2.4.1: Evolution of Genomes

Unit 2.4.1 - Evolution of Genomes

• Please read and watch the following Learning Resources.
• Reading the material for understanding, and taking notes during videos, will take approximately 1 hour.
• Bolded terms are located at the end of the unit in the Glossary. There is also a Unit Summary at the end of the Unit.
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Learning Objectives

• Identify how variations in the size and number of genes in different species can impact evolution;
• Explain the importance of genomic changes in an evolutionary context;
• Recognize the evolutionary implications of observed genome similarities between distant species.

Introduction

Video

This 9-minute video provides an overview of how the genes and genome of a species can change over time to lead to different species.
Questions after watching: How can a gene duplication event fuel evolution? Can you think of another example where something was duplicated and then repurposed by evolution?

Genetic Diversity

Genetic variation is essential for a species to evolve in response to environmental change, including any variation in nucleotides, genes, chromosomes, or genomes of organisms. Each of these levels of genetic diversity adds to the capacity of a population to evolve.

Variations in Number of Genes

DNA is contained in the chromosomes present within the cell; some chromosomes are contained within specific organelles in the cell (for example, the chromosomes of mitochondria and chloroplast). A gene is a discrete section of a chromosome that codes for one or more proteins. Each gene is a hereditary section of DNA that occupies a specific place on the chromosome and controls a particular characteristic of an organism. Genetic diversity at its most elementary level is represented by differences in the sequence of nucleotides (with one of the following bases: adenine, cytosine, guanine) that form DNA (deoxyribonucleic acid) within the cells of the organism. During sexual reproduction, offspring inherit alleles from both parents. These inherited alleles might be slightly different, especially if there has been migration or hybridization of organisms. Also, when the offspring’s chromosomes are copied after fertilization, genes can be exchanged in a process called sexual recombination. Helpful or silent mutations and sexual recombination may allow the evolution of new characteristics.

Sometimes whole genes are duplicated, or deleted. This results in an organism having more or fewer copies of that gene than others. The effect on the phenotype may be to increase or decrease the gene product’s function (e.g., if the gene encoded a protein that makes pigment, and now the organism has two genes encoding pigment, the organism may now be twice as pigmented as its parents with only one copy of the gene). Natural selection may then select for or against organisms with this altered number of gene copies.

Sometimes there is no immediate effect of the gene duplication on phenotype. The additional copy of the gene may accumulate mutations without deleterious effects on the organism. These mutations will likely modify the gene product’s function if it is transcribed. In doing this, the gene may acquire slightly different abilities, such as the factor X blood clotting example the video above.

Variations in Number of Chromosomes

Many organisms are diploid, having two sets of chromosomes, and therefore two copies (called alleles) of each gene. However, some organisms can be haploid, triploid, or tetraploid (having one, three, or four sets of chromosomes respectively). Within any single organism, there may be variation between the two (or more) alleles for each gene. This variation is introduced either through mutation of one of the alleles or as a result of recombination during sexual reproduction.

During replication events, there can be mutations that involve whole chromosomes or whole chunks of chromosomes. Some mutations duplicate a whole chromosome (a whole stretch of DNA with many genes); some may delete a large portion. Some rearrange chromosomes, keeping all of the DNA information but putting it in different places in the genome. It turns out that in some instances, the location matters because it contains sites that initiate and control transcription. In bacteria, this can matter because one transcription factor can control the transcription of many genes in a row.

Changes in chromosome number or structure can affect many genes at the same time. Like changes in individual genes, this can result in some immediate effects because of the change in the number of genes producing the gene products (proteins). Or there can be little to no immediate effect, but the extra copy gives rise to “spare copies” that are free to mutate and acquire new abilities and functions.

Variation in Genome Size

A genome is the total genetic information of a cell or organism. The evolution of the genome is characterized by the accumulation of changes. Analysis of genomes and their changes in sequence or size over time involves various fields.

Genome size is usually measured in base pairs (or bases in single-stranded DNA or RNA). Different species can have different numbers of genes within the entire DNA or genome of the organism. Gene number is the main factor influencing the size of the prokaryotic genome. However, a greater total number of genes might not correspond with greater complexity in the phenotype (behavior, structure, or function) of eukaryotes.

For example, the predicted size of the human genome is not much larger than the genomes of some invertebrates and is far smaller than some species of ferns. However, in humans, more proteins are encoded per gene than in other species. In eukaryotic organisms, this is an observed paradox: the number of genes that make up the genome does not correlate with genome size. In other words, the genome size is much larger than would be expected given the total number of protein-coding genes.

Larger amounts of genetic information may allow for more variation in the overall genome, conferring additional adaptability to environmental change. It may also play a role in speciation. On the other hand, longer genomes take more energy and time to duplicate, so there is an evolutionary trade-off.

Video

In this 8-minute video, evidence of human evolution in the human genome is demonstrated.
Question after watching: What are three lines of evidence from the human genome of our evolutionary past?