A4.1 Evolution and speciation

Evolution

Changes in heritable characteristics of organisms over generations

Speciation

The development of new species from pre-existing species over time

Darwin’s theory of evolution by natural selection

Individuals in a species show variation due to mutations in DNA. Individuals in a population compete for survival due to selection pressures. Individuals with characteristics better suited to the environment have a higher chance of survival and are more likely to reproduce. Advantages alleles are passed down to offspring. Over generations, the advantageous alleles become more frequent

Lamarck theory 

A characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn’t used gradually disappears. The beneficial characteristics are passed to offspring

Limitation of Lamarck’s theory

They lack the component of heritability. Acquired characteristics are not passed to offspring

Sequence data

Can be obtained from DNA, RNA, and proteins, however must come from highly conserved DNA regions. Sequences must come from the same part of DNA. Similarities in different species suggest species share a common ancestor

Selective breeding

A process in which humans choose organisms with desirable characteristics and breed them together repeatedly to increase the expression of these characteristics over many generations

Process of selective breeding

1. Population shows variation

2. Breeders select individuals with desired characteristics

3. Two selected individuals are bred together

4. The offspring which display the desired characteristics are selected for further breeding

5. Process is repeated over many generations

Homologous structures

Anatomical features that have a similar basic structure due to common ancestry, but perform different functions

Adaptive radiation

The idea that organisms with homologous structures have all evolved from a shared, common ancestor but have adapted to different environments

Homologous structure: The pentadactyl limb

Any limb that has five digits

Analogous structures

Characteristics with similar form and function, but with different evolutionary origin

Convergent evolution

The independent development of similar traits in distantly related species, often as an adaptation to similar selection pressures

Examples of analogous structures

Dolphins and sharks. They are both groups of aquatic animals that share a similar body shape, but they belong to different classes (dolphins are mammal, sharks are fish). Their streamlined body shapes evolve separately

Reproductive isolation

Occurs when changes in the alleles and phenotypes of some individuals in a species prevent them from successfully breeding with other individuals

Examples of allele and phenotype changes that can lead to reproductive isolation:

Seasonal changes (different mating or flowering seasons), behavioural changes (differing courtship behaviours means they can no longer attract individuals for mating)

Geographical isolation

Occurs when a population is physically separated from other members of the same species by a barrier like a body of water, which prevents gene flow and interbreeding

Example of a speciation event that has resulted from geographical isolation

The evolution of bonobos and chimpanzees. Chimpanzees are found on the north of the Congo river, while bonobos are found on the south. This suggests at some point, the river caused geographical isolation. Different selection pressures would have acted on them, so differential selection occurred, resulting in different species.

Differential selection

The process where two or more geographically isolated populations of the same species experience different environmental pressures, leading to different selective pressures

Allopatric speciation

When two populations of species are geographically isolated

Sympatric speciation

When two populations of species are living in the same area

Example of allopatric speciation

Trees. A population of trees exists in a mountainous habitat. A mountain range forms, leading to geographical isolation. This prevents interbreeding and gene flow. Differential selection occurs as they experience different environments

Example of sympatric speciation

Fruit flies. A random allele change from a mutation divides the species into two populations. This prevents interbreeding and gene flow. Eventually the two populations can no longer breed

Hybrid

The offspring of individuals of two different species

Speciation in plants

Abrupt speciation in plants can occur because plant cells are viable even when they are polyploid (having more than two sets of chromosomes).

Autopolyploidy

When an individual gains more than two sets of chromosomes from within a single species

Allopolyploidy

When an individual gains more than two sets of chromosomes from two different species