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Observation 1: Populations tend to produce more offspring than the environment can support
Thomas Malthus (a Scottish economist) wrote his Essay on the Principles of Populations (1798). In it he proposed that populations in nature cannot continually increase, sooner or later food supply is insufficient and famine stops further population growth.
Both Charles Darwin and Alfred Russel Wallace had read Malthus and understood the idea of exponential population growth.
Darwin estimated that 1 pair of elephants could produce 19 million elephants in 700 years.
Some organisms can do better than this.
1 pair of cockroaches could produce 164 000 million in 7 months. More than enough to fill an average Parisian apartment!
Observation 2: The numbers of individuals in a population remain stable
In terms of population growth the population at its carrying capacity has zero growth.
Deduction 1: There must be a struggle for survival
Some of the offspring produced in a generation do not survive.
Darwin identified competition as a major factor limiting population sizes (competition for resources such as food, shelter, mates, nesting sites etc.).
Observation 3: Members of a population show variation
Some of these variations are inherited by the offspring. Ironically the mechanism of the inheritance of genes was being worked out at this time (Gregor Mendel produced his findings in the 1850s) but they remained undiscovered by biologists until 1900.
Darwin was however aware that sexual reproduction mixes variations to produce new combinations (recombinants)
Later Mendel’s work was able to explain some of the patterns of inheritance through the mixing that occurs during meiosis and fertilisation.
Darwin could not explain the origin of new variants. This had to wait until the 1920s and 1930s when work began on mutations after the discovery of radiation.
Deduction 2: There will be a struggle for survival between the members of the population. Individuals with advantageous variations will breed and produce more offspring.
As generations pass by, the proportions of the alleles for the different variants will change in favour of those that provide the best adaptations.
This is sometimes referred to as "survival of the fittest".
Natural selection has been observed at work in populations of species over the past century. Examples include pesticide resistance in insects, antibiotic resistance in bacteria, industrial melanism in moths and tolerance to heavy metals in plants.
The Origin of Species by Natural Selection
Darwin and Wallace argued that if natural selection proceeded for a long enough period of time it could bring about the evolution of new species. Darwin himself favoured a long period of slow changes. Recently this has been refined to include the possibility of rapid changes over a short period of time.
Whether fast or slow, observing the evolution of a new species is unlikely in the lifetime of a scientist. That species evolve is a fact but that they evolve by natural selection remains a theory. Other mechanisms exist that can also lead to the evolution of species (e.g. genetic drift).
Convergent and Divergent Evolution
Species exposed to the same selective pressure in different parts of the world tend to develop the same adaptations even though they may be completely unrelated (e.g. the placental wolf and the marsupial thylacine or Tasmanian wolf). This is convergent evolution.
Populations of a species that are separated and evolve under different selective pressures develop different adaptations as they diverge. This is divergent evolution.
Species that populate a new area where there are vacant niches will diverge and specialise as they fill the vacant niches. This is called adaptive radiation (e.g. Darwin’s Finches on the Galapagos).
Example industrial melanism
Note The melanic allele is dominant so if it is present it will be expressed and selection acts on it immediately.
The speckled allele is recessive so it will be carried by heterozygotes. It may remain "hidden" in the population form many generations.
Thus, the melanic allele may not become completely fixed.
Industrial melanism is an example of transient polymorphism where one allele replaces another.
Sickle cell anaemia is an example of balanced polymorphism where two alleles are advantaged in zones infested by malaria.
The sickle allele gives protection to malaria but can lead to a fatal blood disease
The normal haemoglobin allele permits normal transport of oxygen but gives no protection to malaria
The transition from one species to another
But what is a species?
A group of individuals that breed together freely in nature to produce fully fertile offspring.
Does forced mating count?
If populations are geographically separated it is not possible to test this definition.
If populations are separated in time it is not possible to test this (e.g. fossils in different strata are they the same species?)
Some species only show asexual reproduction.
Mechanisms of speciation
Isolation of a population so that it cannot breed freely with others is necessary.
Fragmentation of the range
Populations can become isolated within the range of the parent population.
Differences in food preferences may develop in a part of the population which stop them from breeding freely.
Seasonal isolation may occur, e.g. different flowering times or breeding seasons.
Parts of a population may develop a preference for a particular variety and may not mate with any other. e.g. the snow goose, blue forms tend to mate with blue forms and white forms tend to mate with white forms.
Genetalia or floral parts may be incompatible
Fertilisation may be prevented by:
Failure of the gametes to be attracted to one another.
The sperm cell receptors of the oocyte may be incompatible with the acrosome.
Pollen tubes cannot find or penetrate the embryo sac in flowers.
Hybrid inviability. Hybrid offspring die.
Hybrid infertility. Hybrids survive but are incapable of producing gametes.
© Paul Billiet 2012