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A species is a group of organisms that can interbreed and produce fertile offspring. Members of the same species typically share similar characteristics because their DNA is relatively similar. However, not all organisms that look alike belong to the same species, as seen with monarch and viceroy butterflies, which are genetically distinct despite their similar orange-and-black appearance.

Hybrids result from interbreeding different species but are biologically incapable of further reproduction due to post-zygotic isolation. For example, zonkeys (zebra-donkey hybrids) are infertile because horses and donkeys have different chromosome numbers—64 and 62 respectively. Their hybrid offspring have 63 chromosomes, an odd number that cannot be equally sorted into gametes, preventing reproduction.

Pre-zygotic barriers prevent mating or fertilization from occurring before a zygote forms, such as when two frog species remain active at different times of day. Post-zygotic barriers operate after fertilization, preventing hybrid zygotes from developing into fertile adults. Together, these mechanisms maintain species separation and prevent gene flow between different populations.

Temporal isolation prevents mating by separating species through different timing of daily routines or breeding seasons. For example, the red-legged frog breeds from January to March, while its close relative the yellow-legged frog breeds from late March through May. This timing difference prevents contact and mating between the two species.

Mechanical isolation is a pre-zygotic barrier where physical characteristics of different species prevent mating. Insects display diverse genital morphologies that allow them to mate only with members of their own species. This structural incompatibility ensures reproductive isolation without requiring behavioral or environmental separation.

Hybrid inviability occurs when a zygote forms but the embryo cannot survive due to incompatible genetic material from two distinct species. Hybrid genomes contain genes and chromosomes from different species that do not act harmoniously together, resulting in obvious fitness costs that prevent the hybrid from developing into a viable adult.

Yes. Some hybrids survive to adulthood and can copulate, but non-genetic factors prevent procreation. In Drosophila hybrids, viable gametes are produced, yet sperm from hybrid males cannot survive in female semen receptors of either parent species or other hybrids, preventing successful reproduction despite physical viability.