Ch. 2 Evolutionary Transitions

Evolutionary Ecology

8 major transitions

1. Cells

2. Chromosomes

3. DNA + Proteins

4. Prokaryotes to Eukaryotes

5. Asexual to Sexual

6. Multicellularity

7. Social colonies

8. Language

Evolutionary Transition 1

Individual replicating molecules grouped together in compartments (i.e. cells)

Evolutionary Transition 2

These replicators bound physically together to form chromosomes

Evolutionary Transition 3

RNA gave up being metabolic catalysts and replicator to DNA + proteins

Evolutionary Transition 4

Some prokaryotes (bacteria) transformed to eukaryotes

Evolutionary Transition 5

Asexual eukaryotic clones transformed to sexual population

Evolutionary Transition 6

Some singe-celled protist transformed into multicellular organisms

Evolutionary Transition 7

Solitary individuals began to be social

Evolutionary Transition 8

One species developed language (ours)

3 challenges to accepting the 8 major transitions

1. Phenotypic changes were changes to the genetic systems

   • normally we assume the genetic system to be constant

2. Phenotypic changes were large

   • Each step needed to retain functional integrity and be favored by natural selection

3. Changes were successfully from a macro-evolutionary perspective

   • Transition was retained to the present day and usually retained in abundance

How could the 8 major transitions retained in abundance?

1. Each transition happened on numerous occasions

2. Reversal to ancestral state was limited

3. Extinction of clades possessing the trait was reduced

4. Speciation of clades possessing the trait was increased

How did sex evolve?

Sex likely evolved in eukaryotes from a clonal ancestral state (mitosis)

Syngamy

Fusion of two haploid gametes

Anisogamy

The gametes that are very different in form and behavior

Meiotic life cycle

1. Fusion of two haploid gametes (sperm and egg)

2. A number of mitotic cell cycles may then follow

3. Some homologous chromosomes swap bits of DNA (recombination = crossing over)

4. Replicate and then undergo two rounds of cell divisions (e.g. 4 haploid cells

Evidence from Barbulanympha

• A surviving ancient protist lineage

• Lives inside the gut of insects (roaches)

• Has a cycle that involves endomitosis instead of syngamy

  • Gain diploid state by copying haploid chromosomes

First step for how sex went from asexual eukaryotic clones to sexual pops

acquisition of a life cycle that alternated between diploid and haploid

• Diploid stage acquired via endomitosis

• Haploid stage via a single one step reduction division

Second step for how sex went from asexual eukaryotic clones to sexual pops

Endomitosis replaced by syngamy

One step meiosis as seen in many sporozoans

Third step for how sex went from asexual eukaryotic clones to sexual pops

crossing over, followed by chromosome doubling = Two step meiosis

Fourth step for how sex went from asexual eukaryotic clones to sexual pops

Anisogamy - gained nutrients

Steps in order of how sex transitioned?

1. Endomitosis plus one step meiosis

2. Syngamy

3. Premeiotic doubling, and crossing over

4. Anisogamy

Majority of work on evolution of sex has focused on…

The advantages of crossing over

two processes might have selected for crossing overs evolution

1. Recombination can lower genetic load if mutations act synergistically

2. Selection for change (directional selection) on polygenic traits

frequency of recombination is higher and large long lived organisms why?

• Higher probability that the effects of mutation will be synergistic

• Selective pressure is so much greater

why anisogamy?

One gamete of the pair (the egg) is larger and carries the organelles.

It’s evolved to prevent conflict between organelles from different parents.

• mini organelles contain their own DNA, so if both gametes carried organelles, replicating entities would compete which would be detrimental to the eukaryote cell

Isogamy

Gametes are the same size

Usually in most single celled organisms

Parthogenesis

• an embryo develops from an unfertilized diploid egg = clonal reproduction

• Allows for rapid population increase when environmental conditions are favorable

During parthenogenesis:

• chromosome doubling occurs as normal

• Only one subsequent division = diploid eggs

• Many eggs end up with 2 Z chromosomes = male

how do mammals prevent sex from reversing to a clonal state?

Genomic Imprinting

What is genomic imprinting?

Early zygote development requires genes from both parents and each gene has different levels of activation. As a result parthenogenesis is not possible

how do gymnosperms prevent sex from reversing to a colonial state?

differences in provisioning of essential organs

Gynogenesis

eggs need to be fertilized by sperm of another species for successful development

evidence that parthenogenesis is rare

Found in isolated species in genera that are predominantly sexual

recent study showing that parthenogenesis is rare

example: (guppies)

Age differences

• Sexual species equals 3 million years

• Sperm dependent pathogens equals a few thousand years old

• Oldest clone is a hybridogen

Hybridogen

sperm genome is expressed in the phenotype, but not involved in gamete production

sex has a twofold cost

1. Male offspring cannot give birth to young, but all parts of genetic female offspring can

2. Finding a mate and sexual reproduction is costly

Generational differences between unisexual and sexual

Unisexual can have 4 offspring by gen 3

Sexual can only have 2 offspring by gen 3

given the cost of sexual reproduction, why don’t clones drive sexual species to extinction?

Advantages of recombination

what are three positive effects of combination?

1. Lower loads of delirious mutations

2. Lower parasite loads

3. Mullers ratchet

Mullers Ratchet

• small pops

• in a clone population, the load of slightly deleterious mutations continually cranks up ratchet like

• does not happen in a sexual population - genetic load remains stable or declines over time

how does the evolution of sex compare with the other major evolutionary transitions?

1. Combine together

2. Reversal is difficult

3. conflict between combining entities

4. Division of labor