1. Variation and evolutionBy
Dr. Ali Ghazzay

2. Variation and Evolution
Give a definition of Variation
Variation is the range of differences that there are between individual organisms.

3. Variation and Evolution
Variation can be within species
(Think of all the differences between individual humans)
These are different varieties of the same species

4. Variation and Evolution
Variation can be
CONTINUOUS,
have any value between two extremes.
Examples:
Height of human
Body mass of cats
Trumpet length of daffodils
Leaf width of shrub
Length of bacteria
All these features
show normal
distribution (see graph)

5. Variation and Evolution
Variation can be DISCONTINUOUS , in this case there are only a few possible categories that the characteristic can fall into.
EG:
Flower color in a single species
Human blood group or
Bacteria that are or are not resistant to an antibiotic

6. Variation and Evolution
Genetic variation Each tomato is different because it comes from a plant with different alleles, that code for different characteristics
Environmental variation These plants are genetically identical, the flower color difference is due to the effects of soil pH.

7. Variation and Evolution
GENETIC
ENVIRONMENTAL
Differences that are due to the DNA inside the cells of the organism rather than the effect that its surroundings have on it.
Differences that are due to the external environment. For example availability of water, nutrients, light, prevailing winds.

8. Meiosis creates genetic variation
During normal cell growth, mitosis produces daughter cells that are identical to parent cell.
Meiosis results in genetic variation by mixing of maternal and paternal chromosomes and crossing over.
No daughter cells formed during meiosis are genetically identical to either mother or father.
During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. 8

9. The Origins of Genetic Variation
Offspring of sexual reproduction are genetically different from their parents and from one another. 9

10. Independent Assortment of Chromosomes
In independent assortment, every chromosome pair orients independently of the others during meiosis. 10

11. 11

12. Random Fertilization
The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote. 12

13. Crossing Over In crossing over,
Homologous chromosomes exchange genetic information.
Genetic recombination occurs. 13

14. 14

16. The result of nondisjunction

17. The Gene Pool
Members of a species can interbreed & produce fertile offspring
Species have a shared gene pool
Gene pool – all of the alleles of all individuals in a population 17 17

18. The Gene Pool Different species do NOT exchange genes by interbreeding
Different species that interbreed often produce sterile or less viable offspring e.g. Mule 18 18

19. Adaptations
Many of the differences between species are present because they help the individuals to survive.
We call these differences ADAPTATIONS.
You should understand that there are Adaptations to:
ANATOMY or body FORM,
PHYSIOLOGY or body FUNCTION
BEHAVIOUR

20. Variation leads to ?
Explain the consequences of the four observations made by Darwin in proposing his theory of natural selection.
What are the 4 observations that Darwin made?
1. Offspring generally appear similar to parents.
2. No two individuals are identical. (Why not?)
3. organisms have the ability to produce large numbers of offspring
4. Populations in nature tend to remain fairly stable in size.
SO what are the consequences? Evolution

21. Variation and Evolution
Because individuals over produce, yet populations remain stable COMPETITION must take place.
In COMPETITION there are “winners” and “losers”. Winners are better adapted with more useful characteristics, they are more able to survive and breed, so pass on their beneficial alleles.
Over time when this happens continually, accumulated changes can give rise to a new species.

22. Variation and evolution
Variation in organisms characteristics are caused by the organisms having different alleles of genes.
Some differences enable the organism to survive better (compete more successfully)
The ones with beneficial alleles survive, breed and pass on their alleles to the next generation
Those without beneficial alleles die before they reproduce, so their alleles are less likely to be passed on.
The beneficial alleles increase in frequency in the population and may eventually produce a a new species.

23. Microevolution
Changes occur in gene pools due to mutation, natural selection, genetic drift, etc.
Gene pool changes cause more VARIATION in individuals in the population
This process is called MICROEVOLUTION
Example: Bacteria becoming unaffected by antibiotics (resistant) 23 23

24. Selective Pressures Competition for food, water Predation Disease
Physical and chemical factors
Competition for mates
Competition for space, nesting sites, territory etc
An allele that helps an organism compete better is “selected for”, increases in frequency
An allele that fails to help an organism survive is usually “selected against”, decreases in frequency

25. Evolution of a new species
Speciation occurs when there is a “Reproductive Barrier”, some thing that stops members of an original population passing their alleles freely among all members, this prevents free gene flow. The barrier may be a geographical barrow = Allopatric speciation. The barrier may be behavioural, biochemical, or anatomical = sympatric speciation.

26. In Summary: Causes for Genetic Variation and Evolution
Genetic Drift
- the change in the gene pool
of a small population due to chance of v va
Natural Selection
- success in reproduction based on heritable traits results in selected alleles being passed to relatively more offspring (Darwinian inheritance)
- Cause ADAPTATION of Populations
Gene Flow
-is genetic exchange due to the migration of fertile individuals or gametes between populations 26 26

27. Causes Continue Crossing Over
Exchange of segments of chromosomes during meiosis
Mutations
a change in an organism’s DNA
Mutations can be transmitted in gametes to offspring
Non-random mating
- Mates are chosen on
the basis of the best traits 27 27