Speciation - Allopatric and Sympatric and genetic drift

What is the founder effect?

- A case of genetic drift - The loss of genetic variation that occurs when a new population is established from a very small number of individuals from a larger population

Population bottleneck:

- A sharp reduction in the size of a population due to environmental events (earthquakes, floods, fires) or human activities (hunting, habitat destruction) - Population bottlenecks produce a smaller population with reduced genetic diversity - In subsequent generations, genetic diversity remains lower, only slowly increasing with time as random mutations occur.

Sympatric speciation definition

Formation of two species from one original species due to reproductive isolation whilst occupying the same geographical location. This can be by: - Temporal variation- breeding seasons at different times. - Behavioural variation- mutations affecting courtship. - Mechanical variation- anatomical differences preventing mating. - Gametic variation- results in genetic or biochemical incompatibility. - Hybrid sterility- cannot produce viable gametes.

Example of allopatric speciation is Galapagos finch:

- A single ancestral species colonised one of Galapagos islands. In the absence of competition, its population increased and populations became established in other habitat on same and diff islands. - Each population evolved adaptations to suit new environment, including available food resources e.g. different shapes and sizes of beak for diff seeds - Being geographically separated from mainland population, these changes led to various populations being so different they can't interbreed and so separate species.

Example of sympatric speciation, apple fly maggots:

- Ancestors of apple maggot flies laid their eggs only on hawthorn trees (which are native to America) - Most (about 95%) female maggot flies generally choose to lay their eggs on the type of fruit they grew up in, and males also tend to look for mates on the type of fruit they grew up in -> means there the flies are almost completely reproductively isolated - So hawthorn flies generally end up mating with other hawthorn flies and apple flies generally end up mating with other apple flies. - Means gene flow between parts of the population that mate on different types of fruit is reduced. This host shift from hawthorns to apples may be the first step toward sympatric speciation — in fewer than 200 years, some genetic differences between these two groups of flies have evolved (not 2 diff species just yet)

PPQ: compare natural selection and genetic drift

- Genetic drift occurs in smaller population - However, natural selection proposes that the alleles are selected for a purpose, to enable the organisms to survive and adapt to their environment, whereas genetic drift proposes that allele frequencies change by chance.

Who proposed the idea of genetic drift?

- Motoo Kimura - He proposed the neutral theory; most evolutionary changes and most of the variation with in and between species is not caused by natural selection but by random drift of mutant alleles that are neutral

Sympatric speciation process:

- Populations become reproductively isolated due to differences in their behaviour. Individuals of the same species may not be separated by geographically barriers, but are still unable to reproduce. - Could be because a random mutation within the population could impact reproductive behaviour, for example it may cause individuals to perform a different courtship ritual or for individuals to be fertile at different times of the year. - So these individuals will not reproduce together and there will be no gene flow between the two groups within the populations. - Overtime these reproductively isolated populations will accumulate different mutations, until their DNA is so different that they cannot reproduce to create fertile offspring. - Therefore classes are two different species.

Allopatric speciation process:

- Populations can become separated geographically leading to reproductive isolation. - Within all populations there is genetic variation due to random mutations. - A population could become geographically isolated over time by new mountain ranges etc. Separating original population into two, whom are now unable to reproduce due to the geographical barrier. - Separate populations will have different selection pressures and so will accumulate different beneficial allele mutations over time to help them survival in own environments, which are likely to vary. - Due to this accumulation of genetic differences over time the two populations become so genetically different that they would be unable to reproduce to create fertile offspring. - They are therefore classes are two different species, with own gene pool

Relaxation of selective pressures for genetic drift:

- The random nature of mutations ensures that all genes will be subject to mutations - When natural selection is relaxed, there is no mechanism for weeding out mutations that disrupt the function of that gene - Without selection, disruptive mutations will accumulate in the population - As these mutations accumulate, the proportion of individuals carrying the functional gene drops, so with each subsequent generation, it becomes less likely that the functional gene will be passed on.

What is genetic drift?

- This is the change in the allele frequency within a population between generations due to random sampling- random chance - There will always be genetic drift from one generation to the next, but continual, substantial genetic drift results in evolution. - The smaller a population is the bigger the impact allele frequency changes have proportionally and this is why evolution often occurs more rapidly in smaller populations. - Genetic drift may cause alleles to disappear completely and thereby reduce genetic variation

PPQ: Lord Howe Islands possesses 2 species of palm tree which have arisen by sympatric speciation. The 2 species diverged from each other after island formed 6.5 million years ago. The flowering time of the 2 species differ. Using this info, suggest how these 2 species of palm tree arose by sympatric speciation. (6 marks)

1) The palm trees are both found in the same envrionment 2) But mutations cause them to have different flowering times 3) Therefore, reproduction isolation occurs 4) The different alleles are passed onto their own offspring 5) Disruptive (natural) selection occurs 6) Eventually different species cannot interbreed to produce fertile offspring, thus they are two differenet species.

Speciation definition

The evolution of new species from existing ones

Allopatric speciation definition

The formation of two species from an original one due to geographical isolation, such as physical barriers like oceans or new mountain ranges.

Adaptive radiation definition

a process in which organisms diversify rapidly into a multitude of new forms, particularly when a change in the environment makes new resources available and opens environmental niches