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Evolutionary Biology
The evolution of life histories
| Question | Answer |
|---|---|
| Life history strategies | Variation in demographic traits – Number and size of offspring – Age distribution of reproduction – Life span – Alternative mating strategies – Dispersal – Mode of reproduction |
| Life history strategies | Life history theory addresses the conditions that favour the evolution of variation in these traits |
| Variation in life history strategies | Number and size of offspring • Many small offspring ------- a continuum -------- few large offspring • Blue tit, Parus caeruleus – 14 small eggs • Kiwi, Apteryx – single egg weighing 25% of its mother |
| Variation in life history strategies | Variation in age distribution of reproduction Two insects that differ in generation time and rate of increase • Periodical Cicada, Magicacada septendecim • Milkweed Aphids, Aphis nerii (parthogenic reproduction) |
| Variation in life history strategies | Life span Relatively fixed lifespans - varies between populations/species Fruit flies, Drosophila melanogaster - 26 days Bristlecone pines, Pinus aristata - 4600 years |
| Life history traits and fitness | Differences among genotypes relating to life history traits determine differences in fitness • Fitness = the per capita rate of increase of a genotype = r • r = per capita rate of birth - per capita rate of death |
| Life history traits and fitness | Relative values of r determine the course of evolution by natural selection Evolutionary change in a demographic feature, e.g. reproductive lifespan, causes a change in the components of fitness |
| Life history traits and fitness | Evolution of other features (e.g. body size) occur because they affect one of the demographic traits, i.e. number / size of offspring |
| Which LH traits would maximise fitness | Rates of increase can be calculated from life history tables, using a mathematical model - r will be increased by: 1. Higher survival up to and through the reproductive ages • Natural selection does not favour post-reproductive survival….. Unless? |
| Which LH traits would maximise fitness | 2. Earlier age of first reproduction - higher chance of surviving to reproductive age - reduced generation time - offspring produced at an earlier age contribute more to population growth |
| Which LH traits would maximise fitness | 3. Higher fecundity at each reproductive age 4. Higher fecundity early in life 5. Longer reproductive lifespan |
| Constraints and trade-offs | 1.Phylogenetic constraints • Arise from their evolutionary history - e.g. silkworm moths lack mouthparts 2. Genetic constraints • Lack of genetic variation - or constraints among traits due to genetic pleiotropy |
| Constraints and trade-offs | 3. Trade-offs • Advantage of a change in one life history trait causes disadvantage elsewhere • Type of Antagonistic pleiotropy – negative correlation between traits, • because of allocation trade-offs |
| Constraints and trade-offs | - genotypes (B and B’) differ in investment in reproduction Versus maintenance/growth - increased fecundity….. but decreased growth or survival Complicated – as could also be differences in resource ] Acquisition (A and A’) |
| Trade-offs are central to LHS diversity | Growth vs reproduction Now vs later |
| Cost of reproduction: evidence | 1) Female Anolis sagrei with ovaries removed (OVX) grew bigger than controls (sham) (SVL= snout to vent length) 2) OVX females also lived longer than the Sham females which were still producing eggs |
| Cost of reproduction: evidence | Selection of populations of Drosophila for age at reproduction Partridge et al. (1999) • Those selected to reproduce when old - had lower mortality rates (Fig A) • However, they also had lower egg production (especially when young) (Fig B) |
| Evolution of senescence and life span | Senescence – accelerated physiological degeneration with age - increased likelihood of death (actuarial senescence) |
| Factors | 1) Antagonistic pleiotropy - Williams (1957) Because of greater contribution of earlier age classes to fitness, alleles that provide an advantage in early life have a selective advantage… even if deleterious later in life e.g. alleles for reproduction |
| Factors | 2) Accumulation of deleterious mutations - Medewar (1952) Deleterious mutations that only affect later age classes accumulate in populations because selection against them is weak |
| Why is selection weaker for older age classes? | – better to reproduce earlier – extrinsic mortality mean less individuals survive/breed at older ages – Sometimes referred to as the selection shadow Senescence / lifespan arises partly due to selection for earlier reproduction |
| Age schedules of reproduction | – high extrinsic mortality on adults will select for high reproductive effort early in life – low extrinsic mortality may select for delayed maturation and later reproduction - especially if fecundity increases disproportionately with size |
| Age schedules of reproduction | Semelparity – reproduce once and die Iteroparity – reproduce repeatedly |
| Age schedules of reproduction | Semelparous advantageous if: • Survival increases with body size • Exponential relationship between body mass and reproduction • If reproduction is very stressful or risky Grow to maturity rapidly – then invest all in one reproductive attempt |
| Iteroparous advantageous if: | • Increases chance of success in fluctuating environments • If adult mortality (especially linked to breeding) is low • If greater fecundity achieved by saving some resources and deferring some reproduction |
| Number and size of offspring | Reproductive effort invested in: • many small offspring --------------------------- few large offspring • All else being equal, genotype with higher fecundity will have higher fitness than one with lower fecundity |
| Optimal number of offspring | • Maximises the number of surviving (recruited) offspring • Increasing or decreasing the number of offspring reduces parents fitness |
| Trade-off between the number and size/resources of offspring | • Finite resources to invest egg/seed/embryo/young |
| Evolution of the rate of increase | = Per capita rate of increase of a genotype – measure of fitness Intrinsic rate of increase (rm) – no density dependent (competition) effects on birth/death rates |
| Evolution of the rate of increase | Instantaneous rate of increase (r) – actual rate – reduced by density dependence/competition Different genotypes have different rates of increase at different densities |
| Evolution of the rate of increase | Rate of increase for genotypes A & B - where B invests more per offspring, but with less offspring (less fecund) B has lower rate of increase at low density (lower intrinisic rate = rm,B) |
| Evolution of the rate of increase | but a selective advantage (higher rate of increase) at higher density (nearing carrying capacity = KB) At high density, dead individuals are more frequently replaced by B rather than A individuals |
| r- and k-selected strategies | relate to the selection of traits that allow success in particular environments |
| r - selected organisms | In unstable or unpredictable environments with low density dependence, r-selection predominates - the ability to reproduce Characteristic traits include high fecundity, small size, short generation time, and the ability to disperse offspring widely |
| K - selected organisms | • In stable environments subject to strong density dependence, K-selection predominates - ability to compete for limited resources is crucial • Tend to have long life span, and to produce fewer, well cared for offspring |
| Disadvantages of sex | • Halves the potential reproductive rate • Means you share your genetic reproduction • Break up co-adapted gene complexes • Cost of finding mates , copulating etc |
| Advantages of sex | Half genome transmitted from each parent with recombination • Stops accumulation of deleterious mutations Fisher-Muller hypothesis • Generates offspring variability to combat rapidly co-adapting pathogens - Red Queen hypothesis |
| Effects of sexual selection on life history | Differs between the sexes • Anisogamy - difference in investment in sperm and eggs • Females = limiting sex, males = limited sex • Selection on males to compete... And females to choose |
| Male reproductive success - intense mate competition | • Direct competition between males • Mate choice by females • Leads to different LHS between sexes • E.g. sexual dimorphism |
| Effects of sexual selection on life history | In competition for mates: Males that are larger or invest more energy in competition for females are usually more successful |
| Variant life history strategies within a sex | Alternative mating tactics in males Adopted by males that differ in size or morphological characteristics e.g. territorial males and sneaker (or satellite) males • Making the best of a bad job? • Equally effective alternative strategy? |
| Variant life history strategies within a sex | Sex change – sequential hermaphroditism Female to male – protogyny Male to female – protandry Associated with changes in the relative reproductive success of the two sexes as an individual gets bigger |
| Sex change – sequential hermaphroditism | 2 pathways by which terminal-phase males develop in the bluehead wrasse, Thalassoma fasciatum Sex change – sequential hermaphroditism A terminal-phase male bluehead wrasse (top) and a female (bottom) Initial phase males resemble females |
| From theory natural selection should favour individuals with certain traits | Reproduce early Invest all in reproduction Produce lots of offspring |
| This is often not the case because of different trade-offs due to the ecology of different species | Density dependence and competition Extrinsic mortality Constraints |
Created by:
rose.coo
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