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CHAPTER 2 Ecology’s Evolutionary backdrop Ecology’s evolutionary backdrop 2 “Nothing in biology makes sense except in the light of evolution” – Russian-American biologist Dobzhansky Very little in evolution makes sense except in the light of ecology Why are there so many types of organisms? Why are their distributions so restricted? Why: all species are so specialized that they are almost always absent from almost everywhere 4/30/2017 Evolution by natural selection 3 Darwin, 1859, ‘On the origin of species’ Malthus, 1798, ‘An essay on the principle of population’ Gradually developed the view that the natural diversity of nature was the result of a process of evolution in which natural selection favored some variants within species through a ‘struggle for existence.’ The human population – capable of doubling every 25 years and so… Darwin and Wallace – field ecologists – realized this argument applied equally to plant and animal kingdoms The great fecundity of some species + every species ‘must suffer destruction during some period of its life and during some season or occasional year – otherwise…’ 4/30/2017 Fundamental truths of evolutionary theory 4 1. 2. 3. 4. Individuals that form a population of a species are not identical Some of the variation between individuals is heritable All populations could grow at a rate that would overwhelm the environment – but most individuals die before reproductions and most (usually all) reproduce at less than their maximal rate Different ancestors leave different numbers of descendants (not just offspring): they do not all contribute equally to subsequent generations. those that contribute most have the greatest influence on the heritable characteristics of subsequent generations 4/30/2017 In other words … 1. 2. 3. Variation among individuals Inheritance of that variation Differences in survival and reproductive success (or fitness) related to that variation Human selection vs natural selection 6 Human selection – example? Natural selection – example? Difference? 4/30/2017 Natural selection 7 No aim for the future Past environments may have selected certain characteristics of individuals that we see today – but those traits are ‘suited’ to present day environments ONLY because environments tend to remain the same – or change quite slowly 4/30/2017 Evolution within species 8 To understand the evolution of species – we need to understand evolution WITHIN species Evolution by natural selection does far more than create new species Natural selection and evolution occur within species – and we can study them in action within our own lifetime 4/30/2017 Geographic variation within species 9 The characteristics of a species may vary over its geographic range Note: Evolution forces characteristics of populations to diverge from each other IF: There is sufficient heritable VARIATION on which selection can act; and Forces of selection favoring divergence are strong enough to counteract the mixing and hybridization of individuals from different sites 4/30/2017 10 Reciprocal transplant experiments – what do they tell us? Beware of assuming that all species exhibit geographically distinct variants with a genetic basis Difficult to detect local specialization of animals by transplanting them into each other’s habitat. Why? Can do with invertebrates (corals, sea anemones, …) 4/30/2017 11 Geographic variants of species – i.e. influence of physical environment Also Influence of other selective forces 4/30/2017 Evolutionary experiments 12 The guppy, small freshwater fish from S America Present in all water bodies – subdivided by waterfalls – thus isolating fish populations Populations differ from each other; 47 traits tend to vary with intensity of risk from predators w/o predators: male guppies are brightly decorated. Females prefer but… Controlled field experiment 4/30/2017 13 Variation within a species with human-made selection pressures 4/30/2017 14 Sites in Britain and Ireland where frequencies of the pale (forma typica) and melanic forms of Biston betularia were recorded More than 20,000 specimens Pollutants spread toward the east; unpolluted west 4/30/2017 + Selection and change in melanistic moths (peppered moths) Dark form: more popular in forests near industrialized regions Industrial melanism Field experiment 16 Large numbers of melanic and pale moths reared and released in rural and unpolluted areas 190 moths captured by birds; 164 melanic Experiment repeated near a city Moths of the typical form disadvantaged in the polluted industrial environment Criticism of Kettlewell’s research Moths that were used for the mark – recapture experiments were reared in the lab This might have affected their behavior – eg – choice of resting locations Experimental moths released at unnaturally high densities – might have affected the behavior of predators So? But: with pollution control forests became cleaner frequencies of melanistic moths decreased (as predicted by evolutionary theory) Evolution and coevolution 19 Physical conditions are not heritable; not subject to natural selection What about when two species interact? 4/30/2017 Coevolution 20/34 When populations of two or more species interact, each may evolve in response to characteristics of the other that affect its own evolutionary fitness. This process is referred to as coevolution: plants and animals employ structures and behaviors to obtain food and to avoid being eaten or parasitized: much of this diversity is the result of coevolution: natural selection on the means of food procurement and escape Coevolution is mediated by biological agents. 21 The evolutionary effects of biological agents are unlike those of physical factors in two important ways: biological factors stimulate mutual evolutionary responses; adaptations of organisms in response to changes in the physical environment have no effect on that environment biological agents foster diversity of adaptations rather than promoting similarity Convergence 22 In response to biological factors, organisms tend to diversify: organisms specialize, approaching feeding, avoidance of predators and mutually beneficial arrangements in unique ways In contrast, organisms responding to similar physical stresses in the environment tend to evolve similar adaptations: this familiar process is known as convergence 1. Coevolution involves mutual responses in the two species predator species evolves to be faster, allowing it to catch more results in greater selection on speed in the prey for “regular” evolution, there isn’t any response by the environment 2. Coevolution promotes diversity of adaptations evolution often converges on the same solution for the same problems coevolution involves specific, unique responses to specific challenges The confusing definition of coevolution Species A evolves an adaptation in response to species B Species B evolves in response to the adaptation of species A This isn’t coevolution Species A has some trait unrelated to species B Species B evolves in response to that trait in species A What is a species? 27 (Mayr and Dobzhansky) – two populations were part of the same species if they could – potentially – breed together to produce fertile offspring Not enough time to apply this test before recognizing every species Test recognizes a crucial element in the evolutionary process: 2 parts of a population can evolve into distinct species ONLY IF a barrier prevents gene flow between them Biospecies (species) do not exchange genes 4/30/2017 + Orthodox speciation 28 4/30/2017 + 29 Two species of gull diverged from a common ancestry (from the lesser black-backed gull in Siberia) as they colonized and encircled N Hemisphere Where they occur together in N Europe they are unable to interbreed and are two distinct species They are linked along their ranges by a series of freely interbreeding races or subspecies Islands and speciation: Darwin’s finches -Genetic distance between species is shown by length of horizontal lines -This process happened in less than 3 million years -But is continuing… 31 4/30/2017 Heavy rains during El Nino events support lush plant growth in the archipelago. Evolution in action – Geospiza fortis Finches do not survive or die at random Because the average hardness of seeds increased as the drought intensified and the softest seeds were consumed birds with larger beaks that could generate the forces needed to crack hard seeds survived better than those with smaller beaks (drought in mid-1970s) The average beak size of surviving individuals and their progeny increased significantly In the exceedingly wet year of 1983, small seeds produced; birds with smaller beaks handled smaller seeds more efficiently … average beak size returned to a lower value Evolutionary responses were small – still they illustrate the capacity of a population to respond to changes in the environment – changes in climate What is necessary for such ‘evolution in action’? 35 Test 1 (Chapters 1 and 2) Next Friday – October15 4/30/2017