Heredity and evolution
- The transmission of genetic characters from parents to offspring: it is dependent upon the segregation and recombination of
genes during meiosis and fertilization and results in the genesis of a new individual similar to others of its kind but
exhibiting certain variations resulting from the particular mix of genes and their interactions with the environment.
- The passing of characteristics from parents to children.
Heredity, the sum of all biological processes by which particular characteristics are transmitted from parents to their
offspring. The concept of heredity encompasses two seemingly paradoxical observations about organisms: the constancy of a
species from generation to generation and the variation among individuals within a species. Constancy and variation are
actually two sides of the same coin, as becomes clear in the study of genetics
. Both aspects of heredity can be explained
by genes, the functional units of heritable material that are found within all living cells.
The set of genes that an offspring inherits from both parents, a combination of the genetic material of each, is called the
organism’s genotype. The genotype is contrasted to the phenotype, which is the organism’s outward appearance and the
developmental outcome of its genes. The phenotype includes an organism’s bodily structures, physiological processes, and
behaviours. Although the genotype determines the broad limits of the features an organism can develop, the features that
actually develop, i.e., the phenotype, depend on complex interactions between genes and their environment. The genotype
remains constant throughout an organism’s lifetime; however, because the organism’s internal and external environments
change continuously, so does its phenotype. In conducting genetic studies, it is crucial to discover the degree to which
the observable trait is attributable to the pattern of genes in the cells and to what extent it arises from environmental
Basic features of heredity
Prescientific conceptions of heredity
Heredity was for a long time one of the most puzzling and mysterious phenomena of nature. This was so because the sex
cells, which form the bridge across which heredity must pass between the generations, are usually invisible to the naked
eye. Only after the invention of the microscope early in the 17th century and the subsequent discovery of the sex cells
could the essentials of heredity be grasped. Before that time, ancient Greek philosopher and scientist Aristotle
(4th century bc) speculated that the relative contributions of the female and the male parents were very unequal;
the female was thought to supply what he called the “matter” and the male the “motion.” The Institutes of Manu,
composed in India between 100 and 300 ad, consider the role of the female like that of the field and of the male
like that of the seed; new bodies are formed “by the united operation of the seed and the field.” In reality both
parents transmit the heredity pattern equally, and, on average, children resemble their mothers as much as they do
their fathers. Nevertheless, the female and male sex cells may be very different in size and structure; the mass of
an egg cell is sometimes millions of times greater than that of a spermatozoon.
- Evolution is a process of continuous branching and diversification from common trunks. This pattern of irreversible separation gives life's history its basic
- Evolution is change in the heritable characteristics of biological populations over successive generations.Evolutionary processes give rise to biodiversity at every
level of biological organisation, including the levels of species, individual organisms, and molecules.
Repeated formation of new species (speciation), change within species (anagenesis), and loss of species (extinction) throughout the evolutionary history of life on
Earth are demonstrated by shared sets of morphological and biochemical traits, including shared DNA sequences. These shared traits are more similar among species
that share a more recent common ancestor, and can be used to reconstruct a biological "tree of life" based on evolutionary relationships (phylogenetics), using both
existing species and fossils. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils to fossilised multicellular
organisms. Existing patterns of biodiversity have been shaped both by speciation and by extinction.
History of evolutionary thought
- In 1842, Charles Darwin penned his first sketch of On the Origin of Species.
Main article: History of evolutionary thought
The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and
Empedocles. Such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura (On the Nature of
In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part
of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the
standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond
one-for-one with exact metaphysical forms and specifically gave examples of how new types of living things could come to be.
- Main article: Genetic variation
Further information: Genetic diversity and Population genetics
An individual organism's phenotype results from both its genotype and the influence from the environment it has lived in. A substantial part of the phenotypic variation
in a population is caused by genotypic variation. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The
frequency of one particular allele will become more or less prevalent relative to other forms of that gene. Variation disappears when a new allele reaches the point
of fixation—when it either disappears from the population or replaces the ancestral allele entirely.
Natural selection will only cause evolution if there is enough genetic variation in a population. Before the discovery of Mendelian genetics, one common hypothesis was
blending inheritance. But with blending inheritance, genetic variance would be rapidly lost, making evolution by natural selection implausible. The Hardy–Weinberg
principle provides the solution to how variation is maintained in a population with Mendelian inheritance. The frequencies of alleles (variations in a gene) will
remain constant in the absence of selection, mutation, migration and genetic drift.
Main article: Mutation
Mutations are changes in the DNA sequence of a cell's genome. When mutations occur, they may alter the product of a gene, or prevent the gene from functioning, or have
no effect. Based on studies in the fly Drosophila melanogaster, it has been suggested that if a mutation changes a protein produced by a gene, this will probably be
harmful, with about 70% of these mutations having damaging effects, and the remainder being either neutral or weakly beneficial.
- Mutations can involve large sections of a chromosome becoming duplicated (usually by genetic recombination), which can introduce extra copies of a gene into a genome.
Extra copies of genes are a major source of the raw material needed for new genes to evolve. This is important because most new genes evolve within gene families from
pre-existing genes that share common ancestors. For example, the human eye uses four genes to make structures that sense light: three for colour vision and one for
night vision; all four are descended from a single ancestral gene.
- New genes can be generated from an ancestral gene when a duplicate copy mutates and acquires a new function. This process is easier once a gene has been duplicated
because it increases the redundancy of the system; one gene in the pair can acquire a new function while the other copy continues to perform its original function.
Other types of mutations can even generate entirely new genes from previously noncoding DNA.
Sex and recombination
- Further information: Sexual reproduction, Genetic recombination, and Evolution of sexual reproduction
In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes of other organisms during reproduction. In contrast, the offspring of
sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In a related process called homologous
recombination, sexual organisms exchange DNA between two matching chromosomes. Recombination and reassortment do not alter allele frequencies, but instead change
which alleles are associated with each other, producing offspring with new combinations of alleles. Sex usually increases genetic variation and may increase the
rate of evolution.
The two-fold cost of sex was first described by John Maynard Smith. The first cost is that in sexually dimorphic species only one of the two sexes can bear young.
(This cost does not apply to hermaphroditic species, like most plants and many invertebrates.) The second cost is that any individual who reproduces sexually can only
pass on 50% of its genes to any individual offspring, with even less passed on as each new generation passes. Yet sexual reproduction is the more common means of
reproduction among eukaryotes and multicellular organisms. The Red Queen hypothesis has been used to explain the significance of sexual reproduction as a means to
enable continual evolution and adaptation in response to coevolution with other species in an ever-changing environment.
- Further information: Gene flow
- Gene flow is the exchange of genes between populations and between species. It can therefore be a source of variation that is new to a population or to a species. Gene
flow can be caused by the movement of individuals between separate populations of organisms, as might be caused by the movement of mice between inland and coastal
populations, or the movement of pollen between heavy metal tolerant and heavy metal sensitive populations of grasses.
Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer. Horizontal gene transfer is the transfer of genetic material from
one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance,
as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to eukaryotes such as
the yeast Saccharomyces cerevisiae and the adzuki bean weevil Callosobruchus chinensis has occurred. An example of larger-scale transfers are the eukaryotic
bdelloid rotifers, which have received a range of genes from bacteria, fungi and plants. Viruses can also carry DNA between organisms, allowing transfer of genes
even across biological domains.
- Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and bacteria, during the acquisition of chloroplasts and mitochondria. It is
possible that eukaryotes themselves originated from horizontal gene transfers between bacteria and archaea.
Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of
natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to
selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new
species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.
These outcomes of evolution are distinguished based on time scale as macroevolution versus microevolution. Macroevolution refers to evolution that occurs at or above
the level of species, in particular speciation and extinction; whereas microevolution refers to smaller evolutionary changes within a species or population, in
particular shifts in gene frequency and adaptation. In general, macroevolution is regarded as the outcome of long periods of microevolution. Thus, the
distinction between micro- and macroevolution is not a fundamental one—the difference is simply the time involved. However, in macroevolution, the traits of
the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the
chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated.
In this sense, microevolution and macroevolution might involve selection at different levels—with microevolution acting on genes and organisms, versus
macroevolutionary processes such as species selection acting on entire species and affecting their rates of speciation and extinction.