Processes and Patterns of Evolution

Year 13

Evolution

•Evolution is the change in allele frequencies within a population over time .

•Microevolution: small scale changes within gene pools over generations.

•Macroevolution: evolutionary changes on a large scale involving whole groups of species and genera.

Gene pool

• Sum total of genes in a whole population

Evidence for evolution• Evidence for evolution comes from a

variety of sources:

Early ideas on evolution

Natural selection

Natural selection:

1)All organisms have a high reproductive rate but food supply and other environmental factors are limiting. This means offspring struggle to survive.

2)There is variation among offspring. Some are better adapted to the environment than others.

3) Those organisms with favourable variations will live longer and will pass on favourable characteristics to offspring.

4) Over time, each successive generation will be better adapted to the environment – “survival of the fittest”.

5) This leads to a change in the frequency of alleles within the population and may cause speciation over a long period of time.

Selection Pressures

• These are pressures that result in survival of those organisms with favorable alleles. Selection pressures affect population size and gene pool.

• Selection pressures include:• 1) competition• 2) predation• 3) climatic factors• 4) disease

Three Forms of Natural Selection

Directional Selection

Hominid Brain Size

Beak Depth Changed in a Predictable Way in Response to Natural Selection

Significantly, beak depth is a genetically determined trait.

Human Birth Weight Is Under Stabilizing Selection

Modern medicine relaxes this and other forms of selection.

Stabilizing Selection for the Sickle Cell Allele

In heterozygous form, the sickle cell allele of -globin confers resistance to malaria. Therefore, the allele is maintained, even though it’s harmful in homozygous form.

Disruptive natural selection

The formation of new species = speciation

• Definition of species:

A species consists of groups of similar individuals who can interbreed with each other to produce fertile offspring but do not naturally interbreed with members of other species.

A Population is a group of the same species living in the same area at the same time and who can interbreed

Demes

• A species usually exists as distinct populations may be separated geographically. These local interbreeding populations are called demes.

• Organisms mostly interbreed within the deme rather than with members of other populations, therefore, demes often develop slightly different allele frequencies, giving each different characteristics.

Species tricky to define

• Boundaries of a species gene pool can be unclear .

For example: closely related species of the dog family can interbreed

Also, species can show a gradual change in phenotype over a geographical area. This gradual change is called a cline. This often occurs over the length of a country or continent.

No

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DingoCanis familiaris dingo

CoyoteCanis latrans

SpeciesThe boundaries of a species gene pool can be sometimes unclear, such as the genus to which all dogs,

wolves, and related species belong:

Coyote–red wolf hybrids

Interbreeding

Inter-breeding

Interbreeding

Inter-breeding

Domestic dogCanis familiaris

No

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rbre

edin

g

Interbreeding

Inter-breeding

Interbreeding

Gray wolfCanis lupus

Red wolfCanis rufus

Black-backed jackalCanis mesomelas

Golden jackalCanis aureus

Side-striped jackalCanis adjustus

Clines Species can show a gradual change in phenotype over a geographical area. This gradual change is called a cline. This often occurs over the

length of a country or continent.

Ring species – a special type of cline

A

B

C

D

E

If a cline forms a ring, (eg. across a continent) demes A and E may be unable to breed when they meet, although, the intermediate forms can still interbreed. Are A and E still the same species or two separate species?

Sub-species

• These arise when populations show characteristics that are different from nearby populations. Sub-species can interbreed but this often occurs less frequently.

Reproductive Isolating mechanisms

Reproductive isolating mechanisms prevent successful breeding between different species. They are barriers to gene flow.

Most species have more than one isolating mechanism operating to maintain a distinct gene pool.

Geographical barriers

• Physical barriers such as mountains, rivers, oceans and deserts prevent gene flow between two different species.

Prezygotic (before fertilisation) isolating mechanisms

• These operate prior to mating

• Include:1) habitat preference

2)Timing of mating

3)Behavioural incompatibility

4)Structural incompatibility

5)Gamete mortality

1) Habitat preference

2) Timing of mating

Closely related species may have quite different breeding seasons. This makes them sexually active at different times of the year.

3) Behavioural incompatibility

These behaviours ensure mating within species.

4) Structural incompatibility

Gamete mortality

Even if mating occurs, the sperm of one species may not be able to survive in the reproductive tract of another species.

Postzygotic Isolating mechanisms

• These act after fertilisation to prevent successful reproduction.

Include: 1) hybrid sterility

2) hybrid inviability

3) hybrid breakdown

1) Hybrid sterility

• Even if two species mate and produce offspring, the offspring may be sterile. This is common in the horse family.

• One cause of this sterility is the failure of meiosis to produce normal gametes. This can occur if parents are different in chromosome number.

Zebronky or zedonk

2) Hybrid inviability

• A zygote is formed but fails to develop properly. Sometimes fail to divide because of unmatched chromosome numbers from each gamete.

3) Hybrid breakdown

• The first generation offspring (F1) are fertile but the second generation (F2) are infertile or inviable.

Review questions

• 1. Would having just one pre-zygotic RIM between a species/population be enough to prevent gene flow? Why?

• 2. Would pre-zygotic or post-zygotic RIM have the best chance of preventing hybrids?

• 3. Which forms of RIM would most likely establish first? Pre or post? Why?

• 4. At a biological level why are hybrids necessary, yet also a risk to a species survival?

Types of speciation

• Allopatric speciation

This occurs when species become geographically separated, each being subjected to different natural selection pressures, and finally establishing reproductive isolating mechanisms.

Sympatric speciation

• Occurs when a population forms a new species within the same area as the parent species.

• There is no geographical separation.

• More rare than allopatric speciation.

Two situations where sympatric speciation thought to occur:

A) Speciation through niche differentiation – there may be a change in host preference, food preference or habitat preference. This will lead to disruptive natural selection.

B) Instant speciation as a result of polyploidy (particularly in plants).

Page 242 Biozone

New Zealand examples

• In New Zealand, the genus Melicytus comprises 11 species, including mahoe. Seven of these species are diploid (with 2N = 32). They all have unisexual flowers and grow as trees or tall shrubs. There are two tetraploid species (2N = 64), both of which have hermaphrodite flowers and a divaricating shrub habit, and one hexaploid (2N = 96), which has unisexual flowers but also grows as a divaricating shrub.

SYMPATRIC SPECIATION

In America 200 years ago there lived a species of Hawthorn maggot fly, which lived on Hawthorn fruit.

When apples were introduced a new niche opened up, and some of the Hawthorn flies moves in.

These new flies now were unlikely to breed with Hawthorn flies as they were not in the same places. Therefore no gene flow.

Our new species: Apple maggot flies.

SAME PLACE…

Stages in the development of a new species:

• Do pages 240-245

Patterns of evolution

Divergent evolution

• This is when an ancestral species evolves into two or more species that occupy different ecological niches.

• This may be due to the ancestral species spreading out to occupy new habitats with differing conditions. The populations then become genetically isolated.

Adaptive radiation

• This is an example of divergent evolution, but the ancestral species diverges into a large number of species.

• Adaptive radiation is more common in periods of major environmental change eg. cooling climates. Organisms can develop new adaptations enabling them to exploit different habitats.

• Page 254

Adaptive radiation of the ratites

Page 256 biozone

Convergent evolution

• This occurs when unrelated species evolve similar features .The species DO NOT arise from a common ancestor but have similar ecological roles.

• Similarity of form due to convergence is called analogy.

• http://www.teachersdomain.org/resource/tdc02.sci.life.evo.convergence/

Coevolution

• This is when two (or more) species evolve in response to each other.

• Over time, the parties in the relationship become mutually dependent on each other.

• Individuals who have less favorable alleles will be lost via selection from the other species and therefore only favorable alleles will be passed to the next generation.

Pollination syndromes

• Flower structure has evolved in response to many types of animal pollinators.

• Flowers and pollinators have coordinated traits known as pollination syndromes.

• These improve pollination efficiency.

• See page 252 and 253

Predator and Prey (arms race)

• http://www.teachersdomain.org/resource/tdc02.sci.life.evo.leaf/

• http://www.youtube.com/watch?v=R5piJCyHwtw

Phylogeny Tree

• A phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor. In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the descendants, and the edge lengths in some trees correspond to time estimates.

Homologous structures

• Structural similarities between groups of organisms suggest they descended from a common ancestor.

• Homologous structures are evidence of adaptive radiation.

The pace of evolution – two theories

1) Punctuated equilibrium

Fossil evidence indicates species stayed the same for long periods of time, and then there were short bursts of evolution that produced new species quite rapidly.

This, is the punctuated equilibrium theory.

2) Gradualism

The theory of gradualism assumes that populations slowly diverge from one another by accumulating adaptations in response to different selective pressures.

If species evolved by gradualism there should be transitional forms seen in the fossil record, as with the evolution of the horse.

Extinction

•Extinction is an important process in evolution as it provides opportunities for new species to develop in the vacant free niches.

•Extinction is a natural process in the lifecycle of a species.

•Radiation may follow extinctions but are rarely the cause of extinctions.

• http://www.teachersdomain.org/resource/tdc02.sci.life.eco.bioinvaders/

Background rate of extinction.

This is the steady rate of extinction of a species within a taxanomic group. The duration of complex organisms is estimated to be 1 million years compared with 10-20 million years for simpler organisms.

Mass extinctions

This refers to abrupt increases in extinction rates affecting huge numbers of species at the same time.

Examples

Modern day (holocene)

Cretaceous-Tertiary (65.5mya)-75% of species extinct

Triassic-jurassic (200mya) -50% of species extinct

Permian-Triassic (250mya) -83% of species extinct .

Late Devonian (360mya) – 70% of species extinct

Mitochondrial DNA mtDNA

• DNA is present inside the nucleus of every cell of our body but it is the DNA of the cell’s mitochondria that has been most commonly used to construct evolutionary trees.

• Mitochondria have their own genome of about 16,500 base pairs that exists outside of the cell nucleus.

• They are present in large numbers in each cell, so fewer samples are required.

• They have a higher rate of substitution (mutations where one nucleotide is replaced with another) than nuclear DNA making it easier to resolve differences between closely related individuals.

• They are inherited only from the mother. This means that every individual within the same maternal lineage will possess the same mtDNA which allows tracing of a direct genetic line.

• They don’t recombine. The process of recombination in nuclear DNA (except the Y chromosome) mixes sections of DNA from the mother and the father creating a jumbled genetic history.