Heredity (part 5): Discontinuous and Continuous Variation

1. Discontinuous and Continuous Variation

2. Lesson Objectives (j) describe the difference between continuous and discontinuous variation and give examples of each (k) state that competition which arises from variation leads to differential survival of, and reproduction by, those organisms best fitted to the environment (l) give examples of environmental factors that act as forces of natural selection (m) assess the importance of natural selection as a possible mechanism for evolution (n) give examples of artificial selection such as in the production of economically important plants and animals

3. Variations • Variations are differences in traits of characteristics between individuals of the same species • Discontinuous variations e.g. Mendel’s pea plants either tall or short and had no intermediate forms between these traits • Easily distinguishable and not affected by environmental conditions • Brought about by one or a few genes

4. Continuous variation • When traits do not fall into clear-cut classes • e.g. intermediate skin colour and height • There is a continuous variation from one extreme to the other • This is brought about by the additive (combined) effect of many genes e.g. more dark genes than fair genes the darker the skin tone

5. Discontinuous Variation Continuous Variation Deals with a few clear-cut phenotypes Deals with a spectrum of phenotypes, ranging from one extreme to the other Controlled by many genes Controlled by a few genes Genes do not show additive effect Genes show additive effect, e.g. the more ‘dark’genes, the darker will be the skin colour Not modified by environmental changes Modified by environmental conditions, e.g. greater exposure of the skin to sunlight will produce a darker skin colour

6. Artificial selection

7. Artificial selection • By crossing different varieties of plants and animals to produce more variations in the offspring • By selective breeding, special or improved breeds (with desirable traits) of plant and animals are produced

8. Examples of artificial selection

9. Examples of artificial selection

10. Selective breeding of plants Grow the seeds, self- fertilise and selection of seeds with highest oil content • After many generations, pure-breeding plants that produce seeds with the desired qualities are obtained. The plants are self-pollinated to ensure the desirable genes are inherited by future generations of plants. Self-pollination of pure breeding plants • Breeders analyse seeds from different plants and select plants that produce seeds with high oil content. • ‘Good’ seeds grow into new plants and self- fertilise. Resulting seeds with the highest oil content are selected and used again as parents for the next generation Selecting plants with good gene combinations for oil production

11. Hybridisation in plants • When breeders cross different varieties of plants to produce a new and better variety. • For example, wild type of sugarcane (resistant to disease but low in sugar) with cultivated sugarcane in Java rich in sugar but susceptible to disease to produce a hybrid that is both resistant to disease and rich in sugar content. Disadvantage: Hybrid seeds are costly as hybrids do not breed true i.e. they do not produce offspring identical to their parents. hybrid needs to be reproduced via vegetative propagation or hybridisation process has to be repeated every generation. The the

12. Selective breeding in animals • Cows that produce plenty of milk and good meat are selected and used as female parents for the next generation. • However, unlike plants, animals cannot be self- fertilised. So breeders, mate cows with desirable traits with a bull that has known required traits. This way they are more likely to obtain the desirable gene combination.

13. Selective breeding in animals • Cows that produce plenty of milk and good meat are selected and used as female parents for the next generation. • However, unlike plants, animals cannot be self- fertilised. • So breeders, mate cows with desirable traits with a bull that has known required traits. This way they are more likely to obtain the desirable gene combination.

14. Hybridisation in animals • For example, the Jersey cow is known to produce plenty of good milk, but it does not thrive well in warm climates. Whereas the Brahman breed of cattle thrives in warm climates but their cows are not good milk producers. • Breeders living in warm climates allow the Jersey cow to mate with the Brahman bull, so that the genes with good qualities from the two parents will be passed down to the hybrids.

15. Hybridisation in animals • Many rounds of mating might be required to obtain a pure breed. • So breeders continue to select and hybridise their cattle until they produce a new variety of cattle that contains the good qualities of both the Jersey and Brahman breeds. One this is achieved, the improved breed of cattle must be maintained by inbreeding (breeding among close relatives)

16. Dangers of inbreeding • Inbreeding can result in an accumulation of recessive alleles in the population. • These recessive genes that are not expressed in the heterozygous parents are more likely to be passed down to the offspring. • If the recessive genes codes for a genetic disease, homozygous recessive offspring will suffer from the disease e.g. inbreeding among horses result in a lethal intestinal defect in foals.

17. Natural selection vs Artificial selection Natural Selection Selection occurs when natural environmental condition change Varieties are produced by mutations Artificial Selection Humans select the varieties of organism that suit their needs. Varieties are produced by selective breeding