9 min readPathway sheetPart III (4 marks) or Part IV (7 marks): the main points of Darwinism; speciation as a 2-mark question

Darwinism, natural selection and speciation

Darwin said more young are born than can survive, so they struggle. Those with useful variations survive and breed: this natural selection slowly forms new species.

Proposed byCharles Darwin, an English naturalist

BookOn the Origin of Species by Means of Natural Selection (1859)

VoyageHMS Beagle; he studied the finches of the Galapagos Islands

SpeciationFormation of new species from existing ones

Darwin's theory in one chain

Darwin travelled on HMS Beagle (1831 to 1836) and studied finches with different beaks on the Galapagos Islands.

Main points of Darwinism

1
Overproduction: living things produce far more young than can survive. One pomegranate holds hundreds of seeds.
2
Struggle for existence: food and space are limited, so organisms compete. Struggle is intraspecific (same species), interspecific (different species) or environmental (heat, drought, floods).
3
Variations: no two individuals are exactly alike. Some variations help in the struggle.
4★
Survival of the fittest: individuals with useful variations survive and breed, and the unfit die out. Nature 'selects' the fit: natural selection.
the phrase is Herbert Spencer's
5
Origin of species: useful variations add up over many generations, and a new species forms.

★ key step

Objections to Darwinism

Darwin could not explain how variations arise. Genes and mutations were not known then.
He could not explain how variations are passed to the young.
He did not separate variations that are inherited from those that are not.
Natural selection explains the survival of the fittest, but not the arrival of the fittest.

Speciation

A species is a group of organisms that can breed among themselves and produce fertile young. Speciation is the formation of new species from existing ones. It often starts when a river, mountain or sea splits a population. This is geographical isolation. Each group lives in different conditions. Mutations and natural selection make the groups different over many generations. In time they cannot interbreed, even if they meet. This is reproductive isolation, and two species have formed. Darwin's finches on the Galapagos Islands formed many species with different beaks in this way.

Lamarckism vs Darwinism

LamarckismChange comes from use and disuse and the animal's own needs. Acquired characters are inherited. The giraffe stretched its neck, and the longer neck was passed on.

DarwinismChange comes from natural selection of useful inborn variations. Giraffes that happened to be born with longer necks got more leaves, survived and passed the trait on.

In real life

When the same mosquito spray or antibiotic is overused, a few resistant ones survive and multiply. This is natural selection happening in our homes and hospitals.

Exam angle

2 marks: “What is meant by 'survival of the fittest'?” Individuals with useful variations survive the struggle for existence and breed, while the unfit die. Herbert Spencer coined the phrase; Darwin called it natural selection.

2 marks: “Define speciation.” The formation of new species from existing ones, mainly through geographical isolation followed by reproductive isolation.

7 marks: “Explain Darwin's theory of natural selection. Write its objections.” Overproduction, struggle (three kinds), variations, survival of the fittest, origin of species; giraffe or finch example; objections: no explanation of how variations arise or are inherited.

Remember it

Many born, few survive, the fit win, new kinds form: Darwin in one line.