Evolution and Speciation TEST NOTES

Seite 1

Patterns of Evolution:

Divergent Evolution

Convergent Evolution

Co-Evolution

Adaptive Radiation

Punctuated Equilibrium

Phyletic Gradualism

Divergent Evolution:

Ancestral species evolve into 2 or more species

Become specialised to occupy different niches

Involves formation of a large number of species from ancestral species

Due to the availability of many niches

Associated with Homologous Structures

Different functions

Same ancestral origins

Evolve differently

Exposed to different selection pressures

Exposed to different ecological niches

Convergent Evolution:

2 or more unrelated species evolve to resemble each other

Develop similar features in response to similar niche requirements

Associated with Analogous Structures

Different ancestral origins

Similar functions

Similar appearances

Evolve similarly

Exposed to similar selection pressures

Exposed to similar ecological niches

Adaptive Radiation:

Different groups able to occupy a wide variety of different ecological niches recently become available

A form of divergent evolution

The rapid evolution of a number of different species from the same ancestral origin

Co-Evolution:

2 different species ecologically intimate

Influence each other’s evolution

Evolutionary changes in one species

That species acts as a selection pressure on another

Vice Versa

Has 3 forms

Predator-Prey

Predator and prey evolve together

Predator evolves whatever necessary to eat prey

Speed, camouflage, heightened sense, immunity to prey’s poison, own poison

Prey evolves whatever necessary to avoid the predator

Speed, camouflage, heightened senses, own poison, thorns

Mutualistic

A relationship develops because of selection pressures

Develop adaptations for the benefit of each other’s niche requirements - gain survival advantages

Mutual benefits for both species

Greater reproductive success

Pass on their favoured alleles

Parasitic

Parasite causes harm

Takes nutrition from the host

Lifecycles in or on the host

Lives outside the host (external parasites)

Lives inside the host (internal parasites)

Phyletic Gradualism:

Evolutionary model that occurs as a slow steady process

Organisms change and develop slowly over time

Small periodic changes in the gene pool

Natural selection gradually changes the average features of species

Removes less fit individuals

Increases fit individuals

Not concentrated in lineage-splitting evolution

Have non-straight lines in morphology graph

Single species gradually change into new species - original goes extinct

Punctuated Equilibrium:

Evolutionary pattern

Connected to speciation events

E.g RIM's

Species undergoing very little evolution for long periods of time

Before suddenly evolving rapidly

Statis (straight line)

Lack of substantial change over millions of years

Happens to small populations that have become isolated from original species

Causes of Punctuated Equilibrium:

Mutations

Change in environment

Mutations - Punctuated Equilibrium:

Mutations in the genes of few individuals cause a rapid change

Very small population

Chances of evolution increase rapidly

Individuals evolve independently

Exposed to different selection pressures

Different alleles selected for and passed on

Gene pool drastically changes in small time

Sudden divergence

Change in environment - Punctuated Equilibrium:

Stage 1: statis

Organism living, dying

Stage 2: isolation

A sudden change in environment e.g drop in sea level forms lake

A small number of population of organism isolated

Stage 3: selection pressures

Small population experiences strong selection and rapid change

New environment formed - new selection pressures

Stage 4: no preservation

Seite 2

Processes of Evolution:

Natural Selection

Natural Selection:

Organisms with structural, physiological, behavioural features

More likely to survive

Reproductive success

Better suited to the environment

Contribute more alleles to the gene pool

Pass favourable traits to next generation

Organisms have adaptations

Enables survival

Selected for by the environment

Increased chance of survival

Increased likelihood of passing favourable genes to next generation

The mutation causes a new allele to form and increases the allele diversity.

Mutations provide the "raw material" upon which the mechanisms of natural selection can act.

If the mutation increases fitness, the individuals with this adaptive phenotype are more likely to reproduce and pass on the favourable allele.

The favourable allele will increase in frequency within the population over time.

Types of Evolution:

Stabilising Selection

Intermediate phenotype selected for - has the highest fitness

Two or more extreme phenotypes selected against

Reduces diversity within gene pools

Weak intraspecific competition cause disruptive selection

Weak selection pressures cause disruptive selection

Disruptive Selection

Two or more extreme phenotypes selected for - has the highest fitness

Intermediate phenotype selected against

Increases diversity within gene pools

Strong intraspecific competition cause disruptive selection

Strong selection pressures cause disruptive selection

Leads to speciation

Splits population into two groups with distinct phenotypes

Directional Selection

One extreme phenotype selected for

Other phenotypes selected against

Shifts allele frequency over time towards the selected phenotype

Advantageous alleles increase in frequency

Increased fitness of individuals with those phenotypes (reduces competition)

Seite 3

Polyploidy:

When an organism has more than two complete sets of chromosomes in its somatic cells

Occurs due to non-disjunction

Chromosomes are not pulled apart correctly (by spindle fibres) during cell division (meiosis or mitosis)

Gamete cells have extra pairs of chromosomes

Results in 2n gametes

If these 2n gametes fuse - a tetraploid (4n) may result

Results in instant speciation

Allopolyploidy:

Results from the mating between 2 different but closely related species

Hybrid is formed and may be sterile

Does not have homologous pairs of chromosomes

Cannot form gametes

Autopolyploidy:

One species contributes all the chromosomes

Polyploidy is where an organism has more than two homologous sets of chromosomes in their somatic cells. The polyploids are fertile because they have homologous pairs / even number of chromosomes so can form viable gametes. Polyploidy can be caused by the spindle pulling an incorrect number of chromosomes to one end. This nondisjunction may result in 2n gametes. If two 2n gametes fuse, a tetraploid (4n) may result. Postzygotic RIMs such as hybrid infertility / hybrid sterility may keep polyploids isolated. E.g. tetraploid with 18 chromosomes in gametes + hexaploid with 27 chromosomes in gametes results in hybrid with 45 chromosomes, which cannot form viable gametes because the chromosomes cannot pair at meiosis because some have no homologues. Changes in morphology resulting from polyploidy contribute to habitat / niche differentiation. These new physiological tolerances allow them to exploit novel niches. Different habitats / niches have different selection pressures such as water availability which may lead to (sympatric) speciation. For example, polyploids have lower stomata density and thicker epidermis which reduces water loss / transpiration and maintains turgor. The wider xylem reduces the chances of air blockage in drought conditions, which gives an adaptive advantage in these conditions and allows photosynthesis to be maintained.

1) The definition of polyploidy

2) For hybrids to form, parents need to be closely related.

3) Hybrid infertility (sterility) is a result of NOT having enough chromosomes from either parent. Inheriting only 1 set from each parent means that homologous chromosomes cannot form.

4) In order for hybrids to become fertile, one of two things needs to happen:

- self-fertilisation occurs

or

- non-disjunction event occurs which involves the duplication of chromosome numbers

Both of these occurrences result in chromosomes being able to pair up (homologous pairs)

5) In order for a hybrid to become fertile, homologous chromosomes from each parent are required.

6) Why does this result in a new species? No longer is this hybrid able to reproduce with original species to produce fertile offspring due to an incorrect number of chromosomes.

Seite 4

Speciation:

Allopatric Speciation

Sympatric Speciation

RIM's

Allopatric Speciation:

Formation of 2 different species

One population divides by a physical barrier

Two or more geographically isolated populations form

Prevents gene flow between populations

Subject to different selection pressures

Selection pressures favour phenotypes suited to each environment

The allele frequency of favourable alleles increase over time

Sympatric Speciation:

Formation of 2 different species

In the same geographical area without prior geographical isolation

Diverged from a common ancestor

Occupy separate niches in that area and rarely come into contact

Prevents gene flow between the species due to competition avoidance

Subject to different selection pressures

Seite 5