Formation of offspring without the fusion of egg and sperm
The offspring genetically identical to parent
Sexual Reproduction
Formation of offspring by fusion of a female gamete (egg) with a male gamete (sperm)
Offspring inherit features from both parents, not identical to either
Evolution will not occur in a population if
Mating is random
No natural selection
No mutation
Large population size
No migratoin
Types of asexual reproduction
Budding
Fission
Parthenogenosis
Budding
Offspring develop as outgrowth of parent
Fission
Parent splits into two organisms of roughly equal size
Parthenogenosis
Unfertilized eggs develop into offspring
Whiptail lizards: the entire species is female -> reproduce this way
They still display courtship/mating behaviors -> associated with hormone levels during breeding season
Females mounted by "male-like" females more likely to ovulate and lay more eggs
Asexual reproduction vs. sexual reproduction
Asexual:
-All individuals can bear offspring
-Reproduction NOT limited by number of females
-More offspring by Generation 4
Sexual:
-Half of the individuals can bear offspring
-Require 2 individuals to produce offspring
-Reproduction limited by number of females
Reproduction in isolation?
Asexual: yes
Sexual: no, must find a mate
-Isolation or inability to attract a mate limits reproductive output
What evidence do we have that sex is evolutionary advantageous?
Ubiquity: seen in most eukaryotes (prokaryotes also have modes of recombination)
Historic Extinction Rates: asexual lineages have higher extinction rates because if there is a lethal disease then there will be no adaptation.
How can beneficial alleles be spread through asexually vs. sexually reproducing populations?
Asexual: offspring are clones of parents, so beneficial alleles not combined in offspring.
Each lineage would have to independently gain beneficial alleles since mutation is random, it takes much longer
Sexual: Beneficial alleles rapidly combined in offspring. so it can spread through population quickly
Sex produces new combinations of alleles
Why is variation beneficial?
Variation allows population response to changing environment via selection
Advantages of Sex
Variation
Fewer Deleterious Alleles (in asexual reproduction all mutations are passed to offspring)
Mutation Meltdown
Over time in asexual populations, proportion of low fitness individuals with many deleterious alleles increases
-eventual consequence=extinction
What explains greater extinction rates of asexual lineages?
Mutation meltdown
Lower variation
If both parents are heterozygous have deleterious alleles, how many of the offspring will express the deleterious phenotypes?
In asexual reproduction, all of the offspring express both deleterious phenotypes.
In sexual reproduction, 56% of offspring will express both, 38% of offspring will express one allele, and 6% of them will express none
-In a dihybrid cross ~50% have lower negative allele proportion. The people without it will have higher fitness and reproduce
Sexual Selection
Individuals with certain inherited characteristics are more likely than others to attract mates and therefore pass on their alleles at a greater frequency than individuals without those phenotypic characteristics
A form of natural selection
Why is sexual selection typically males competing for females? (gamete production)
Female gametes: eggs are limiting, so females can produce limited number per breeding opportunity
Male gametes: excess of sperm, so they are not limited by sperm available during breeding season
Results in male skewed operational sex ration (OSR)
Operational Sex Ratio (OSR)
Ratio of males in populations that are ready to mate with females in that population that are ready to mate
Why is sexual selection typically males competing for females? (parental care)
Parental care often falls on the female-- gestation, lactation, guarding, feeding. There is a greater energetic cost to raising young
Differential parental investment (PI): measured as each parent's contribution to rearing offspring
Female mating strategy
Find a mate with beneficial alleles or good resources so offspring have better chance of survival
This is because gametes are limited with high PI
Male mating strategy
Mate with as many partners as possible (and leave rearing to them if possible)
This is because gametes are not limited with low PI
On average, which sex will have higher reproductive output (number of offspring)? (assuming roughly equal numbers of males and females in the population)
B) Females and males will have the same average reproductive success
If in a deer population produces 30 offspring, they all came from a male and a female
Considering your knowledge of sexual selection, which sex will have greater variation in reproductive output among individuals of that sex?
B) Males will have greater variation
Why is female reproductive success relatively uniform?
Not limited by mating opportunities -> male strategy to gain as many mates as possible
Limited egg (gamete) resources -> fertilized
Limited by resources required to raise young
Why is male reproductive success variable?
Limited by mating opportunities -> female strategy to mate with high quality mates only
Males that can demonstrate quality preferred
Given this, selection will act more strongly on which sex?
A) Selection will be strongest on males
Why does selection vary?
Selection acts on variation: stronger in males because of greater variation in reproductive success
Selection for characteristics that allow greater access to mates
-Characteristics that allow defense of territories/resources and thus greater access/opportunity to mate
-Characteristics that result in greater female choice
