CIE IGCSE Biology Unit 18: Variation and Selection

variation

differences between individuals of the same species

phenotypic variation

the variability in phenotypes that exists in a population caused by both genetic and environmental factors

genetic variation

diversity in gene frequencies caused by mutation, sexual reproduction and genetic drift

continuous variation

results in a range of phenotypes between two extremes, e.g. height in humans

discontinuous variation

caused by genes alone, e.g. A, B, AB and O

blood groups in humans, results in a limited number of phenotypes with no intermediates, e.g. tongue rolling

mutation

genetic change, is the way in which new

alleles are formed

gene mutation

change in the base sequence of DNA

Factors of rate of mutation

ionising radiation and some chemicals increase the rate of mutation

sickle-cell anaemia

A genetic disorder affecting the structure of the haemoglobin which in turn affects the shape of the red blood cells, making them sickle shaped so they don't carry oxygen efficiently.

Cause of sickle cell anemia

change in the base sequence of

the gene for haemoglobin results in abnormal

haemoglobin and sickle-shaped red blood cells

Resistance to malaria

people who are heterozygous (HbS HbA) for the sickle-cell allele have a resistance to malaria

Explain the distribution of the sickle-cell allele

in human populations

People with sickle cell produce only sickle shape haemoglobin which is not good in transporting oxygen, common in areas with malaria because people with heterozygous have both normal and abnormal forms of haemoglobin and have a resistance to malaria due to disease cannot enter their red blood cell

adaptive features

the inherited functional features of an organism that increase its fitness

fitness

the probability of an organism surviving and reproducing in the environment in which it is found

adaptive features of hydrophytes

-Stomata on top of leaf for gas exchange

-Have large air spaces in leaf for leaf to be buoyant

adaptive features of xerophytes

-Stomata in pits to reduce water loss

-Leaves rolled up or turn into thorns to reduce surface area for evaporation

-Thick waxy cuticle

natural selection

- variation within populations

- production of many offspring

- competition for resources

- struggle for survival

- reproduction by individuals that are better

adapted to the environment than others

- passing on of their alleles to the next

generation

selective breeding

- selection by humans of individuals with

desirable features

- crossing these individuals to produce the

next generation

- selection of offspring showing the desirable

features

evolution

the change in adaptive features of a population over time as the result of natural selection

process of adaptation

the process, resulting from natural selection, by

which populations become more suited to their

environment over many generations

example of evolution by natural selection

the development of strains of antibiotic resistant bacteria

artificial selection vs natural selection

Artificial selection:

-selection due to human influences

-produces organisms different form native generations

-does not result in new species

-Inbreeding causes loss of vigor

-Fast

-Proportion of heterozygous individuals is reduced

Natural selection:

-Selection due to enviromental factors

-Produces greater biodiversity

-May lead to new species

-Outbreeding is common leading to hybrid vigor

-Slow process

-Proportion of heterozygous individuals remains high