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How to find optimal offspring size

Multiply number by survival probability for each size


Pros/cons of one vs many bouts of reproduction

One is good because all resources can be put in at once
Many is good because more offspring are likely to survive


Pros/cons of early vs late start of reproduction

Early can produce more offspring and individuals have a lower risk of dying before reproduction
Delaying allows time for individual to grow


Take home messages on life history strategies

All strategies involve tradeoffs
We expect strategies to evolve that optimize AVERAGE lifetime fitness
Optimal strategies vary across species and even populations



A group of organisms of the same species living in a particular area


Characteristics of populations

Age structure
Survivorship curve



Population size
Can be estimated by mark and recapture studies


Some sources of error for mark and recapture studies

Trap shyness
Tag affects survival
Assumes no changes in population size (no reproduction or death)
No migration


Replacement level fertility

2.1 children/family


Type 1/ Convex curve

Common in organisms that produce few offspring, but take good care of them
Humans, elephants, whales, etc.


Type II/ Constant curve

Common in organisms with relative constant change of death throughout life
Rodents, songbirds, hydra


Type III/ Concave curve

Many die early in life but those who survive usually live long
Lots of offspring but little care
Many plants, turtles, tadpoles


What can be done to slow population growth?

Education to delay age of first reproduction


How do scientists identify genes that are (or were recently) under selection?

As a favored version of a gene becomes more common in a population, genomes will look increasingly alike in and around the gene. Because variation is brushed away, the favored gene’s rise in popularity is called a sweep


Why did the scientists find more evidence of recent selection on genes in populations in Europe and Asia than in Africa?

One could argue that we are adapted to the African environment and that most signals are seen when people adapt to new environments
The people who left Africa were then forced to adapt to different environments.


Why is there greater genetic variation among people living in Africa than among people living in other parts of the world?

Diversity decreases steadily the further a population has migrated from the African homeland, since each group that moved onward carried away only some of the diversity of its parent population.


According to the article, is light colored skin homologous or analogous between people of European and East Asian ancestry? Why?

Analogous- 2 different genes contribute to light skin color in each population
Presumably, different mutations were available in each population for natural selection to work on. The fact that the two populations took independent paths toward developing lighter skin suggests that there was not much gene flow between them.


What is the difference between a hard sweep and a soft sweep? We talked in the past about selection on new mutations vs selection on standing genetic variation. Which scenario is associated with a hard sweep vs. a soft sweep?

Hard sweeps= substantial sweeps, with a new version of a gene being present in a large percentage of the population. These hard sweeps are often assumed to start from a new mutation. (Less common)

Soft sweeps= work on traits affected by many genes, like height. (more common) act on standing genetic variation


The title of the article makes reference to 'very recent evolution.' Based on the examples in the article, what time-scale is considered 'very recent' in the context of human evolution - tens, hundreds, thousands, or millions of years?

3,000 years ago, if confirmed, would be the most recent known instance of human evolution. (monks evolving genes to better withstand low oxygen levels)


People can be short either because they grow slowly or because they stop growing at an earlier age than others. Which explanation for short stature do the authors derive from the data on Pygmy populations?

The authors conclude that Pygmy populations stop growing at an earlier age.


What is the life-history trade-off that the authors postulate has resulted in short stature in these populations?

There is a life history tradeoff between extended growth and early reproduction, with the balance pending toward the latter because of their exceptionally high mortality rates.


The authors point out that selection isn't favoring short stature per se. What life history trait does selection favor in these populations? Why is it favored?

Selection favors earlier onset of reproduction, because the earlier one starts reproducing, the more likely they are to pass on genes to subsequent generations because they have a greater opportunity to have more children.


What are the characteristics of stages 1-4 of the model in terms of birth rates, death rates, and changes in population size? Make sure you understand how these characteristics are represented in the top figure, how they change over time, and how a population at the start of stage 1 differs from a population at the end of stage 4.

Stage 1: Pre-modern; balance between birth and death rates; very slow population growth (High Stationary Stage)

Stage 2: Urbanizing/Industrializing; Birth rate stays high with declining death rate; increasingly rapid rise in population growth, young age structure

Stage 3: Mature Industrial; decline in birth rate, population growth

Stage 4: Post Industrial; Stability; no population change; population age structure has become older, population decline may set in quickly


Look at the third figure, illustrating survivorship curves for different countries at different times. Compare the survivorship curves of 17th century England with modern Great Britain. Are you surprised by the difference? What are the factors that have caused this change?

17th century England shows a Type III (Concave) curve while Modern GB shows a Type I (Convex) curve. In 17th century England, there were very high child mortality rates. So few females lived to reproduction, so only high fertility rates could sustain the population.


Why do populations start growing in size during stage two?

There are declines in death rates. Due initially to 2 factors: 1) improvements in food supply from greater agricultural yield (crop rotation, selective breeding, seed drill technology) and 2) significant improvements in public health that greatly reduced childhood mortality (improvements in water supply, sewage, food handling, personal hygiene, etc.)


What caused the decrease in deaths from scurvy in the 20th century?

General improvements in human well-being, an increase in public health awareness, and a decline in poverty.


Why is there typically a time lag between the decline of the death rate and the decline in the birth rate as societies move through the demographic transition?

Continued decline in childhood death means that at some point parents realize they need not require so many children to be born to ensure a comfortable old age


Why do populations continue to grow in size for a period of time after birth rates have fallen to replacement level fertility?

Population momentum- tendency for population growth to continue beyond the time that replacement-level fertility has been achieved because of a relatively high concentration of people in the childbearing years. Population momentum occurs towards the end of Stage Three


According to the population clock, what is the current world population, rounded to two significant digits?



What two countries have the highest population? Where does the US rank?

1. China
2. India
3. U.S.