Ask A Scientist General Science Archive

name        Ahmed
status       student
grade        other
location     N/A

Question -   My Question is in Evolutionary Biology:

If we assume that all species have decended from a common ancestor,
then why don't they have the ability to mate together.  And if they
had it, then they would be of the same species by definition... For
instance, If we assume that man have evolved from apes, then why a
human sperm does not have the ability to fertilize an ape egg?  Taking
into consideration the fact that variations that lead to evolution
does not necessarily change the structure of the receptors on the egg
that recognize the sperm.. And if so, then we would have a numberless
of individuals belonging to the same species but with different
morphological and anatomical structures!!!!! Why does this not
happen?!!!
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Thanks for your question, Ahmed… The reason for the rarity of
cross-species breeding in nature is, essentially, a three-fold
problem; there are behavioral barriers, anatomical barriers, and
genetic barriers.  Firstly, differing species of organisms are, quite
simply, not usually interested in mating, and (in most cases) are only
apt to mate at very specific times of the year, which decreases the
likelihood of cross-species copulation.  Secondly, the anatomies of
two, sexually-reproducing organisms of differing species are likely to
be reproductively incompatible.  Thirdly (and probably most
importantly), the genetic variations (the number of chromosome pairs,
for example) among different species dictates that their sex cells are
incapable of successful fertilization.

The evolution model predicts that, as a consequence of genetic
mutations, populations of organisms will, over a sufficiently long
period of time, naturally acquire physical traits that make them
distinctly different from their ancestors, so much so that they may no
longer be sexually compatible with those ancestors.  Your observation
that any two organisms who can mate are, by definition, members of the
same species very nearly answers your own question… with one
problem, however.  It is true that, traditionally, a species was
defined as a mating group.  We know today that this definition does
not hold, however, as cross-species mating (a lion and a tiger, for
example) is, in fact, possible.  The point I would like to make is
that our taxonomic classification of organisms is purely a convention;
there is nothing in nature that actually dictates that two organisms
that are classified by scientists as belonging to different species
should necessarily be classified as such.  In short, nature makes no
attempt to classify organisms as being related or unrelated.  As far
as nature is concerned, every organism on this planet is unique and
can only be truly classified by itself.  That you and I are both
classified as belonging to the same genus and species (Homo sapiens)
is purely artificial, and only done so for the purpose of making
biological information more easily organized and the search for
patterns more easily accomplished.  We both individually represent
separate, unique attempts on the part of nature to build a more
survival-prone organism.

Humans and “great apes” (e.g. orangutans, chimpanzees, and
gorillas) cannot successfully produce offspring due to a difference in
the number of chromosomes in their gametes… human gametes (sex
cells) each have 23 chromosomes (for a total of 46) and ape gametes
have 24 chromosomes (for a total of 48).  This difference in the
number of chromosomes prevents a sperm cell of either species from
successfully fertilizing an egg from the other species.  It would be
possible, as you suggest, for an organism to develop unique physical
traits that do not necessarily interfere with its ability to be
fertilized by or to fertilize another; yet, eventually, that ability
is likely to change as a consequence of mutation, too, in the
successive generations following.  When those mutations occur, the
resulting offspring are no longer sexually compatible with their
predecessors and we arrive at our “traditional” definition of a
new species.

I hope that this has helped answer your questions…

Scott J. Badham
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The main reason separate species form is because of reproductive isolation,  
in other words something happens that they can't reproduce together  anymore.  
This is due to some mutation event.  There are many  scenarios in which this 
could happen.  For instance, humans and chimpanzees  have about 98.5% of our 
DNA in common.  But humans have 46 chromosomes and  chimps have 48.  If you 
take the chromosomes of a chimp and those of a  human and pair them up together, 
ie. their number 1 by our number 1, and so on,  it appears that somewhere 
chimp chromosome #2 split into 2 giving them an  extra pair.  If you pair our 
number 2 and the two halves of whichever one  of theirs that split (I'm sorry I 
can't remember which ones!) the pattern  of bands on both match up almost 
perfectly. (There are actually some other  mutations that have also happened in this 
pair).  The fact that humans have  46 and chimps have 48 makes it so our 
species cannot mate together  anymore.  Most species have different chromosome 
numbers and even if they  are the same the genes that are on each chromosome 
might be different.   Sometimes related organisms that are different species can 
reproduce together  but their offspring can't mate, ie. they are sterile.  A 
good example of  this is the offspring of the donkey and the horse which is 
called a mule.   One has 62 chromosomes and one has 64.  This gives the mule 63, 
an uneven  number.  Dogs and wolves can have offspring which are fertile and 
for this  reason some scientists argue that they really shouldn't be classified 
as  separate species, but we may be witnessing a speciation event in progress.
Another reason organisms speciate is due to geographic isolation.   Once 
organisms can't get to each other to mate, over time they get different  mutations 
in their DNA and adapt differently to their environments.  If  enough time 
passes and enough mutation has happened in theory they may not be  able to mate 
again.  Think of the animals that are found in  Australia.  Before continental 
drift began during the Mesozoic Era, mammals  had started to evolve.  Once 
Australia split off and became an island, they  were geographically and 
reproductively isolated from the mainland and developed  different mutations.  The 
environments were different and the traits  that were selected for in each 
environment were different.  There are  mammals found there that are found nowhere 
else.  They would not be able to  mate with other mammals from the mainland 
because they have become too  different.
I understand that it is hard to comprehend that all the  biodiversity of the 
earth resulted from a common ancestor.  Our lifetimes  are relatively long and 
evolution doesn't happen in individuals once they are  born.  We don't SEE 
evolution happening except in short lived  organisms such as bacteria.  It is 
also hard to comprehend the vastness of  geologic time.  But if you accept that 
the earth is perhaps 6 billion years  old and that life has been evolving for 
4.5 billion years, and that organisms  have been changing over time in 
response to changing environments, then it is  easier to accept that these mutations 
could have occurred.
 
vanhoeck
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Firstly, it's important to keep in mind that the concept of species is
artificial and that there are several different definition of species.
The most common working definition is that a species is a group of
individuals with the potential for interbreeding.

Using that definition, any time a population gets split into two
separate subpopulations that may no longer interbreed, we say that the
two subpopulations are reproductively isolated, and we call them
different species.  For many closely related populations of organisms,
it's not at all clear whether they should be considered the same or
different species.  
An excellent example is domestic dogs and wolves.  Huskies interbreed
readily and successfully with wolves.  Chihuahuas, on the other hand,
would probably not interbreed with wolves; although I don't know this
for certain, I suspect that a wolf would sooner eat a Chihuahua than
mate with one.  And yet we have traditionally lumped Chihuahuas and
Huskies together as one species (Canis familiaris), while considering
wolves to be a different one (C. lupus).  The reason for this is more
historical than biological.

The point is, there is no perfect definition of species, because it is
an artificial concept.  And yet there are clearly certain groups of
individuals that freely interbreed, to the exclusion of other groups.
There are many factors that prevent any two populations from
interbreeding.  These are called reproductive barriers; they range from
behavioral isolation (when individuals from the two populations are not
sexually attracted to one another) to mechanical isolation (when the
parts don't fit) to gametic incompatibility (when sperm from population
A can't fertilize egg from population B) to zygotic inviability (when a
zygote fails to develop), to hybrid infertility (when a hybrid
individual is sterile), etc.  

Horses and donkeys are an instructive example.  A horse and a donkey
will happily mate, and they produce very healthy offspring: mules.
However, it turns out that mules are sterile.  Because of this, the
horse gene pool and the donkey gene pool are effectively separate.
Thus, horses and donkeys are reproductively isolated, and even though
they can have offspring, it makes perfectly good sense to consider them
different species.  

It may be that human and chimpanzee gametes are compatible; I don't know
that anyone has ever tried.  It seems clear to me, though, that humans
and chimpanzees are behaviorally isolated, and I feel comfortable
defining them as different species.

Christopher Perkins
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