Processes of Evolution

Introduction

Biological Evolution - the genetic change in a population from one generation to another

Speciation - The formation of a new species, the result of reproductive isolation

Gene Pool - The number of alleles that occurs in a population

Populations are the units of evolution and changes in allele frequency in the gene pool of a population indicates that evolution is occurring.

Deme - A local population that has no or limited gene flow with members of other populations.

Cline - A gradient of variation

Clinal Variation - Must be linked in some way to an environmental gradient. There are two possible mechanisms:

1. Due to environmental differences - in this case the cline would be purely phenotypic

2. Due to genetic differences resulting from selection

Ring Species - A special case of cline in which the two ends of the cline join to form a loop or ring. (When it is hard to achieve geographic isolation)

Gene Flow

When individuals migrate between populations and interbreed. Introduces their alleles into populations.

Gene Flow - The movement of alleles carried by individuals between populations.

Immigration - Individuals move into a population

Emigration - Individuals move out of a population

Gene flow can:

• Increase genetic differences between populations by introducing new alleles. Significant if a barrier isolates populations once migration has happened so that no further gene flow can occur

• Decrease genetic differences between populations - If migration is regular and involves large numbers of individuals, accumulated genetic differences between populations can be reduced by gene flow

Genetic Drift

Genetic Drift - A change in allele frequency due to the accumulated effects of chance

Genetic drift occurs in small populations and with alleles not greatly affected by selection.

Founder Effect

Founder Effect - When a small number of individual animals or plants are dispersed to a distant place, the new arrivals carry only a small portion of the gene pool of the ‘parent’ population. The alleles of the colonists are therefore only a sample (not representative) of the parent gene pool. It is a largely a matter of chance which alleles are present in the pool.

Bottleneck Effect

Occurs when a large population is suddenly reduced in size, due to either a catastrophic environmental event or human impact. Bottleneck may randomly alter allele frequencies and/or remove alleles so that when the population recovers, allele frequencies may not be representative of the original population and genetic diversity is likely to be reduced.

Inbreeding and Outbreeding

Inbreeding

Commonly used in selective breeding. It increases the chances of favourable alleles coming together and resulting in desirable phenotypes.

Inbreeding has a large affect on small populations such as a founder population.

1. Increases the proportion of homozygotes in the population similar to self-fertilisations

2. results in rare recessive alleles being expressed.

3. results in less diverse genetics in a population.

Outbreeding

Increases the number of heterozygotes in a population and reduces the chances of unfavourable and harmful recessive alleles coming together.

Offspring produced by outbreeding show hybrid vigour when they contain a variety of alleles from their genetically different parents. These organisms tend to have greater evolutionary fitness compared to inbred offspring.

Natural Evolution

Where inheritable traits that make an individual more likely to survive long enough in its environment to reproduce become more common in the population over successive generations.

Abiotic and Biotic environmental factors influence which individuals survive (they exert selection pressures on the population).

Modes of Evolution

Stabilising Selection

Extreme phenotypes are selected against and average phenotypes are selected for

Disruptive Selection

Individuals at both extremes of the distribution are selected for simultaneously. Average phenotypes are at a disadvantage and are selected against

Directional Selection

A single phenotype is selected for and the allele frequency shifts in one direction.

Sexual Selection

Males (or females) compete for partners with the heritable characteristics of the ‘winners’ being passed on.

Phylogenetic Trees

Evolutionary trees drawn as a branching diagram. Shows the inferred evolutionary relationships between taxa.

Inferred relationships are based on:

1. Similarities and differences in their molecular biology (DNA)

2. Similarities and differences in their physical characteristics.

Where taxa join at their bases in a phylogenetic tree (nodes) indicates the existence of a common ancestor that the groups have descended from. Each line of the tree represents time.

Phylogenetic tres shows Divergent Evolution.

Ploidy

Refers to the number of chromosomes in a cell. Uncommon in animals. Numerical changes in chromosomes have been important in the evolution of over a third of all flowering plant species.

Genome - The entire component of genetic material in an organism

Haploid - The number of chromosomes in the gamete

Diploid - The number of chromosomes in the zygote

Monoploid - The number of chromosomes in each set (Haploid and Monoploid are interchangeable)

Aneuploidy - Only certain chromosomes are represented an unusual number of times

Euploidy - Variation in chromosome number involves the entire genome.

Polyploidy

Autopolyploidy

Involves the multiplication of the entire genome within a single species.

Autotriploid has 3 sets of chromosomes. Autotetraploid has 4 sets.

Results from the failure of chromosomes to separate during the first or second division of meiosis or in mitosis (results in a tetraploid) because the spindle does not function properly. All the chromosomes finish up in the same nucleus, which has twice as many chromosomes as it should have.

Reduces infertility

Allopolyploidy

More important in evolution than autopolyploids.

Results from hybridisation between species.

Rare in Animal Kingdom.

Occurs in animals that reproduce parthenogenetically (without fertilisation)

Patterns of Evolution

Divergent - Occurs when species become different. It leads to Diversity in Species. Does share common ancestor.

Convergent - Occurs when species become similar. It leads to lack of genetic diversity. Doesn’t share common ancestor. Similar due to selection pressures. Results in analogous structures.

Parallel Evolution - when two or more related species evolve along similar lines independently.

Co-Evolution - occurs when two species form a mutalistic relationship. It leads to Extinction as if one species dies, the other will also die. Doesn’t share common ancestor. A change in feature of one species acts as a pressure for the change in another species.

Vicariance - Large-scale allopatric speciation of a taxonomic group by a significant geophysical event

Adaptive Radiation

a type of divergent evolution. Involved rapid evolution of a large number of species from an ancestral group to occupy a variety of different ecological niches that may have become available suddenly.

Can arise in two different ways

• evolution of a new structure, physiology or behaviour enabling exploitations of a different aspect of habitat

• geographical isolation from competitors or predators

Rates of Evolution

Gradualism

One form to another.

The transitional forms in the fossil record in response to natural selection

Punctuated Equilibrium

Involves the formation of many different species that are no longer closely related.

Homologous Structures

features similar in structure and origin but different in function. Similiarity in structure indicates common ancestry, while difference in function indicates adaption to different selection pressures in different environments. Can be seen in the limbs.

Analogous Structures

Structures with different evolutionary orignings that appear very similar because they carry out the same or similar functions.

Speciation

Introduction

The formation of new species resulting from populations becoming reproductively isolated; gene flow no longer occurs.

Speciation can occur:

• gradually by the slow accumulation of small changes

• instantly by the changes in chromosome number through polyploidy

Sympatric Species - are (closely) related species that live in the same geographic location.

Allopatric Species - are (closely) related species that live in different geographic location.

Reproductive Isolating Mechanism (RIM) - any factor that stops members of populations of the same spceices or members of different species from breeding together; it acts as a barrier to gene flow.

RIMS can be;

• Prezygotic - act before the egg is fertilised

• Postzygotic - act after the egg is fertlilised

Reproductive isolation may result from just one or a combination of prezygotic and/or postzygotic isolating mechanisms.

Prezygotic Isolating Mechanisms

Geographical Isolation

Geographical isolation results from physical barriers (rivers, mountains, oceans) separating populations. If the population come together again later, they are sufficiently different (as a result of selection pressures) that they do not/ unable to breed.

Structural (Morphological) Isolation

Results from the fact that the differences in structure of the external reproductive organs do not allow for the transfer of sperm between sexes.

Ecological Isolation

Resutls from differences in habitat within the same geographical area, so that the populations rarely come in contact with each other.

Temporal Isolation

Results from breeding behaviour occurring at different non-overlapping times. This can be saily or seasonal differences in timing of mating.

Behavioural Isolation

Results from differences in behaviour, typically mating behaviour such as courtship

Can also result from different activity patterns.

Gametic Isolation

Results from the incompatibility of gametes. Sperm may not be able to fertlise an egg of another species because:

• The egg’s surface does not have the correct chemical receptors

• Sperm cannot penetrate the surface of the egg

• Sperm cannot survive in the chemical environment of the female reproductive system

Postzygotic Isolating Mechanisms

Hybrid Inviability

the zygote, embyo or offspring is inevitable so it dies early in its development

Hybrid Sterility

The hybrid reaches maturity but is infertile so it cannot breed

Hybrid Breakdown

Occurs when the hybrid reaches maturity and is able to breed but the next generations are infertile or have reduced reproductive capability

Allopatric Speciation

Allopatric speciation is the process where speciation cannot occur until a population has divided into two or more geographically isolated populations.

Sympatric Speciation

Most common form of sympatric speciation is instant speciation. This involves the formation of polyploid therefore is more common in plants.

Less important that allopatric speciation

If populations become sympatric three things can happen:

1. Interbreeding

2. Populations do not hybridise

3. They hybridise freely and merges to form the original single species.

Instant Speciation

Through polyploid. Results in the evolution of many plant species. Results in a new species in a single generation due to the consequent formation of the postzygotic reproductive barrier of hybrid sterility.

Allopolyploidy is the more important of the two as it results in the hybrids containing the genes of two different species which can be advantageous to the polyploid should the selection pressures change.

Formation of Geographical Barriers

Mountain building and changes in sea level