E volution

1. Evolution Patterns of Selection

2. Recall… • Last day we looked at how mutations provide a continuous supply of new genetic variations, which can be inherited and expressed as different phenotypes • Today: how do these factors relate to natural selection: genetic variation, competition within populations and diverse environments

3. Sickle-cell anemia • This allele is different from the normal gene for hemoglobin by having a single base-pair mutation • Homozygous individuals are severely affected • Heterozygous “ “ mildly affected but are much more resistant to malaria than those with the normal hemoglobin • Carriers of the allele are at a disadvantage in areas where malaria does not occur, while they are favoured in areas where malaria is common • Therefore they are much more likely to survive and pass on their genes to the next generation • In this case the environment provides the selection pressure

4. Sickle-cell anemia • The allele is only common where it provides an overall advantage to the individual • This establishes a relationship between mutations and evolution: • Harmful mutations are frequent, but are selected against and thus these mutant alleles remain rare • Beneficial mutations though rare, when selected for, accumulate over time • Genes provide the source of variation but not the selective forces

5. Stabilizing selection • Occurs when the most common phenotypes within a population are the most favoured by the environment==conversely any trait that deviates from this is selected against • Example: human birth weights are variable and partially inheritable • 3 kg is the norm for a human baby • Babies that weigh less are often premature, and those that weigh more often have complications affecting theirs and their mother’s survival • Natural selection has eliminated the extremes, so that today most babies are near the ideal weight

6. Stabilizing selection • By far the most common form of selection

7. Directional selection • Occurs when the env’tfavours individuals with more extreme variations of the trait, which could result in an observable change in the population

8. Directional selection • Humans can cause directional selection as well=salmon have large populations and short generation times

9. Disruptive selection • Occurs when the env’tfavours individuals at the extreme ends of the trait, as opposed to the intermediate variations

10. Black-bellied seedcracker finch • They depend on soft seeds and hard seeds for food • Finches with small bills are efficient feeding on the soft seeds, and those with large bills are able to crack the hard seeds • Important because it forms distinctive forms within a population that may become isolated breeding populations with separate gene pools

11. Sexual selection • Favours the selection of any trait that confers an advantage in terms of the mating success of the individual • This is associated with sexual dimorphism: which is the physical (often extreme) differences in the appearance of males and females • The most common forms of sexual selection are the results of female mate choice and male to male competition

12. Sexual selection • Females can chose based on physical traits, colouration, or behavioural traits such as courtship displays and songs • Sometimes males develop features that enable them to establish and defend a territory from other males=sometimes detaining the females • How would you be able to tell these are not env’tal selective pressures? • Both sexes would possess the features • MHC=sweaty shirts

13. Sexual selection • Some features are a compromise between mating and remaining conspicuous to predators==bright colours and song • Sometimes leads to extremes==runaway selection==stalk eyed fly % favouring long % favouring short Female Line of Origin

14. What about plants? • Sexual diversity is not limited to just animals • Plants do not select mates but they do need to attract suitors to assist in pollination • Flowers and scents are the most obvious examples of sexual features that have evolved==maximize pollination

15. References Pgs 556-564 Finish up lab HW questions