More on plant species (and animal species)

Joe Shaw
Wed, 22 Nov 2006 15:15:54 PST
Hi Gang,


I was thinking more about plant species, and why/how they sometimes 
interbreed, etc.  I checked with friends in ecology departments and did some 
reading.  It turns out that plants just have looser sterility barriers than 
do animals.  But, animal species are not so exempt from inter-fertility as 

Fortunately for hybridizers and gardeners everywhere, plants do have some 
elasticity in their sterility barriers.  Oaks, cacti, and liliaceous plants 
come to mind when I think of species  that can interbreed, but which 
typically don't in the wild.

2.  What is a species? (or, I don't know what a species is)

Before I get to the animal discussion I want to point out that surely, some 
confusion over inter-specific is because we (we humans) can only do the best 
we can, and we're not always able to discern a species from a subspecies or 
a variety.  Charles Darwin remarked on this over 150 years ago and his 
suggestion was to be careful.

Darwin said, "From these remarks it will be seen that I look at the term 
species as one arbitrarily given, for the sake of convenience, to a set of 
individuals closely resembling each other, and that it does not essentially 
differ from the term variety."

In a letter to a colleague Darwin also wrote, "It is really laughable to see 
what different ideas are prominent in various naturalists' minds when they 
speak of species; in some, resemblance is everything and descent of little 
weight - in some, resemblance seems to go for nothing, and Creation the 
reigning idea - In some, descent is the key, - in some, sterility an 
unfailing test, with others it is not worth a farthing. It all comes, I 
believe, from trying to define the indefinable."

So, the point is that (especially it seems with plants) you are in very good 
company if it seems hard to known what is a species and what is just a 
variant within a species.  (LINKS 3 and 4)

2.  Mammal species that interbreed

I like John Hawks' Weblog on all things related to evolution, genetics, and 
paleoanthropolgy.  His discourses are well thought out, and he uses the 
current literature to guide, illuminate, and flesh out his arguments.

One interesting point he makes is that many mammalian species are known that 
can interbreed freely if they have no other options, e.g., if they are 
isolated from their normal mates.  He points out that members of the 
wolf/dog/coyote/jackal clade can and do interbreed and produce viable and 
fertile offspring, especially in areas where the various animals are thrust 
into contact with each other.  The point is that these species don't 
normally interbreed, and they are easily distinguished from each other, but 
they can and do interbreed upon occasion. Factors that keep them from 
interbreeding include lack of overlapping ranges and behavioral details.

For instance, the four recognized jackal species are found in Asia and 
Africa where they seldom overlap with the distribution of wolves and they 
seldom have a chance to hybridize (in nature).  Similarly, frequency and 
season of estrus can prevent some animals from interbreeding unless they are 
forced together in zoos, etc.  Additionally, various species that might be 
able to interbreed treat each other as "different;" and they are more likely 
to attack or flee than to mate.

In any event, Dr. Hawks reports, "Wolves and coyotes (and dogs) mate fairly 
extensively wherever they are sympatric. Bison had a time in history when 
they received lots of genes from cattle."  (LINK 2)

LINK 1:  John Hawks
LINK 2:  On Introgression
LINK 3:  Charles Darwin, Origin of a Species by Natural Selection (1856, 
Chapter 3, p. 56)…
LINK 4:  Darwin letter to Joseph Hooker…



Conroe TX

Leaves are showing on early Narcissus types (Tazetta hybrids). 

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