Chromosome numbers vary enormously among species. Humans have 46 chromosomes in most body cells, dogs have 78, fruit flies have 8, and some plants have hundreds. Yet chromosome number does not neatly predict how complex an organism is. A species with many chromosomes is not necessarily more complex than one with fewer.
The reason is that chromosome number is mainly a consequence of how genomes have been reorganized and inherited over evolutionary time. Chromosomes can fuse, split, duplicate, or disappear without necessarily changing the total amount of genetic information very much. In other cases, whole genomes can be duplicated, producing organisms with many more chromosomes and copies of their genes.
Understanding chromosome number therefore requires separating two ideas that are often confused: how much DNA an organism has and how that DNA is packaged into chromosomes.
What is a chromosome?
A chromosome is a long DNA molecule packaged with proteins. DNA contains genes and other sequences that help control how cells function and develop. Packaging the DNA into chromosomes allows the cell to organize, copy, and distribute its genetic material.
In species that reproduce sexually, chromosomes usually occur in pairs in most body cells. One member of each pair comes from the mother and the other from the father. These are called homologous chromosomes because they carry corresponding sets of genes, although the particular versions of those genes can differ.
Humans, for example, normally have 23 pairs of chromosomes, for a total of 46. Twenty-two pairs are called autosomes, while the 23rd pair consists of the sex chromosomes.
The number scientists usually report for a species is its diploid chromosome number, meaning the number found in its ordinary body cells. Sex cells such as eggs and sperm are haploid: they contain one set of chromosomes rather than two.
Chromosome number is not the same as genome size
A crucial point is that chromosome count tells us relatively little by itself about the amount of DNA in an organism.
Imagine a genome containing 100 units of DNA. Evolutionary changes could package those units into 10 chromosomes, 20 chromosomes, or another number without necessarily changing the total amount of DNA.
This is possible because chromosomes are not fixed biological units in the way that individual genes often are. During evolution, chromosomes can undergo structural rearrangements.
Two chromosomes can fuse into one, reducing the chromosome count. Conversely, one chromosome can split into two, increasing the count. Large sections of chromosomes can also be rearranged, duplicated, deleted, or moved.
As a result, two closely related species can have different chromosome numbers even when their genomes contain broadly similar sets of genes.
A human example: chromosome fusion
Human chromosome 2 provides a well-known example of how chromosome structure can change during evolution. Humans have 46 chromosomes, whereas our closest living great-ape relatives have 48.
The difference is associated with the fusion of two ancestral ape chromosomes into the chromosome 2 found in humans. In other words, a change in chromosome number did not require humans to acquire an entirely new set of genes. Instead, two ancestral chromosomes became joined into one.
This illustrates an important evolutionary principle: chromosome number can change through changes in genome organization rather than through a proportional gain or loss of genetic information.
How chromosome numbers change
Several evolutionary processes can alter chromosome number.
Chromosome fusion and fission
A chromosome can become joined to another chromosome, a process known as fusion. If the resulting chromosome is stable and the organism can reproduce successfully, the new chromosome arrangement can persist and spread through a population.
The reverse can also happen. A chromosome may become divided into two separate chromosomes, called fission.
These changes can alter chromosome number without necessarily adding or removing large amounts of DNA.
Chromosome rearrangements can sometimes affect fertility, however. If individuals with different chromosome arrangements mate, their chromosomes may have difficulty pairing correctly during meiosis, the specialized cell division that produces eggs and sperm. Whether such a rearrangement becomes established therefore depends partly on its effects on reproduction and survival.
Whole-genome duplication
Another major route to different chromosome numbers is polyploidy, in which an organism has more than two complete sets of chromosomes.
Whole-genome duplication can occur when cell division or the formation of reproductive cells goes wrong, producing cells with extra chromosome sets. If such an event occurs in an organism’s evolutionary history and the resulting lineage survives and reproduces, the additional genome copies can become a permanent feature of the species.
Polyploidy is particularly important in plants. Some plant species have multiple copies of their entire genomes, and genome duplication has played a major role in plant evolution.
Because an entire set of chromosomes is duplicated at once, polyploidy can produce a dramatic increase in chromosome number and DNA content.
Why doesn’t natural selection force all species to have the same chromosome number?
There is no single chromosome number that is inherently optimal for all organisms.
Evolution does not generally select for a particular chromosome count simply because the number itself is high or low. What matters is how genetic and chromosomal changes affect an organism’s ability to survive and reproduce.
A chromosome fusion that has little harmful effect can persist. A rearrangement that improves reproductive success under particular circumstances may become more common. A change that severely disrupts gene function or chromosome segregation is much less likely to become established.
Chromosome number can therefore drift as genomes change, while natural selection acts on the consequences of those changes.
In some cases, chromosome rearrangements may contribute to the formation of new species. If populations accumulate different chromosome structures, individuals from the two populations may have reduced fertility when they interbreed. Chromosomal differences can thus become one factor helping populations remain reproductively separate.
Why don’t extra chromosomes automatically make an organism more complex?
Because chromosomes are containers for genetic information, not independent measures of biological sophistication.
One chromosome can contain many genes, while another can contain relatively few. Chromosome counts also do not tell us how many genes an organism has, how those genes are regulated, or how genes interact with one another.
Biological complexity depends on many features of a genome and its regulation, including gene content, gene expression, regulatory DNA, alternative processing of genetic information, interactions among genes, and the development and physiology of the organism.
This is why chromosome number is a poor measure of complexity. A species can have twice as many chromosomes as another without having twice as many genes or being twice as complex.
What happens when chromosome number changes within a species?
Chromosome number is usually remarkably consistent within a species, but exceptions occur.
A change involving an individual chromosome can result from errors during cell division. Nondisjunction, for example, occurs when chromosomes fail to separate properly during meiosis or cell division. The resulting cells can have an abnormal number of chromosomes.
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In humans, an extra copy of chromosome 21 causes Down syndrome, also called trisomy 21. Here, the issue is not that humans as a species normally have a different chromosome number, but that particular cells or an individual’s cells contain an additional chromosome.
Changes involving entire chromosome sets are different. An individual may sometimes inherit or develop an additional complete set of chromosomes, producing a polyploid organism. Such events are much more common and evolutionarily significant in many plants than in humans.
Does chromosome number determine whether species can interbreed?
Not by itself.
Species with different chromosome numbers can sometimes produce offspring, and species with the same chromosome number are not necessarily closely related. What matters is whether their chromosomes and genomes can function together during reproduction and development.
For two individuals with different chromosome numbers, successful reproduction can become difficult if their chromosomes cannot pair and segregate properly during meiosis. But chromosome number alone does not tell us whether this will happen.
Two species could have the same number of chromosomes but substantial genetic and chromosomal differences. Conversely, closely related organisms can sometimes differ in chromosome number because of a relatively small number of chromosome fusions or splits.
Why are chromosome numbers so stable once established?
Although chromosome number can change over evolutionary time, it cannot change casually in every generation without consequences.
During meiosis, homologous chromosomes must pair and then separate in an orderly way so that reproductive cells receive the appropriate chromosome complement. Large chromosomal differences can interfere with this process.
That creates a form of evolutionary constraint. A chromosome rearrangement may arise, but for it to become common in a population, individuals carrying it generally must remain capable of producing viable offspring.
Once a particular chromosome arrangement becomes widespread, it is inherited through generations and can become characteristic of a species.
This helps explain why chromosome numbers can remain stable for long periods while still differing substantially among related lineages.
The deeper evolutionary picture
Differences in chromosome number are best understood as differences in the history of genome organization.
One lineage may retain an ancestral chromosome arrangement. Another may experience chromosome fusion. A third may undergo chromosome splitting. Yet another may experience duplication of an entire genome. Over many generations, these changes can produce species with strikingly different chromosome counts.
The number itself is therefore not the main story. What matters is what happened to the genome to produce that number.
A species with more chromosomes may simply have its DNA divided into more packages. A species with fewer chromosomes may have chromosomes that contain larger amounts of DNA. And a species with dramatically more chromosomes may have gained them through whole-genome duplication.
Chromosome number is consequently a useful clue to evolutionary history, but it is not a simple score of genetic complexity, intelligence, or biological advancement. To understand what makes one species different from another, scientists have to look beyond the chromosome count to the DNA those chromosomes contain, how that DNA is organized, and how genes are regulated and used.
