Ontogeny in the context of Fertilization

Natural selection is the differential survival and reproduction of individuals due to differences in the relative fitness endowed on them by their own particular complement of observable characteristics. It is a key law or mechanism of evolution which changes the heritable traits characteristic of a population or species over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.

For Darwin natural selection was a law or principle which resulted from three different kinds of process: inheritance, including the transmission of heritable material from parent to offspring and its development (ontogeny) in the offspring; variation, which partly resulted from an organism's own agency (see phenotype; Baldwin effect); and the struggle for existence, which included both competition between organisms and cooperation or 'mutual aid' (particularly in 'social' plants and social animals).

In biology, a biological life cycle (or just life cycle when the biological context is clear) is a series of stages of the life of an organism, that begins as a zygote, often in an egg, and concludes as an adult that reproduces, producing an offspring in the form of a new zygote which then itself goes through the same series of stages, the process repeating in a cyclic fashion. In humans, the concept of a single generation is a cohort of people who, on average, are born around the same period of time, it is related though distinct from the biological concept of generations.

"The concept is closely related to those of the life history, development and ontogeny, but differs from them in stressing renewal." Transitions of form may involve growth, asexual reproduction, or sexual reproduction.

Behavioral ecology, also spelled behavioural ecology, is the study of the evolutionary basis for animal behavior due to ecological pressures. Behavioral ecology emerged from ethology after Niko Tinbergen outlined four questions to address when studying animal behaviors: what are the proximate causes, ontogeny, survival value, and phylogeny of a behavior?

If an organism has a trait that provides a selective advantage (i.e., has adaptive significance) in its environment, then natural selection favors it. Adaptive significance refers to the expression of a trait that affects fitness, measured by an individual's reproductive success. Adaptive traits are those that produce more copies of the individual's genes in future generations. Maladaptive traits are those that leave fewer. For example, if a bird that can call more loudly attracts more mates, then a loud call is an adaptive trait for that species because a louder bird mates more frequently than less loud birds—thus sending more loud-calling genes into future generations. Conversely, loud calling birds may attract the attention of predators more often, decreasing their presence in the gene pool.

Ernst Heinrich Philipp August Haeckel (/ˈhɛkəl/; German: [ɛʁnst ˈhɛkl̩]; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new species, mapped a genealogical tree relating all life forms and coined many terms in biology, including ecology, phylum, phylogeny, ontogeny, and Protista. Haeckel promoted and popularised Charles Darwin's work in Germany and developed the disproven but influential recapitulation theory ("ontogeny recapitulates phylogeny"), wrongly claiming that an individual organism's biological development, or ontogeny, parallels and summarizes its species' evolutionary development, or phylogeny, using incorrectly drawn images of human embryonic development. Whether they were intentionally falsified, or drawn poorly by accident is a matter of debate.

The published artwork of Haeckel includes over 100 detailed, multi-colour illustrations of animals and sea creatures, collected in his Kunstformen der Natur ("Art Forms of Nature"), a book which would go on to influence the Art Nouveau artistic movement. As a philosopher, Ernst Haeckel wrote Die Welträthsel (1895–1899; in English: The Riddles of the Universe, 1900), the genesis for the term "world riddle" (Welträtsel); and Freedom in Science and Teaching to support teaching evolution.

A mouth also referred to as the oral is the body orifice through which many animals ingest food and vocalize. The body cavity immediately behind the mouth opening, known as the oral cavity (or cavum oris in Latin), is also the first part of the alimentary canal, which leads to the pharynx and the gullet. In tetrapod vertebrates, the mouth is bounded on the outside by the lips and cheeks — thus the oral cavity is also known as the buccal cavity (from Latin bucca, meaning "cheek") — and contains the tongue on the inside. Except for some groups like birds and lissamphibians, vertebrates usually have teeth in their mouths, although some fish species have pharyngeal teeth instead of oral teeth.

Most bilaterian phyla, including arthropods, molluscs and chordates, have a two-opening gut tube with a mouth at one end and an anus at the other. Which end forms first in ontogeny is a criterion used to classify bilaterian animals into protostomes and deuterostomes.

The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive adult stages in the evolution of the animal's remote ancestors (phylogeny). It was formulated in the 1820s by Étienne Serres based on the work of Johann Friedrich Meckel, after whom it is also known as the Meckel–Serres law.

Since embryos also evolve in different ways, the shortcomings of the theory had been recognized by the early 20th century, and it had been relegated to "biological mythology" by the mid-20th century. New discoveries in evolutionary developmental biology (Evo Devo) are providing explanations for these phenomena on a molecular level.

In the history of biology, preformationism (or preformism) is a formerly popular theory that organisms develop from miniature versions of themselves. Instead of assembly from parts, preformationists believed that the form of living things exist, in real terms, prior to their development. Preformationists suggested that all organisms were created at the same time, and that succeeding generations grow from homunculi, or animalcules, that have existed since the beginning of creation, which is typically defined by religious beliefs.

Epigenesis (or neoformism), then, in this context, is the denial of preformationism: the idea that, in some sense, the form of living things comes into existence. As opposed to "strict" preformationism, it is the notion that "each embryo or organism is gradually produced from an undifferentiated mass by a series of steps and stages during which new parts are added" (Magner 2002, p. 154). This word is still used in a more modern sense, to refer to those aspects of the generation of form during ontogeny that are not strictly genetic, or, in other words, epigenetic.

Tinbergen's four questions, named after 20th century biologist Nikolaas Tinbergen, are complementary categories of explanations for animal behaviour. These are commonly called levels of analysis. It suggests that an integrative understanding of behaviour must include ultimate (evolutionary) explanations, in particular:

• behavioural adaptive functions
• phylogenetic history; and the proximate explanations
• underlying physiological mechanisms
• ontogenetic/developmental history.

In fungi, an ascospore is the sexual spore formed inside an ascus—the sac-like cell that defines the division Ascomycota, the largest and most diverse division of fungi. After two parental nuclei fuse, the ascus undergoes meiosis (halving of genetic material) followed by a mitosis (cell division), ordinarily producing eight genetically distinct haploid spores; most yeasts stop at four ascospores, whereas some moulds carry out extra post-meiotic divisions to yield dozens. Many asci build internal pressure and shoot their spores clear of the calm thin layer of still air enveloping the fruit body, whereas subterranean truffles depend on animals for dispersal.

Development shapes both form and endurance of ascospores. A hook-shaped crozier aligns the paired nuclei; a double-membrane system then parcels each daughter nucleus, and successive wall layers of β-glucan, chitosan and lineage-specific armour envelop the incipient spores. The finished walls—smooth, ridged, spiny or gelatinous, and coloured from hyaline to jet-black—let certain ascospores survive pasteurisation, deep-freezing, desiccation and ultraviolet radiation. Dormant spores can lie inert for years until heat shock, seasonal wetting or other cues trigger germ tube emergence. Such structural and developmental traits are mainstays of fungal taxonomy and phylogenetic inference.

Phoresis or phoresy is a temporary commensalistic relationship when an organism (a phoront or phoretic) attaches itself to a host organism solely for travel. It has been seen in ticks and mites since the 18th century, and in fossils 320 million years old. It is not restricted to arthropods or animals; plants with seeds that disperse by attaching themselves to animals are also considered to be phoretic.

Phoresis is rooted in the Greek words phoras (bearing) and phor (thief). The term, originally defined in 1896 as a relationship in which the host acts as a vehicle for its passenger, clashed with other terminology being developed at the time, so constraints on the length of time, feeding, and ontogeny are now considered. Phoresis is used as a strategy for dispersal, seasonal migration, transport to new host/habitat, escaping ephemeral habitats, and reducing inbreeding depression. In addition to the benefits afforded to individuals and species, its presence can add to the ecological diversity and complexity of an ecosystem.

Huntingtin (Htt) is a human protein encoded by the HTT gene, also known as IT15 ("interesting transcript 15"). Pathogenic expansions in HTT (disease-causing repeat length increases) cause Huntington's disease (HD), and the protein has also been implicated in mechanisms of long-term memory storage.

HTT is expressed in many tissues, with the highest levels in the brain. Expression is developmentally regulated and required for embryogenesis. Huntingtin normally consists of 3,144 amino acids and has a predicted mass of ~350 kDa, depending on the length of its polyglutamine tract. Polymorphisms in HTT alter the number of glutamine residues: the wild-type allele encodes 6–35 repeats, whereas pathogenic expansions in HD exceed 36, with severe juvenile cases reaching ~250 repeats. The name huntingtin reflects this association with disease; IT15 was its earlier designation.

The T-cell receptor (TCR) is a protein complex, located on the surface of T cells (also called T lymphocytes). They are responsible for recognizing fragments of antigen as peptides bound to major histocompatibility complex (MHC) molecules. The binding between TCR and antigen peptides is of relatively low affinity and is biologically degenerate (that is, many TCRs recognize the same antigen peptide, and many antigen peptides are recognized by the same TCR).

The TCR is composed of two different protein chains (that is, it is a heterodimer). In humans, in 95% of T cells the TCR consists of an alpha (α) chain and a beta (β) chain (encoded by TRA and TRB, respectively), whereas in 5% of T cells the TCR consists of gamma and delta (γ/δ) chains (encoded by TRG and TRD, respectively). This ratio changes during ontogeny and in diseased states (such as leukemia). It also differs between species. Orthologues of the 4 loci have been mapped in various species. Each locus can produce a variety of polypeptides with both constant and variable regions.