Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics. In humans, eye color is an example of an inherited characteristic: an individual might inherit the "brown-eye trait" from one of the parents. The complete set of observable traits of the structure and behavior of an organism is called its phenotype. These traits arise from the interaction of its genotype with the environment. For example, suntanned skin comes from the interaction between a person's phenotype and sunlight;  thus, suntans are not passed on to people's children.
However, some people tan more easily than others, due to differences in their genotype:  a striking example is people with the inherited trait of albinism , who do not tan at all and are very sensitive to sunburn. Heritable traits are known to be passed from one generation to the next via DNA , a molecule that encodes genetic information. The sequence of bases along a particular DNA molecule specifies the genetic information: this is comparable to a sequence of letters spelling out a passage of text.
A portion of a DNA molecule that specifies a single functional unit is called a gene ; different genes have different sequences of bases. Within cells , the long strands of DNA form condensed structures called chromosomes. Organisms inherit genetic material from their parents in the form of homologous chromosomes , containing a unique combination of DNA sequences that code for genes. The specific location of a DNA sequence within a chromosome is known as a locus. If the DNA sequence at a particular locus varies between individuals, the different forms of this sequence are called alleles.
DNA sequences can change through mutations , producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism.
However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes within and among organisms. Recent findings have confirmed important examples of heritable changes that cannot be explained by direct agency of the DNA molecule.
These phenomena are classed as epigenetic inheritance systems that are causally or independently evolving over genes. Research into modes and mechanisms of epigenetic inheritance is still in its scientific infancy, however, this area of research has attracted much recent activity as it broadens the scope of heritability and evolutionary biology in general. For example, ecological inheritance through the process of niche construction is defined by the regular and repeated activities of organisms in their environment.
This generates a legacy of effect that modifies and feeds back into the selection regime of subsequent generations. Descendants inherit genes plus environmental characteristics generated by the ecological actions of ancestors.
When Charles Darwin proposed his theory of evolution in , one of its major problems was the lack of an underlying mechanism for heredity. Blending inheritance would lead to uniformity across populations in only a few generations and then would remove variation from a population on which natural selection could act. Darwin's initial model of heredity was adopted by, and then heavily modified by, his cousin Francis Galton , who laid the framework for the biometric school of heredity. The inheritance of acquired traits was shown to have little basis in the s when August Weismann cut the tails off many generations of mice and found that their offspring continued to develop tails.
Scientists in Antiquity had a variety of ideas about heredity: Theophrastus proposed that male flowers caused female flowers to ripen;  Hippocrates speculated that "seeds" were produced by various body parts and transmitted to offspring at the time of conception;  and Aristotle thought that male and female fluids mixed at conception.
Ancient understandings of heredity transitioned to two debated doctrines in the 18th century. The Doctrine of Epigenesis and the Doctrine of Preformation were two distinct views of the understanding of heredity. The Doctrine of Epigenesis, originated by Aristotle , claimed that an embryo continually develops.
The foundation of this doctrine was based on the theory of inheritance of acquired traits. In direct opposition, the Doctrine of Preformation claimed that "like generates like" where the germ would evolve to yield offspring similar to the parents. The Preformationist view believed procreation was an act of revealing what had been created long before.
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However, this was disputed by the creation of the cell theory in the 19th century, where the fundamental unit of life is the cell, and not some preformed parts of an organism. Various hereditary mechanisms, including blending inheritance were also envisaged without being properly tested or quantified, and were later disputed. Nevertheless, people were able to develop domestic breeds of animals as well as crops through artificial selection.
The inheritance of acquired traits also formed a part of early Lamarckian ideas on evolution. During the 18th century, Dutch microscopist Antonie van Leeuwenhoek — discovered "animalcules" in the sperm of humans and other animals. These scientists formed a school of thought known as the "spermists".
They contended the only contributions of the female to the next generation were the womb in which the homunculus grew, and prenatal influences of the womb. Ovists thought women carried eggs containing boy and girl children, and that the gender of the offspring was determined well before conception. The idea of particulate inheritance of genes can be attributed to the Moravian  monk Gregor Mendel who published his work on pea plants in However, his work was not widely known and was rediscovered in Fisher 's paper, " The Correlation Between Relatives on the Supposition of Mendelian Inheritance " Mendel's overall contribution gave scientists a useful overview that traits were inheritable.
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His pea plant demonstration became the foundation of the study of Mendelian Traits. These traits can be traced on a single locus. In the s, work by Fisher and others resulted in a combination of Mendelian and biometric schools into the modern evolutionary synthesis. The modern synthesis bridged the gap between experimental geneticists and naturalists; and between both and palaeontologists, stating that:  .
The idea that speciation occurs after populations are reproductively isolated has been much debated.
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