section 3b_Adaptation.pdf

2024/08/03 ADAPTATION 1 ADAPTATION Living things are adapted for life on Earth Well adjusted (suited) for living...

2024/08/03 ADAPTATION 1 ADAPTATION Living things are adapted for life on Earth Well adjusted (suited) for living in their natural environment e.g. woodpecker’s beak, camouflage in moths, etc. 2 ADAPTATION Explanations: Inheritance of acquired characters “Lamarckism” Challenged: Weismann, late 1800s: (most) acquired characters not inherited Can’t explain where adaptive mechanism comes from Assumes, rather than explains, adaptation Natural selection Only explanation for adaptation Not all change relies on natural selection e.g. random drift in molecules But only natural selection results in adaptation 3  2024/08/03 ADAPTATION Can natural selection explain all known adaptations? Criticism against natural selection (as the only force for adaptation): 1. Co-adaptations 2. Functionless / disadvantageous rudimentary stages 4 ADAPTATION | CO- ADAPTATION Darwinian response: Complex adaptations Do not evolve in one sudden step Darwin => evolution is gradual Evolve in many small steps Each step simple increase in fitness Colour change in peppered moth, beak size in finches “Gradualism” – fundamental property of evolutionary biology Not rate over time, but way in which changes happen Only need to show adaptation could have evolved in many small steps e.g. eye - light-sensitive organs (not all complex) 5 ADAPTATION | CO-ADAPTATION 6  2024/08/03 E.g. eye (cont.) Nilsson & Pelger (1994) – computer model study Crude light sensitive organ – 3 layers Change at random ~ 1% any step New generation formed from optically superior eyes ADAPTATION | Optical physics – easily quantified = visual CO- acuity – ability to resolve objects in space ADAPTATION Complete formation of eye in ± 2000 steps Estimates of heritability and strength of selection ± 400 000 generations If generation time 1 year → less than ½ My Evolution of complex structures not difficult 7 ADAPTATION | CO-ADAPTATION 8 ADAPTATION | FUNCTIONLESS RUDIMENTARY STAGES Functionless / disadvantageous rudimentary stages Criticism: Adaptation could not have been advantageous in rudimentary form “What is the use of half a wing?” Darwinian reply: Suggest ways in which the character could have been advantageous in rudimentary form e.g. partial wing Broken fall from tree Gliding from cliff tops / between trees Early stages would not have required all the characteristics of a fully formed wing 9  2024/08/03 NEW ADAPTATIONS Darwin theorised evolution as a gradual process… Gradualism implies continuity between all forms of adaptation So, assuming natural selection can lead to the development of complex structures – How do new / novel adaptations evolve? 1. Gradual transformation of a structure with constant function 2. Function of an adaptation may change with (some) change in form. 3. Combination of unrelated parts 10 NEW ADAPTATIONS 1. Gradual transformation of a structure with constant function Structures arise by small incremental steps from pre- existing structures. e.g. the eye evolving from photo-receptive cells (changed but essentially similar function). Eventually recognizable as a structure that did not exist before But, still evolved in continuous small stages 11 2. Function of an adaptation may change with little change in form Pre-adaptation NEW When the function of a structure/adaptation happens to ADAPTATIONS be able to change with little change in structure. e.g. feathers, limbs 12  2024/08/03 NEW ADAPTATIONS 13 NEW ADAPTATIONS e.g. feathers Found in modern birds, enable flight Fossil non-avian dinosaurs feathers or rudimentary feathers Originally evolved for other reason – thermoregulation / display, but changed gradually to take on new function (flight) e.g. tetrapod leg Fish lack legs Fossil evidence suggest legs (paddles) originally evolved for underwater swimming Bone structure of swimming paddles in one group turned out to be appropriate for a leg that could walk on land. 14 3. New adaptation may evolve by combining unrelated parts Two pre-existing parts combined e.g. production of lactose Milk contains large amounts of lactose NEW Mammals evolved new enzyme – lactose synthase – to produce milk ADAPTATIONS Catalysing the conversion of glucose to lactose lactose synthase: modified version of two pre-existing enzymes: Galactose transferase: functions in Golgi apparatus of eukaryotic cells α-lactalbumin used in vertebrate antibacterial defences 15  2024/08/03 Two whole species merge by symbiosis One species takes up residence inside another Evolve into new species with new combined physiology endosymbiosis NEW e.g. mitochondria (chloroplasts) ADAPTATIONS One bacterial cell engulfed another bacterial cell New cell had a more complex metabolism than either ancestor e.g. cells with mitochondria can burn carbohydrates in oxygen for energy More efficient than anaerobic respiration 16 Adaptations in nature are not perfect Some adaptation appear imperfect May not be possible for an adaptation to be simultaneously perfect for all levels of organization IMPERFECT May be morphologically advantageous, but maybe not at cellular, genetic or group level ADAPTATION Theory of evolution predicts: Quality of adaptations will progressively improve for as long as there is genetic variation to work on 17 IMPERFECT ADAPTATION Constraints to perfection… 1. Time lags Reasons why adaptations 2. Genetic constraints may be imperfect 3. Developmental constraints 4. Historic constraints 5. Trade-off between different adaptive needs 18  2024/08/03 Reasons why adaptations may be imperfect 1. Time lags Evolution takes time Environment of all species changes more or less continuously IMPERFECT Evolutionary fortune of species they compete / cooperate with ADAPTATION Must evolve to keep up with events, but Will lag some distance behind optimal adaptation to their environment Adaptation considered imperfect when natural selection cannot operate as fast as environment of species changes 19 e.g. fruit of some Central American plants Many flowering plants produce fruit to attract animals as dispersal agents - adapted their fruit to the animals that are around If fauna changes, plants must adapt - fruit IMPERFECT North & Central America: large herbivores until ± 10 000 ya ADAPTATION Giant ground sloth, giant bear, large horse, mammoths, gomphotheres Now all extinct But some Central American trees still carry fruit that are adapted to large herbivores Large size Hard external coverings 20 IMPERFECT ADAPTATION Megathurian: giant ground sloth Gomphotheres (elephant relative) Mammoth 21  2024/08/03 IMPERFECT ADAPTATION Not enough time has passed to adapt to smaller contemporary mammals 22 2. Genetic constraints When heterozygous advantage exists Heterozygous genotype = selective advantage Homozygote(s) = deleterious High mortality rate appears ineffective Evolution of homozygous forms that can survive IMPERFECT should be more advantageous => Requires new mutation that could perform ADAPTATION the function of the heterozygous form: e.g. sickle-cell anaemia A new mutation producing haemoglobin resistant to malaria, and that can survive as homozygote Eliminate the cost of heterozygous advantage (i.e. deleterious homozygotes) 23 IMPERFECT ADAPTATION 3. Developmental constraints Different groups of organisms have evolved distinct developmental mechanisms The way an organism develops will influence the kind of mutations it is likely to generate e.g. a plant may be more likely to mutate to a form with more branches than would a vertebrate 24  2024/08/03 IMPERFECT ADAPTATION 4. Historic constraints Theory predicts that evolution by natural selection proceeds in small steps Each change must be advantageous in the short term Natural selection cannot favour a disadvantageous change now because it will ultimately be better Natural selection may push population to a local peak in adaptive topography As environment changes, adaptive topography changes Graphic representation of the various fitness levels of different genotypes New global peak may arise (peaks = high fitness). But population may be stuck at local peak: Because natural selection does not favour evolution towards less advantageous state 25 IMPERFECT ADAPTATION Adaptive topography of a hypothetical species: “x” optimal state – natural selection drives adaptation to that point Adaptive topography changes….new global optimum Any change from “x” = less adaptive state Population stuck (natural selection will not favour weaker adaptation even if it would allow increased adaptation in future). 26 IMPERFECT ADAPTATION 4. Historic constraints… e.g. recurrent laryngeal nerve [page 281, Ridley 2004] The laryngeal nerve is the = 4th vagus nerve (one of the cranial nerves) First evolved in fish-like ancestors Pass behind 6th arterial arch through gills In mammals: 6th gill arch became ductus arteriosus (close to heart) Nerve still runs behind modified gill arch Modern mammal – from brain, down neck, round dorsal aorta, back up to larynx Humans: detour 1 – 2 ft Modern giraffe: all the way down neck and back up 27  2024/08/03 IMPERFECT ADAPTATION 28 IMPERFECT ADAPTATION Probably imposes a cost More nerve needs to grow Increased signal time Fault arose because natural selection operates in short term Modification of what is already present 29 IMPERFECT ADAPTATION 5. Trade-off between different adaptive needs Many organs adapted to perform more than one function Adaptations for each function a compromise E.g. mouth used for feeding & breathing in tetrapods Trade-offs in behaviour Allocate time to different activities e.g. foraging vs. finding a mate Trade-offs over whole lifetime Life-history strategy Reproduction early in life vs. reproduction later on Observing one trait in isolation, it may appear imperfect, when it really is a compromise between different trade-offs 30