This lecture What is evolution, and how does it work? Why might evolution matter for species’ responses to climate change? How can we predict whether evolution will occur?

What is Evolution? Any change across successive generations in the genetic makeup of biological populations.

What is Evolution? Any change across successive generations in the genetic makeup of biological populations. Biston betularia Another common misconception about evolution is that it always goes in one direction.

Darwin’s Logic Fact 1: Every species would increase exponentially if all offspring survived. Fact 2: Most populations are relatively stable. Fact 3: Natural resources are limited.

Darwin’s Logic Fact 1: Every species would increase exponentially if all offspring survived. Fact 2: Most populations are relatively stable. Fact 3: Natural resources are limited. Inference 1: Since more individuals are produced than can be supported by the available resources, there must be a ‘struggle for existence.’ Inference 2: Survival is not random, but depends in part on the hereditary constitution of surviving individuals. This unequal survival constitutes the process of natural selection

Darwin’s Logic Fact 1: Every species would increase exponentially if all offspring survived. Fact 2: Most populations are relatively stable. Fact 3: Natural resources are limited. Inference 1: Since more individuals are produced than can be supported by the available resources, there must be a ‘struggle for existence.’ Inference 2: Survival is not random, but depends in part on the hereditary constitution of surviving individuals. This unequal survival constitutes the process of natural selection Fact 4: No two individuals are exactly the same; populations have enormous variability Fact 5: Much of this variation is heritable.

Darwin’s Logic Fact 1: Every species would increase exponentially if all offspring survived. Fact 2: Most populations are relatively stable. Fact 3: Natural resources are limited. Inference 1: Since more individuals are produced than can be supported by the available resources, there must be a ‘struggle for existence.’ Inference 2: Survival is not random, but depends in part on the hereditary constitution of surviving individuals. This unequal survival constitutes the process of natural selection Fact 4: No two individuals are exactly the same; populations have enormous variability Fact 5: Much of this variation is heritable. Inference 3: Over the generations, the process of natural selection will lead to a continuing gradual change of populations, that is, to evolution.

Demographic effects of adaptation: a thought experiment So how could adaptation buffer the demographic effects of environmental change? Let’s start with a thought experiment. Imagine, first, that we have a population with no variation. A population of clones. This population has a carrying capacity of 100, and an intrinsic growth rate of 20% N t+1=Nt+0.2*Nt(100-Nt/100) generation 1

New stressor kills 50% per generation Now a new stressor comes along, say, a change in temperature. And this new stressor kills 50% of the population every generation. Because the mortality rate exceeds the intrinsic growth rate, the population will begin to decline. N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 2 generation 1

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 3 generation 2

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 4 generation 3

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 5 generation 4

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 6 generation 5

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 7 generation 6

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 7 generation 7

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 8 generation 8

New stressor kills 50% per generation N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 9 generation 9

N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 10 generation 10 Under these circumstances, it will take this population about 10 generations to reach extinction N t+1=Nt+0.2*Nt(100-Nt/100) – 0.5Nt generation 10 generation 10

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N Without genetic variation With genetic variation With variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Now lets compare that scenario to the scenario where there is variation. On top I’m showing you the same population of clones, on the bottom a population with the same demographic parameters, but where 10% of the individuals are resistant to the new stress. Now if we play these two scenarios out over 10 generations…. resistant susceptible generation 1

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation generation 2

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation We see that both populations initially decline generation 3

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation generation 4

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation generation 5

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation generation 6

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation But after a while the second population begins to recover generation 7

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation generation 8

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation And has regained 50% of the original size while the first population declined to extinction. generation 9

What if there is genetic variation? Without variation N t+1=Nt+0.2*Nt(k-Nt/k)-0.5N With N t+1=Nt+0.2*Nt(k-Nt/k)-0.5NS Without genetic variation With genetic variation What I’d like to point out about these two scenarios is that we cannot look out into a population and “see” genetic variation with our naked eyes. And the majority of the biological consequences of climate change has been taking place on this part of the curve. We see that a new stressor kills 50% of the population, reduces fecundity by 10%, etc. and we simply project these effects into the future. These projections ignore the capacity for adaptation, but at the same time we know that the capacity for adaptation may radically alter the demographic effects of environmental change. generation 10

Evolutionary rescue occurs when a population survives because of evolution, when it would otherwise have gone extinct A central goal of this class is to understand how, and under what circumstances evolutionary rescue could occur in the context of climate change

What is Evolution? Any change across successive generations in the genetic makeup of biological populations. Year one Drift Natural selection Year two

Natural Selection Natural selection – individuals with certain trait values will leave more descendents than others. Differential reproduction or survival causes some genotypes to increase in frequency, others to decrease Evolution is a change in the genetic makeup of a population, but natural selection acts on phenotypes.