Mechanisms of Speciation

1. Isolation

(a) Geographical isolation (Allopatric speciation)
– Involves a geographical barrier
– Physically isolates populations and thus block gene flow (reproduction btw populations)
– Once isolated, allopatric populations accumulate genetic differences due to natural selection, genetic drift, and mutations.

Note: Sympatric speciation which does not involve a geographical barrier can occur in result of non-random mating (i.e. host shift isolates parasite populations reproductively) and polyploidy, e.g. autopolyploidy (doubling of chromosome number in the same species) and allopolyploidy
(Refer to part (iii) hybridisation)

(b) Reproductive isolation

(i) Prezygotic barriers
* Ecological – different habitats
* Temporal – different breeding season
* Behavioural – different mating behaviour
* Mechanical – morphological/structural differences in reproductive organs

(ii) Postzygotic barriers
* Zygote/embryo dies before reaching maturity
* Hybrids are mature but sterile

2. Genetic drift

– Alteration in allele frequency that usually occurs in a small population and results from chance/random processes only, e.g. natural disasters (not from natural selection/mutation/immigration).
– Bottleneck effect
– Founder effect
– Small genetic variation within the small population

3. Hybridisation (Alloploidy)

– Process of combining different varieties or species of organisms to create a hybrid.
– Alloploidy is the result of hybridisation of two different species (usually plants) in the same area.
– The hybrid is sterile, but after doubling of chromosomes, it become fertile but cannot mate with the parental plants.
– Example: Triticum aestivum (also known as bread wheat)

4. Adaptive radiation (divergent evolution)

– Development of new species from a single recent ancestor and are adapted to each new environment
– Selection pressure acts on populations; population which is well adapted to the niche continue to survive and reproduce.
– As competition grow, some finches fly to find new ecological niches which has less competition and this allow them to survive.
– In this new environment, a new selection pressure apply onto the population and thus the most adapted finches survive and continue to contribute their alleles to the gene pool.
– Since finches of different islands rarely interbeed, eventually the alleles in each gene pool become significantly different, result in genetic differences and reproductive barriers.


How (early) taxonomists define species?

(i) Similar morphologies (phenotype)

concept/basis: Organisms sharing the similar characteristics and morphologically alike are of the same species.

problems: This definition is applicable only if evolution does not exist. Considering the fact that species have evolved from a common ancestor, large morphological differences sometimes exist between very closely related populations; conversely, since organisms are known to adapt to the changing environment, genetically distinct populations may look very similar.

(ii) An interbreed of population

concept/basis: Organisms belong to the same species if they can mate and produce viable offspring

problems: This definition relies on reproductive behaviour, therefore cannot be applied to fossil species and asexual organisms.

Modern definitions: by comparing the genetic constituents of organisms, e.g. DNA base sequences, amino acid sequences.

Plant Hormones

Cell elongation?

Auxin (Phototropism & Geotropism)

Gibberellins (Increases length of internodes)

Cell division?

Auxin (development of root & fruit)


Inhibits growth?

Auxin (in high concentration)

Abscisic acid

Promotes dormancy?

Abscisic acid

Break dormancy?




Promotes abscission?

Abscisic acid


Inhibits abscission?


Promotes flowering?


Promotes ripening of fruit?


Promotes apical dominance?


Hormonal Control During Pregnancy (Q&A)

What is the role of trophoblast?

Secretes HCG.

What is the role of HCG?

Maintains corpus luteum & stimulates corpus luteum to continue producing oestrogen & progesterone.

What is the role of placenta?

Secretes oestrogen & progesterone.

What is the effect of increasing level of oestrogen & progesterone?

They maintain the thickness of endometrium.

They also inhibit the secretion of oxytocin & prolactin by pituitary gland.

Thus they maintain a relatively low level of oxytocin & prolactin.

Why oxytocin levels still continue to rise?

Oestrogen stimulates the build-up of oxytocin receptors on foetus.

What is the role of oxytocin?

It causes the smooth muscle of uterus to contract so that the foetus is expelled out of the uterus.

Its level reach the peak when parturition occurs, that is, when placenta no longer there to secrete progesterone & oestrogen, thus releasing the inhibition to the contraction of myometrium.

What is the role of prolactin?

It works together with oxytocin, when breastfeeding occurs, to stimulate contraction of smooth muscles of mammary gland to expel the milk. The level spikes whenever breastfeeding occurs—it is stimulated by the sucking action by the baby.

Ovarian Cycle

*numbers displayed in figure does not correspond to the numbers below.

1. Hypothalamus releases GnRH.

2. GnRH stimulates anterior pituitary to secrete FSH & LH.
3. FSH stimulates follicle growth. LH stimulates follicles to produce oestrogen.
4. Cells of growing follicles (theca cells) start to produce oestrogen.
5. When oestrogen secretion by the growing follicles begin to rise steeply, FSH & LH levels increase markedly. (High concentration of oestrogen stimulates hypothalamus to produce more GnRH)
*note: LH increases more steeply than FSH because high concentration of oestrogen increases the GnRH sensitivity of LH-releasing cells in pituitary.
6. The maturing follicle, in response to a peak in LH level (LH surge), releases the secondary oocyte. (Ovulation)
7. LH also stimulates the follicular tissue left behind in the ovary to transform into corpus luteum.
8. Under continued stimulation by LH, corpus luteum secretes oestrogen and progesterone.
9. As oestrogen & progesterone levels rise, the combination of these hormones inhibits the secretion of GnRH by hypothalamus, hence reducing the secretion of FSH and LH to very low levels.
10. Low FSH & LH levels cause the corpus luteum to disintegrate, thus a sharp decline in oestrogen and progesterone levels too.
11. The decreasing levels of oestrogen & progesterone therefore stop inhibition on hypothalamus to secrete GnRH.
12. The pituitary can then begin to secrete enough FSH to stimulate growth of new follicles in the ovary, initiating the next ovarian cycle.
What if fertilisation happens?
Though step 9 occurred (which causes low FSH & LH levels), but HCG produced by blastocyst (which formed from zygote) will take over the role of LH to maintain corpus luteum & stimulate corpus luteum to continue the production of oestrogen and oestrogen.
Hormonal control during pregnancy will be discussed in detail in the next post.

Menstrual cycle? Ovarian cycle?
Menstrual cycle is the monthly preparation of uterus to receive an embryo. It is marked by a discharge of blood from disintegration of endometrial lining of uterus when no embryo is formed. The cycle can be divided into 3 phases: Menstrual (day 1~5); Proliferative (day 5~14, before ovulation); Secretory (day 14~28, after ovulation).
Ovarian cycle is a series of events in ovaries that occur during & after maturation of oocyte. It is marked by the development of Graafian follicle and secondary oocyte and also the formation of corpus luteum. The cycle can be divided into 3 phases: Follicular phase (before ovulation); Ovulation; Luteal phase (after ovulation).
Do other animals have menstruation too??
Most female mammals undergo ovarian cycle.
But only human and certain other primates have menstrual cycle.
Other mammals have oestrous cycle instead.
Human females may engage in sexual activities anytime during menstrual cycle, but other mammals only mate during ovulation of oestrous cycle.

Electrochemical Series

  • The more positive the E° value, the further the position of equilibrium lies to the right.

  • That means that the more positive the E° value, the more likely the substances on the left-hand side of the equations are to pick up electrons.

  • A substance which picks up electrons from something else is an oxidising agent.

  • The more positive the E° value, the stronger the substances on the left-hand side of the equation are as oxidising agents.

Mechanism of Hormone Action