Biodiversity: The range of species in an ecosystem.
Species: Group of organisms which can interbreed to produce fertile offspring.
EXAMPLE 1: The dogs are the same species
Reasons:
1. The offspring are fertile
2. The number of chromosomes in the gametes from both
parents are the same
EXAMPLE 2: A horse and a donkey are not the same species.
Reasons:
1. The donkey is infertile
2. The number of chromosomes in the gametes from both
….parents are not the same so chromosomes cannot form …
. homologous pairs
A stable ecosystem: Has a wide range of species and food webs.
Removal of one/more species in a foodweb has consequences on all other organisms/populations.
Species are always changing over time as they adapt to their environment
Within a community, the species present are in one of three states:
1. Moving towards extinction
2. In a stable relationship
3. SPECIATION
Speciation is the formation of a new biological species.
Cause: Interrupting gene flow between 2 populations
causing their gene pools to diverge AND result in
Evolutionary change.
2 speciation types:
Ecological niche: The role an organism plays in its environment
3. Undergoing speciation
(1) Allopatric speciation
ISOLATION MECHANISM: Gene flow between 2 or more populations is prevented by GEOGRAPHICAL BARRIER
A. Mountain range
B. River
C. Desert
D. Sea
MECHANISM:
1. Members of a large population of one species occupy an
environment. They share the same gene pool and interbreed
freely but show variation in their species.
2. The population become completely split into two
SUB-POPULATIONS by a GEOGRAPHICAL ISOLATION BARRIER,
which prevents gene exchange and interbreeding.
3. MUTATIONS occur at random. Mutations in sub-population-1
are different from mutations in sub-population-2. RESULT:
NEW VARIATION in each group which is not shared by both
sub-populations
SELECTION PRESSURES acting on each sub-population are
different depending on the local conditions e.g. climate,
predators, disease.
Factors affecting the gene frequencies of the sub-populations:
(a) NATURAL SELECTION: Affects each subpopulation
in a different way, by favouring members of the
sub-population who are best adapted to their
environment.
(b) GENETIC DRIFT: If one population is small, it may not
possess the complete range of alleles.
4. Over many generations (a long time), stages 3 and 4 cause the
2 gene pools to become so altered that the groups become
genetically distinct and ISOLATED.
5. If the GEOGRAPHICAL ISOLATION BARRIER is removed,
members of the sub-populations cannot interbreed.
Reason: TWO DISTINCT SPECIES HAVE EVOLVED
(a) Chromosomes cannot form pairs
(b) Anatomical differences
Examples:
1. European wren on island isolated by sea from
…,the mainland.
2. Darwin’s finches
(adaptations of beak shape and size) on
….Galapagos Islands compared with South America
3. Desert plants
(adaptations of roots, reduced leaf surface area
…and presence of thick waxy cuticle on leaves).
(2) Sympatric speciation
Two or more populations live in close proximity in the same environment but are GENETICALLY ISOLATED.
ISOLATION MECHANISM
Gene flow is prevented by the presence of a BEHAVIOURAL or ECOLOGICAL BARRIER or the occurrence of
POLYPLOIDY in plants.
PROMOTED BY DISRUPTIVE SELECTION.
EXAMPLES:
(a) Fruit flies
Original population live on Hawthorn trees and larvae feed on berries
Settlers to North America introduced a new apple tree species.
Some flies ate the new food source and began to become adapted to the new ecological niche
Ecological barrier: where mating and egg laying occurs is the same as the food source for larvae.
The difference in behaviour created an ecological barrier between berry eating and apple eating larvae.
(b) Polyploidy (Gaining one or more complete sets of extra chromosomes)
Polyploid plants cannot breed with the plant (s) that produced it.
It will be genetically distinct even if they are found in the same area
Example: Sweetcorn and pod corn (popcorn)
Hybrid zones
An environment may contain several sub-populations of a species which cannot all interbreed.
A
B
C
D
Hybrid zones
Each sub-population can breed with its neighbour but may not be able to breed with more distant members
of the species.
Hybrid zones exist where interbreeding is possible and as a result genes are able to flow between the subpopulations.
If populations B or C become extinct, gene flow is disrupted and populations A and D cannot breed together
and therefore become two separate species .
A
C
D
Hybrid zones
Example: hybrid speciation is seen in sunflowers from Texas.
One (diploid) species arose via POLYPLOYDY in one parent.
All species could interbreed due to hybrid lineage until the polyploidy was ecologically (and
geographically) isolated from its progenitors.
1.
1.
All species interbreed until polyploid is removed or isolated
polyploid
1.