The Evolution of Complexity

Lecture `11

Complexity

The complexity of a biological system reflects the number of parts it has

Major Transitions in Evolution hypothesis

Greater complexity arises from greater ‘cooperation’ among previously independent units

What is the ‘unit of selection’?

Selection acts at a specific level of biological organization when:

• There is variation among units at that level

• Units exhibit heritability across generations

• Units have differential fitness (survival and/or reproduction)

Genes as the ultimate target of selection

Because genes are the unit of inheritance, ultimately the target of selection is the gene

Why does multi-level selection pose a problem for complexity?

• Selection at a given level of organization means that units compete to maximize fitness

• Competition among lower-level units of organization may reduce fitness at higher levels

A solution to the multi-level selection problem

If lower-level units of organization cooperate rather than competing, higher-level fitness costs can be avoided

How do biological subunits stay so cooperative?

• Many features of individual organisms prevent competition WITHIN an individual

  • Prevent evolution within individuals

  • Align fitness interests across levels of organization

• Ensures that many genes succeed by enhancing the fitness of the individual

Mitosis and meiosis

• Mitosis ensures that alleles don’t compete within an individual

• Meiosis ensures fair representation of gene variants from each parent among daughter cells

Development and multicellularity

Starting from a single cell prevents initial competition among cell lineages

Positive natural selection of alleles

Alleles spread through a population by increasing individual fitness

Fair meiosis

Meiosis provides a fair representation of an allele’s fitness effects on individuals

Meiotic drive

If an allele can bias its own transmission

• Then it can spread to higher frequency

• Even while reducing individual fitness

• “Selfish” genetic elements relative to organism’s fitness interests

By enhancing its own transmission, meiotic drive allele (a) can spread 

Increased frequency of a even while reducing individual fitness

Cheating Mendel’s Law of Segregation

Almost all (95-99) offspring are Ss! Expect 50% Ss offspring with fair meiosis

What is the evolutionary response to cheating Mendel’s law?

When cheating alleles spread, there is a strong selection on rest of genome for suppression of cheating

Transposable Elements: Cheating Mendel’s Law through over-replication

• TEs are self-replicating segments of DNA (aka ‘transposons’)

• TE replication separated from cellular replication

• Ensure their own over-representation in offspring

How do genomes not ‘explode’ from transposition?

• Alleles arising elsewhere in genome that silence TEs will be favoured by individual selection

• Transposition-selection balance

Transposition-selection balance

• Transposition is a form of mutation that can disrupt a gene

• Natural selection against harmful effects on the organism reduce abundance of chromosome copies with most TEs

• Abundance of TEs in an organism results from a balance between these offspring forces

How do individual genomes stay so cooperative?

• Many features ensure that the variance in fitness WITHIN an individual is minimized

• Ensures that many genes succeed by enhancing the fitness of the organism

• BUT: countless ways to evade cooperation

• Presence of strong selection on rest of genome (‘policing’) seems essential to maintain higher-level cohesion

How do collections of cells stay cooperative?

• Starting from a single cell reduces competition within individuals

• Separation of germline with limited numbers of cell divisions inhibits transmission of selfish cell lineages

• Tumour suppressors, other features inhibit unregulated cell division

Cancer: selfish cell lineages evolving within an individual

• Spreads commonly in tissue that is relatively undifferentiated

  • Evolves resistance to treatment/immune system

• Illustrates the “short-sightedness” of the evolutionary process