Before studying this unit you should read and understand the

contents of this website:http://evolution.berkeley.edu/evolibrary/article/evo_01

What is a species?Members of a species are similar to each other but different to members of another species

This similarities can be:–Physical–Biochemical–Immunilogical–Developmental–Ecological

What is a species?Dictionary definition 1:

‘A class of individuals or objects grouped by virtue of their common attributes and assigned a common name; a division subordinate to a genus’

The American Heritage® Dictionary of the English Language

Dictionary definition 2:

‘A group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding’

Oxford English Dictionary

Biological definitionTwo organisms are the same species if:

a) They have the potential to breed

b) Their offspring are fertile

‘A population or group of populations whose members have the potential to interbreed to produce viable, fertile offspring’

How do species occur?

Often called EVOLUTION: the species we see and know now have evolved from other species

This idea is central to biological classification

The formation of new species from existing species due to an increasing genetic difference of a population that leads to a lack of interbreeding

Speciation

HybridisationThe production of offspring from parents of two genetically different species

PolyploidyA random mutation that causes a sterile hybrid to double its chromosome number and become fertile

Occurs very rarely in animals

More common in plants especially angiosperms (flowering plants)

How speciation occurs

Isolation

Reproductive barriers

Genetic Isolation

New species formation

Reproductive isolation

• Organism cannot interbreed or mate with another group.

• This could be because: incompatible gametes, wrong courtship, anatomical differences, geographical barrier, etc

Allopatric Speciation

A mode of speciation that occurs when the ancestral population becomes segregated by a geographical barriers

Mountain range

Rivers/Oceans

Glaciers

Geographical speciation process:

• Populations separated by physical barrier• No mixing of gene pools• Different selection pressures• Adaptation to local environment• Survival and reproduction• Mutations in one group different from

mutations on the other group• Change in allele frequency• RESULT : 2 DIFERENT SPECIES THAT

CAN NOT INTERBREED

Sympatric speciation•A mode of speciation that occurs without geographical isolation.

•Relatively quick

•Genetic differences occur through strong forces of natural selection or mutations

•New species arise within the range of the parent population

Charles Robert Darwin1809 Born in Shrewsbury, England

1831-1836

Naturalist onboard HMS Beagle on a worldwide voyage.

Sep 1835

Reached the Galapagos Islands and collected various finches among many animal and plant samples

July 1837

Starts notes on ’The Origin of species’

1858 Alfred Wallace sends Darwin his own manuscript

1859 Darwin publishes ‘On the Origin of the Species’ which was highly controversial

1882 Dies and is buried in Westminster Abbey

Darwins Finches

Different beak sizes developed from eating different food sources:

•Nuts

•Insects

•Fruits

•Blood!

Pre-mating barriers to interbreeding

Explanation

Geographical separation Inhabit different continents, islands or sides of a canyon

Habitat Isolation Populations inhabit different local habitats within one environment

Temporal Isolation Populations use the same environment but are reproductively active at different times (times of day/seasons)

Behavioural Isolation Courtship rituals and songs of birds may be specific to a particular species and do not attract similar species

Mechanical Isolation Fail to mate due to physical incompatibility (pollinators for flowers)

Post-mating barriers to interbreeding

Explanation

Gametic Isolation Female might kill sperm of wrong genotypes or pollen of wrong genotype fails to germinate

Hybrid not viable Hybrid fails to develop maturity, zygote may be aborted during development

Hybrid sterility Hybrid grows to maturity but is sterile (e.g. mule)

Hybrid breakdown First generation may be viable but 2nd generation are sterile, become weak or defective.