ecology Flashcards
(28 cards)
What is the fundamental principle of Darwinism?
Natural selection acts on continuous variation. The differential success of various phenotypes linked to particulate units of inheritance results in gradual, continuous changes within a population as that population grows better adapted to particular biotic and abiotic interactions. This form of microevolution is expected to produce a normal distribution of divergence within a population.
What is the theory of discontinuous evolution?
Proponents of discontinuous evolution, or mutation theory, argue that species divergence and fixation of beneficial phenotypes do not occur through the gradual accumulation of minor mutations (microevolution) but through large genetic disturbances or ‘jumps’.
How did biometricians (e.g. Weldon) approach the study of evolution?
Biometricians utilised statistical analysis of large population datasets to study continuous trait variation. Weldon’s study of shore crab morphology (e.g. measurement of shell dimensions) primarily showed data closely correlating with a normal distribution leading him to favour microevolutionary theory. Weldon did identify one characteristic (shore crab forehead width) which displayed an asymmetric distribution - but he interpreted this as the result of two overlapping normal distributions, reflecting the active process of divergence in the shore crab population.
How did Mendelians (e.g. Hugo de Vries) approach the study of evolution?
Mendelian views were influenced by the observation of discrete, qualitative trait changes in Mendel’s controlled experiments and thus favoured mutation theory. Hugo de Vries, for example, performed controlled breeding crosses of evening primroses, producing discrete morphologically divergent forms in rapid ‘jumps’ across generations.
What are the primary issues with Mendelian studies of discontinuous evolution?
The publication of Mendel’s original experimental data excluded ‘anomalous’ data points inconsistent with his hypotheses. His reporting is also primarily recorded as lecture transcripts - and not in the form of rigorous scientific documentation - limiting reproducibility. The ecological validity of controlled laboratory experiments is also limited.
What is the theory of evolution by natural selection?
Heritable traits associated with increased reproductive success will accumulate within populations, resulting in individuals appearing ‘designed’ for maximal fitness.
How did Francis Galton’s work provide evidence for microevolutionary theory?
Francis Galton observed regression to the mean in studies of the influence of mid-parent height on offspring phenotype, observing that ‘extreme’ parent phenotypes often resulted in ‘less extreme’ offspring phenotypes - representing an evolutionary shift towards the population average, thus fitting a normal distribution.
How did William Castle’s work provide evidence for microevolutionary theory?
William Castle studied the ‘hooded’ character in domesticated ‘fancy’ rats. The ‘hooded’ character often displays continuous variation as to the proportion of the rat body covered with dark fur. Within thirteen generations of selective breeding, Castle had pushed this continuous trait outside of the range of natural variation, producing two ‘extreme’ phenotypes - ‘fully hooded’ dark rats and rats with very minimal hoods. This experiment provided evidence for directional selection acting on continuous variance to induce significant evolutionary change, as was further reinforced by his reversal experiments - wherein he found considerable difficulty in breeding the rats back to their ancestral phenotype.
What is ‘the modern synthesis’?
The modern synthesis reconciles Darwin’s theory of evolution by natural selection with Mendel’s particulate inheritance. Proponents of the modern synthesis hold that evolution refers to changes in allele frequencies within populations over generations, with gradualism as the primary mode of evolutionary change (macroevolution as the result of microevolution on extended timescales). Mendelian particulate inheritance maintains genetic diversity on a population level, whilst selection acts on small variations to stimulate the evolution of novel phenotypes.
How are genetic dynamics within populations altered by biased sex ratios?
If there is a biased sex ratio within a population, offspring fitness is partially determined by sex. In a male-diminished population, male offspring have a higher reproductive value (and vice versa). Populations are complex and thus ‘number of offspring’ is not the only measure of reproductive fitness.
What is the comparative method?
The comparative method entails the comparison of variation across species, or species groups. This method is very valuable for identifying broad trends across species but is inherently observational, implying difficulty explicating correlation v causation. Researchers can form inferences about adaptation and evolutionary relationships through meta-analyses of multiple single-species studies - e.g. Darwin’s study of sexual selection, exploring behavioural and morphological patterns across monogamous and polygamous species. Using the comparative method, Darwin identified higher levels of sexual dimorphism in polygamous species - linked to the increased intensity of sexual selection associated with competition for multiple mates.
How did John Crook utilise the comparative method in his study of red-headed and southern masked weavers?
Crook correlated phenotypic differences across species (e.g. monogamy v polygamy, cryptic nests v conspicuous colonial nest structures) with aspects of social organisation and ecology. He found that insectivorous weavers were typically solitary and monogamous with cryptic nest structures, whilst seed-eaters were polygamous and colonial with conspicuous nest structures. The random distribution of motile insect prey across the environment favoured solitary, territorial behaviour, with no deterministic benefit to clustering. On the other hand, seed sources (trees) are clustered, increasing the carrying capacity of a given patch. Colonies result in more efficient exploitation of clustered resources and polygamy - which increases genetic dispersal - becomes an available option in colonial social structures.
What are some key limitations of Crook’s research?
Closely related species often share traits due to shared evolutionary descent - and not independent trait evolution, meaning that they cannot represent independent data points in studies of evolutionary transitions. If they are treated as such, this becomes pseudoreplication - whereby multiple samples from the same experimental unit are presented as independent replicates, artificially inflating sample size. The weavers were closely related and thus phylogenetically non-independent. Additionally, the tips of a phylogenetic tree do not provide valid information about evolutionary transitions. Crook also did not quantify ecological variables (i.e. neglected to quantify the diets of the waver species) and did not consider bidirectional causality (i.e. sociality as determining diet).
What is the independent contrasts method?
The independent contrasts method studies divergence in trait value between sister species. This divergence in trait value can be used to reconstruct ancestral states (i.e. to estimate the trait values of a common ancestor). Through the formation of multiple ‘independent contrasts’ (calculated differences in trait value between sister groups, inferred to stem from independent evolutionary events), researchers can generate independent data points for statistical analysis. Essentially, the quantification of divergence between two closely related species becomes a single data point.
How was the comparative method used to research sexual dimorphism in primates?
There is no sexual dimorphism in the indri lemur species and there is extreme sexual dimorphism in mandrills (with the male double the size of the female). Comparative studies of primate sexual dimorphism have hypothesised that dimorphism is a result of increased sexual selection, or resource competition. Higher dimorphism correlates well with a lower male-to-female ratio (which could link back to intense sexual selection, with aggressive competition potentially increasing male mortality).
What did comparative studies of fig wasp populations reveal?
The pollinating female fig wasp is more well-armoured and less violent than the non-pollinating wasp, which has an extensive ovipositor for nectar extraction without fig entry. This implies the action of differing selection pressures on different fig wasp clades.
What is macroecology?
Macroecology is the study of broad scale ecological patterns and processes (e.g. metabolic scaling and diversity gradients). Species-area curves (shifts in species richness over area) and latitudinal gradients (analyses of the trend toward increasing diversity from the poles to the tropics) represent macroecological phenomena.
What is the species-area curve?
The species-area curve describes how species richness increases with the area surveyed. This relationship follows a power function, such that a tenfold increase in area doubles the number of species. The species-area curve is the most widely supported rule of species diversity, fairly universal across spatial scales (e.g. from archipelagos to continents). Early studies were concentrated on island biogeography (discrete areas of habitat, straightforward for accumulation of species data). On log-transformed axes, this relationship appears linear.
What are the three main mechanisms proposed to describe species-area relationships?
These mechanisms are not mutually exclusive but emphasise different ecological processes; the habitat diversity hypothesis (a larger habitat sustains a wider range of subhabitat types and thus a wider range of species), the passive sampling hypothesis (a null model, assuming species presence is proportional to island area) and the equilibrium hypothesis (the number of species on an island is a dynamic equilibrium between immigration and extinction rates, influenced by island area and isolation).
How can each of the three hypotheses for explaining the species-area curve be tested?
The habitat diversity hypothesis can only be tested where area and habitat diversity can be separated (e.g. through examination of homogeneous habitats and comparison to heterogeneous habitats). Habitat diversity often but not universally explains patterns in species richness. The passive sampling hypothesis fails to account for species richness on small islands or species turnover, but provides a baseline expectation. MacArthur and Wilson’s Island Biogeography (Equilibrium) Hypothesis predicts that immigration rate decreases as island species richness increases (as there are fewer potential niches for colonisation). Isolated islands have lower immigration rates. Extinction rate increases with species richness due to increased competition and smaller population sizes (more prone to stochasticity). This predicts that closer mainland islands have higher species richness, and larger islands maintain higher species richness.
What assumptions underly Macarthur and Wilson’s Island Biogeography Hypothesis?
Realistic models show that emigration and immigration functions plateau over time, impacted by the strength of dispersing species. Strong dispersers colonise easily, creating variability in colonisation rates. However, this hypothesis assumes that evolution does not influence species richness and that extinction and colonisation rates persist independently of island species richness. This model also accounts for turnover in species composition only when species richness remains constant over time. Increasing isolation should result in decreasing species richness, which is not always accurate.
What studies provide evidence for MacArthur and Wilson’s Island Biogeography Hypothesis?
Across the Channel Islands, the number of bird species between 1917 and 1968 remained fairly constant but identities changed through turnover (with the Los Coronados Island sustaining four extinctions and four colonisations, maintaining a stable species richness of eleven). Experimental invertebrate defaunation of mangrove islands resulted in a restoration of species richness to a consistent plateau/asymptote within two years, indicating that rates of colonisation balanced the extinction rates of existing species.
Can the equilibrium hypothesis be used to improve conservation planning?
Yes, through prediction of extinction rates following habitat land area reduction. These predictions can be used in reserve design when species richness is the primary object of conservation interest (as area is the main predictor of variation in richness). The SLOSS controversy has been partially resolved as a consequence of island biogeography research, supporting the idea that larger reserves or more connected patches can better maintain species richness when natural communities sustain high levels of species overlap. However, the ‘several small’ option can be beneficial if there is high beta diversity across natural habitats (species turnover across patches).
What is the latitudinal diversity gradient?
The latitudinal diversity gradient is a fundamental macroecological pattern, with species richness per unit area decreasing with distance from the equator. Ecological guilds at tropical latitudes tend to have higher species richness irregardless of land area (e.g. coral reefs comprise one percent of global land area and house over a quarter of marine fish species). While some clades (e.g. ichneumoid wasps) show ‘quirky’ patterns of diversification, the trend is robust across taxa.
