metaphase I, homologous chromosomes are arranged in the center of the cell facing opposite poles. The homologous pairs orient randomly at the equator. For example, if two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This determines the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. This is called independent assortment. 

Random fusion of gametes from different parents 

When male gamete and a female gamete meet, each result in immense number of genetic possibilities created during independent assortment and crossing over. Human diploid cells have 23 pairs of chromosomes. When independent assortment occurs during meiosis I, there are 223 or 8.4 million possible gametes that are created if no crossing overtakes place. In humans, about 2 to 3 crossovers per chromosome occur, and different crossovers occur in each meiotic division. 

Summary

• Variation exists among individuals of one species. Variation is due to crossing over, recombination, mutation, and environmental effects on the expression of genes present on chromosomes.

• Natural selection is a mechanism of evolution. In this the organisms that are able to reproduce and survive the changing environmental conditions are selected by nature.

• The giraffes had shorter necks. Their feeding habits led to the elongation of their necks and the genes from such animals were transferred to future generations.

• The theory of natural selection was proposed by Charles Darwin.

• Mutations – Changing the genetic composition of gametes (germline mutation) leads to changed characteristics in offspring.

• Meiosis – Either crossing over (prophase I) or independent assortment (metaphase I).

• Sexual reproduction leads to many possibilities in genetic variations. Sexual reproduction results in offspring that are genetically unique. They are different from parent

• The homologous chromosomes pair during prophase 1 of meiosis., Crossing over also occurs during this phase. Crossing over is exchange of genetic material and it results in new combination of genes.

• During meiotic division homologous chromosomes are distributed to daughter cells, and different chromosomes segregate independently of each other. This is called independent assortment.

• In sexual reproduction, two gametes produce an offspring. It is known as random fertiltudion.