L5 Natural selection

What was the idea of special creation of organisms?

- states species didnt change

- each species was separately created

- earth and life are young

What is descent with modification?

- States species change over time

- Species derive from common ancestors

- Earth and life are old

What did Carolus Linnaeus state?

• Species are immutable (unchanging)

• Fits with Genesis

• No evidence for an alternative explanation

What is Lamarckism?

• 1809 "Philosophie Zoologique"

• 3 elements

1. Fact

2. Course

3. Need

• Inheritance of acquired characteristics

Why was Lamarckism wrong?

• Confusion between physiology and evolution

• Evolution towards perfection

eg. he suggested that giraffes necks had stretched over time

What was correct about Lamarckism?

Species do evolve

Is evolution goal orientated?

No, and it does not lead to perfectly adapted organisms

What evidence shows that evolution is not goal orientated?

Left recurrent laryngeal nerve

Mammalian larynx

in humans it loops round aorta

Which if we could choose its not a good idea for it to loop around

but we originally got this because of the anatomy of when we were fish

How is evolution shaped by natural selection?

Many processes shape evolution but only natural selection has a systematic tendency to build up the elaborate functions that are needed to maintain reproductive success (fitness)→ leads to adaptation

• Natural selection is the inevitable consequence of inherited variations in fitness

What is the definition of natural selection?

Any consistent difference in fitness (reproductive success) among phenotypically (genetically) different biological unit (individual, genotype, gene) within a population

What was Darwins idea?

• Darwin: preservation of favourable variations and the rejection of unfavourable variations

• The fitness of an individual is defined as the relative contribution of its genotype to the next generation relative to the contributions of other genotypes and is determined by the number of offspring it manages to rear successfully aka Darwinian fitness

What does natural selection depend on according to Darwin?

Inherited variation in fitness

What were Darwins 4 premises of natural selection?

1. Organisms are variable

2. At least some variation is heritable

3. A pair of parents produces more than just 2 offspring

4. Population size is generally stable → Differential survival →Differential reproduction

What is an Example of selection in response to an environmental pressure?

Variation in chilli spice

Hot chillies result in hot chillies etc

Predator can eat some of the chillies but only eats the mild ones, so over time only the hot ones are left, so there is a loss in mild chillies so the population gets hotter

What does natural selection do to the fitness of genotypes?

• Selection increases the frequency of "fit" genotypes

Over time adaptive evolution

It increases mean fitness

What factors can effect the increase in fitness caused by natural selection?

• Mutation, migration, drift and recombination decrease mean fitness

• Natural selection is therefore the only process that causes adaptation

• However, other processes may facilitate natural selection

What is an adaptation?

'An adaptation' is a phenotypic trait moulded by natural selection. The trait could be physiological, behavioural, developmental or morphological, or it could be a life history trait

What is adaptation?

'Adaptation' is a process of evolution in which traits in a population are modified by natural selection to meet better the challenges presented by the local environment

What is evolution?

Evolution = Change in allele frequencies over time

What does natural selection result in over time?

evolution

What are the 2 conditions in which natural selection will result in evolution?

1. Variation in lifetime reproductive success

2. Trait variation must be heritable

• Environmental variation (provides selection pressures) →adaptive radiation

What was the idea of Darwins finches?

They famously evolved to have different beaks which are suited to different food types

Darwin's work with finches helped him understand how species can change over time through natural selection

What are different modes of selection?

1. Directional

2. Disruptive

3. Stabilising

4. balancing

What is directional selection?

favours individuals at one end of the phenotypic range

invasion of a new trait

What is disruptive selection?

favours individuals at both extremes of the phenotypic range

Can cause adaptive radiation

eg. overdominance

What is stabilising selection?

selection favouring average individuals

No evolution

What are types of directional selection?

- Purifying selection = directional selection against deleterious mutations

- Truncation selection = allows only those with highest trait values to reproduce

What is balancing selection?

selection that maintains polymorphism (e.g. over dominance and frequency-dependent selection)