* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Evolution
Survey
Document related concepts
On the Origin of Species wikipedia , lookup
Punctuated equilibrium wikipedia , lookup
Sympatric speciation wikipedia , lookup
Theistic evolution wikipedia , lookup
Genetic drift wikipedia , lookup
Evidence of common descent wikipedia , lookup
Sexual selection wikipedia , lookup
Inclusive fitness wikipedia , lookup
Hologenome theory of evolution wikipedia , lookup
The Descent of Man, and Selection in Relation to Sex wikipedia , lookup
Saltation (biology) wikipedia , lookup
Transcript
Evolution Covered This Darwin’s Theory of Natural Selection Summary of Key Points Examples of Evolution by Natural Selection What influences Phenotype - Nature vs. Nurture? - “Common Garden” Experiments - Molecular Methods Types of Selection Genetic Drift Not Covered Yet Speciation Readings: Chapter 4 Charles Darwin (1809-1882) Published On the Origin of Species (1859) (Full Title: On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life) Provided a mechanism for evolution (“natural selection”): • Individuals vary • Not all survive • Those with favored traits tend to survive • Traits are inherited • Population changes (species evolve) Excerpt from Darwin’s Book As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form. – Charles Darwin Darwin’s theory provided a coherent explanation consistent with geological observations The fossil record is littered with “fossils” successes that later became failures! B.P. 17.9 http://evolution.berkeley.edu/evosite/evo101/ Science 27 February 2009: Vol. 323. no. 5918, pp. 1197 - 1201 Darwin’s theory was also consistent with observable biogeographic patterns Voyage of the Beagle (27 Dec. 1831 to 2 Oct. 1836) http://en.wikipedia.org/wiki/Second_voyage_of_HMS_Beagle HMS Beagle near Tierra del Fuego Painting of the Beagle by Conrad Martens - http://en.wikipedia.org/wiki/Hms_beagle Darwin’s Finches • 14 finch species found on the Galapagos Islands today. • All originated from ≈ 30 individuals of the same species of seed-eating finch that colonized the islands 2 to 3 million years ago, and evolved into different species on different islands (adaptive radiation). • Each species has unique beak adaptations that have allowed it to survive and reproduce in its specific environment. • Beak size reflects a wide variation in diet: seeds, fruit, nectar, blood, eggs, insects, etc. http://www.geo.cornell.edu/geology/GalapagosWWW/GalapagosGeology.html Nothing makes sense, except in light of evolution - Theodore Dobzhansky • 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 most wonderful have been, and are being, evolved. - Charles Darwin Variation within species • All species show variation • Variation is due to – Genetic characteristics – Environment • Genetic traits can be inherited http://www.okc.cc.ok.us/biologylabs/Documents/Evolution%20Tutorial/variation.htm Competition – not all individuals survive to reproduce • In the wild, almost all species produce many more offspring than survive to adulthood • The offspring compete for food resources, favourable locations, safety from predators etc. • Many die before maturity Salmon fry: offspring of two parent salmon http://www.robertclarkphoto.com/site/index.php?set_albumName=darwin&id=darwin_08&option=com_gallery&Itemid=83&include=view_photo.php Natural selection • Statistically, offspring that survive to reproductive age are those best suited to their environment • The environment selects those variations that give best survival chances • Variations that increase survival or reproductive chances are most likely to be passed on to next generation • This process is called natural selection http://evolution.berkeley.edu/evolibrary/article//bergstrom_02 Artificial selection • Variation has long been exploited by farmers and breeders • By breeding selected animals it has been possible to produce diverse strains or breeds of crops and livestock http://en.wikipedia.org/wiki/Image:IMG013biglittledogFX_wb.jpg http://www.cz-pes.cz/really-irish-wolfhound/dovolena/foto_uvod.jpg Darwin – a retrospective analysis • Darwin’s basic mechanism (natural selection) has withstood the test of time. • Since Darwin’s time, our understanding of this mechanism has expanded greatly, aided by: – Mendelian genetics (“genes” as mechanism of inheritance; “alleles” as alternate forms of the same gene) – Molecular genetics (DNA, RNA, proteins…) – Ecological theory and observations (island biogeography, etc.) • Darwin’s ideas have also been supplemented: – Genetic drift – (Endo)symbiosis Peppered moth – a visible example of evolution in action Speckled form Natural selection in response to environmental change Black form http://ukmoths.org.uk/show.php?bf=1931 Example of evolution: peppered moth • • • • • • • Peppered moth Biston betularia was well camouflaged on forest trees Rare black variants were conspicuous: eaten by birds Trees were blackened during industrial revolution BP 17.7 Black variant became less conspicuous than the speckled form Black variant became more dominant in industrial areas Species has evolved in response to pressure from another species (Homo sapiens) With recent improvements in air quality – speckled form has reappeared http://ukmoths.org.uk/show.php?bf=1931 Phenotype – “the observable manifestation of a species” Adjustment to an environment can occur two ways: 1) During the life of an individual (e.g. physiological adjustment or “acclimation)” 2) Over several lifetimes, genetically, via natural selection (e.g. “adaptation” by evolution) Key question in ecology: What is the role of genetic vs. environmental factors in determining “phenotype?” (“Nature vs. Nurture”) Two examples: 1) Clausen Keck & Heisey’s “Common Garden” studies 2) Ted Case’s desert chuckwalla studies Key question in ecology: What is the role of genetic vs. environmental factors in determining “phenotype?” Two examples: 1) Clausen Keck & Heisey’s “Common Garden” studies. 2) Ted Case’s desert chuckwalla studies Common Garden Experiment Clausen, Keck & Heisey (1940) Timberline (3050 m) Mather (1400 m) Stanford (30 m) Fig. 4.25 & 4.26, Molles & Cahill 2008 Fig. 4.26, Molles & Cahill 2008 Summary of Common Garden Results Fig. 4.27, Molles & Cahill 2008 Take-home messages: • The common garden studies of Clausen, Keck and Heisey proved to be a powerful experimental tool for isolating and quantifying genetic and environmental influences on phenotype. • These experiments helped define the concept of “ecotype” – a locally adapted population (subspecies) What is the role of genetic vs. environmental factors in determining “phenotype?” Two examples: 1) Clausen Keck & Heisey’s “Common Garden” studies 2) Ted Case’s desert chuckwalla studies Variation Within Chuckwalla Populations • Natural selection works on variation in a heritable trait associated with fitness. Fig. 4.3, Molles & Cahill 2008 – The relative genetic contribution of individuals to future generations. • Phenotypic variation in a desert lizard – Chuckwallahs are large herbivores lizards found in SW US and NW Mexico – Ted Case studied body size variation at 12 sites. – Higher elevations = more rainfall – More rainfall = more food. – Lizards from higher elevations were • 25% longer • 2x heavier Fig. 4.5 Molles & Cahill 2008 Variation Within Chuckwalla Populations • Phenotypic variation in a desert lizard – Do these size differences arise because food availability differs or because populations are genetically distinct? – Christopher Tracy’s experiments: • Reared juvenile chuckwallahs from 6 different populations under identical conditions = common laboratory conditions (c.f. common garden experiment). • Individuals collected from higher elevations consistently grew larger (Size must also be genetically determined). Fig. 4.6, Molles & Cahill 2008 We now have tools to understand these principles at a molecular level Fig. 4.28 Molles & Cahill The current challenge is to link the emerging “molecular” understanding with our “ecological” understanding – a question of putting information into context. Types of Natural Selection • Stabilizing Selection • Directional Selection • Disruptive Selection Stabilizing Selection • Stabilizing selection acts to impede changes in a population by acting against extreme phenotypes and favouring average phenotypes. • Most common in a stable environment. Stabilizing Selection • The incidence of human infant mortality is higher for very heavy as well as for very light babies. Directional Selection • Directional selection leads to changes in phenotypes by favoring an extreme phenotype over other phenotypes in the population. • Most common in a changing environment. Directional Selection • Boag and Grant (1981) studied the effect of drought on beak size in a population of medium ground finches on Daphne Major. • Drought resistant plants produced large seeds that were difficult to crack. Sample size Beak length Beak depth (mm) (mm) Before 642 10.68 9.42 After 85 11.07 9.96 Directional Selection due to Human Pressures (pp. 97-99, Molles & Cahill 2008) • Atlantic Cod • Mountain sheep Disruptive Selection • Disruptive selection creates bimodal distributions by favoring two or more extreme phenotypes over the average phenotype in a population. • Common in heterogeneous environments. Disruptive Selection • Black-bellied seedcracker in Cameroon. [Stopped here] Evolution • Change in gene frequencies within a population over time. • Can occur through – Natural selection • Can favour, disfavour, or conserve the genetic make-up of a population • 3 types of selection – Genetic drift • Change in allele frequencies in a population due to chance or random events • Especially in small populations Hardy-Weinberg Principle • Stable populations tend to behave according the Hardy-Weinberg principle (see pp. 92-95) – maintain stable allele frequencies. – Large population size – Random mating – No mutations • What happens when these assumptions are violated (e.g. small, isolated populations, or non-random mating)? “Genetic Drift” • Affects small, isolated populations • Random events cause change in allele frequencies over time What is a Species? • A group of individuals that – are morphologically & reproductively more similar to one another than to other populations, – share a singular ancestordescendant heritage, Hermit warbler – and are more or less reproductively isolated. Townsend's warbler Speciation • The process in which one or more new species evolve from a single ancestral species, through the acquisition of reproductive isolation. • Example: Common Yellow-throats • Seven distinct subspecies or races of yellow-throats in the US. • Reproductively isolated. • Would merge into a single reproducing population if they occupied the same territory. • Over time if they continue to be isolated they may become separate species. What Causes Speciation? • Allopatric speciation = a single population becomes spatially subdivided into multiple subpopulations. • Parapatric speciation = a population expands into a new habitat within the range of its parent species. • Sympatric speciation = a single population forms genetically distinct subpopulations with no spatial isolation. – Can occur due to assortative mating. • Individuals choose to mate with individuals that are similar to themselves. Types of Speciation: Figure 4.21, p.105