How does evolution operate?. Darwin’s natural selection and adaptation Since organisms produce...

How does evolution operate?

Darwin’s natural selection and adaptation

• Since organisms produce more young than can be supported

• Since most populations are normally stable• Since natural resources are limited• Therefore a struggle for existence takes place• Since individuals vary extensively• Since much variation is heritable• Therefore individuals with heritable traits that

increases their survival leave more offspring• Therefore unequal ability to survive and reproduce

leads to a gradual change in population gene frequency.

Remember

• Selection operated at the level of the individual

• Populations evolve (change in gene frequency).

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H-W assumptions…return to violations

• Small populations– Bottle necks– Genetic drift– Isolation (no

migration)

• LEAD TO ALLELE CHANGE—BUT IT IS RANDOM

• Selection

• LEADS TO ALLELE CHANGE—NOT RANDOM i.e. directional

Variation

• Variation may be genetic or non-genetic.

• Genetic traits - heritable• Epigenetics eg sex determination in turtles• maternal effects

– nutrition effects– intra-uterine effects– environmental effects

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How does genetic variation come about?

Types of selection

Types of selection

• Stabilizing selection – but remember Hardy-Wienberg.

• Directional selection• Disruptive or diversifying selection

• Group selection• Kin selection • Sexual selection

Group Selection

• Originally proposed by Wynne-Edwards• Birds• Individuals do what is good for the group• Has been rejected

Kin Selection

• is the evolutionary mechanism that selects for those behaviors that increase the inclusive fitness of related individuals.

Variation: the special case of polymorphism

Australian snails Bankivia fasciata

High level of polymorphism

- Adaptive selection theory (Lewontin)

- Neutral theory (Kimura) balance between drift and mutation

No agreement on what maintains polymorphism

What drives selection?

• Biotic and abiotic factors

• Remembering from ecology—what are the ways that abiotic factors might cause change?

• Factors driving selection– Abiotic

• Climate• Nutrient availability• Other?

• Change in gene frequency since some genes convey higher fitness to individuals – no competition involved.

Biotic pressures

• Remembering from ecology—what are the different types of intra specific relationships that occur between animals?

• —what are the different types of inter specific relationships that occur between animals?

Inter – specific interactions

• Back to ecology—recall Gause’s principle of competitive exclusion— ‘no two species can occupy the same niche indefinitely when resources are limiting’

• Terms—– ‘allopatric’: species occupy different areas– ‘sympatric’: species occupy same areas

Average beak depth of two different species of finch on two different islands

Complications on a theme…Mimicry

Poisonous butterflies

Non-poisonous mimics

Hypothesis testing

• The interesting story of the giraffe’s neck…not all adaptations are for improved health.

Feeding behaviour during dry season

In combat over mates, males can actually kill a rival

Within species interactions

• The puzzle of sexual dimorphism

– It cannot be due to natural selection

Male white turkey Female white turkey

Sexual selection

• Two important components– Mate choice

– Competition for mates

Sexual selection

• Size differences between males and females. Very common.

• How do we explain it?

Male

Female

Sexual Selection

Social system

Harem typeMonogamous pairs Female

territoriality