The biological species concept was proposed by Ernst Mayr in 1942. It defines a species as a group of actually or potentially interbreeding natural populations that can produce fertile offspring and that are reproductively isolated from other such groups.'Potentially' matters: two populations on separate islands never meet, but if they could breed together and produce fertile offspring, they are still one species.Species stay separate because of isolating mechanisms:
• prezygotic mechanisms stop mating or fertilisation happening: for example different breeding seasons, different courtship displays or songs, living in different habitats, or genitalia or flowers that do not fit together.
• postzygotic mechanisms act after fertilisation: the hybrid may die, or it may survive but be infertile.Worked example: a horse has 64 chromosomes (32 pairs), so its gametes contain 32. A donkey has 62 chromosomes, so its gametes contain 31. A mule therefore has 32 + 31 = 63 chromosomes. The horse and donkey chromosomes cannot form homologous pairs in meiosis, so the mule cannot make viable gametes and is sterile. The horse and donkey are separate species.The biological concept is useful because it is based on gene flow: members of a species share a gene pool, and reproductive isolation stops alleles passing between species, so each species evolves separately.
• prezygotic mechanisms stop mating or fertilisation happening: for example different breeding seasons, different courtship displays or songs, living in different habitats, or genitalia or flowers that do not fit together.
• postzygotic mechanisms act after fertilisation: the hybrid may die, or it may survive but be infertile.Worked example: a horse has 64 chromosomes (32 pairs), so its gametes contain 32. A donkey has 62 chromosomes, so its gametes contain 31. A mule therefore has 32 + 31 = 63 chromosomes. The horse and donkey chromosomes cannot form homologous pairs in meiosis, so the mule cannot make viable gametes and is sterile. The horse and donkey are separate species.The biological concept is useful because it is based on gene flow: members of a species share a gene pool, and reproductive isolation stops alleles passing between species, so each species evolves separately.