Transcript Lecture 2 - Organic Origins Debate

The Problems of
Parenting
Offspring
“Vehicles” transporting copied genes to
succeeding generations
 Offspring reproductive success is the major
determinant of fitness
 Parenting may be selected to optimise fitness

Sex Differences in Parenting

Hypotheses explaining why human fathers
invest less parental effort than mothers
 Paternity uncertainty
 Abandonability
 Mating opportunity costs
Paternity Uncertainty
Due to internal fertilisation, men cannot be
certain of their offspring’s relatedness
 Cuckolding is very costly to men, so men will
be reluctant to invest
 For paternal care to exist:
 Benefits of other investment < benefits of
raising own offspring

Abandonability
The first parent that can abandon the
offspring should be more likely to do so
 Greater prevalence should be found in
species with internal fertilisation
 However
 This relates back to paternity certainty
 In simultaneous gamete release, a 50/50
split should occur, but does not

Mating Opportunity Costs


By investing in offspring, males miss more
mating opportunities (minimum investment)
When mating opportunity costs are high,
paternal care should be low
 Sex ratio bias
Mating & Parental Effort


A reminder
 Mating effort: % of reproductive effort
invested in acquiring & maintaining mates
 Parental effort: % of reproductive effort
invested in ensuring survival of offspring
Sex differences in parental effort
 Partner variety benefits men
 Paternity uncertainty
Discriminative Parental Solicitude
Basically, the idea of parental favouritism
 Based on three factors
 Genetic relatedness of offspring
 Conversion of parental care to fitness
 Alternative use of resources

Genetic Relatedness


Resemblance of offspring is crucial to father
 One year-olds tend to resemble fathers
Men tend to invest less $ in education of
stepchildren than genetic children
 Investment in current stepchildren appears
to be a form of mating effort
Abuse & Child Homicide
Stepchildren are 40 times more likely to be
abused than genetically-related children
 Stepchildren are 40 - 100 times more likely
to be killed
 “Stepparenthood per se remains the single
most powerful risk factor for child abuse that
has yet been identified” (Daly & Wilson,
1988, p. 87-88)

Child Victims per Thousand
Risk of Child Abuse
14
12
10
8
Natural Parents
One Stepparent
6
4
2
0
0-4
5-10
11-16
Age of Child
Adapted from Daly & Wilson (1988)
Child Victims per Million
Risk of Child Homicide
700
600
500
Natural Parent
Perpetrator
Stepparent
Perpetrator
400
300
200
100
0
0-2
3-5
6-8
9-11 12-14 15-17
Age of Child
Adapted from Daly & Wilson (1988)
Conversion of Parental Care


For parental care to be selected, it must
increase offspring reproductive success
Most likely affected by two factors
 Birth abnormalities
 Child age
Congenital Abnormalities
Children with congenital abnormalities
probably have decreased reproductive value
 Large proportion of children with serious
illnesses are institutionalised
 Abuse rates
 Base rate of 1.5%
 7.5-60% in children with serious illnesses

Infant Health


Mann (1992)
 Healthy-unhealthy twin dyads
 At four months, 50% of mothers showed
positive bias to healthy twin
 At eight months, 100% of mothers showed
positive bias
Discriminative parental solicitude does not
imply that parents will only invest in healthy
children
Child Age

Reproductive value changes with age
 Infants are low in reproductive value,
because of high mortality rate
 Infants killed if birth interval too short or
family size too large
 As child increases in age, reproductive
value increases
40
35
30
25
Natural Parent
Nonrelative
20
15
10
5
16
14
12
10
8
6
4
2
0
0
Child Victims per Million
Interaction of Relatedness & Age
Age of Victim
Daly & Wilson (1988)
Alternative Use of Resources


Maternal age
 Cost of missed reproductive opportunities
increase with age
Maternal marital status
 Single mothers have less resources than
married ones
Effects of Age & Marital Status
Infanticides per Million
160
140
120
100
Married
Unmarried
80
60
40
20
0
<19
20-24
25-29
30-34
>35
Maternal Age
Daly & Wilson (1988)
A Word on Adaptation
Remember, adaptation is an onerous concept
 Daly & Wilson argue that stepparental abuse
& homicide is not adaptive, but a “reverse
assay” of parental care
 Performed irregularly & inefficiently
 Great cost to perpetrator
 No known direct benefits

Parent-Offspring Conflict


If offspring are vehicles for parental genes,
aren’t parent & offspring interests the same?
No, they are not
 Human offspring may share 50% unique
genes with parents, but they also differ by
50%
 Hence, interests will not always coincide
Differing Interests

Parental and offspring interests typically
differ with relation to resources
 Intrauterine conflicts
 Extrauterine conflicts (e.g., weaning)
 Sibling value, cooperation, & competition
Abnormal Zygotes

A woman has several chances to get
pregnant, but a zygote has only one chance
to be born
 Up to 78% of all fertilised eggs fail to
implant or are spontaneously aborted,
likely due to abnormalities
 Women appear to have developed a fetal
screening mechanism
Intrauterine Conflict
Om
Of
Nutrient Provided
Williams (1997)
Genomic Imprinting

Kinship theory of imprinting (Haig)
 Whether an allele entered a zygote by
sperm or by egg affects the relatedness of
the zygote to the parent at that locus
 A locus will converge on either symmetric
or asymmetric evolutionarily stable
strategies (ESS)
 Cost/benefit is usually maternal investment
Imprinting: An Example

The case of insulin-like growth factor II (Igf2;
Constância et al, 2002)
 Igf2 is paternally-expressed
 Controls placental growth
 Studies on mice with placental Igf2
deletions demonstrate reduced placental
growth and smaller offspring
Loudest Voice Prevails


The “loudest-voice-prevails” principle refers
to the escalating competition between fetus
genes & mother
May have health implications (e.g.,
gestational diabetes, spontaneous abortions,
preeclampsia)
The Wrap-Up
Offspring as genetic vehicles
 Sex differences in parental investment
 Discriminative parental solicitude
 Stepparenting as a risk factor
 Health & age of offspring
 Age & marital status of mother
 Parent-offspring conflict & genomic
imprinting

Things to Come

Kinship
 Hamilton’s rule
 Evidence of inclusive fitness applications
 Grandparental investment
 Sex differences
 Evolution of the family unit