Ch. 15 Sex And Reproductive Success

Front 1

Define genetic system

Back 1

How an organism reproduces: whether it is sexual/asexual, fertilizes or outcrosses, or hermaphroditic/separate sexes

Front 2

Define sex

Back 2

The union of 2 gametes followed by reduction. Often involves outcrossing between 2 individuals but can also occur through self-fertilization. Almost always includes segregation of alleles and recombination to varying degrees between species.

Front 3

Define isogamous.

Back 3

Uniting cells are the same size so these species have mating types, not distinct sexes. An example is Chlamydomonas

Front 4

Define dioecious.

Back 4

Also called gonochoristic. These species have distinct sexes.
Examples: willow trees and mammals

Front 5

Define hermaphroditic

Back 5

Also called cosexual. One individual can produce both kinds of gametes. 2 types:
1. Sequential: individual born as one sex but can later change into the alternate sex. Protandry: male to female; Ptrotogyny: female to male.
2. Individual born with both sets of gametes

Front 6

Describe asexual reproduction: parthenogenesis, vegetative propagation, and bacterial conjugation

Back 6

Vegetative propagation: offspring develops from a cluster of cells
Parthenogenesis: offspring develops from a single cell. Most common type is apomixis: meiosis is suppressed and the offspring develops from unfertilized egg. Genetically identical to mother except for new mutations
Bacterial conjugation: transfer of genetic material from cell to cell contact or through a bridge-like structure called pilus. Can have horizontal or lateral gene transfer

Front 7

What are the long-term advantages of sex?

Back 7

Increases rate of adaptive evolution of a species and thereby reduces the risk of extinction. Creates genetic diversity
Evidence: most asexual lineages of eukaryotes have arisen quite recently from sexual ancestors
Since there are so few ancient sexual forms we can deduce that most asexual lineages from a long time ago have become extinct

Front 8

What is the advantage of asexuality?

Back 8

Only one parent is required and all of that parent's genes are pass on to its progeny

Front 9

What are short-term disadvantages of sex and recombination?

Back 9

1. Recombination destroys adaptive combinations of genes: ex. primrose: recombination of GA and ga produces Ga, which is less successful in pollination
2. Cost of sex: Sexual females will only have half as many grandchildren as asexual females, which means that the rate of increase of an asexual genotype should be twice as great as that of a sexual genotype

Front 10

Describe the hypothesis of Mullers ratchet.

Back 10

In an asexual population, individuals carry either the same or more mutations that their ancestors did, but never less. The class with the lowest mutation load is lost due to genetic drift or by acquisition of new mutations. This causes an accelerated reduction of fitness: mutational meltdown, that results in extinction. In a sexual species, however, recombination reconstitutes the lowest mutation class by creating progeny with new combinations of favorable alleles.

Front 11

Explain the deleterious mutation hypothesis

Back 11

Sex exists to purge a species of damaging genetic mutations.
Deleterious mutations are eliminated from sexual population quicker than an asexual population because recombination brings them together and eliminates them simultaneously
This increase in fitness more than balances out the cost of sex

Front 12

Explain how sex can accelerate the rate of evolution:

Back 12

If you have mutations A,B, C that are advantageous when they come together:
-asexual: second mutation is necessary for lineage bearing the first mutation, etc
-sexual: independent mutations are brought together more rapidly through recombination so adaptation is more rapid

Front 13

Explain the balance argument

Back 13

Suggests that if sex were maladaptive at the individual level, it would be quickly lost from species with asexual/sexual alternation such as plants, aphids, and sponges.
These species reproduce sexually for periods of environmental uncertainty and asexually when conditions are more stable.
Both types of reproduction must have balanced advantages otherwise the inferior one would be lost

Front 14

How can environments change rapidly enough to be advantageous every generation?

Back 14

Coevolution of parasites and hosts can produce rapid environmental changes that make sex advantageous in the short term. In this case the environment for the parasites is the host's resistance mechanism and for the hosts it is the parasite's method of penetrating its defense. Sexual reproduction persists and is advantageous for hosts because it allows quick evolution of new genetic defenses against parasites

Front 15

Explain the Red Queen hypothesis.

Back 15

1.Continuing adaptation is needed to maintain relative fitness amongst the systems being co-evolved with. Explains the advantage of sex at the individual level
2. Refers to arms race: evolutionary struggle between competing sets of co-evolving genes that develop adaptations and counter-adaptations against each other