Selection
It is a process whereby some organisms successfully transfer their genes to the next generation
while others fail to do so. Organisms that manage to reach reproductive stage and successfully
compete for reproductive mates become parents of the next generation.
Natural Selection
This is selection based on the law of the jungle (survival of the fittest). Fitness is the probability
of the organism surviving and reproducing in the environment in which it is found. Natural
selection operates through the following:
1. Variation
Differences that exist among individuals of the same species. Some variations make some
organisms more adapted to their environment than others. Organisms that are less or not
adapted fail to transfer their genes to the next generation. Variation is mainly caused by
gene mutation, meiosis and fertilization.
2. Overpopulation
A lot of eggs and sperms are produced. Some of the eggs are eaten by predators. Some
seeds get eaten by birds while others fall in places where they cannot germinate. The
production of large numbers of eggs and seeds ensures that enough offspring make through
early stages of life to maintain the population from year to year.
3. Competition
Organisms that survive the early stages of life compete for food, water, space, mating
partners, sunlight and nutrients. When organisms like bacteria increase in numbers, they
get poisoned by their own excreta.
4. Adaptation
Surviving the competition is a result of adaptive features. An adaptive feature is an
inherited feature that helps an organism to survive and reproduce in its environment.
Adaptive features include: camouflage, rapid growth, long roots, resistance to disease,
anaerobic respiration and spore formation
Adaptive features of Xerophytes
These are plants which are adapted to survive very arid/dry conditions (where water is in
short supply)
-leaves have a smaller surface area so that less water will evaporate from them. Some
plants like cacti have leaves in the form of thorns.
-have swollen stems for water storage
-thick waxy cuticle to reduce transpiration
-hairy leaves which trap a layer of moist air next to their surfaces
-fewer stomata than usual which are on the underside or sunk in deep pits in leaves in order
for less water to evaporate.
-a deep root system or spreading root system to efficiently absorb any little water from rain
or overnight condensation
-produce abscisic acid which closes the stomata most of the time
-fleshy underground stems e.g cactus, acacia, oleander, prickly pear
Adaptive features of Hydrophytes
These are plants adapted to grow in wetlands
-wide, flat leaves to increase the rate of evaporation and absorb as much light as possible
-small roots which are only there for enchorage as there is no need for an extensive root
system because of the abundance of water
-very thin cuticle because there is no need to conserve moisture
-have hollow spaces in them (no xylem) that are filled with air to help them float on top of
water (buoyance) where they can get plenty of sunlight
-have stomata on both sides of the leaves
e.g hyacinth plant and Canadian pond weed
5. Evolution
This is the change in adaptive features of a population over time as a result of natural
selection. When the environment changes, the individuals whose features are favoured by
the new environment would start to compete successfully. Normally, individuals at the
extreme range of variation do not usually survive. E.g In case of bucks in the wild, those with
small body sizes are bullied from mating while those too big are exposed to predators
(easily seen). Their evading speed is also compromised by their very large sizes. It is the
medium sized bucks that survive and become parents of the next generation
