Bisc307 Lecture 2 1 11 12 Topics covered Evolutionary Perspectives What is evolution Descent with modification Outcome is an organism matched adapted to its environment Matching optimizes reproductive success fitness Reproductive success the probability that one s genes will be passed on to the next generation This is the concept of fitness Evolution does not optimize lifespan or general health It does not optimize happiness or wellbeing Within a population evolution optimizes fitness in the environment If the environment changes the population s fitness will suffer Driving force is natural selection Mechanisms of Evolution Evolution is based on 3 main observations 1 Species produce more offspring than resources can support 2 Individuals vary in ability to compete for resources survive and reproduce variation 3 Some of this variation is heritable inheritance Inherited variability is due to 3 main sources mutations recombination during meiosis and epigenetic modifications modifications of DNA that affect gene expression and are inherited but do not involve changes in nucleotide sequences e g histones binding to DNA or methylation of DNA A fourth source is developmental plasticity the environment during embryonic and postnatal development that affects the phenotype which is especially important in the development of the CNS in utero and post natal Darwin took these observations and made them into the following two conclusions 1 Differences in survival and reproduction lead to over representation of more successful forms in next generation selection 2 Selection over generations leads to change in composition of the population evolution And how long it takes depends on generation time and selective pressures But over time it will lead to changes in composition of the population Can be small microevolution or large macroevolution includes production of a new species Natural Selection Survival of the Fittest Selection maximizes reproductive fitness not health or longevity Ex There are insects that hatch mature breed and die in a day and they are very reproductively fit Natural selection doesn t CARE about the well being or survival of the individual It only cares about the successful transmission of genes to the next generation Evolutionary success of humans did not require a long life as part of the human reproductive strategy Looking at the graph on the left before the marked increase in human longevity starting in the 1800 s the human life span was estimated to be 30 years In the past most people didn t live long enough to contract many of the popular chronic diseases that occur during middle or advanced age Historical life span of humans also makes sense if you look at human fertility as a function of age next page Fertility peaks in the 20 s and falls precipitously after that This is a good match with the past lifespan Reproductive fitness is maximized in humans in their 20 s and therefore diseases that affect people in their 40 s and beyond have little to no effect on our reproductive fitness Natural selection cannot optimize things that occur AFTER reproductive period Ex Heart failure Why hasn t evolution fixed this In young individuals the mechanisms that heart and kidney use to compensate for heart failure work very well in a young healthy reproducing person But heart failure is a disease of middle and old age past the reproductive fitness period in which evolution can take effect These flaws can t be removed by natural selection because they do not affect reproduction Humans Are Bipedal and Have Large Brains These are the two most important characteristics that set humans apart Being bipedal means we can stand up on hind limbs And our brains specifically our neocortexes are huge Graph on left brain weight vs body weight Most primates fit on the line But you can see that humans are above the line red circle We have unusually large brains relative to our bodyweight Evolution results in compromises Ex 1 Having our spines upright is a bad design because all of our weight and pressure is on our spine so our knees and ankles are easily injured but being upright is a big enough evolutionary advantage to compromise for the vulnerable design of the human knee Bipedalism narrow pelvis vs newborn survival large head This is another example of an evolutionary tradeoff Our pelvises need to be narrow so we can walk upright and balance efficiently However babies need to be born as developed as possible with big brains to maximize chances of survival But it is difficult to have a huge brain coming out of a narrow pelvis right The human head can only barely fit through the pelvis and only if it is in the correct orientation Without assistance the success rate is only 85 How could evolution have resulted in a system that is 15 faulty It is all about compromise The female pelvis is as wide as it can be to allow efficient walking and running And the baby s brain is as large as it can be but can still barely fit through the pelvis Having a large brain allowed for the evolution of language culture society Moms giving birth had the assistance of older members who could help them This tradeoff for a big brain and bipedalism was good enough to ensure the survival of our species Babies are altricial they need extended parental care Large brain size has other ramifications the head cannot be fully developed at birth We are not a precocial species like zebras a newborn zebra can be walking with the herd within an hour Babies are altricial opposite of precoccial They need a lot of help because their brains are born very immature Humans cannot survive independently before 8 years after birth Humans had to invest a lot of effort into caring for their young As a result women could not have very many children and humans tended to live in groups Anthropologists looked at sizes of groups It turns out that the ideal group size for humans is 150 Beyond that we lose ability to keep track of social structure Evolving this ability to work in social group is one of the strategies to deal with evolutionary compromises like taking care of babies Human Body is Full of Design Flaws Evolution only works by modifying what was there before It can t start over Characteristics are locked in from the beginning As a result design flaws exist 1 The inside out retina Our retina is inside out light has to pass through our blood vessels before it hits our retina Blood is not transparent so you have the shadow of the blood