Darwinian Natural Selection

Darwinian Natural Selection Evidence of Evolution • Direct observation: species change • Fossils show intermediate forms • Extant species show structural, developmental and genetic homology • Vestigial traits reveal ancestral condition – Structural, developmental, genetic • That is, descent with modification becomes fact But HOW does life evolve • What mechanism(s) account for the fact of evolution? • Darwin’s answer: natural selection Artificial selection Evolution by natural selection is an outcome • If the following are true: – Individuals vary (within a species) – Some of that variation is genetic – More offspring are produced than will themselves reproduce – Reproduction is not random Survival and reproduction • ‘Survival of the fittest’ • It is reproduction that counts, not survival • Survival is often necessary for reproduction Differential Reproduction • Differential: some more than others • So if some reproduce more than others, then the traits of those that reproduce more will become more frequent • Everyone alive today had ancestors that reproduced Populations change over time • If some genotypes reproduce more than others, • THEN the population will change • That is a necessary outcome, and that is natural selection This is a testable idea • Natural selection relies on four things being true • Each of those things can be independently tested and verified (or refuted) • No belief or faith is required – You can simply understand this and think that it is true Natural selection occurs if: • • • • Individuals within a population vary Some of that variation is genetic Not everyone survives and reproduces Reproduction is not random with respect to traits that vary genetically An example: Galapagos finches • Peter and Rosemary Grant have studied finches on the Galapagos islands since 1973 Galapagos Archipelago Daphne Major 40 hectares (hectare = 10,000 square meters, or 2.471 acres) Finches 14 species Phylogeny based on DNA sequence variation Lots of beak variation among species We focus on Geospiza fortis, medium ground finch Do individuals in a population vary? • Population of G. fortis averages 1200 individuals • Almost all have been captured, marked, measured. Is some of that variation genetic? • Heritability: the proportion of phenotypic variation in a population that is due to genetic variation • Proportion ranges from 0 to 1. • So variation could be: – Completely due to environment: h2 = 0 – Completely due to differences among genes: h2 = 1 – Due to both genetic and environmental factors • 0 < h2 < 1 Heritability estimated from resemblance among relatives Does everyone survive and reproduce? • Finch example: 89% of G. conirostris die without breeding • In 1977 severe drought caused crash in G. fortis population Drought, food, and finches Beyond finches….think • What would happen to the population size if the average female produced more or less than one surviving reproducing daughter? – More than 1 ? – Less than 1 ? • So in sexual species, if each male and female couple produces two surviving reproducing offspring OVER THEIR ENTIRE LIFE then the population is stable Example • Female cricket mates with a male, lays 100 eggs over her life • What % of those survive and reproduce (on average) if the population is stable? • Male and female robin make a nest with four eggs year after year for 7 years = 28 eggs • On average, only 2 of 28 survive and reproduce – One replaces the female, one replaces the male Potential for reproduction OK, the majority of reproductive potential is not met • Death • Failure to reproduce • The question is, is the reproduction that does occur random with respect to heritable traits or not? Non-random finch survival 1977 What caused non-random survival? • That is, why were finches with bigger beaks favored by selection? • Average seed size increased – Large hard fruits of Tribulus cistoides became important for survival Available Seed Size Summary of Finch Example • Individuals within a population vary – True for about everything ever measured • Some of that variation is genetic – – – – Also true for hundreds of traits measured Morphological traits typical h2 ~ 0.5 Behavioral traits typical h2 ~ 0.3 Life history traits typical h2 ~ 0.2 Summary of Finch Example • Not all individuals survive and reproduce – Also true for all stable populations with potential reproduction greater than 1 daughter per female • Survival and reproduction not random – Finches: birds with thicker beaks favored because environmental conditions changed – Also true in hundreds of documented cases Evolution by natural selection results • What a simple idea! • T. H. Huxley reported to have said ‘How stupid not to have thought of that before’ Natural Selection recap • Evolutionary change via the mechanism of natural selection is a necessary result of – Variation among individuals within a population – A genetic basis to that variation – Excess reproductive potential – Non-random reproduction (entailing survival) • Clarifications Selection versus evolution • Selection is differential reproduction, it: – Occurs within a generation – Acts on individuals – Acts on phenotypes (expressed traits) • Evolutionary change – Occurs across generations – Within a population – Is change in genotype frequency Selection within a generation • Finches – Some birds in 1977 lived some died • HIV virus – Some virions with AZT resistant RT active site mutations reproduced, others without the mutation did not Evolutionary change across generations • Finches – 1978 population of finches bigger and with deeper beaks than 1976 finches • HIV – The population of virions late in infection is AZT resistant, whereas few virions are resistant in the early virion population Selection acts on individuals • Some individual finches died without reproducing • Some individual virions could not transcribe RNA to DNA using reverse transcriptase in the presence of AZT so those individual virions did not reproduce (but others did) Selection depends on phenotype • It is the interaction with the environment that causes success or failure • That interaction is mediated through the phenotype…….. • But the change in the population as a whole over time is genotypic Alternatively • Within a generation, individuals either reproduce or not depending on their phenotype • Across generations, population changes in genotypic frequency result Natural selection is not predictive • Evolution by natural selection is a consequence of the fitness of phenotypes in preceding generations • It is impossible for organisms to forecast useful mutations • Natural selection does not directly create populations composed of well adapted individuals, it does it indirectly by killing off less fit alternatives How can new traits appear • Selection acts to weed out less successful variants based on existing phenotypes….so how do new traits arise? • Selection removes variation • Mutation adds variation Change over time • 60 generations of artificial selection • Produced new trait values beyond natural range What about new traits? Producing imperfection • Natural selection maximizes reproduction • Takes what it can as starting material • Optimizes individual features subject to constraints • Constraints may be – Developmental – Correlated characters – Trade-offs Producing complex adaptation • Objection of some religious groups – How can a random process create complex adaptations? – Valid point is that intermediate steps need to be favored by selection as well • A complex adaptation can not originate because it will be useful once it is formed – Evolution does not anticipate – Intermediate forms need explanation What good is an imperfect eye? • Biologists answer on several fronts: – Who has perfect eyes? • An intermediate eye needs function – But it is competing with non-eyes – An intermediate eye may well be worse that a good eye, but it isn’t competing with good eyes (yet) • First eyes may simply sense light Mollusc eyes a) Pigment spot b) Pigment cup c) Optic cup (abalone) d) Complex eye with lens (snail Littorina and octopi) There is no such thing as evolutionary progress • There is no goal • All organisms currently alive had each and every one of their ancestors survive long enough to reproduce • There is no purpose to evolution • Life has no extrinsic meaning fitness • • • • Means reproductive success Not stronger, faster, smarter Not longer lived Just producing more surviving offspring that the alternative • ‘Survival of the fittest’ means that genes that promote reproduction will survive, genes that reproduce less will disappear Levels of Selection 1: Not Species • Evolution by natural selection is NOT different species competing for survival – Cheetahs chase, sometimes catch and eat gazelles • Selection by cheetahs favors faster gazelles • Faster gazelles selectively favor faster cheetahs – Gazelles compete for survival with gazelles – Cheetahs compete for survival with cheetahs Levels of Selection 2: Not Groups • Groups are not a significant evolutionary unit (for most purposes) – Selection acts on individuals within a population – So selection does not produce adaptations that are “good for the group” or “good for the species” – If a trait promotes the reproduction of the individual that has it, then it is favored Problems with Darwin’s formulation • In 1859, the genetic basis of inheritance was unknown – Darwin could not provide mutation as a mechanism adding variation depleted by selection – Darwin could not explain how beneficial traits can be inherited intact rather than be lost • Mendel’s experiments showing independent segregation and assortment unknown to Darwin • Traits thought to be mixed, or blended together • Lord Kelvin calculated the earth to be 15-20 million years old – Based on assumption of radiant cooling The Modern Synthesis • Integration of genetic understanding – Individuals differ because mutation creates new alleles which are found in novel combinations because of independent assortment and segregation – Individuals pass alleles to offspring – In most generations more offspring produced than will themselves reproduce – The individuals that do reproduce have a nonrandom subset of alleles that make them well adapted to their environment Why is evolution controversial? • It isn’t….scientifically that is. • Some religious fundamentalists object to the idea of evolution. • There is a basic difference between science and religion. The bases of science and religion • Science is fundamentally skeptical – Evidence is required – Constantly questioning – Modifies views in light of evidence • Religion is fundamentally faith based – Belief is required, evidence is not Darwin’s outlook (1859, p. 490) “There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and wonderful have been, and are being, evolved.”