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Lecture 5

BIOLOGY 1M03 Lecture 5: 5- Evolution (Constraints and HWE)

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Ben Evans

Bio 1M03 January 14, 2016 Evolution Myth: Natural Selection results in the most optimal phenotype - Not true: selection is not able to optimize all aspects of a trait due to genetic correlations and various types of constraint (fitness tradeoffs, historical, formal, temporal) - Selection also can only act on available variation, so the level of molecular variation is also a constraint for adaptation Fitness Tradeoff - Compromise between traits in terms of how those traits are adapted for the environment - Humans were evolved from 4 legged animals, humans developed bipedalism and the pelvis underwent very large changes compromising the cranial size - Compromised the pelvis - Brain size and human pelvis are examples of trade-offs - Humans evolved a brain that is constrained by the birth canal width - Parental care: effort per offspring versus number of offspring - Peacock tails: flight ability versus reproductive success - Sickle Cell anemia: reduced oxygen transport versus malaria resistance - Allele when homozygous causes red blood cells to lose their biconcave shape, if you have this disease then it is beneficial since it allows resistance to malaria - Malaria is one of the largest killers - Morning sickness: mother feels sick but potential benefit to fetus - Those who duffer from morning sickness, have kids that grow up to be healthier Genetic correlation - Occurs when selection favoring alleles for one trait causes a correlated but suboptimal change in an allele for another trait - Other repercussions since the gene has multiple functions - Changing one feature (or gene) often affects multiple feature - During evolution, reduction in jaw size associated with the increased size of the skull led to suboptimal tooth arrangement in humans including wisdom teeth - The number of teeth did not change, but the amount of space is decreased causing the need for wisdom tooth removal and braces - Many genes have pleiotropic effects- they influence more than one characteristic - Pleiotropic means the gene is capable of multiple functions - Pleiotropic genes act as a drag to prevent certain optimal features - Change in the jaw and cranial structure - Compromise structure Historical constraints - Evolution modifies ancestral traits - Must begin at the ancestral starting point - Presents variation biases future possibilities - The evolution of all the appendages was constrained by ancestral appendage - Homologous bone structures - Constrained by the opportunities they could reach due to ancestral starting point - In both the morphology and genetic limitations affect the historical constraints - Limits the world of opportunities as they must start from one starting point Formal constraints - Evolution needs to work within the laws of physics Temporal constraints -9 - Rate of mutation is usually very slow (1x10 mutations per generation) - Most mutations occur in non-coding areas in the genome - The mutations that have effects are very rare - Most mutations have no effect, but most mutations are deleterious - Most mutations are either not advantageous, or have a small phenotypic effect - It generally takes a long time to wait for a new mutation that actually produces a useful phenotype - More often, natural selection works on variation that is already present in a polymorphic ancestor - Evolution occurs by mutation and it takes time for a series of useful mutations to occur - Rate of mutation is usually very slow - Mutations can be categorized 2 way: affects gametes and affects somatic - Most mutations occur in somatic which cannot be passed on - Somatic mutations cannot affect fitness and do not affect every cell in the body - Those that occur in the germ line do not affect the person carrying them they affect offspring - Mutations take time - Most mutations are neutral or have small phenotypic events - Natural selection acts on mutations and variation that is already present Consequences of Natural Selection - Natural selection is not a clean cut process - The outcome cannot be predicted that easily - It takes generations to see a change - Outcome is not always what is expected - More often natural selection creates a compromise rather than creating the optimal, wanted effects Hardy-Weinberg Equilibrium - Five factors that tend to change allele frequencies o Natural selection: increase or decrease in the frequency of certain alleles o Genetic drift: causes allele frequencies to change stochastically
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