Evolution

Evolution • • • • • • • • • • • • • • • • • • • • Variety is the spice of life As Darwin realized, evolution depends on variation Individuals with adaptive variations are more likely to survive and reproduce They tend to pass on those adaptive variations to their offspring Darwin knew that animals could pass particular traits on to their descendants Farmers could even breed animals for specific characteristics They did so by selecting individuals with particular desirable traits to breed together But what force in nature could substitute for the hand of the farmer? In 1838, Darwin read An Essay on the Principle of Population, published in 1798 by Thomas Malthus Malthus wrote to counteract arguments that social progress could be achieved through a better understanding of nature Malthus agreed that nature had a lot to say But what nature really had to tell us was not very pleasant Nature demonstrates that progress is only possible with enormous suffering and sacrifice of life Wars, famines, plagues…these were nature’s way of balancing the books on the excess human population Malthus reasoned that populations would increase geometrically, but resources could only increase arithmetically Over time, this would lead to a growing gap between too many people and too few resources Out of this gap came what Malthus called “the struggle for existence” Ideas were a major influence on Darwin, who reasoned that what was true of humanity must also apply to other animals as well - a struggle for existence In any struggle, there would be winners and losers The winners must be those individuals better equipped to survive, what Herbert Spencer was to later call survival of the fittest • • • • • • Darwin realized that evolution must be tied to variation Species were really just local groups of individuals, all of whom varied from one another in certain ways Just as farmers selected the best varieties to breed, nature must somehow select those individuals best fit to survive In every natural population, some varieties must be better equipped to prevail in the struggle for existence Those well adapted individuals would have more offspring than others, passing on their variation to the next generation Darwin realized that evolution was a selective process, what he called natural selection Theory of Evolution by Natural Selection • • • • Variation exists between individuals in a population The number of individuals in a population tends to increase geometrically Resources increase arithmetically, leading to a struggle for existence in which many individuals are eliminated or don’t reproduce This elimination is selective > those individuals who are better adapted survive to reproduce > those less well adapted fail to reproduce, or reproduce in smaller numbers The differential reproduction of these well-adapted individuals leads to a change in the proportion of certain traits in the next generation Darwin did not understand the physical basis of heredity The answer that Darwin sought was actually found in his lifetime! Gregor Mendel, in 1866, experimented with garden peas, discovered the mechanism of heredity Began his famous experiments on garden peas in the monastery garden Darwin never heard about Mendel’s results Mendel showed that: > The physical units of heredity came in pairs, one unit from each parent > Heredity was particulate, not blending of fluids (contrast with Darwin) • • • • • • • • • • • • • • • • • • • • • As geneticists later discovered, new variations are constantly being created by mutations Genes coding for adaptive traits will occur with more often in the next generation Evolution is simply a change in gene frequency over time Species begin when small parts of a larger population are geographically isolated from the parent population Differences in local conditions favor different varieties in the isolated population, whose gene pool is a subset of the original population (founder effect) Because they are geographically isolated, members of this small populations can only interbreed with one another Mutations and other genetic changes within this isolated population can lead to entirely new varieties, and new combinations of existing traits The population may experience genetic drift, a random fluctuation in the proportion of a particular allele in a small inbred population Natural selection favors some combinations of alleles over other combinations Over time, the effect of natural selection, genetic drift, mutation, and recombination can become isolating mechanisms Isolating mechanisms are any force or factor that slows or stops the flow of genes between populations (interbreeding) When the isolated population is reunited with the parent population, they can no longer interbreed They have become reproductively isolated, and can now be considered a new species Many types of isolating mechanisms > Change in courtship timing or courtship behavior > Change in body size or in the size or shape of sex organs > Changes in habitat – forest vs. meadow ex. (don’t meet, don’t mate) One of the most interesting examples of evolution is the case of the peppered moth, Biston betularia Until 1845, all known specimens were light-colored moths In 1845 a black peppered moth was captured - Biston betularia carbonaria • • • • • • • • • • • • • • • • • • • • • • • The black form was caught near Manchester, England, heavily polluted area At the height of the industrial revolution, Manchester was badly polluted The trees and grasses were covered with a thick layer of soot, killing the lichens that had camouflaged the light moths As pollution increased, more of the black form were captured In the most heavily polluted areas, the black form became the dominant variety Industrial melanism - replacement of a light morph by a dark morph in an industrialized area - nearly 200 species of Lepidoptera Natural selection seemed to favor the dark morph in polluted areas, and the light morph in cleaner areas What was the selective pressure? H.B.D. Kettlewell thought that the color protected the moths from hungry birds Kettlewell marked a sample of moths of each color under their wings He released equal numbers of each morph in a polluted area (Birmingham) and a clean area (Dorset) He later returned and recaptured as many moths as he could at both sites Dorset (clean) Birmingham 6% black 12.5% light 40% black 19% light Twice as many light moths survived in clean areas, twice as many black moths survived in polluted areas To determine if hungry birds were the selective pressure, he put equal numbers of moths on light and dark tree trunks Dorset (clean) Birmingham 164 black 15 black 26 light 43 light Each variation was adapted to its own local conditions Both morphs were well adapted in one environment, and poorly adapted in the other • • • • • • • • • • • • • • • • • • • • • • Britain finally cleaned up its environment, by the mid 1970’s the soot was mostly gone Proportion of black moths dropped, until the lighter form was the most common one The black form never disappeared, it stabilized at 20% of the total population In this dimorphic species, natural selection shifted the balance back and forth in direct response to changes in the environment Shows us something very important about evolution Evolution is not progressive! Evolution means change, but the direction of that change depends on what is adaptive in a particular time and place Thick bunny fur is a great idea in the Yukon, but would be the kiss of death in Southern Louisiana in July This textbook example is not quite as clear cut as most textbooks suggest It turns out that peppered moths don’t like to sit on tree trunks at all! Cyril Clarke collected 17,300 peppered moths over a 35 year period - only two were on the trunk of a tree!! So where do the moths like to hang out? In the shade - where even the lighter colored moths are well hidden Light colored moths have translucent wings, so light passes through and bounces back to highlight the wings from below So light colored moths show up fairly well on light trees in the open sunlight - not as well protected as we might assume And black moths are not very easy to find when they’re hiding in the shade on light trees - better protected than we might assume Populations can change very rapidly in response to strong selective pressures One of the best-documented examples involves one of Darwin’s finches, the medium ground finch - Geospiza fortis Studied by Peter Grant - The Beak of the Finch (Pulitzer Prize winner) Grant was studying finches on Daphne Major, trying to determine what factors shaped their communities Darwin’s finches differ mainly in the size and shape of the beak These differences are adaptations to the local food supply • • • • • • • • • • • • • • • • • • • • • • • • • • The medium ground finch, G. fortis, eats insects and small to medium size seeds Following a severe drought in 1977, most seeds and insects were quickly eaten Birds began to starve - 85% of the adults died, only one nestling lived out of 388 The survivors were all larger birds, with larger beaks They were able to eat the larger seeds that were too tough for other birds to crack Within a single generation, average body size and beak size increased significantly One of the first well-documented cases of evolution in the wild When the rains resumed, body size and beak size gradually returned to their previous average Evolution is not progressive The ground finch is a great example of directional selection Directional Selection - average value of a trait is shifted in a particular direction (higher or lower) Sometimes natural selection favors the average value of a trait, and selects against either extreme Stabilizing Selection - acts to stabilize the population around some average value Average size duck and chicken eggs have the highest hatching rate Average size human babies have the highest survival rate Stabilizing selection may be the most common type Sometimes there are conflicting forces acting on a population Disruptive Selection - the environment selects for the two extremes, against the average, splitting the population in two or more types Bradshaw and Jowett studied bentgrass Wales has many lead and copper mines Mines produce heaps of tailings (wastes) Tailings are toxic - heavy metals Bentgrass grows on mine wastes Bentgrass on mine wastes has evolved tolerance to heavy metals Transplanted grass from mine wastes to normal soil , and grass from normal soil to mine wastes Wild type did better in normal soil, metal-resistant plants did better on mine wastes • • • • • • • Two separate races of bentgrass have evolved in about 400 years of mining Plants in California evolved a copper resistant race in less than 70 years Evolution can happen very quickly Easy to demonstrate evolution at the leve of individual species Demonstrating evolution above the level of species is much harder Have to rely on several lines of circumstantial evidence Proofs of Evolution > Biodiversity > Biogeography > Fossil record > Embryology > Comparative anatomy > Molecular evolution Biodiversity - large number of species, many similar forms Biogeography -similar forms in different environments, shaped by local conditions Fossil record - shows pattern of divergence from common ancestors Embryology - similarities in early development imply a common ancestry > Chordate embryos very similar > Invertebrate larvae often similar – trochophore links annelids and mollusks – nauplius larva common to all crustaceans Comparative anatomy - similarities in basic anatomical structure among diverse groups of animals Vertebrate forelimbs - same bones modified to do different things in different animals (walk, run, swim, fly…) Molecular evolution - line of evidence that wasn’t known in Darwin’s day Slow evolution of proteins and other biomolecules (like ribosomal RNA) gives us a molecular clock to measure evolution Amino acid sequence of same proteins in different species show many changes Many strong lines of evidence that point to the underlying pattern in nature that Darwin perceived The nature and distribution of organisms is a simple consequence of descent with modification from a common ancestor • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Understanding the history of life requires untangling the web of life, sorting out relationships and origins Much of that information comes from the study of taxonomy Taxonomy is the description, naming, and classification of living organisms About 1.4 million species now named, maybe as many as 10 million or more We classify organisms by their similarities These can be physical, physiological, even behavioral We classify organisms by their similarities These can be physical, physiological, even behavioral Taxonomy is the description, naming, and classification of living organisms About 1.4 million species now named, maybe as many as 10 million or more We classify organisms in an attempt to understand their phylogeny Phylogeny is the evolutionary history of organisms (their lineage) Every scheme of classification is arbitrary Our current scheme of classification is called cladistic analysis or cladism Each taxon is a clade, a branch on the tree of life (cladogram) Clades are determined by traits they share, traits that are different from their ancestors Monophyletic - taxon contains the common ancestor and all of its descendants Cladists only recognize monophyletic groups Cladists try to avoid paraphyletic and polyphyletic groups Monophyletic - contains the common ancestor and all of its descendants Paraphyletic - contains common ancestor but only some descendants (most similar) Polyphyletic - contains some descendant species but no common ancestor (may even come from different ancestors)