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

2. Industrial melanism
The melanic allele has a selective advantage in polluted environments
More individuals with the melanic allele are reproduced
The next generation will have a higher proportion (frequency) 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.
© 2016 Paul Billiet ODWS

3. 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 for many generations
Thus the melanic allele may not become completely fixed.
© 2016 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 (Hbn & Hbs) 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.
© 2016 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 = speciation
Many generations are needed
Therefore, long periods of time are needed.
© 2016 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
© 2016 Paul Billiet ODWS

7. Mechanisms of speciation
Madagascar Google earth
Isolation of a population so that it cannot breed freely with others is necessary
Geographic = allopatric
Sympatric =
Ecological
Behavioural
Mechanical/anatomical
Physiological
Genetic
Ringtailed lemurs
(Lemur catta)
© 2016 Paul Billiet ODWS

8. Geographic: allopatric speciation
Fragmentation of the range
Changes in climate isolate populations on mountain tops 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).
© 2016 Paul Billiet ODWS

9. Sympatric speciation
Formation of a new species in the same geographical area.
© 2016 Paul Billiet ODWS

10. 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 occur e.g. different flowering times or breeding seasons.
SawFly (Tenthredo livida)
© 2016 Paul Billiet ODWS

11. 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.
Snow geese (Chen caerulescens)
© 2016 Paul Billiet ODWS

12. Mechanical/anatomical
Genetalia or floral parts may be incompatible.
© 2016 Paul Billiet ODWS
Pin and thrumb primroses (Primula vulgaris)

13. 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
© 2016 Paul Billiet ODWS

14. Genetic Hybrid inviability Hybrid offspring die
Hybrid infertility Hybrids survive but are incapable of producing gametes
Zedonk
Liger
© 2016 Paul Billiet ODWS

15. Polyploidy
An extra chromosome is aneuploidy but a whole extra set of chromosomes is polyploidy
Tolerated in plants (cabbage family, ferns) more than animals (salmon, salamanders and goldfish)
Most flowering plant species show some signs of polyploidy in their evolution
Can be detect easily these days using flow cytometry
Example Allium species.
Christmas Fern
© 2016 Paul Billiet ODWS

16. Mechanism Problem in anaphase results in extra set
Asexual reproduction increases the numbers of polyploids
Polyploids reproduce amongst themselves (selfing).
© 2016 Paul Billiet ODWS

17. Advantages of polyploids
Better capabilities to survive harsh environments
Polyploid species seem to increase in numbers with latitude, altitude and areas of high salinity.
© 2016 Paul Billiet ODWS

18. Allium fistulosum cell culture
© 2016 Paul Billiet ODWS

19. Adaptive radiation
A single species migrating into a newly formed area (e.g. a lake, a volcanic island)
Lots of niches are vacant
The species diverges and fills the niches
Divergent evolution.
Darwin’s finches

20. Divergent Evolution
Populations of a species that are separated and evolve under different selective pressures develop different adaptations as they diverge
They are usually geographically separated so that there is a barrier to the mixing of genes
Divergent evolution produces homologous organs
Organs where the same anatomical structure is adapted for different purposes.
e.g. the pentadactyl limb
© 2016 Paul Billiet ODWS

21. Convergent evolution
Species in different areas evolve the same adaptations
The selective pressures are the same.
Cactus
New Mexico
Euphorbia
S Africa
© 2016 Paul Billiet ODWS

22. Convergent evolution Thylacine Thylacinus cynocephalus
Wolf Canis lupus
© 2016 Paul Billiet ODWS

23. Convergent v divergent
Convergent same adaptations evolve Divergent different adaptations evolve
Convergent unrelated species Divergent related species
Convergent same niche in different areas Divergent different niches in the same area.
© 2016 Paul Billiet ODWS