The first two links are irrelevant because they describe physical characteristics and not a well defined line. The quotes you copied are vague descriptions at best.
The last quote you emphasized says “it can be defined” which shows it’s not a well defined line. Also, there are quite a few cross-species hybrids that have produced fertile offspring.
African forest elephants and African bush elephants are known to hybridize with each other where their ranges overlap.[17] Evidence shows that they can form hybrid zones where their ranges overlap, particularly in the Democratic Republic of Congo and Uganda. Hybrids have been found to be fertile, and a significant number of elephants in hybrid zones, such as Kibale National Park in Uganda, show intermediate physical characteristics.
These birds aren’t even in the same family and have hybridized
The red-crested cardinal (Paroaria coronata, family Thraupidae) has hybridized between the northern cardinal (Cardinalia cardinalis, family Cardinalidae), shiny cowbird (Molothrus bonariensis, family Icteridae), and chestnut-capped blackbird.
The concept of a species is a lot more flexible and less-defined than you think.
The first two links are irrelevant because they describe physical characteristics and not a well defined line. The quotes you copied are vague descriptions at best.
The last quote you emphasized says “it can be defined” which shows it’s not a well defined line. Also, there are quite a few cross-species hybrids that have produced fertile offspring.
Here’s a list of some
These birds aren’t even in the same family and have hybridized
The concept of a species is a lot more flexible and less-defined than you think.