Life History Strategies

An organism’s life history is the pattern of growth, development, and reproduction over the course of its life.

Since fitness is a measure of the organism’s reproductive success, we know that natural selection will favour the life history strategy that results in the greatest number of offspring that survive to produce offspring of their own. This has important implications on both survivorship (mortality) and birthrate (fecundity) of an individual in a population, and these are related to the amount of energy that is used to reproduce over give time, known as reproductive effort.

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Where Do New Species Come From?

A fundamental question in ecology is the question of how the vast diversity of species that we find on this planet originate.


Key to this is the idea of reproductive isolation, or the group of mechanisms which prevent two groups of living organisms from breeding with each other. Reproductive isolation can be subdivided into two majour groups: prezygotic mechanisms whereby species physically do not mate or have fertilization occur, such as in species living in different habitats or in species choosing not to mate with one another, and postzygotic mechanisms, where fertilization and conception occur, but the embryo either has very low fitness, may be sterile (they are unable to have offspring), or the offspring of the new young are sterile. An example of the latter are the offspring of donkeys and horses, known as mules which, while able to survive on their own, are unable to have viable offspring. Postzygotic mechanisms can occur due to changes in the chromosome number of the offspring, rearrangement of the chromosomes themselves, Haldane’s rule (the sex which has only one of each sex chromosome, such as male humans with their single X and single Y chromosome compared to the females with two X chromosomes, is more susceptible to harmful mutations since they only have one copy of each chromosome), and the Dobzhansky-Muller incompatibility (when random mutations in the genes of two different populations that have divided due to some, potentially geographic, barrier cause the hybrids to not be viable when the populations are mixed again).


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What is a Species?

The task of classification of organisms has remained a daunting task ever since we humans began to group what we consider to be closely related living beings together.


In the mid 1700s, Swedish botanist, zoologist, and physician Carolus Linnaeus wrote the System Naturae, a way of organizing living beings into a hierarchical fashion. Combined with Darwin’s mechanism behind the divergence of species, German biologist Will Hennig established the modern approach to hierarchical classification which we call phylogenetic systematics, or the classification of organisms according to their evolutionary histories.


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Origin of Life

While Darwin and Wallace’s theory of evolution explains the vast diversity of species we see today, their theory doesn’t offer an explanation as to HOW life came to be on our planet 3.2 billion years ago.

We know from looking at extant species today that certain properties of life are maintained and thus have been results of the process of natural selection, i.e. adjusting and balancing the internal environment known as homeostasis, the ability to maintain discrete parts known as structural organization, the ability to control chemical reactions within the internal environment known as metabolism, growth and reproduction, and the ability to actively respond to environmental cues.

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The History of Evolution and Natural Selection

Within much of western science prior to the 6th century BC, creation myths pervaded much of the answers to the creation of the universe and the species within it.

Aristotle had proposed a taxonomy of nature. Amongst the Ancient Greeks, Empedocles had proposed an early form of spontaneous generation, where individuals were assembled from parts that, if unable to function together, became extinct, while Aristotle detailed the diversity of life in his Great Chain of Being, in which all species existed on a fixed landscape in which one would eventually become the next, with humanity positioned at it’s peak.

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Natural Selection and Adaptation

Natural selection is the key mechanism through with evolutionary change occurs and has led to the near infinite modifications that we see in the biological diversity of the world today.

In order for natural selection to occur, three conditions must be met. First, there must be variation – individuals differing from each other – within the population. Second, these differences must be inheritable, that is, they can be passed down from one generation to the next. Thirdly, there must be differential reproductive success among individuals in the population. In other words, individuals with some sets of traits must be more successful at surviving and reproducing in their environment that some other individuals of the same species.

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Nutrient Cycling and Decomposition

The source of all essential nutrients is either in the atmosphere or in the form of rocks and minerals around us.

These nutrients make their way through living and inorganic forms through nutrient cycling. As organisms are develop, they take in these nutrients and store them in their tissues and/or use them for life’s processes, effectively locking them up until such a time as they are released back into the global pool of nutrients from which other organisms can draw from through a process known as decomposition.

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Energetics and Ecological Productivity

All energy on the planet is governed by the laws of thermodynamics.

The first law states that energy can be transferred but cannot be created nor destroyed. The second law states that while energy is transferred, a portion of it will be lost as heat energy.

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Organisms and their Environment

Each environment an organism inhabits holds a different set of constraints for processes related to the survival, growth, and reproductive ability of a species.

Different patterns of temperature, precipitation, seasonality, depth, salinity, pH, and dissolved oxygen among terrestrial and aquatic ecosystems are reflected in the different characteristics that an organism exhibits. These characteristics include the physiology, morphology, behaviour, and lifetime pattern of development (known as life history) of an organism. Any of these characteristics which can be transferred from one generation to the next and serves to increase the ability for the organism to survive under a given environmental condition is known as an adaptation. Not all characteristics can be considered adaptations as the characteristic which enable an organism to succeed in one environment may prove to be detrimental in another. As the environment is constantly shifting, this leads to a game of cat and mouse, with organisms adapting to try and keep up with the rapidly changing environment. The patchwork of environmental conditions present on the earth lead to gradual changes in characteristics associated with these environmental gradients and is known as a cline. Amongst this patchwork will be distinct environmental conditions, such as mountaintops and grasslands, and populations which are highly adapted to these local environments are known as ecotypes.

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What is Ecology?

Ecology is the study of the relationship between organisms and their environment

Ecology is the scientific study of the relationships between the processes behind an organism’s ability to survive, grow and reproduce, and the physical/chemical components (abiotic – temperature, moisture, oxygen concentration, light availability) and the biological/living components (biotic) of their environment. Stemming from the Greek root work “Oikos” (family household), modern-day ecology has strong roots to the plant geography and natural history movements of the 1800’s.

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