Biology 1107 Lecture #42
Friday April 23, 2010 (04-23-10)
Lecture based on CHAPTER 24
THE ORIGIN OF SPECIES
A. What Is a Species?
In 1942 Ernst Mayr enunciated the
biological species concept
to address biological diversity.
A species is a population or group of populations whose members have the potential to interbreed with
each other in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring
with members of other species.
A biological species is the largest set of populations in which genetic exchange is possible and that is
genetically isolated from other populations.
For example, the eastern and western meadowlarks may have similar shapes and coloration, but
differences in song help prevent interbreeding between the two species.
In contrast, humans have considerable diversity, but we all belong to the same species because of our
capacity to interbreed.
1. The biological species concept emphasizes reproductive isolation.
2. Prezygotic and postzygotic barriers isolate the gene pools of biological species
Reproductive barriers can be categorized as prezygotic or postzygotic, depending on whether they
function before or after the formation of zygotes.
impede mating between species or hinder fertilization of ova
if members of
different species attempt to mate.
These barriers include habitat isolation, behavioral isolation, temporal isolation, mechanical isolation, and
Two organisms that use different habitats even in the same geographic area
are unlikely to encounter each other to even attempt mating. This is exemplified by the two species of
garter snakes, in the genus
, that occur in the same areas but because one lives mainly in
water and the other is primarily terrestrial, they rarely encounter each other.
Many species use elaborate behaviors unique to a species to attract mates.
For example, female fireflies only flash back and attract males who first signaled to them with a species-
specific rhythm of light signals. In many species, elaborate courtship displays identify potential mates of
the correct species and synchronize gonadal maturation.
Two species that breed during different times of day, different seasons, or
different years cannot mix gametes.
For example, while the geographic ranges of the western spotted skunk and the eastern spotted skunk
overlap, they do not interbreed because the former mates in late summer and the latter in late winter.
Closely related species may attempt to mate but fail because they are
anatomically incompatible and transfer of sperm is not possible.
To illustrate, mechanical barriers contribute to the reproductive isolation of flowering plants that are