Main content
Course: MCAT > Unit 6
Lesson 10: Evolution and population dynamics- Evolution and population dynamics questions
- Selective pressures on cacti needle length
- Human impact on animal populations
- Evolution and natural selection
- Fitness and fecundity
- Alternative selection
- Genetic drift, bottleneck effect, and founder effect
- Inbreeding
- Reproductive isolation
- Evolution: Natural selection and human selection article
© 2024 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Reproductive isolation
Species are groups of organisms that can reproduce together; reproductive isolation keeps species separate. Pre-zygotic isolation prevents mating or fertilization (through temporal, habitat, behavioral, or mechanical isolation), while post-zygotic isolation affects zygotes or hybrids (via zygote mortality, hybrid inviability, or hybrid sterility). Although sexually-reproducing species are easier to classify, the species problem makes classifying asexually-reproducing organisms difficult. Created by Ross Firestone.
Want to join the conversation?
- What about horses and donkeys: animals from two different species but capable of producing a mule (or a hinny)? Shouldn't they be of the same species by the definition at around0:45?(5 votes)
- No because their chromosomes don't line up, making the mule infertile or sterile.(3 votes)
- What is love? My friend Haddaway asked so many times, so I was curious as to what it is.(6 votes)
- Well, love for close family is a cocktail of hormones used to make you have a close bond, mostly evolving from the usefulness of working well with others in your band/tribe you would be in(which was normally made of close relatives). Love for a person outside of your family is a different mixture of hormones (made of more happy hormones like endorphins or dopamine) used to make the two of you mate and reproduce to spread your genes. This evolved from the how people who had this "love" did spread their genes more, unlike the non-mate loving counterparts. So, in general, love is bunch of hormones made to make you survive or mate, like any other adaptation.(3 votes)
- In the case of bactéria, the capacity to transfer some parts of their genome isn't a source of diversity (maybe bigger than sexual reproduction could produce)?(2 votes)
- It is a source of diversity for bacterias but it's just not through sexual reproduction.(3 votes)
- So what exactly is the difference between gametic isolation and hybrid sterility? Thanks!(2 votes)
- Gametic isolation occurs when the two organisms/species mate successfully but can not form a zygote (pre zygotic mechanism)> In the case of hybrid sterility, the zygote is formed and grown into a new hybrid organism but the hybrid is not able to reproduce. For example, mule which is a hybrid of mare and jack (two different species).(2 votes)
- So is reproductive isolation the barriers that unable a species to breed successfully with other species either with pre-zygotic factors or post-zygotic factors? Is it preventing the interbreeding between the same species or different species?(2 votes)
- Reproductive isolation is defined as "the inability of a species to breed successfully with related species due to geographical, behavioral, physiological, or genetic barriers or differences."
Both pre-zygotic and post-zygotic barriers lead to reproductive isolation and thus prevent mating between different (but perhaps related) species.
Note that this must be the case, as we conventionally consider two organisms to be of different species if they are unable to successfully produce viable, fertile offspring with each other (i.e., if they are reproductively isolated).(2 votes)
- If a species reproduces asexually, does that mean all organisms in that species are the same? How can there be any genetic diversity if they never breed with each other? Could all organisms in that species be descended from one organism thousands or millions of years ago?(2 votes)
- You're correct; if an organism reproduces asexually, their offspring will basically be clones of the parent. But because the offspring are clones, there will be little to no genetic diversity in the population. Mutations are still possible in these populations and can lead to diversity; however, they will not happen as quickly or make changes as drastically as what is seen with sexual reproduction. Unless an organism can reproduce extremely quickly in this manner, there will be little, if any, diversity in the population.
It is possible for all organisms living on Earth right now to have arisen from one species of organism billions of years ago. Scientists call this ancestor the LUCA, or the Last Universal Common Ancestor. Every organism shares a couple of genes with every other organism on the planet, meaning that there must have been a common ancestor to everything. The LUCA probably would have looked like a small bacterium. It gets a little messy, however, if you account for horizontal gene transfer, where organisms like bacteria share their genes with each other rather than passing them down through reproduction. If, say, bacteria passed around some of their genes between plants or archaea, then you could have something that looks like common ancestry when it was really just something giving their genes to another organism. It is a very complex topic, so I encourage you to do some further reading on horizontal gene transfer and the LUCA.
Does this help?
(As a side note: prokaryotes that reproduce asexually, like bacteria, do not take much time at all to reproduce. E.coli, for instance, only takes 20 minutes to double! This can lead to entirely new mutant strains or even new species popping up within a matter of weeks. You don't see that happen too often with eukaryotes, because of their slower reproduction times.)(2 votes)
- what would be responsible for the divergence of two species from a common ancestor?
Natural selection or reproductive isolation?(2 votes) - Question I've always had... Grolar bears. A mix between a polar bear and a grizzly bear. I believe that they are more common now that the polar bear territory is further south. There have also been cases of grolar bears successfully reproducing (in the wild). What does that make grolar bears, grizzly bears and polar bears.(2 votes)
- Excuse me, so does reproductive isolation deal with the same species or different species - my question is, at2:07, is the teacher referring to the same species, two organisms or two different species and different organisms? Thanks!(1 vote)
- Reproductive isolation is what causes a populations of a species to genetically diverge and eventually become distinct species.(1 vote)
Video transcript
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.