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SPECIES AND SPECIES FORMATION
What is a species?
The biological species concept is the most widely accepted definition of species:
It states the species are … a group of related organisms that are able to interbreed
and produce fertile offspring.
Appearance isn’t everything
a. Organisms may appear to be alike and be different species. For example,
Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks
(Sturnella magna) look almost identical to one another, yet do not interbreed
with each other—thus, they are separate species according to this definition.
The Western meadowlark (left) and the Eastern meadowlark (right) appear to be
identical, and their ranges overlap, but their distinct
songs prevent interbreeding.
a. Organisms may look different and yet be the
same species. For example, look at these ants.
You might think that they are distantly related
species. In fact, they are sisters—two ants of the
species Pheidole barbata, fulfilling different roles
in the same colony.
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Defining speciation
The formation of new species as a result of geographic, ecological and behavioral
separation that prevents previously interbreeding
populations from breeding with each other.
As long as a population has the opportunity to interbreed and exchange genes, they
remain one species. A population of one species can only evolve into more than
one species if groups within the population become isolated from each other by
barriers that prevent exchange of genes.
There are three types of isolating barrier:

Geographical
Features such as rivers or mountain ranges isolate groups. Movement of landmasses by continental drift led to geographical isolation millions of years ago.

Ecological and behavioral separation
Although groups are not geographically isolated from each other they may be
isolated by such things as occupying different habitats or breeding areas, pH,
salinity. Breeding between groups within a population may not be possible because
of differences in courtship behavior, physical differences which prevent mating, or
failure of gametes to fuse.
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The diagram illustrates what could happen to populations of animals, which
become geographically isolated. Once two groups are isolated different mutations
occur in each group. If the environments differ, different adaptations are favoured
by natural selection. This leads to different characteristics evolving in each group
as time passes. Eventually the groups become so different that if they come
together again they are unable to interbreed and are now separate species.
A common pattern resulting from isolation and speciation is adaptive radiation.
This is where a common ancestor evolves into a number of species. These have
adaptations allowing them to occupy different niches.
A niche is the role of an organism in its community and consists of features such
as its feeding methods and habitat.
Extinction
In biology, disappearance of species of living organisms. Extinction usually occurs
as a result of changed conditions to which the species is not suited. If no member
of the affected species survives and reproduces, the entire line dies out, leaving no
descendants.
Reasons for extinction include:
 Habitat destruction - if the natural home or environment of an animal, plant
is destroyed, during this process, the organisms that previously used the site
are displaced or destroyed, reducing biodiversity. E.g. the Cuban macaw or
the golden toad in Costa Rica.
 Hunting– Overharvesting' and 'overhunting' contributed to the extinction of
the Caribbean monk seal in the mid-20th century; fisherman considered seals
to be competitors for fish, so they killed them in large numbers.
 Invasive or introduced pest species- have caused many native animals and
plants to become extinct across the world.
 Pollution- Contaminates the natural environment with harmful substances
produced by human activity.
 Disease - The spread of disease may be both a natural and human factor
behind extinction. E.g. a fungal infection caused significant decreases in the
populations of the mountain chicken in Dominican and Montserrat.