Week 1 - BIO1022

Natural selection and the tree of life

what is the DNA within an organism called?

Genome

what do genes encode?

a single product

what is a variant of a gene called?

allele

Disruptive selection

selection against intermediate forms and favours the extreme ends leading to U-shaped distribution.

Directional selection

Traits at one extreme are favoured over all other types, causing the overall average trait of a population to shift in a single direction over time.

Stabilising selection

A type of selection which favours average traits and acts against extreme variations. it reduces overall genetic diversity and keeps a population over time.

Balancing selection

Keeps multiple versions of a gene (alleles) active in a population, primarily through heterozygote advantage and frequency - dependent selection

Intrasexual selection

evolutionary and biological competition where members of the same sex fight or compete directly with each other to gain access to mates.

Intersexual Selection

Evolutionary process where one sex choose mates from the other sex based on specific traits.

Principle of parsimony

all things are connected or behave in the simplest way to produce a given outcome.

How is parsimony used in predicting evolution?

as evolutionary change is slow, a tree that contains the fewest number of change from ancestor to descendent is considered most parsimonious, and therefore most likely correct.

Parsimony in phylogeny - lungfish

If a phylogeny was constructed where fishes shared a more recent ancestor with lungfish than tetrapods, it would mean either:

1) lungs evolved twice in vertebrates, once for lungfish, and once for the common ancestor of tetrapods.

2) The common ancestor of most vertebrates evolved lungs, which were lost in certain lineages of fishes.

Both of these phylogenies require at least two evolutionary changes. The most parsimonious explanation is that lungfish have a more recent ancestor with tetrapods, than it does with the other fishes, and this common ancestor developed lungs, as it only requires one change.

Complications - homologous characters

As the evolution of unique characters is rare, the vast majority of characters are traits inherited from a common ancestor. we call these homologous characters.

• i.e. characters between two organisms are similar because they were inherited from a common ancestor.

Complications - analogous characters

The construction of a phylogenetic tree is complicated by analogous characters. These are characters that are similar in their structure due to independent evolution across two seperate lineages.

complications - how to analogous characters occur?

analogous characters come about via convergent evolution where two seperate lineages evolve to resemble one another to fill a similar niche. A classic example of an analogous trait is the development of wings in bats and birds.

Complications - the gain and then loss of characters

while divergent evolution (the change of related species to fit different niches) provides the many synapomorphies useful for constructing trees, gained traits can subsequently be lost, which further complicates construction of phylogenetic trees.

synapomorphies

A shared, derived trait found in two or more taxa that originated in their most recent common ancestor.

plesiomorphy

ancestral or primitive evolutionary trait shared by a group of organisms, which has been retained from a more distant common ancestor.

Molecular characters

Molecular characters in phylogeny are the most powerful. Any mutations that occur in the DNA sequence in one ancestor will then be passed directly to its decedents. Using this logic, we can assume that organisms with a more recent common ancestor would contain fewer DNA changes between them. We can therefore use the similarity of genetic sequences as a measure of evolutionary distance; the more similar the DNA sequence between two organisms, the more recent their common ancestor.