BioL2X: Genomes to Ecosystems (G2E) Lecture PDF

BioL2X: Genomes to Ecosystems (G2E) [email protected] https://llewellynlab.com/  Lecture Structure Pattern and process: evolution and selection Principals of natural selection Natural selection and the modern synthesis How can selection act on phenotypes...

BioL2X: Genomes to Ecosystems (G2E) [email protected] https://llewellynlab.com/  Lecture Structure Pattern and process: evolution and selection Principals of natural selection Natural selection and the modern synthesis How can selection act on phenotypes Quantitative genetics and heritability Special cases: sexual selection https://www.sli.do/ #X387  Evolution: Pattern and process Pattern is the change we see in the fossil record through time Infer evolutionary relationships between different fossil organisms and their living descendants Assumption that all organisms, extinct or extant are in some way related Who do these belong to ? Evolution: Pattern and process Infer ancestry via homology What is the process driving evolutionary change ?  Keeping on your toes…. Which of the following is true ? A. Pakicetus evolved into modern whales B. Pakicetus is more closely related to modern whales than it is to hippos C. Pakicetus is the ancestor of modern whales and hippos  Evolution: Pattern and process Ecological opportunity in the shallow equatorial Tethys ocean Abundant marine food sources in the shallows - what process is driving whales into the ocean ?  Evolution: Pattern and process The pattern of change we see in organisms over time – most conveniently on the fossil record** - tells us only that change is happening **As we’ll see later – our genes are another kind of fossil record The process by which evolution is happening (the ‘how?’) - has been the subject of considerable debate These days we broadly accept ‘natural selection’ as the model that best fits the evidence of evolution contained 1) within our genes, 2) within the homologies between living organisms or 3) within the fossil record Evolution: Pattern and process **the record in our genes  Principles of Natural selection: Darwin’s wasn’t the only theory Catastrophism Scalae Naturae Lamarckianis Series catastrophic m events defines the A solid march Individuals loose fossil record towards ‘perfection’ characters they Georges Cuvier Slime moulds at the don’t require (1812) bottom, us at the Acquired traits are top heritable JB Lamarck (1809) Mechanisms of Mutationalism evolution Species emerge in Theistic large jumps Also ‘saltationism’ Orthogenesis evolution Divine creation Hugo de Vries 1900 God generating Directional force beneficial driving evolution mutations ….the evolution Evolution is non- Asa Grey and reticulate of evolutionary Various proponents others thought…. ural selection and the modern synthesis: Darwin’s big ‘On the Origin of Species’ published 1859 Broad support among scientists (scientific naturalism) Skepticism from the church and the establishment (natural theologists) The result of 30 years of careful natural observation and Demographer synthesis…. Geologist Malthus Lyell Paleontologist Owen Transmutationist D-boy Darwin Sr.  Natural selection and the modern synthesis: Darwin’s theory in a nutshell Every species is fertile enough that if all offspring survived to reproduce, the population would grow Despite periodic fluctuations, populations remain roughly the same size Resources such as food are limited and are relatively stable over time A struggle for survival ensues Individuals in a population vary significantly from one another and much of this variation is heritable Individuals less suited to the environment are less likely to survive and less likely to reproduce; individuals more suited to the environment are more likely to survive and more likely to reproduce and leave their heritable traits to future generations, which produces the process of natural selection This slowly effected process results in populations changing to adapt to their environments, and ultimately, these variations accumulate over time to form new species. Natural selection and the modern synthesis: Mendel meets Darwin Darwin posited that a portion of inter-individual variation must be heritable Darwin developed ‘pangensisis’ theory of evolution involving ‘gemmules’ which were produced by all tissues. Pangenesis had a Lamarckian twist Mendel studies inheritance of seven phenotypic traits Developed two key ‘rules’ of inheritance 1) law of segregation (gametes), 2) law of independent assortment (prophase 1) Described genes as ‘factors’  Natural selection and the modern synthesis: the modern synthesis is born Mutation Genetic Variation Mendelian inheritance Malthusian Modern competition synthesis Natural selection Variation  How does selection act on phenotypes ? Frequency of individuals (note the selection does act on phenotypes, not genotypes) A hypothetical population of organisms Population ‘Trait’ is the phenotype under selection (e.g. tail length) Variation in trait values might be normally distributed without Value of a trait selection Selection type 1 – Stabilizing or ‘purifying’ selection Selection against extreme trait values Population after selection Population Phenotypic variation lost from before selection population Value of a trait Mean trait value stays the same  How does selection act on phenotypes ? (note the selection does act on phenotypes, not genotypes) Frequency of individuals Frequency of individuals Selection against extremes Selection type 2 – Directional Selection Population after selection Mean trait value moves in response Population to the direction and intensity of the before selection selection Value of a trait Selection against extremes Selection type 3 – Disruptive Selection Population after selection Selection against mean trait Population before selection The result is a multimodel trait distribution Value of a trait  How does selection act on phenotypes ? – some examples – you tell me… A) B) C) Halfway point evolutionary click-bait ntitative genetics and heritability: nature vs nu How much can we really blame on our parents ? The genetic component of any given trait is hard to disentangle from the environmental one. (Complicated by the fact we are often Broadly speaking, any given phenotype is affected by its genetic bais born in the same (the heritable environment bit), as as our well as an environmental component parents….) Genetics Phenotype Environment Thus the heritability (h2) of any given trait is the ratio of the genetic variation to total phenotypic variation h =V /V 2 G P Ignore the 2 Variability against the same genetic background = phenotyic plasticity  Quantitative genetics and heritability: using heritability to predict evolutionary change The essence of the modern synthesis is that selection due to environmental pressures can be propagated via heritability through evolutionary time G1 – Mean height 4 meters G2 – Mean height 4.5 meters  Quantitative genetics and heritability: using heritability to predict evolutionary change A way of directly measuring heritability is to look at how well trait values correlate between generations – i.e. how much do Offspring trait value F2 (phenotype) Offspring trait value F2 (phenotype) we look like our parents ? 2 Gradient = h 0 p Mean of parental trait value Mean of parental trait value Strong heritability Weak heritability  Quantitative genetics and heritability: using heritability to predict evolutionary change The greater the heritability of a trait, the faster a species can respond to selective pressure – the breeders equation Selection R=hS differential Evolutionary 2 response Change in phenotype due to Change in selection phenotype between generations Transmissibility of Heritability phenotype Confused ? Lets talk Giraffes…  Quantitative genetics and heritability: using heritability to predict evolutionary change G1 before selection 4 m G1 after selection 4.5 m G2 = ?  Quantitative genetics and heritability: using heritability to predict evolutionary change h2 = 1 IGNORE THE 2 New trait value 4.5 R = h 2S G1 before selection 4 m G1 after selection 4.5 m How much longer (R) ? S = 4.5 - 4.0 = 0.5 R = 1 x 0.5 = 0.5  Quantitative genetics and heritability: using heritability to predict evolutionary change h2 = 0.5 New trait value 4.25 R = h 2S G1 before selection 4 m G1 after selection 4.5 m How much longer (R) ? S = 4.5 - 4.0 = 0.5  Special cases: sexual selection Darwin noted that some characteristics in sexually dimorphic species could not readily be explained by natural selection. What good is a peacock’s tail ? Darwin recognized two mechanisms of sexual selection 1) Intrasexual selection, or competition between members of the same sex (usually males) for access to mates (accepted immediately) 2) Intersexual selection, where members of one sex (usually females) choose members of the opposite sex (ridiculed for 80 years!) Special cases: sexual selection – investment For example: sperm egg Special cases: sexual selection – variance in reproductive success  Special cases: sexual selection – and its all about variance in reproductive success Males compete for Little competition – low SS Females compete females, strong SS for males, strong on the males SS on the females  Take homes 1. The briefest of introductions to evolution and natural selection 2. You should appreciate the difference between pattern and process in evolution 3. Understand the principles of natural election and the modern synthesis 4. Understand how different types of selection can affected phenotypes 5. Understand what variance (G&E) underpins phenotypes 6. Understand heritability and the breeders equation