NATURAL SELECTION AT THE LEVEL OF THE ALLELE

1. Sickle cell anemia NATURAL SELECTION AT THE LEVEL OF THE ALLELE Biston betularia

2. Industrial melanism • The melanic allele has a selective advantage in polluted environments • More individuals with the melanic allele are reproduce • The next generation will have a higher proportion of the melanic allele compared to the other (speckled) • In successive generations it will become predominant until it is said to be fixed in the population © 2008 Paul Billiet ODWS

3. Recessive alleles can hangon • The melanic allele is dominant(M) • If it is present it will be expressed and selection acts on it immediately • The speckled allele is recessive (m) so it will be carried by heterozygotes (Mm) • It may remain “hidden” in the population form many generations • Thus the melanic allele may not become completely fixed © 2008 Paul Billiet ODWS

4. Balanced and transient • Industrial melanism is an example of transient polymorphism where one allele replaces another • Sickle cell anaemia is an example of balanced polymorphism • Two alleles are advantaged in zones infested by malaria • The sickle allele gives protection to malaria but can lead to a fatal blood disease • The normal haemoglobin allele permits normal transport of oxygen but gives no protection to malaria © 2008 Paul Billiet ODWS

5. From one species to another • An accumulation of many advantageous alleles • Eventually individuals with different alleles can no longer breed together • Separate species are formed that are genetically incompatible • Many generations are needed • Therefore, long periods of time are needed © 2008 Paul Billiet ODWS

6. But what is a species? • A group of individuals that breed together freely in nature to produce fully fertile offspring • Does forced mating count? • If populations are geographically separated it is not possible to test this definition • If populations are separated in time it is not possible to test this e.g. fossils in different strata • Some species only show asexual reproduction • A potentially interbreeding population having a common gene pool © 2008 Paul Billiet ODWS

7. Madagascar Google earth Ringtailed lemurs (Lemur catta) Mechanisms of speciation Isolation of a population so that it cannot breed freely with others is necessary • Geographic • Ecological • Behavioural • Mechanical/anatomical • Physiological • Genetic © 2008 Paul Billiet ODWS

8. Geographic Fragmentation of the range • Changes in climate isolate populations on mountain tops cause a rise in sea level creating islands • Geological changes which raise mountains or create new seaways Migration • Migration of a population to a new area • If the population is small it may not represent the gene pool of the parent population left behind (the founder effect) © 2008 Paul Billiet ODWS

9. SawFly (Tenthredo livida) Ecological • Populations can become isolated within the range of the parent population • Differences in food preferences may develop in a part of the population that stop them from breeding freely • Seasonal isolation may occure.g. different flowering times or breeding seasons © 2008 Paul Billiet ODWS

10. Snow geese (Chen caerulescens) Behavioural • Parts of a population may develop a preference for a particular variety • They may not mate with any other • e.g. The snow goose blue forms tend to mate with blue forms and white forms tend to mate with white forms © 2008 Paul Billiet ODWS

11. Pin and thrumb primroses (Primula vulgaris) Mechanical/anatomical • Genetalia or floral parts may be incompatible © 2008 Paul Billiet ODWS

12. Fertilisation Physiological Fertilisation may be prevented by: • Failure of the gametes to be attracted to one another • The sperm cell receptors of the oocyte may be incompatible with the acrosome • Pollen tubes cannot find or penetrate the embryo sac in flowers © 2008 Paul Billiet ODWS

13. Zedonk Genetic Hybrid inviability • Hybrid offspring die • Hybrid infertilityHybrids survive but are incapable of producing gametes Liger © 2008 Paul Billiet ODWS