Have you ever wondered how we
classify different organisms into different species? Well, before we
look at that, let's go over the difference
between asexual reproduction and sexual reproduction. In asexual reproduction,
one organism, like a single bacterium, will
divide into two daughter cells that are both genetically
identical to the original cell. In sexual reproduction, two
members of the same species will reproduce together
in order to form genetically unique offspring. Now, in general, we
say that organisms that reproduce asexually usually
have low genetic diversity, whereas sexually
reproducing species have high genetic diversity. So what is a species? Now, this can be a very
difficult question to answer. For sexually
reproducing organisms, we can say that two organisms,
like this cat and this human, are members of
different species. They're unable to have
offspring together. However, for asexually
reproducing organisms, like bacteria,
protists, and archaea, it's a little more confusing. These species don't mate
with other organisms. So we have a difficult
time classifying them into different categories. And we call this
the species problem. But in this video, we're
going to spend time just looking at those sexually
reproducing organisms. And these are separated
into different species by different forms of what we
call reproductive isolation. And this is the idea that
there are many forces that stop two different organisms
from having offspring together. And we can divide these forces
into two separate categories, pre-zygotic forms and
post-zygotic forms. Pre-zygotic isolation refers
to all the different forces that prevent two organisms
from having offspring together that occur prior to the
formation of a zygote. And remember that a zygote
is a single cell that is made up of the genetic
material of both organisms that have reproduced together. Post-zygotic forms of isolation
we'll get into a little bit later. So the first type of
pre-zygotic isolation is temporal/habitat isolation. And temporal isolation
refers to the fact that not all organisms
mate at the same time. Some may mate at night, while
others mate during the day. Some may mate in spring,
while others mate in winter. If two organisms do not
find mates at the same time, then they are
temporally isolated. Habitat isolation refers to the
place where the organisms mate. Some may prefer
mating in the forest, while others prefer
mating in the mountains. And if two organisms don't
find mates in the same place, then they are also isolated. If time and place
aren't a problem, then the next barrier
is behavioral isolation, which refers to mate
selection and how organisms go about
attracting a mate. Now, not all organisms will
attract a mate the same way. Perhaps one animal, like a
bird, will attract a mate by singing a song, whereas
this bunny rabbit may do a little dance
to attract a mate. So we have behavioral isolation. And now we have
mechanical isolation, which deals with the physical
inability of two organisms to mate, even if they wanted to. Now, a great example of
this is a huge animal like an elephant being unable
to mate with a tiny mouse. If two organisms do
mate successfully, they may still encounter
gametic isolation, which is when fertilization
between the two gametes to form a zygote is impossible. Now, once the zygote
has been formed, we can move on and look
at post-zygotic forms of reproductive isolation. And the first form
is zygote mortality. And this occurs when even if
the two gametes from the two organisms can fuse
successfully and form a zygote, that zygote would have
a high mortality rate and be unable to develop
into a mature offspring. Next we have hybrid
inviability, which occurs when a zygote is able to
grow into a mature offspring, but that offspring will
have a high mortality rate and won't be able to
grow into a mature adult. Finally, we have the last form
of reproductive isolation, which is hybrid sterility. And this is when the offspring
can grow into a mature adult. But that mature adult
is not able to mate and have offspring of its own. So if two sexually
reproducing organisms are not isolated by
any of these barriers, then we can generally
say that they are members of the same species. So what did we learn? Well, first we learned
about the species problem and how classifying
different organisms into different species
can be quite difficult. We have a pretty good
definition for sexually reproducing organisms,
but not really for asexually
reproducing organisms. And next we learned about
reproductive isolation and how we can say that two
sexually reproducing organisms are reproductively isolated
if they are unable to freely produce fertile
offspring together.