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

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 ODWSODWS

Recessive alleles can hang on 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 ODWSODWS

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 ODWSODWS

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 ODWSODWS

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 ODWSODWS

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

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 ODWSODWS

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 occur e.g. different flowering times or breeding seasons SawFly (Tenthredo livida) © 2008 Paul Billiet ODWSODWS

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 Snow geese (Chen caerulescens) © 2008 Paul Billiet ODWSODWS

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

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 Fertilisation © 2008 Paul Billiet ODWSODWS

Genetic Hybrid inviability Hybrid offspring die Hybrid infertility Hybrids survive but are incapable of producing gametes Liger Zedonk © 2008 Paul Billiet ODWSODWS