T11: Animal Reproduction

what is sexual reproduction?

 the fusion of haploid gametes forms a diploid cell, the zygote.

what is asexual reproduction?

Asexual reproduction: new individuals are generated without the fusion of egg and sperm

what are the mechanisms of asexual reproduction?

• budding, fission, and fragmentation with regeneration.

 

how are Variations on the mode of reproduction achieved?

• are achieved through parthenogenesis, hermaphroditism, and sex reversal.

what is parthenogenesis?

• Parthenogenesis: form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization.

what is hermaohroditism?

• Hermaphroditism:  each individual has both male and female reproductive systems

what are cycles of reproduction?

Cycles of reproduction: Hormones and environmental cues control reproductive cycles

• These cycles are controlled by hormones, whose secretion is in turn regulated by environmental cues.

• In this way, animals conserve resources, reproducing only when sufficient energy sources are available and when environmental conditions favour the survival of offspring.

what is ovulation?

Ovulation, the release of mature eggs, occurs at the midpoint of each cycle.

what is internal fertilisation?

In species with external fertilisation, the female releases eggs into the environment, where the male then fertilises them.

what is external fertilisation?

In species with internal fertilisation, sperm deposited in or near the female reproductive tract fertilise eggs within the tract.

how do animals ensure the survival of their offspring?

Typically, animals that fertilise eggs internally produce fewer gametes than species with external fertilisation, because a higher fraction of their zygotes survive.

Better zygote survival is due in part to the fact that eggs fertilised internally are sheltered from potential predators.

what is sexual selection?

Sexual selection is a natural selection process where certain traits increase an individual's chances of attracting mates, often leading to increased reproductive success.

what is sexual dimorphism?

Sexual dimorphism refers to the differences in physical characteristics between male and female members of the same species, often related to reproductive roles and mate attraction.

what are secondary sexual characters?

secondary sexual characters: are not required for breeding, but they are involved in the process of reproduction e.g. the peacock’s tail

• Darwin noticed that there exist in males certain characters that are apparently "costly": he called these secondary sexual characters

what are primary sexual characters?

primary sexual characters: are organs such as genitalia, and are needed for breeding

where does the selective pressure come from in sexual selection?

from the females: they choose to mate only with certain males

what kind of traits does sexual selection enhance?

features that are useful in:

1) Male-Male Competition – Combat between males as they compete for access to females.

2) Female Choice – Features that evolve to make males more attractive to females so that they actively choose them.

what is anisogamy?

Anisogamy refers to a form of sexual reproduction involving the fusion of two distinct types of gametes, specifically larger female eggs and smaller male sperm, which leads to differential investment in reproduction by sexes.

what are male and females like in anisogamy?

‘Males’ = small, motile cells produced in large numbers

‘Females’ = large, high provisioned cells produced in low numbers

anisogamy diagram

What is the reproductive success like for females?

• The potential for maximum reproductive success is small

• Therefore, reproductive success for females is limited more by the number of eggs she can produce than by the number of males she mates with.

What is the reproductive success like for males?

• the potential for reproductive success is large

• Therefore, success is limited by the number of females he mates with rather than the number of ejaculates.

how do both males and females maximise their reproductive success?

 Selection favours male traits that increase fertilisation success

• Males of many taxa can produce large amounts of sperm

Selection favours female traits that increase fecundity and offspring quality

• Females are limited by egg production

what is the runaway process/selection?

The runaway process, or runaway selection, is a phenomenon in sexual selection where traits that are favored by one sex become exaggerated over generations due to a positive feedback loop between female preference and male ornamentation.

Simplest model of Runaway Sexual Selection

Haploid organism (one set of chromosomes)

 

2 types of female genotypes

The male trait is genetically linked to these two

 

1) Choosy females that prefer elaborate ornament

2) Non-choosy females who pick ornamented males half the time and plain males half the time

 

F1

1/2 the offspring have genes for Ornament and Choosy female gene

1/4 of the offspring have genes for Ornament and Non-Choosy female gene

1/4 of the offspring have genes for Plain males and Non-Choosy female gene

 

Therefore, Ornamented males have a fitness of 3/4

Plain males have a fitness of 1/4

 

So ornamented males are favoured and a `Runaway Positive Feedback Loop' occurs such that the trait and the female preference for it charge along in tandem.

why do females choose males on the basis of elaborate ornaments?

• Females choose males on the basis of elaborate ornaments because such ornaments are STRONG INDICATORS of a superior male genotype.

•  In this scenario a female peacock would choose a male with an elaborate train not just because she has a genetic preference for it (R SS) but because it accurately reflects how good his genotype his.

what is sperm competition?

Sperm competition is the competition between the ejaculates of different males for the fertilization of a given set of ova.

when does sperm competition occur?

SC occurs when a female is inseminated by two or more males – it can be inferred from either detection of mixed paternity or direct observation of copulations

what is sperm expenditure?

 The amount of reproductive effort spent by a male on a given ejaculate.

It is assumed that sperm is cheap, but ejaculates are expensive. Increased ejaculate effort increases gains from a given mating but reduces opportunities for more matings.

what happens when the risk for sperm competition is high?

• Where the risk of sperm competition is high, and numerical superiority of sperm confers an advantage in fertilisation, there will be selection for males to deliver large quantities of sperm.

• Either mating multiple times with the female

• Or by increasing the size of individual ejaculates

what is the correlation between sperm, testes and sperm velocity?

• The bigger the testes, the more sperm produced and stored.

• It is predicted that the higher the level of SC, the larger the testes.

• Additionally, increased sperm velocity is associated with higher sperm competition, as faster sperm are more likely to reach and fertilize an egg.