18.1Variation and Selection
Variation — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions
5 min read 10 parts Lesson 1 of 3 3 quizzes
Lesson summary
18.1 Variation — IGCSE BIOLOGY notes: Describe continuous and discontinuous variation, genetic and environmental causes, and mutation.Variation = differences between individuals of the same species.Phenotypic variation — differences in features between individuals of a species.Fe…
- Genetic variation
- Environmental variation
- Inheritance
- New alleles
- Most mutations
- Harmful mutations
Section 1 of 10Types of variation
Types of variation
Variation
Variation differences between individuals of the same species.
Phenotypic variation
- Phenotypic variation differences in features between individuals of a species.
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Phenotypes | A range between two extremes with many in-between values | A limited number of distinct categories with no intermediates |
| Usually caused by | Genes and the environment | Genes alone |
| Examples | Height, body mass, finger length | ABO blood group, sex, tongue rolling, seed shape in peas |
| Graph | Smooth bell-shaped curve | Step-like bar chart |
Section 2 of 10Causes of variation
Genetic variation
- Genetic variation caused by differences in genes.
- Examples are blood group, eye colour, sex, tongue rolling and free or fixed ear lobes.
Environmental variation
- Environmental variation caused by factors outside the organism, such as climate, diet, accidents, culture and lifestyle.
- Examples are scars, weight gain from diet, the language a person speaks, and a plant growing taller in shade.
Inheritance
- Inheritance only genetic variation is inherited.
Section 3 of 10Genes and environment together
Genes and environment together
Genes decide what characteristics can be inherited.
The environment affects how these characteristics develop.
Children of tall parents have the genetic potential to be tall, but a poor diet limits their growth.
Section 4 of 10Mutation
Mutation
Mutation a genetic change.
New alleles
- New alleles mutation is the way new alleles are formed.
Most mutations
- Most mutations have no effect on the phenotype.
Harmful mutations
- Harmful mutations some have dramatic effects, for example sickle cell anaemia.
Beneficial mutations
- Beneficial mutations occasionally give a survival advantage, for example a feather colour change that attracts mates.
Gene mutation
- Gene mutation a random change in the base sequence of DNA.
Section 5 of 10Causes of increased mutation
Ionising radiation
- Ionising radiation gamma rays and X-rays damage bonds and change base sequences.
Ultraviolet light
- Ultraviolet light can also change base sequences.
Chemical mutagens
- Chemical mutagens such as tar in tobacco smoke.
Cancer
- Cancer higher mutation rates raise the risk of cancer.
Section 6 of 10Sources of genetic variation in populations
Mutation
- Mutation forms new alleles.
Meiosis
- Meiosis forms new combinations of alleles in the gametes.
Random mating
- Random mating determines which partners reproduce.
Random fertilisation
- Random fertilisation determines which sperm fuses with which egg.
Section 7 of 10Sickle cell anaemia and malaria
Cause
- Cause a mutation changes the haemoglobin protein.
HbS HbS
- HbS HbS sickle cell anaemia.
HbA HbS
- HbA HbS normal health and resistance to malaria.
Selection
- Selection heterozygotes survive malaria better, so the allele stays common where malaria is common.
Section 8 of 10More from the specification
1
Statistical variance in biological phenotypes
- Statistical variance in biological phenotypes computing standard deviation and variance across quantitative continuous traits provides a robust means to quantify phenotypic variation.
2
Continuous versus Discontinuous Phenotypic Variation
- Continuous variation exhibits a complete unbroken gradient across phenotypes governed by polygenic inheritance and environmental factors (e.g., height),
- whereas discontinuous variation displays distinct categorical phenotypes controlled by single genes.
- (e.g., blood groups)
3
Physiological Adaptations to Arid Environments
- Desert rodents like gerbils possess remarkably elongated loops of Henle in their renal nephrons, enabling extreme hyperosmotic urine concentration to minimize metabolic water loss.
- (up to 5400 mOsm/L)
4
Sources of Heritable Biological Variation
- Genetic diversity originates primarily through random gene mutations, meiotic crossing over, independent assortment of chromosomes,
- and the random union of gametes during fertilisation.
5
Natural selection
- Survival and reproduction of the best-adapted individuals.
6
Selective breeding
- Humans choosing which individuals breed.
7
Artificial selection
- Selective breeding of organisms with desirable traits by humans.
8
Adapted
- Having a structure suited to carrying out a particular function.
9
Development
- The changes as an organism becomes more complex and mature.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Variation | Differences between individuals of the same species. |
| Continuous variation | A range of phenotypes between two extremes. |
| Discontinuous variation | A limited number of distinct phenotypes with no intermediates. |
| Mutation | A genetic change. |
| Ionising radiation | Radiation such as X-rays and gamma rays that increases mutation rate. |
| Allele | A version of a gene. |
Section 10 of 10Exam technique
1
Saying all variation is inherited — environmental variation such as scars is not.
2
Saying mutations are always harmful — most have no effect and a few are beneficial.
3
Saying continuous variation is caused by genes only — it is caused by genes and the environment.
4
Saying mutations are caused by need — they are random.
Marked practiceQuizzes for this lesson
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